A - Z of Powder Metallurgy 1856174298, 9781856174299

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Foreword The idea of a quick, alphabetical guide to powder metallurgy has appealed to me since my earliest teaching days. I was infatuated with the concentrated information available in Henry Hausner뭩 Handbook of Powder Metallurgy. Subsequently, larger handbooks arrived, such as those from ASM International, but they were subdivided into sections that might be 10 to 20 pages of details. Moreover, these handbooks were organized along the logic of powders-processes-properties-applications; thank heavens for the index, since that is about the only way to find a tidbit of needed information. In many instances we just want to refresh our memory or check some facts or find a useful relation. Along those lines, one of my favorite final exam questions is for the student to demonstrate expertise on a subject on one page; for example what is HIP, how is it used, what is a typical cycle, and so on. Most students complain because they are restricted to a single page and cannot ramble in an unstructured way. Ken Blanchard and Spencer Johnson provided another model in their famous One Minute Manager. I have long felt we need the One Minute Powder Metallurgist. So when I was approached by Bernard Williams about a book to be called the A to Z of Powder Metallurgy, the idea resonated with my interest as a user, teacher, and contributor. It was hard work, enlightening, and hopefully provides a good balance between definitions and background needed as a starting point. Much effort was put into including all of the terms you might see in a class, textbook, or conference. The text has been reviewed repeatedly to ensure it is an accurate reflection of the state of affairs and the language we use in powder metallurgy. Help on this project came from many sources, starting with Bernard Williams who suggested the topic and arranged the contract with Elsevier. Important text editing was provided by Susan Ferchalk. The line drawings were generated by Rick Sharbaugh while the microscopic images were largely provided by Louis Campbell, two outstanding individuals. Several students and staff helped on supplying reviews and proofing draft manuscripts. Of course the many prior publications listed in the references were most helpful. The book is dedicated to my upbeat partners, Jessie and Sara - male and female, brother and sister, black (tricolor) and white (blue merle) - two Aussies.

Symbols

Page 1

A to Z of Powder Metallurgy

Symbols These are some of the common symbols used in powder metallurgy. There is no accepted standard, but these tend to be the more popular nomenclature and definitions for the mathematical symbols. Also shown are the typical units. In some cases the units depend on the context. For example, density can be commonly expressed in g/cm3, but is often given as a percent or fraction of theoretical, and in true SI units should be in kg/m3 or Mg/m3.

lower case a = crack size, m a = lattice constant, nm b = Burgers vector, nm c = concentration, wt. % d = diameter, m dP = diameter of a pore, 탆 ef = elongation to fracture, % f = fraction of a phase f = fractional packing density g = geometric constant g = gravitational acceleration, m/s2 h = height, m i = current, A i = intensity k = Boltzmann's constant = 1.381@10-23 J/K l = length, m m = mass, kg m = Herring scaling law exponent (1, 2, 3, or 4) m = Weibull modulus n = counting number n = work hardening exponent n = sintering neck growth exponent (2, 3, 4, 5, 6, or 7) p = neck filet radius, 탆

Symbols

Page 2

q = probability r = radius, m t = time, s u = friction coefficient v = velocity, m/s w = width, m x = coordinate y = coordinate z = coordinate z = friction factor upper case A = area, m2 AR = aspect ratio Aw = atomic weight, g/mol B = kinetic terms in sintering, various units and subscripts possible B = magnetic response, T B = X-ray peak broadening, deg BM = maximum magnetization, T BR = remanent magnetization, T C = concentration, various units such as kg/m3 Ci = cost of operation i, $/h CR = compression ratio CSS = solid-solid contiguity Cg = connectivity or contacts per grain in two dimensions CP = heat capacity, J/(EC kg) CP = process capability CPk = process control capability CR = coercive force, A/m CS = segregation coefficient CT = coefficient of thermal expansion, ppm/EC CV = coefficient of variation, % D = particle size, 탆 D10 = particle size corresponding to 10 % point on the cumulative size distribution D50 = particle size corresponding to 50 % point on the cumulative size distribution D90 = particle size corresponding to 90 % point on the cumulative size distribution DA = equivalent spherical particle diameter based on projected area, 탆 DS = equivalent spherical particle diameter based on surface area, 탆 DV = equivalent spherical particle diameter based on volume, 탆 Di = diffusivity for atomic motion by process i (i = V, GB, SD for volume, grain boundary, and surface), typical units of m2/s E = energy, J

Symbols E = elastic modulus or Young뭩 modulus, GPa F = force, N F = free energy, J/mol FB = breaking or fracture load, N G = grain size, 탆 G = Gibbs free energy, J/mol H = heat, J H = enthalpy, J/mol H = hardness, various units H = magnetic field strength, T H = tool height, fill depth, or size, m I = nucleation frequency, 1/(m3 s) I = impact energy, J/cm2 J = flux, various units such as mol/(s m2) K = equilibrium constant K = grain growth rate constant, 탆 3/s K = strength factor for cylindrical bearings, MPa KIc = fracture toughness, MPa(m)½ L = macroscopic length, m M = magnification M = mixture homogeneity M = molecular weight, g/mol N = number No = Avagadro뭩 number, 6.02A1023 Nc = coordination number in three dimensions P = pressure, Pa or MPa Pg = gas pressure, Pa or MPa Q = activation energy, kJ/mol Q = flow rate, m3/s Q = heat flow, W/m2 R = extrusion area ratio R = gas constant, 8.31 J/(K mol) R = ratio of maximum to minimum stress in fatigue R = resistance, Ω RN = Reynolds number S = saturation S = specific surface area, m2/g S = solubility, various units such as kg/kg or kg/mol S = entropy, J/K SW = particle size distribution width T = temperature, usually EC or K T = tortuosity

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Symbols U = shear modulus, GPa V = velocity, m/s V = volume, m3 V = voltage, V VS = volume fraction of solid VL = volume fraction of liquid VP = fractional porosity W = weight, kg W = work, J WN = Weber number X = depth or distance, m X = neck diameter, 탆 Xm = absorption capacity Y = shrinkage, % or fractional Z = atomic number Z = relative ductility Greek symbols α = permeability, m2 α = thermal expansion coefficient, 1/EC or 10-6/EC or ppm/EC or ppm/K β = inertial flow resistance, m β = fraction transformed γ = surface energy (subscripts identify the two phases - SS, SL, SV, LV), J/m2 γ = shear strain δ = grain boundary width, m δ = tolerance ε = strain εt = true strain ζ = angle of repose η = viscosity, PaAs θ = contact or wetting angle, degree θ = X-ray diffraction angle, degree κ = thermal conductivity, W/(m EC) λ = mean free spacing, 탆 λ = secondary dendrite arm spacing, 탆 λ = wavelength, m μ = magnetic permeability ν = Poisson's ratio ξ= geometric term π = 3.14159 ρ = density, fractional or g/cm3 or Mg/m3 ρA = apparent density, g/cm3

Page 4

Symbols ρF = fluid density, g/cm3 ρM = metal density, g/cm3 ρT = tap density, g/cm3 σ = standard deviation, units depend on parameter being measured σ = stress, MPa σC = compressive strength, MPa σF = fatigue endurance strength, MPa σY = yield strength, MPa σRCS = radial crush strength, MPa σTRS = transverse rupture strength, MPa σU = ultimate strength, MPa τ = shear stress, MPa φ = dihedral angle, degree Φ = fractional solids loading Ψ = complexity Ψ = densification, % ω = frequency, 1/s Γ = time constant, s Θ = measured angle, degree Ω = atomic volume, m3/atom

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Page 1

Section Numbered Items

A to Z of Powder Metallurgy

Numbered Items These are numbered alloys. The numbering systems are not uniform by region or technology. However, in the common powder metallurgy literature and practice, these are some of the most frequently encountered numerical designations. In some cases they might have prefix designations such as AISI, ASTM, P/F, MIM, CP, or IN relating to the technology or organization, and in some instances suffixes are used to indicate alloy modifications such as L, LN, AB, or ULC.

17-4 A precipitation hardenable stainless steel with excellent strength and hardness, and moderate corrosion resistance; the alloy is also known as AISI 630 stainless steel. The nominal composition in powder metallurgy, most typically formed by injection molding, is iron with 17 wt. % chromium, 4 wt. % nickel, 4 wt. % copper, and 0.3 wt. % of combined niobium and tantalum. The hardness of the material makes it too hard for die compaction, so consolidation is usually by injection molding or hot isostatic pressing. Best properties are achieved after sintering by subjecting the material to an aging heat treatment, usually denoted as Hxxxx, where xxxx gives the aging temperature in degrees Fahrenheit (900EF would be indicated by H900). Typical as-sintered properties are as follows: 17-4 PH stainless density

7.84

g/cm3

melting onset temperature

1340

EC

heat capacity

460

J/(kg EC)

thermal expansion coefficient

10.8

ppm/EC

thermal conductivity

18

W/(m EC)

electrical resistivity

80

µΩ-cm

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Section Numbered Items elastic modulus

197

Poisson뭩 ratio

0.29

hardness

>28

HRC

yield strength

900

MPa

elongation to fracture

10

%

1100

MPa

ultimate tensile strength

GPa

fracture toughness 23 MPa(m)½ Most commonly the heat treatment is tailored for high hardness, up to 38 HRC, with a concomitant increase in yield strength, ultimate tensile strength, but lower ductility.

18-8 The common commercial designation for 304 or 304L stainless steel - indicating 18 wt. % chromium and 8 wt. % nickel content. [see 304, 316]

201 An aluminum alloy custom designed for the powder metallurgy process consisting of aluminum with 4.4 wt. % copper, 0.8 wt. % silicon, 0.5 wt. % magnesium, and up to 0.3 wt. % iron. It can be die pressed and sintered with a post-sintering heat treatment to attain a yield strength of 328 MPa, tensile strength of 335 MPa, but just 2 % elongation.

300 A series of stainless steels that are austenitic (face-centered cubic crystal structure), nonmagnetic, with more than 16 wt. % chromium and 8 wt. % nickel. All of the alloys used in powder metallurgy have low carbon levels to avoid precipitation of grain boundary carbides and carry the L designation, requiring below 300 ppm in the final product.

303 A nonmagnetic austenitic stainless steel with a nominal composition of iron with 18 % chromium, 8 to 13 % wt. % nickel, up to 2 wt. % manganese, and intentional nitrogen alloying (0.2 to 0.6 wt. %). It has good strength and hardness in combination with acceptable corrosion resistance. It is mostly used for powder metallurgy components that are machined after sintering.

304 An austenitic stainless steel with a nominal composition of iron with 18 to 20 wt. %

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Section Numbered Items

chromium, and 8 to 12 wt. % nickel; sometimes simply called 18-8 stainless. In powder metallurgy formulations it usually contains up to 2 wt. % manganese and up to 1 wt. % silicon. With the L suffix there is no more than 300 ppm of carbon allowed in the final product. Like other 300-series of powder metallurgy stainless steels, it is nonmagnetic when properly processed, but the low alloying levels often result in some magnetic phases. The alloying levels are lower than used in 316L, resulting in slightly lower corrosion resistance. When sintered to full density it has mechanical properties as tabulated here: 304L stainless steel 8.0

g/cm3

melting onset temperature

1400

EC

heat capacity

500

J/(kg EC)

thermal expansion coefficient

17.2

ppm/EC

thermal conductivity

16.2

W/(m EC)

electrical resistivity

72

µΩ-cm

elastic modulus

193

GPa

Poisson뭩 ratio

0.29

density

hardness

99

BHN

yield strength

220

MPa

elongation to fracture

57

%

ultimate tensile strength 590 MPa In many powder metallurgy applications sintered stainless is porous with significantly lower properties. In some cases, such as restrictors and filters, controlled porosity is essential and the device should be designed to avoid tensile loading. In other cases, the sintering conditions are not pushed to the point where full density is realized and the mechanical properties are degraded by the residual pores. For example, at a sintered density of 6.9 g/cm3 the yield strength for 304L is 180 MPa and at 6.6 g/cm3 the yield strength is 120 MPa. Other properties such as ductility and corrosion resistance are even more dramatically degraded by pores.

316 This is a nonmagnetic austenitic stainless steel, nominally iron with 16 to 18 wt. % chromium, 10 to 14 wt. % nickel, 2 to 3 wt. % molybdenum, up to 2 wt. % manganese and up to 1 wt. % silicon. Almost always in powder metallurgy the L grade is used,

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Section Numbered Items

implying no more than 300 ppm of carbon. The powder metallurgy composition differs slightly from that of wrought alloy and the properties are significantly different, since wrought 316L is deformed to increase strength while sintering results in an annealed material with low strength. This alloy is one of the most popular stainless steels and is a favorite with both the press-sinter and injection molding routes. When sintered to full density the material has the following properties: 316L stainless steel density

8.05

g/cm3

melting onset temperature

1400

EC

heat capacity

500

J/(kg EC)

thermal expansion coefficient

15.9

ppm/EC

thermal conductivity

16

W/(m EC)

electrical resistivity

74

µΩ-cm

elastic modulus

193

GPa

Poisson뭩 ratio

0.29

hardness

91

BHN

yield strength

255

MPa

elongation to fracture

55

%

ultimate tensile strength 530 MPa This stainless steel has slightly better corrosion resistance versus sintered 303L or 304L, and is much better in pitting corrosion resistance due to the molybdenum alloying. It is the first choice for general purpose corrosion applications. Atmosphere reactions during sintering degrade the properties, especially if the atmosphere reacts with the chromium to form a compound. Further, pores degrade all of the properties. For example after press and sinter processing to a density of 6.6 g/cm3, the yield strength is 140 MPa with less than 20 % elongation to fracture.

400 The series of stainless steels that are ferritic, or possibly martensitic if they contain carbon and are rapidly cooled; these alloys are mostly iron with at least 12 wt. % chromium. They are magnetic. Common powder metallurgy grades tend to be low in carbon as designated by an L in the composition.

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Section Numbered Items

409 A magnetic ferrous alloy that nearly qualifies as a stainless steel, nominally consisting of iron with 11 wt. % chromium, up to 1 wt. % manganese, up to 1 wt. % silicon, and small alloying levels of niobium as a carbide scavenger. Almost always the L suffix is used to designate the maximum carbon level must be below 300 ppm (0.03 wt. % carbon). The chromium content provides corrosion resistance, but it is below the nominal 12 wt. % level needed to be classified as a stainless steel. Modified versions of this alloy reach up to 13.5 wt. % chromium and up to 0.5 wt. % nickel, thereby satisfying the criteria for a stainless steel.

410 A magnetic stainless steel based on iron with 11.5 to 13.5 wt. % chromium, up to 1 wt. % manganese, up to 1 wt. % silicon, up to 0.25 wt. % carbon, and 0.2 to 0.6 wt. % nitrogen. This is one of the few stainless steels used in powder metallurgy that is not a low carbon variety. However, if the 410 is followed by an L, then the maximum carbon level must be below 300 ppm (0.03 wt. %). When processed to full density and heat treated, the mechanical properties show a high hardness (260 VHN) corresponding to a high tensile strength of 800 MPa and 27 % elongation to fracture. The thermal expansion coefficient is 9.9 ppm/EC and the thermal conductivity is 25 W/(m EC). At lower sintered densities, such as 6.9 g/cm3, the yield strength is just 180 MPa with 16 % elongation to fracture.

420 A heat treatable martensitic stainless steel that relies on carbon levels up to 1 wt. % for strengthening (and might include a suffix of C to indicate the desire for carbon). The nominal composition is iron with 12 to 14 wt. % chromium, and up to 1 wt. % each of manganese and silicon. Because of the high hardness, it is mostly processed using powder injection molding, hot isostatic pressing, or high temperature sintering. At full density in the tempered condition this stainless steel has the following properties: 420 stainless steel 7.7

g/cm3

melting onset range

1450

EC

heat capacity

475

J/(kg EC)

thermal expansion coefficient

11

ppm/EC

thermal conductivity

55

W/(m EC)

electrical resistivity

74

µΩ-cm

density

Page 6

Section Numbered Items elastic modulus

200

Poisson뭩 ratio

0.28

hardness

570

VHN

yield strength

1500

MPa

8

%

ultimate tensile strength

1700

MPa

fatigue endurance limit

600

MPa

elongation to fracture

GPa

430 A ferritic stainless steel, nominally iron with 16 to 18 wt. % chromium, up to 1.5 wt. % manganese, and up to 1 wt. % silicon. It is magnetic with reasonable corrosion resistance. When the L suffix is affixed (430L) the carbon level must be held to less than 0.04 wt. %. One version is designated 430N2 to indicate the use of nitrogen for strengthening, requiring from 0.2 to 0.6 wt. % nitrogen. Like all powder metallurgy alloys, porosity degrades properties. At a sintered density of 7.1 g/cm3 the yield strength is 210 MPa, ultimate tensile strength is 340 MPa, and elongation to fracture is 20 %.

434 A magnetic stainless steel with some corrosion resistance, nominally based on iron with 16 to 18 wt. % chromium, about 1 wt. % molybdenum, up to 1 wt. % manganese, and up to 1 wt. % silicon. When used as a suffix, the letter L designates a low carbon level at less than 0.03 wt. %. One version is designated 434N2 to indicate the use of nitrogen for strengthening, requiring from 0.2 to 0.6 wt. % nitrogen. At a sintered density of 7.0 g/cm3 the yield strength is 240 MPa, ultimate tensile strength is 410 MPa, and elongation to fracture is 5 %.

440 Another of the few stainless steels that contains carbon, which is usually indicated by a C suffix. This alloy consists of iron with 16 to 18 wt. % chromium, 1 wt. % manganese, 1 wt. % silicon, 0.75 wt. % molybdenum, and from 1.0 to 1.2 wt. % carbon. When sintered to a closed pore condition at 96 % density and 43 HRC hardness, the material has a yield strength of 410 MPa, ultimate tensile strength of 620 MPa, and at least 2 % fracture elongation.

600 A common Inconel alloy consisting of nickel with approximately 16 wt. % chromium, 7 wt. % iron, and small contents of manganese, silicon, and copper. It has a theoretical

Page 7

Section Numbered Items

density of 8.48 g/cm3, melting onset temperature of 1354EC, room temperature elastic modulus of 214 GPa, and tensile strength of 550 MPa. The 600 alloy is often used for retorts and furnace hardware and even belts in powder metallurgy sintering furnaces.

601 An aluminum alloy custom designed for the powder metallurgy process consisting of aluminum with 1 wt. % magnesium, 0.6 wt. % silicon, 0.25 wt. % copper, and up to 0.3 wt. % iron. It can be formed using mixed powders. When compacted and sintered to 95 % density, this alloy delivers a tensile strength after heat treatment up to 255 MPa. Such a low strength is no longer attractive, since plastics deliver superior strength to weight at a lower price; thus, the alloy is used infrequently.

630 The AISI (American Iron and Steel Institute) formal designation for 17-4 PH stainless steel. [see 17-4 PH]

718 A nickel-base superalloy developed for demanding applications such as in aircraft engines. The nominal composition is nickel with 19 wt. % chromium, 18 wt. % iron, 5 wt. % niobium, 3 wt. % molybdenum, 1 wt. % titanium, and 0.5 wt. % aluminum. It is used in the hot isostatic pressed form, but might first be formed by powder injection molding. The properties of the fully dense and heat treated alloy are as follows: 718 superalloy density

8.19

g/cm3

melting onset temperature

1250

EC

heat capacity

427

J/(kg EC)

thermal expansion coefficient

13

ppm/EC

thermal conductivity

11

W/(m EC)

electrical resistivity

125

µΩ-cm

elastic modulus

205

GPa

Poisson뭩 ratio

0.285

hardness yield strength

404

VHN

1190

MPa

Page 8

Section Numbered Items elongation to fracture

21

%

ultimate tensile strength

1430

MPa

fatigue endurance limit

465

MPa

impact toughness

25

J

1020 A simple steel consisting of iron and 0.2 wt. % carbon. The last two digits indicate the carbon level, and higher carbon contents result in designations of 1040, 1060, or 1080 corresponding to 0.4, 0.6, or 0.8 wt. % carbon. It is generally formed from mixed iron and graphite powders, which are pressed and sintered to form a porous structure. The low carbon and residual porosity degrade mechanical properties in comparison with wrought material.

2200 A designation given to ferrous alloys with about 2 wt. % nickel, low carbon contents (below 0.1 wt. %), and up to 1 wt. % silicon. It is often formed by powder injection molding to provide good soft magnetic properties due to the low carbon.

2700 An iron alloy containing nominally 7 wt. % nickel, up to 0.5 wt. % molybdenum, and less than 0.1 wt. % carbon. In addition it might contain up to 1 wt. % silicon. This low alloy steel is often used because of its magnetic properties or in applications where only a hard case is desired, attained after sintering by case carburization. It can be formed by both powder injection molding and die compaction.

4205 A low alloy steel consisting of an iron matrix with about 0.4 wt. % nickel, 0.7 wt. % molybdenum, and 0.5 wt. % carbon. In some designations it is termed 4250.

4220 A group of alloys used in powder forging that nominally consist of 0.45 wt. % nickel, 0.6 wt. % molybdenum, 0.3 wt. % manganese, and less than 0.15 wt. % copper and 0.1 wt. % chromium. The carbon level is adjustable, and the last two digits can be changed to reflect the carbon content, and might be 40 (4240) or 60 (4260) for higher carbon levels. To indicate the fabrication by powder forging the full designation might be P/F-4220.

4340 A low alloy steel similar to many of the heat treated powder metallurgy alloys, containing

Section Numbered Items

Page 9

1.8 wt. % nickel, 0.8 wt. % chromium, 0.7 wt. % manganese, 0.3 wt. % molybdenum, and 0.4 wt. % carbon. At full density it has a yield strength of 860 MPa and ultimate tensile strength of 1280 MPa with 12 % elongation to fracture.

4405 A sintered steel based on iron with about 0.8 wt. % molybdenum, and carbon specified at 0.5 wt. %. It is not heat treated, so the sintered mechanical properties simply depend on sintered density, with yield strength ranging from 290 MPa at 6.9 g/cm3 to 400 MPa at 7.3 g/cm3. Like other alloys in this series, the former designation is still also used, that being 4450 to reflect the carbon content.

4408 A sinter-hardenable grade of steel used in powder metallurgy, it contains mostly iron with about 2 wt. % nickel and 0.8 wt. % molybdenum, and carbon specified at 0.8 wt. %. Depending on the powder production process and other details, it might have a prefix such as FLN4, so the full designation would be FLN4-4408. The prefix indicates possible copper and nickel additions to the basic chemistry.

4500 A steel with almost no carbon or other alloying except for 0.4 to 0.5 wt. % phosphorous. Also, sometimes known as 45P to indicate the 0.45 wt. % phosphorous level. It is used for soft magnetic applications. Often combined with a prefix of FY and designated as FY-4500.

4605 This alloy formerly was designated 4650, but there was confusion on the actual carbon content. It is an iron alloy with approximately 2 wt. % nickel, from 0.2 to 0.5 wt. % molybdenum, 0.4 to 0.6 wt. % carbon, and up to 1 wt. % silicon. It is a general purpose steel that can be heat treated or case carburized to tailor properties and is mostly selected for structural applications. It can be formed by any of several powder metallurgy routes. Mechanical properties improve with a higher sintered density and post-sintering heat treatments. For example, the sintered yield strength at 6.75 g/cm3 is 290 MPa with almost no measurable ductility. But when sintered to 7.2 g/cm3 and subjected to a post-sintering heat treatment, then the yield strength is more than 1000 MPa, but there still is no significant ductility.

4608 A sinter-hardenable grade of steel used in powder metallurgy that contains mostly iron with about 4 wt. % nickel and up to 1.1 wt. % molybdenum, and carbon specified at 0.8 wt. %. Depending on the powder production process and other details, it might have a prefix such as FLN, so the full designation would be FLN-4608. An alternative might be

Page 10

Section Numbered Items FLC-4608 to indicate added copper in the 1 to 3 wt. % range.

4640 A common heat treatable steel consisting of 2 wt. % nickel, 0.3 wt. % manganese, 0.5 wt. % molybdenum, and 0.4 wt. % carbon. In the full density condition the material has excellent properties, but these properties are degraded by porosity as is commonly found in a powder metallurgy product. The full-density product is characterized by the following properties: 4640 steel density

7.86

g/cm3

melting onset temperature

1410

EC

heat capacity

450

J/(kg EC)

thermal expansion coefficient

13.6

ppm/EC

thermal conductivity

45

W/(m EC)

elastic modulus

211

GPa

Poisson뭩 ratio

0.29

hardness

46

HRC

yield strength

855

MPa

elongation to fracture

16

%

ultimate tensile strength

965

MPa

fracture toughness 50 MPa(m)½ The impact toughness is low in the notched condition, so when this alloy is prepared with residual porosity via the press-sinter route, it has almost no measurable impact toughness using the notched Charpy test, so only the unnotched data are reported.

4908 A sinter-hardenable grade of steel used in powder metallurgy that contains mostly iron with 1 to 3 wt. % nickel and 1.3 to 1.7 wt. % molybdenum, and carbon specified at 0.8 wt. %. Often 2 wt. % copper is added to this composition, and in such a case the full designation would be FLC-4908.

5000 An alloy of approximately equal parts iron and nickel, used for soft magnetic

Section Numbered Items

Page 11

applications. In some situations it might have a prefix of FN, giving a full designation of FN-5000. Although soft, the material provides excellent magnetic response for applications such as in solenoids.

6061 A common aerospace aluminum alloy that consists of low concentrations of magnesium, silicon, manganese, and chromium (for example 1 wt. % Mg, 0.6 wt. % Si, 0.28 wt. % Mn, and 0.2 wt. % Cr). At full density the alloy has a density near 2.7 g/cm3, melting range from 562EC to 651EC, yield strength of 275 MPa, ultimate tensile strength of 310 MPa, and 17 % elongation to fracture. It has a high thermal expansion coefficient at nearly 24 ppm/EC and a thermal conductivity of 177 W/(m EC). It can be heat treated to promote higher strength levels. One frequent use in powder metallurgy is as a matrix phase for silicon carbide whiskers, where an injection molded SiC preform is pressure infiltrated with the molten alloy.

8000 The designation sometimes used for a higher phosphorous iron alloy containing up to 0.8 wt. % phosphorous, used mostly for soft magnetic applications.

Section A

Page 1

A to Z of Powder Metallurgy

A ab initio calculation A new form of material property estimation based on computer calculations that start with simple atomic clusters and very few assumptions on the materials, structure, and properties other than those associated with the atomic bonding. These calculations are used to predict new phases, compounds, or interesting material property combinations, usually in man-made materials that have not previously been tested.

absolute pore size The maximum opening in a porous material, such as a filter, through which no larger particle will pass. It is usually measured using the bubble point test. Also, called the maximum pore size.

absorption The chemical bonding of a gas or molecule or contaminant to the surface of a powder is termed absorption, not adsorption. The species is difficult to remove in comparison with adsorbed species. Because of the chemical bonding, absorbed species are more permanent. Often measured by the capacity of a solid to retain a liquid or gas, such as by capillary, osmotic, or solvent action.

acceptable quality level (AQL) A concept that arises in acceptance sampling, indicating the maximum fraction of nonconforming items, at which the process average can be considered satisfactory. It is the process average at which the risk of rejection is called the producer뭩 risk.

accuracy In testing a material or product this is the degree of agreement between the true value of the attribute being measured and the average of many observations made according to the test method. It is essentially the location of the mean with respect to the specified size. As illustrated in Figure A.1, accuracy is different from repeatability, which is also known as precision. The latter parameter determines how closely scattered the determinations are. A test might be accurate, giving nearly the true value on average, but low in ability to attain the same result in the observations so it is low in precision. Most desirable is testing and manufacturing that are both accurate (gives the true value)

Section A

Page 2

and precise (has a low scatter between observations).

acetal A backbone polymer that is used for powder metallurgy applications due to its high strength and unique depolymerization attributes, consisting of a chain with CH2O repeat units - where every other atom on the backbone is carbon with two side hydrogens and oxygen is in between: H H H H ... C - O - C - O - C - O - C - O ... H H H H Polyacetal melts at 175EC and has a density of 1.42 g/cm3. In the pure form acetal has a strength from 65 MPa to 95 MPa. During extraction, the backbone breaks down in an atmosphere of nitrogen doped with nitric acid to produce formaldehyde, a process termed catalytic debinding. [see catalytic debinding]

acicular powder Needle-shaped particles.

acid insoluble test A test applied to iron or copper powder to indirectly determine the content of inclusions, such as oxides, most of which are insoluble when the powder is digested in acid.

Acrawax™ A trademarked name for ethylene-bis-stearamid. [see ethylene-bis-stearamid]

activated liquid phase sintering A process used to sinter with a liquid in the pores, but where the liquid is ineffective due to a low solubility of the solid in the liquid. An activator is added to promote solid sintering of the solid skeleton and the liquid simply fills the remaining void space. An example of activated liquid phase sintering is in the W-Cu system where Co additions promote high sintered densities. This is because the Cu-Co system has no interaction and Co has an activating effect on tungsten. A cobalt content of about 0.3 to 0.5% gives the optimal benefit.

activated sintering Activated sintering is an enhanced sintering process that includes any treatment which reduces the activation energy for atomic motion. The increase in diffusion rate allows faster sintering, lower sintering temperatures, or improved properties. Classic examples

Section A

Page 3

of activated sintering occur when tungsten is doped with about 0.3 wt. % nickel, iron, cobalt or similar transition metal. Other metals exhibit a similar significant change in sintering rate with proper additives - including tantalum, rhenium, molybdenum, and even titanium. For example, when 1 μm chromium powder is sintered at 1400EC the sintered density is 78%. But the addition of 1% Pd to the same powder, using the same sintering cycle, produces a sintered density of 96%. Similar changes in sintering of refractory metals are possible with other transition metal additions. As the sintered density increases due to activated sintering, the strength, hardness and other physical and mechanical properties also increase. However, concomitant grain growth can degrade the sintered properties due to microstructure coarsening.

activation energy The energy required to break atomic bonds to move an atom to a new lattice site or to evaporate an atom. Usually the activation energy scales with the atomic bond strength, so a high melting temperature, high elastic modulus, or high sublimation enthalpy are positively correlated with a high diffusion activation energy. Various correlations exist between the crystal structure (coordination number) and the activation energies for surface diffusion, grain boundary diffusion, and volume diffusion, all being related to the atomic bond strength and bonding coordination.

activity-based costing An accounting process that proves useful in determining the true cost for component fabrication in both the quotation stage and during production. The concept breaks the manufacturing process into several small steps and for each step calculates the annual cost by summing up the rent, utilities, labor, depreciation, and other factors. Then the annual production, in terms of number of parts or mass of material, is used to assign the cost per part or cost per unit mass. Each step is provided a cost assessment, so total production cost is simply the summation of all of the steps. One consequence of activity-based costing is that operations that are not utilized fully have a higher unit cost. Likewise, busy operations then are driven to minimize the dwell time (increase the number of parts or amount of annual mass) to minimize costs. The concept is fairly simple, but proves controversial since there is a bias toward busy operations being able to quote the lowest price.

adaptive control Control strategy for watching production and making a correction after a deviation occurs, with some loss of good quality until the process is back into specification.

adhesion The attraction of atoms across the interface between two different solids. This attraction provides a weak strength. In powder metallurgy, adhesion is the key to retaining a

Section A

Page 4

desired compact shape in the temperature range between polymer burnout and the onset of sintering. Even loose particles form random contacts with each other and adhesion at these contact points leads to shape retention during heating. The typical weak forces associated with van der Waals bonding and at lower temperatures adhesion can originate from adsorbed liquids. The adhesion stage of sintering occurs spontaneously with the formation of a sinter bond at these contact points.

adhesive bonding Bonding between clean surfaces by the action of an acrylic or epoxy giving a low strength chemical bond. Super glue is a common temporary adhesive used to assemble powder metallurgy green bodies prior to sintering.

adiabatic forming The use of a frozen powder-water feedstock that melts and flows under pressure, but re-freezes when pressure is released. Unlike traditional powder injection molding, no significant amount of heat is added or extracted to change the phase, simply pressure changes are used to melt and freeze the mixture.

admixed powder A small, discrete powder mixed with another powder for lubrication, bonding, or alloying.

adsorption The temporary adherence of a gas or contaminant molecule to the surface of a powder. The species is added to the surface by physical forces, but is not chemically bonded. This is a distinct differentiation from absorption, where the surface is involved in chemical bonding. Adsorbed species can be removed by heating.

Ag The chemical symbol for silver.

agar Also known as agarose, it is a binder ingredient derived from seaweed used in gelation systems for injection molding or slurry casting. Agar is also commonly added to foods to adjust foaming, flow, or gelation.

age hardening A heat treatment that increases hardness by a precipitation reaction; the same as ageing or aging. Excessive age hardening results in degraded properties.

ageing or aging

Section A

Page 5

Heating a material to a temperature where precipitation or ordering reactions come to completion to adjust the properties of the material. Aging treatments are first preceded by some form of solution treatment to take all of the alloying ingredients into solution, with a possible quenching treatment to prevent precipitation during cooling. For some powder metallurgy materials the aging cycle can be included in the cooling step after sintering. There is no change in chemical composition during aging.

agglomerate Several particles adhering together in a cluster that usually can be milled to form discrete particles. Agglomerates are held together by weak forces that can be overcome by small shear stresses, stresses far below the material strength. When mud dries, the soil particles agglomerate into dirt clods that can be broken into particles. Agglomerates can be disintegrated prior to powder characterization, but aggregates cannot be dispersed. Weak forces, such as the van der Waals force, provide a cohesive strength that causes agglomeration and retards packing and flow.

agglomerating agents Usually these are polymers added to a powder to form agglomerates. For example polyvinyl alcohol and polyethylene glycol are water-soluble polymers used to agglomerate small powders. For die compaction, small powders are intentionally formed into large, agglomerated spheres to provide rapid and homogeneous flow into a die cavity. The actual agglomeration step is performed using spray drying or electrostatic agglomeration to form spheres which are sometimes hollow. The large and spherical agglomerate shape is important for high-speed pressing. With the exception of paraffin wax, most of the polymers used as agglomerating agents are water soluble.

agglomeration A tendency for small particles to stick together and appear as larger particles. Agglomeration is associated with weak forces such as water films or van der Waals bonds. Moisture is a common culprit. Water condenses at the particle contacts and forms capillary bridges known as pendular bonds. The attractive force F between contacting particles varies with the liquid-vapor surface energy γLV, and particle size D, as follows:

aggregate A hard, difficult to disperse cluster of particles held together by strong cohesive forces, chemical bonds, or possibly even sinter bonds. Unlike agglomerates that can be broken into particles, aggregates are very hard and are difficult to reduce into individual particles under normal mixing or ultrasonic agitation conditions.

Section A

Page 6

air classification Separation of moving particles into narrow particle size fractions by means of differential settling in a high velocity air stream. Air classification is a variation of the elutriation and sedimentation techniques. Air classification achieves a separation into selected size ranges using a cyclone or a spinning disk (up to 12,000 RPM) and a cross-current air flow. The centrifugal force provides a constant particle velocity, while the air flow achieves size separation by Stokes? law effects. When the two effects are not aligned, particle size separation occurs. Small particles are deflected by the air flow and separated from the larger particles. Control of the disk rotational speed and air flow velocity provides a means of altering the particle size separation. Air classifiers are generally applicable to the size range from 1 to 150 탆 , although the actual behavior varies with the inverse square-root of the material density. A few efforts have been made to use air classification for particle size distribution analysis, but these slow approaches have fallen out of favor in recent years. [see elutriation]

air jet sieve A device in which a powder is fluidized on top of a sieve using a rotating slit that passes air upwards to overcome gravity. At the same time a negative pressure is applied to the bottom of the sieve to pull the small particles into a collection device. Because fresh powder is presented to the sieve on each rotation of the pressurized slit, the approach accelerates sieve classification by reducing sieve blinding.

AISI American Iron and Steel Institute. A body that sets composition and other standards for many steels.

alginate Polymers derived from natural fibers that are usually mixed with water for use in various forms of powder processing. Some of the more popular forms are hydrocolloids derived from marine algae, such as kelp harvested off the coast of San Diego. There are both ammonium and sodium alginates in use for powder extrusion, injection molding, slurry casting, and slip casting.

alloy A mixture of atomic elements, at least one being a metal, to form a material with metallic properties. Metals come in pure elements such gold, copper, iron, and cobalt. Mixtures of these elements at the atomic scale are known either as ceramics, composites, cermets, intermetallics, or alloys. Alloys are metal-like in properties, for example they are electrically conductive, but the properties derive from more than one element.

Page 7

Section A

Example alloys are stainless steels (iron-chromium), steels (iron-carbon), brass (copper-zinc), bronze (copper-tin), and sterling silver (silver-copper). Most alloys have a range of compositions over which beneficial properties are attained.

alloy powder A powder in which each particle is composed of the same mixture of two or more elemental constituents to form an alloy, especially common in gas or water atomized powders such as stainless steels. Can also be called a prealloyed powder.

AlN The chemical symbol for aluminum nitride.

alnico A group of magnetic alloys based on iron with various levels of aluminum (from 7 to 10 wt. %), cobalt (from 5 to 35 wt. %), nickel (from 14 to 28 wt. %), copper (from 0 to 6 wt. %), and titanium (up to 5 wt. %). The name comes from the aluminum-nickel-cobalt alloying - Al-Ni-Co. The materials are typically sintered to densities in the 6.8 to 7.0 g/cm3 range. Tensile strengths after sintering range from 275 MPa to 425 MPa, depending on the composition. When processed to near full density, the remanence is 0.76 T and coercive force is 125,000 A/m. The sintered products retain their magnetic properties up to almost 500EC and compete with cast compositions.

Al2O3 The chemical symbol for alumina, also known as aluminum oxide.

Al-SiC A powder metallurgy composite consisting of an aluminum alloy matrix and dispersed particles or whiskers of silicon carbide. It is frequently processed using hot isostatic pressing or hot extrusion from mixed powders. Various levels of silicon carbide are possible, with 20 to 40 vol. % being most common. As the ceramic phase content increases the composite becomes stiffer, but lower in ductility and fracture toughness. As an example of the possible properties, listed here are the characteristics of a composite consisting of 6061 aluminum alloy matrix and 20 vol. % silicon carbide: Al - 20 vol. % SiC

Al-SiC

density

2.9

g/cm3

heat capacity

800

J/(kg EC)

thermal expansion coefficient

12.4

ppm/EC

thermal conductivity

125

W/(m EC)

Page 8

Section A elastic modulus

108

GPa

as-sintered yield strength

379

MPa

6

%

as-sintered elongation to fracture

as-sintered ultimate tensile strength 441 MPa Because of the expense, this alloy is used mostly for speciality applications, such as racing bicycle frames.

alpha iron Another name for the room-temperature stable form or iron, a body-centered cubic crystal structure as shown in Figure A.2, that is stable up to 910EC. Also, known as ferrite. At full density it has a density of 7.87 g/cm3, elastic modulus of 196 GPa, yield strength (unalloyed) of 150 MPa, elongation to fracture of 45 %, ultimate strength of 260 MPa, thermal expansion coefficient of 12.3 ppm/EC, and thermal conductivity of 75 W/(m EC).

alumina (Al2O3) The compound of aluminum and oxygen Al2O3 that is the most common material used in ceramics and sintering furniture. Alpha alumina, with a trigonal structure, is most common. It has a molecular weight of 101.94 g/mol, density of 3.98 g/cm3 when sintered to full density, and strength (in three point bending via the transverse rupture tests) from 350 MPa to 550 MPa. The elastic modulus at full density is 405 GPa with a hardness of 1500 VHN (corresponding to a Mohs hardness of 9), but the fracture toughness is only 2 MPa(m)½. At room temperature the thermal expansion coefficient is 5.5 ppm/EC while the thermal conductivity is 39 W/(m EC). It has a dielectric constant of 10.1 and a very high electrical resistivity, estimated at 108 ?Ω-cm. Whiskers of alumina have strengths over 2 GPa. Generally, alumina comes in various forms that include calcined alumina (heated to remove impurities), tabular alumina (usually alpha phase with low sodium levels), hydrated alumina (corresponding to Al2O3A3H2O), and boehmite alumina (corresponding roughly to Al2O3AH2O).

aluminide Compounds of aluminum with other metals are called aluminide intermetallics, and those of iron (FeAl), titanium (TiAl and Ti3Al), and nickel (NiAl and Ni3Al) are some of the most popular. Most are produced using various powder metallurgy techniques, including reactive sintering and reactive hot isostatic pressing.

aluminothermic process Reduction of a material based on adding aluminum powder, where the aluminum reacts with the oxygen to form alumina, leaving a reduced metal. In many cases the reaction is

Page 9

Section A

exothermic once initiated. For example a mixture of iron oxide and aluminum powder will react when heated to generate sufficient heat to melt the reduced iron to form a steel slag in the famous thermite reaction.

aluminum (Al) Element 13, it is a face-centered cubic metal with low strength and high ductility that is mostly used as a powder for pyrotechnics, explosives, and rocket fuel. Powder production is mostly via gas atomization, often using air to ensure a passivating oxide on the powder. Besides aluminum powder use in combustion events, a few aluminum alloys are processed by hot isostatic pressing, die compaction, injection molding, or freeform fabrication into engineering components. The properties of pure aluminum are summarized as follows: aluminum

Al

atomic number

13

atomic weight

26.98

g/mol

density

2.70

g/cm3

melting temperature

660

EC

boiling temperature

2467

EC

heat of fusion

10.7

kJ/mol

heat capacity

917

J/(kg EC)

thermal expansion coefficient

23.8

ppm/EC

thermal conductivity

237

W/(m EC)

electrical resistivity

2.7

µΩ-cm

elastic modulus

71

GPa

Poisson뭩 ratio

0.345

hardness

19

BHN

yield strength

25

MPa

elongation to fracture

50

%

ultimate tensile strength

48

MPa

fatigue endurance limit

20

MPa

Section A

Page 10

aluminum alloys Popular light weight materials by powder metallurgy are based on aluminum alloyed for strength or dispersion strengthened using various ceramic phases. Most of the alloys contain chemical agents that dissolve the surface oxides typically found on aluminum powders. These compositions are modified from classic wrought metallurgy, and when processed to full density exhibit mechanical and physical properties similar to handbook values - density 2.7 g/cm3, elastic modulus near 70 GPa, as-sintered strength between 230 MPa and 450 MPa, and up to 10 % elongation. As the porosity increases, these properties fall. Ceramic phase additions are used to lower density while increasing strength and stiffness. By itself aluminum is very weak, has a high thermal expansion coefficient and high thermal conductivity; however, alloying is typical in powder metallurgy to increase strength, but with a loss of thermal conductivity. For example, the 6061 alloy drops to a thermal conductivity of 177 W/(m EC), but the ultimate strength increases to 310 MPa.

aluminum-alumina Various composites consisting of interlaced and connected aluminum and alumina phases, usually produced by powder metallurgy techniques such as internal oxidation, infiltration, or mixed powder compaction. A popular composition is 50 vol. % alumina which has a density of 3.28 g/cm3, elastic modulus of 159 GPa, strength of 370 MPa, fracture toughness of 18 MPa(m)½, and thermal conductivity of 66 W/(m EC).

aluminum nitride The equiatomic compound of aluminum and nitrogen. It is a covalent ceramic widely recognized for a high thermal conductivity, ranging up to 270 W/(m EC), but it is an electrical insulator. It is fabricated by sintering techniques, often in a pressurized nitrogen atmosphere at temperatures in excess of 1700EC. To promote sintering densification, additives such as yttria are used with a small reduction in thermal conductivity. When sintered to full density, aluminum nitride has a density near 3.26 g/cm3, breaking strength in the 300 to 400 MPa range, and a fracture toughness in the 3 to 4 MPa(m)½ range.

aluminum oxide The formal name for alumina.

aluminum-silicon carbide Composites produced from either mixed powders or an injection molded silicon carbide preform infiltrated with molten aluminum. Figure A.3 is an example microstructure. A popular composition is 30 vol. % silicon carbide that has a density of 2.8 g/cm3, elastic modulus of 125 GPa, strength of 300 MPa, thermal expansion coefficient of 14.1 ppm/EC, and thermal conductivity of 158 W/(m EC). Usually the aluminum contains

Section A

Page 11

alloying ingredients such as silicon and magnesium to promote wetting. When the silicon carbide content reaches 70 vol. %, the density is 3.08 g/cm3, elastic modulus is 265 GPa, strength is 255 MPa, thermal expansion coefficient is 6.2 ppm/EC, and thermal conductivity is 170 W/(m EC). This latter composition has a fracture toughness of 10 MPa(m)½.

amalgam The product from transient liquid phase sintering using a precious metal such as silver and liquid mercury, leading to the formation of a solid mercury compound. The most famous amalgam is formed by mixing silver powder with liquid mercury, and prior to loss of the liquid the semisolid mixture is compacted into a tooth cavity to provide a filling. The final product is a compound of silver and mercury. Today the amalgam powder often includes tin and copper to control the reaction time and long-term corrosion response.

American Society for Testing and Materials (ASTM) A US body for setting standards for all varieties of materials and testing protocols. Many of the standards used in powder metallurgy were first approved or established by ASTM.

ammonia dissociator A conversion reactor that first vaporizes liquid ammonia (NH3), then heats that vapor in the presence of a dissociating catalytic agent to produce a sintering atmosphere consisting of three parts hydrogen and one part nitrogen.

ammonium carbonate A volatile compound with the chemistry (NH4)2CO3H2O, sometimes used as a pore former when mixed with metal powders. After consolidation, the ammonium carbonate decomposes at about 58EC to leave an artificial pore. The pore size is determined by the particle size of the carbonate and the porosity is determined by the content of carbonate. The vapor pressure and smell make this an undesirable pore former, and simple waxes and polymers are often used instead.

ammonium paratungstate (APT) A commodity precursor chemical used in the production of tungsten powder and tungsten carbide, consisting of a composition given as 5(NH4)2OA12WO3A5H2O, it is usually prepared as a white powder for calcining into WO3. Subsequently, tungsten powder is obtained by roasting the oxide in hydrogen, and carbides are formed by mixing and heating tungsten powder and graphite.

amorphous

Page 12

Section A

A solid that lacks detectable crystallinity. Amorphous materials appear to be solids, but are disordered and have an atomic arrangement that is similar to a liquid.

amorphous metal A material lacking crystallinity with an atomic arrangement that is similar to that of a liquid. For alloys, amorphous structures are obtained by rapidly quenching molten droplets or mechanical alloying. Rapid solidification processing is intent on forming an amorphous structure at room temperature via rapid heat extraction. Once atomic motion is frozen at low temperatures, the cooled liquid-like structure is preserved. Only on reheating is atomic motion sufficiently active to rearrange the atoms to nucleate crystals. There are many new compositions discovered and most are considered experimental. As an example of the properties, below are listed the attributes for a zirconium alloy that contains 23 wt. % beryllium, 14 wt. % titanium, 13 wt. % copper, and 10 wt. % nickel: amorphous metal

glass

density

6.1

g/cm3

thermal expansion coefficient

8.5

ppm/EC

elastic modulus

96

GPa

Poisson뭩 ratio

0.36

hardness

534

VHN

yield strength

300

MPa

2

%

elongation to fracture

ultimate tensile strength 1900 MPa This property combination is anomalous in powder metallurgy.

Anderson process A means to produce titanium, and other reactive or refractory metals, using a molten sodium loop reactor and a metal chloride feed material. For example, if TiCl4 gas is fed into molten sodium, then a controlled reaction produces titanium and salt (NaCl). The reaction products are harvested and water washed to extract a titanium sponge which is then milled and classified to the desired particle size.

Andreasen size distribution A broad particle size distribution that provides a high packing density. It is named after the colloidal chemist who first identified these characteristics. The cumulative particle

Section A

Page 13

size distribution in terms of weight fraction F(W), fractional weight of powders with a size less than particle size D, is described as follows:

For the highest packing situations the exponent q tends to range between 0.5 and 0.67.

anelastic Literally this means not elastic. The properties of solids, by virtue of which strain is not a single-value function of stress, in the low-stress range where no permanent plastic deformation is occurring. Anelastic processes have a different loading stress-strain path that is not exactly followed on unloading. Often observed in high frequency testing and attributed to atomic level changes brought about by stress, for example interstitial atom jumps into new positions due to the loading curve.

angle of repose The angle from the horizontal plane that a pile of loose powder will assume when freely poured through an orifice. The angle of repose provides a measure of the interparticle friction and ease of shape retention during debinding. The angle of repose is evident in several aspects of powder handling, such as the angle from the horizontal when a powder is poured, tipped, or rotated. Figure A.4 shows a few illustrations of this characteristic behavior. Smaller particles and irregular particles have higher repose angles than large, spherical particles. Large spherical particles exhibit an angle of repose near 30E. Generally the angle of repose will not exceed 38E for free-flowing powders. When the angle of repose exceeds approximately 45E, the powder is cohesive. In some cases agglomeration forces are so strong that the angle of repose exceeds 100E.

angstrom (D) A unit of linear measure equal to 10-10 m or 0.1 nm. It was a former favorite in measuring atomic size, atomic spacing, and atomic motion distances, but has been displaced by the nanometer.

anneal A heat treatment associated with softening a metal or alloy, to remove residual strains, by heating to a high temperature followed by slow cooling. Cycles associated with annealing depend on the alloy, its prior history, and the desired properties. Further, the cooling cycle is often important to the properties after annealing. Other heat treatment steps might be applied to an annealed material.

Section A

Page 14

annealed powder After powder fabrication, the powder is heated to an intermediate temperature to relieve any residual strain, giving a soft powder for compaction. Reduction might also occur during annealing to remove oxides or other contaminants.

anode The positive electrode in an electrolytic cell from which electrons emerge and metal cations pass into the electrolyte. Electrons in the external circuit flow away from the anode toward the cathode. Metal ions enter the electrolyte as a consequence of the anodic reaction, to subsequently be deposited at the cathode where they are harvested to form electrolytic powders.

anodizing A coloration process that creates a surface oxide while capturing a dye to provide coloration; aluminum is the most common material subjected to an anodizing treatment. Anodization is also used on materials that form stable oxides, such as alloys containing aluminum, titanium, zirconium, or chromium. One application is in custom titanium jewelry, which relies on surface treatments to impart a wide variety of colors.

anvil press Die compaction in tooling where there is no upper punch, simply a lower punch pressing the powder against an upper anvil that covers the top of the die. Figure A.5 is a sketch of the layout of punch, die, and upper support during anvil pressing.

APMI A term that no longer means anything, since the former American Powder Metallurgy Institute changed it name to APMI International. The original name was descriptive, but the current APMI International name fails to convey any meaning. The organization is run by the powder metallurgy trade association in the USA, supposedly for individuals. Unfortunately, the key positions, including president, are taken from the ranks of the Metal Powder Industries Federation companies, so it fails the basic litmus test of being a professional society.

apparent density The mass of a unit volume of powder in the loose condition, usually expressed in g/cm3. Standard tests for measuring the apparent density rely on the Hall, Arnold, or Scott meters. The Hall meter is used for the larger particles since it provides data on both the flow rate and the apparent density. It consists of a 60E funnel with a 2.5 mm diameter hole. A modified test with the Carney funnel increases the hole diameter to 5 mm for slower flowing powders. The Scott device is applied to small powders which do not flow, such as refractory metals. The Arnold meter minimizes the need for the powder to flow,

Section A

Page 15

since it slides a ring filled with powder over a 20 cm3 cylindrical hole. That technique best correlates with the filling of die cavities in powder compaction operations. These tests are generally repeatable to ? .1 g/cm3, and reproducible at different labs at ? .2 g/cm3. [see Hall meter, Arnold meter, Scott meter]

apparent hardness The hardness value obtained by testing a sintered material with standard indentation hardness equipment. This reading is a composite of pores and solid material; hence, it is usually lower than the hardness of the solid material.

APT A common abbreviation for ammonium paratungstate.

AQL The abbreviation for acceptable quality level.

arc welding A joining process based on forming molten material at the joint. There are two forms, consumable (MIG) or nonconsumable (GTA) arc. Both create a melt at the contact, the metal flows and bonds the objects prior to solidification.

Archimedes density Density determined by a series of mass measurements. This approach prevents water intrusion into pores to extract the volume. First, the sample is weighed dry (W1), then again after oil impregnation into the evacuated pores (W2), and finally the oilimpregnated sample is immersed in water for the final weight (W3). Usually a wire is used to suspend the sample in the water and its weight Ww must be measured in water too. Then the density can be calculated from the weight determinations as follows:

where Φw is the density of water (in g/cm3), which is temperature dependent, Φw = 1.0017 - 0.0002315 T with T being the water temperature in EC. Dividing the measured density by the theoretical density gives the fractional density. One variant uses water impregnation instead of oil.

Section A

Page 16

Arnold meter A powder test device that determines the powder apparent density in a situation similar to die filling. As shown in Figure A.6, a 20 cm3 volume in a flat block is filled by sweeping a powder-filled cylinder over the opening. When the flat block is lifted, the powder metered into the test volume collects on weighing paper placed below the hole, allowing determination of the powder apparent density analogous to normal powder filling in a die cavity.

Arrhenius relation A mathematical description of the probability for atomic motion based on the statistical distribution of atomic energies. It leads to the exponential temperature dependence for many of the events in sintering, creep, hot pressing, and heat treatment. For example, in sintering the rates of atomic diffusional motion is described by an Arrhenius relation where the volume diffusion rate DV is determined from the atomic vibrational frequency DO, absolute temperature T, universal gas constant R, and the activation energy Q which corresponds to the energy required to induce atomic diffusion, giving,

Fundamentally, the Arrhenius relation is an approximation to an integral of the Boltzmann energy distribution for atoms, giving the fraction of atoms with an energy of Q or greater at any time. The relation is named after Svante Arrhenius who won the Nobel prize in Chemistry in 1903 for his work in physical chemistry, especially linking electrical activity to ionic dissociation in solutions, as now captured in the concept of chemical activity.

ash The residue after burnout of a pure polymer, giving an indication of the degree of contamination expected from that polymer.

aspect ratio A straightforward descriptor of particle shape, defined as the maximum particle dimension divided by the minimum particle dimension. It is usually obtained from computer image analysis using optical microscopy. For a sphere, the aspect ratio is 1, while for a ligament it is often near 3 to 5. A flake particle can have an aspect ratio as high as 200. Many common powders have aspect ratios ranging from 1.05 (near spherical) to 1.4.

ASTM The abbreviation for the American Society for Testing and Materials. A body for setting standards for all varieties of materials and testing protocols.

Section A

Page 17

atmosphere The process gas used in a sintering furnace. Options range from air for ceramics to vacuum for certain reactive materials such as titanium.

atomization The dispersion of molten material into droplets by a rapidly moving stream of gas (usually nitrogen or argon), liquid (usually water), or centrifugal force. The droplets solidify into particles. In some cases compressed air can be used to atomize a metal if oxidation is not a concern; however, usually atomization is performed using rapid quenching and a non reactive atmosphere or even vacuum. Atomization relies on a melt and disintegration of that melt into droplets that freeze into particles. Commercial atomization units operate at production rates as high as 400 kg/min. The process is mostly used for metals, alloys, and intermetallics, with some recent applications in polymers and ceramics. It is applicable to several materials with good process control. [see water atomization, gas atomization, centrifugal atomization]

atomized powder A powder produced by the disintegration and subsequent solidification of a molten liquid stream.

attrition A mechanical milling or grinding process that typically employs a stirred, vibrated, or tumbled container filled with small balls that collide against particles mixed with the balls.

attritor mill A high intensity ball mill with a stationary drum and moving central agitator paddle. The high speed rotation of the agitator induces turbulent motion of the balls that in turn reduces the particle size, forms flakes, or induces mechanical alloying, depending on the operating parameters. [see mechanical alloying]

Au The chemical symbol for gold.

Auger electron spectroscopy A technique for elemental chemical analysis of surface layers that relies on incident Xrays or electrons to identify the atoms present on the surface with some information on the atomic bonding state as well. Surface layers often have compositions that are

Section A

Page 18

significantly different from what is observed with bulk samples. In Auger electron spectroscopy, an incident electron or X-ray beam is used to induce electron exchanges in the near-surface atoms. In some cases as electron cascades occur to return the atom to a stable electron configuration, the excitation energy is transferred to an outer electron, causing it to be ejected from the atom. This ejected electron is called an Auger electron and can be captured and analyzed to determine the near-surface chemistry of the material. Higher energy X-ray emissions lead to X-ray fluorescence. Since the ejected Auger electrons are easily absorbed, only those produced in the first few atomic layers can be detected. Further, the process is better suited to lower atomic number species, but that is often a benefit in powder metallurgy since oxygen, carbon, and similar low atomic number atoms tend to be most prevalent on the surfaces of metal powders.

ausforming Thermomechanical treatment of steel in the metastable austenite conditions below the recrystallization temperature, followed by quenching to obtain martensite or bainite. One application in powder metallurgy is to roll in a precise gear profile in a sintered material during cooling. The austenite is soft and easily deformed prior to its transformation into martensite.

austempering A heat treatment for ferrous alloys where the material is quenched from the austenitizing temperature at a rate fast enough to avoid formation of ferrite or pearlite, and then is held at a temperature just over the start of the martensite transformation temperature for the alloy to allow for the formation of bainite.

austenite The face-centered cubic crystal structure of pure iron (often called gamma iron). The crystal structure is shown in Figure A.7 and it is not stable below 910EC. Alloys of iron and other materials, such as nickel, are also designated as austenitic. With alloying this nonmagnetic form of iron can be stabilized at room temperature and even cryogenic temperatures; austenitic stainless steels are a common example where sufficient nickel is employed to stabilize the nonmagnetic phase at room temperature.

austenitic stainless steel A stainless steel, usually containing at least 18 % chromium, where the face-centered cubic crystal structure is stable to room temperature. Usually this stabilization is via alloying by nickel, but might involve nitrogen, manganese, or other austenite stabilizers. [see 300, 304, 316]

automatic press

Section A

Page 19

A compaction press that runs without an attendant, giving repeated die compaction or sizing.

Avagadro뭩 number The number of atoms or molecules associated with a mole, that being 6.02A1023 and that mass of atoms equals the atomic weight of the material.

Avrami equation An equation used to describe the rate of reaction or phase transformation where first there is a nucleation of the new phase followed by rapid transformation and then a progressively slower reaction as the source species are exhausted. This results in a generalized lazy-S curve of the percent transformed versus time that is best fit by an equation of this form:

where y is the fraction transformed, t is the time, n and Κ are constants for a given reaction. Typically the parameter K is temperature dependent and n ranges from 1 to 4.

axial loading Force applied along the pressing direction through the axis of symmetry.

Section B

Page 1

A to Z of Powder Metallurgy

B B The chemical symbol for boron.

back pressure The hydraulic pressure applied to the injection molding ram during the plastication phase of the injection molding cycle. It has a role in removing air from the feedstock.

backscatter electrons All primary electrons that become scattered out of the original or incident direction to be retransmitted through the surface of a solid are termed backscattered electrons. Most of these electrons have an energy that allows sorting from the secondary electrons. In scanning electron microscopy, most of the electrons are secondary, generated by incident beam collisions with sample electrons, while the production of backscatter electrons depends on the atomic mass. Thus, filtering of the electrons to image based on backscatter electrons provides images that show atomic number contrast.

bainite A form of iron and carbide produced by a special cooling cycle that is initially fast but subsequently slow, allowing transformation of austenite into ferrite and cementite without the formation of the platelet microstructure known as pearlite (too slow) or the formation of hard, brittle martensite (too fast).

ball mill A simple means of deagglomerating or attritioning a powder by rolling a jar on its side, where the jar is partially filled with balls allowing the balls to fall and impact on the powder. This is the same technology as implied by jar milling. See Figure B.1 for a schematic of a ball mill. The optimal rotation speed varies with the inverse square root of the mill diameter. For optimal milling, ? the ball diameter should be approximately 30 times the diameter of the powder ? the balls should fill about 50 % of the jar volume ? the fill of the input material should be about 25 % of the jar volume. The input powder fills the interstitial voids between the balls. This ratio of ingredients gives a good balance between the ball mass and the number of ball contacts or impacts

Section B

Page 2

that give milling. It is also termed jar milling.

barrel The heated portion of the feedstock flow path in an injection molding machine, located just before the nozzle. The barrel holds the feedstock under pressure while providing heat to melt the binder system. A screw operates inside the barrel to both prepare hot feedstock (plastication) and to induce feedstock flow (fill).

barrier filter A sintered porous metal used to capture debris in a flowing fluid, where 100 % of the fluid passes through the porous metal and the pores are sized to collect the debris at a preset size and all particles larger than that size. Fluid flow is perpendicular to the filter surface in contrast with cross-flow filters where the only a portion of the fluid passes through the filter and flow is tangential to the porous metal surface. The smaller the pore size (filter rating) the smaller the particles used to fabricate the filter, with about 0.5 탆 being a typical lower limit. Contrast this with a cross-flow filter. [see cross-flow filter]

base plate A steel flat plate filled with holes and attachment and alignment guides that allows for clamping or bolting or attachment of the various tool components, ejector systems, and other actions custom designed for each part. Base plates are usually purchased as a starting point for the construction of a mold.

batch furnace A closed or box-type furnace that accepts green compacts and performs the desired heating and cooling cycles; also known as a periodic furnace. Figure B.2 is a sketch of a front-loading batch furnace. They are designed for operations that have uneven sintering demands, change materials frequently, have a wide variety of sizes and shapes that require cycle adjustments, and for materials that require long cycles or vacuum sintering. Since only a limited number of components can be processed at one time, batch furnaces tend to be used in speciality operations.

batch sintering The processing of powder compacts through its sintering cycle (time, temperature, and atmosphere) where all of the components are heated in a furnace at the same time. They must be cooled and removed before another cycle can be initiated.

batch size The quantity of material or the number of components fabricated during the same process or in one continuous process that produces the same characteristics; it is also

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Page 3

the lot size that proves justified to form at a single time. Determination of the batch size might come from the customer in terms of the amount ordered, low purchasing frequency or shipment rate, changes in the raw materials, or may reflect equipment capacity limitations. For example, if a batch sintering furnace can only accommodate 5000 components at a time, this becomes the batch size.

Be The chemical symbol for beryllium.

bearing A device that functions between stationary and rotating components that provide a thin fluid lubrication layer. Porous powder metallurgy bearings store oil in the open pores for lubricity as the bearing rotates and heats. Various alloys used for bearings include bronze, bronze-graphite, bronze-iron, and steel compositions.

bearing strength A crush test applied to straight wall cylindrical bearings, as sketched in Figure B.3. The calculated value is often termed the K-factor which is really the radial crush strength, measured by crushing the bearing between two parallel platens. The bearing strength is determined from the maximum load FB in crushing, giving the K-factor as follows:

where L is the cylinder length, D is the outer diameter, and T is the wall thickness.

bed The stationary portion of a press that rests on a foundation, in a pit, or the floor, forming support for the remainder of the press. Also a layer of powder, for example some debinding and sintering cycles rely on a substrate consisting of a ceramic powder bed.

bell (jar) furnace Typically a vacuum furnace where the outer chamber lifts off of the work, which rests on a hearth plate. The name comes from the fact that this shell often looks like a bell with a domed top and a bottom flange lip that provides the seal ring. In this manner the weight of the chamber holds the sealing surface in compression.

belly band A feature that comes with spherical shapes produced by die compaction, as shown in Figure B.4. In forming a perfect sphere, the tip of the punches would be too thin along the equator, so the option is to form a belly band using thicker, stepped punches. After sintering the excess material can be removed by grinding to give a sintered spherical

Section B

Page 4

shape.

belt furnace A continuous sintering furnace that uses a constant mesh belt to transport powder compacts through the hot zone. As sketched in Figure B.5, the belt travels on a hearth and conveys the components from one cold end to the other, then returns by looping back under the furnace. The belt is usually fabricated from a woven stainless steel wire, but might be formed from refractory metals, ceramics, or graphite to reach higher temperatures. Belt furnaces account for more than 80 % of all installed ferrous sintering capacity. Typical widths range from 15 cm to 60 cm and typical belt velocities range up to 20 cm/min. Belt speed and hot zone length determine the time the components are at temperature; typical combinations give times at the peak temperature between 10 and 40 min. Preheat zones burnout the polymers and bring the work up to the sintering temperature on its way to the high heat zone. A final zone is used to cool the components before exit from the furnace. The maximum loading is about 50 kg per m2 of belt, giving typical production rates about 70 to 300 kg/h. Lower loads are necessary at higher temperatures to extend the belt life. Occasionally, the floor of the furnace is inclined from both ends to form a humpback, where the highest point is also the hottest.

bend test Another name for the transverse rupture test used to measure the strength of brittle compacts, typically green compacts in powder metallurgy or brittle sintered materials, including ceramics. A rectangular brittle sample is fractured in 3-point or 4-point bending (note the test is invalid for ductile metals). Fracture must initiate in the outer fiber that is in pure tension. The transverse rupture strength σT is calculated from the specimen geometry and failure load FB as,

where T is the thickness, W is the width, and L is the span distance between the lower support rods. Typical sample dimensions are T = 6 mm (0.25 in), L = 25 mm (1 in), and W = 12 mm (0.5 in). Over a broad array of brittle materials, the bend test strength averages just 25 % of the compressive strength. In powder metallurgy, this bend strength is commonly referred to as the transverse rupture strength while in ceramics it is commonly referred to as the modulus of rupture. The test is invalid for ductile metals, although a value is frequently reported by powder metallurgists not familiar with the underlying assumptions behind the stress calculations.

beryllium Element 4, this light metal is almost always first formed into a powder and that powder is consolidated using hot isostatic pressing or hot pressing. It is a favorite in the

Page 5

Section B

aerospace industry due to the high elastic modulus and low density, and beryllium is alloyed with 38 wt. % aluminum to form an alloy with a high stiffness per unit weight. At low temperatures beryllium is a hexagonal close-packed crystal structure, but transforms to a body-centered cubic crystal just prior to melting. The pure element has the following properties when consolidated by hot pressing or hot isostatic pressing (strength variation depends on the oxide inclusions): beryllium

Be

Be-38Al

atomic number

4

atomic weight

9.01

density

1.85

2.09

g/cm3

melting temperature

1289

644

EC

boiling temperature

2970

EC

heat of fusion

9.8

kJ/mol

heat capacity

1880

1650

J/(kg EC)

thermal expansion coefficient

12.3

16.2

ppm/EC

thermal conductivity

194

220

W/(m EC)

electrical resistivity

4

g/mol

µΩ-cm

elastic modulus

290

200

Poisson뭩 ratio

0.029

0.18

hardness as-sintered yield strength as-sintered elongation to fracture as-sintered ultimate tensile strength

80

GPa

BHN

240-370

428

MPa

3-16

10

%

290-530

468

MPa

fracture toughness 10 MPa(m)½ There are concerns with handling beryllium powder, but the high strength to weight ratio is of great interest for high stiffness aerospace components.

beryllium-aluminum An aerospace alloy that delivers high stiffness per unit density (high specific modulus) based on a composition nominally consisting of beryllium with 30 wt. % aluminum. It can

Section B

Page 6

be fabricated by either hot isostatic pressing or by casting. The density is 2.19 g/cm3, elastic modulus is 207 GPa, yield strength is 213 MPa, fracture elongation is 4 %, ultimate tensile strength is 290 MPa, and thermal conductivity is 108 W/(m EC).

BET surface area The specific surface area as measured by gas adsorption according to the Brunauer, Emmett and Teller theory developed in 1938. Under equilibrium, the rate of adsorption equals the rate of evaporation. It is possible to calculate the free surface area of a powder by changing the amount of gas available for absorption on the surface. Changes are made to the pressure of the absorption gas (usually nitrogen) at a temperature where it condenses on the powder surface (usually at liquid nitrogen temperatures). Calculations allow for estimation of the surface area from the relation between pressure and amount absorbed. A schematic of the measurement is shown in Figure B.6. A glass holder immersed into a chilling fluid such as liquid nitrogen contains the sample.

BHN The abbreviation for Brinell hardness number.

bimetallic component A single component that has a designed change in chemical composition with location to customize the material placement to the intended function. Early examples included wear alloys bonded onto lower cost steels. Recent variants have been formed by powder injection molding to give combinations such as porous and dense, magnetic and nonmagnetic, and corrosion resistant and high strength. The technology is an adaption from plastics technology known as two-color molding. In production the molding machine consists of two barrels, each containing a different material. The tool cavity rotates between the two barrels, allowing simultaneous filling with both feedstocks into two different cavities. Since the component is formed in two steps; the first step is associated with molding the more 밿nner? material, immediately followed by a second molding step to form the second material over the first. The tool cavity is expanded after filling with the first feedstock, thereby giving precise placement of the second portion.

bimodal powder A particle size distribution exhibiting two mode sizes. Such a powder can result from mixing two different particle size distributions. The key to improved packing with a bimodal powder rests with the particle size ratio. Small particles are selected to fill the interstices between large particles without forcing the large particles apart. In turn, even smaller particles can be selected to fit into the remaining voids, giving a further improvement in packing density. The basic behavior is sketched in Figure B.7. The fractional density is shown as a function of composition for a mixture of large and small

Section B

Page 7

spherical particles. At the maximum packing composition there is a greater volume of large particles than small particles. The relative improvement in packing density depends on the size ratio of the large and small particles. Beginning with the large particles, the packing density initially increases as small particles are added to fill the voids between the large particles. That corresponds to the right-hand side of the figure. Eventually, enough small particles are added to fill all of the spaces between the large particles. Any higher concentration of small particles forces the large particles apart and no longer improves the packing density. In contrast, starting with the small particles, clusters of small particles and their associated voids are replaced by large particles. Since the large particles are fully dense, a porous region is replaced with a full density region everywhere a large particle is added. The packing benefit of replacing small particles with large particles continues until a concentration where the large particles contact one another. This is shown on the left-hand plot. The point of maximum packing density corresponds to the intersection of those two curves; the large particles are in point contact with one another and all of the interstitial voids are filled with small particles. The optimal composition in terms of the weight fraction of large particles X* depends on the amount of void space between large particles, which equals 1 ! fL, where fL is the fractional packing density of the large particles,

with the packing density at the optimal composition f * given as,

where the fractional packing density for the small particles is fS. For two spherical powders with a large size difference and an ideal fractional density of 0.64, the corresponding weight fraction of large particles for maximum packing is 0.734, or 26.6 wt. % of the smaller particles. The expected fractional packing density would be 0.87 or 87 %.

binder Substances which serve as cementing and lubricating agents to improve green strength; a polymer added to the powder prior to compaction or molding to increase the fluidity during forming or to improve the green strength of the compact. Binders used in powder metallurgy include simple glues, such as rubber cement, and natural polymers, such as starch, and thermoplastics such as paraffin wax or polyethylene. A favorite binder system for metal powders is 65 % paraffin wax, 30 % polypropylene, and 5 % stearic acid. This binder is expelled during heating to the sintering temperature; thus, it is a temporary vehicle for packing a powder into the desired shape and holding the particles in that shape until the beginning of sintering. In some cases the term is applied to materials added to a powder for the specific purpose of cementing together the grains

Section B

Page 8

during sintering, thereby providing a strong sintered body. In pressing hard particles, the polymer binder might be a wax and in sintering a hard particle the cementing phase might be cobalt. Thus, in WC-Co processing there are both polymer and metal binders. Some of the more common binders in metal powder processing include paraffin wax, polyethylene glycol, polyvinyl alcohol, methylcellulose, and on rare occasion some silicates. [see paraffin wax, methylcellulose, polyvinyl alcohol, polyethylene glycol]

Bingham flow Viscous flow of a feedstock with an initial yield strength, meaning the applied stress must exceed the yield strength prior to initiation of viscous flow. In the Bingham model, a yield strength must be overcome to obtain flow. From that point on, the viscosity is essentially proportional to the shear stress, less the initial yield strength. Compared with the Newtonian model, the Bingham response is more realistic for powder-polymer mixtures. To a point, faster shear rates and higher temperatures enable easier forming of powder systems. [see Newtonian flow]

biocompatible materials A broad group of materials commonly used for body implants, artificial bones or tissue attachment materials, including platinum, cobalt-chromium, tantalum, titanium, and various ceramics such as zirconia, alumina, and hydroxyapatite. These materials share a common resistance to corrosion and tissue reaction over long exposure times. Further, biocompatible materials do not release toxic substances and do not irritate contacting tissue. Generally problems with corrosion, nickel sensitivity, and toxicity lead to exclusion of several stainless steels from the list of accepted biocompatible materials in recent times. Thus, porous tantalum and titanium are more commonly used for tissue ingrowth. After a device is implanted, it becomes fixed to the surrounding tissue over time as tissue grows to fill the pores.

blank A pressed, presintered or fully sintered compact, usually in the unfinished condition, requiring cutting, machining or some other operation to give a final shape.

blended powders Powders of the same composition that are intermixed to homogenize the particles to ensure a consistent particle size at the forming machine. Mixing is a very similar process, but it implies the particles are different in composition.

blending The thorough intermingling of powders of the same nominal composition but from

Section B

Page 9

different production lots or particle size ranges. Blending is intended to remove segregation that might be induced by vibration during transport. As sketched in Figure B.8, powders separate by size when vibrated, similar to mixed nuts where the largest rise to the top during vibration. Such segregation leads to uneven compaction and sintering. Although there are three causes of powder segregation (differences in particle size, density, and shape), size segregation is dominant. A powder will easily segregate by size if the small particles pass through the voids between the large particles. One consequence of size segregation is that the overall apparent density decreases with size segregation. An irregular particle shape will inhibit size segregation. Likewise, less size segregation occurs with particle sizes below approximately 100 μm because of the greater interparticle friction. Blending is the normal process to remove size segregation after transport. Optimal blending is for a short time, using dry powder.

blind pore A dead-ended pore connected to the surface. [see closed pore]

blinding The building up of small particles on a screen surface due to blocking or partial filling of the openings by particles that are too large to pass through the screen openings.

blister A defect on the component surface resulting from gas entrapment or gas generation inside the material during sintering. Figure B.9 shows blisters formed during sintering of a tungsten heavy alloy that generated water vapor as a reaction product inside the compact. In this case sintering was performed in hydrogen, which reacted with remaining oxygen in the powder to produce steam that caused the blisters.

boat (sintering) A container or tray for transport of a material through a sintering furnace, usually with a side lip to create a depression in which the powder or compacts are place. Very similar to a sintering substrate.

boiling water test A test for corrosion. It consists of immersing a test sample in boiling, distilled water for 30 min. After 30 min the heat source is turned off, the sample is cooled and remains in the water for an additional 3 h. Then the specimen is removed, and left to dry for 2 h prior to inspection for signs of corrosion.

Boltzmann뭩 constant A fundamental energy increment associated with atomic motion, equal to 1.38A10-23 J/K

Section B

Page 10

for each atom; when multiplied by Avagadro뭩 number gives the universal gas constant.

bonded powders Mixed powders are sometimes treated with a polymer to reduce separation during transport and handling due to differences in particle, shape, or density. Magnets are some of the most common structures formed from bonded powder-polymer mixtures. In this situation the composite is magnetic but not electrically conductive. Ferrites used in refrigerator magnets are a prime example of polymer bonded powder magnets.

bonding The joining of compacted or loose particles or components into a single component, most commonly by the application of heat, usually termed sintering.

borides Compounds that involve boron as the nonmetallic species, such as titanium diboride (TiB2). Often they are very hard, brittle, and difficult to process. Generally considered to be ceramic compounds, but not intermetallic compounds. The unique behavior of the borides gives rise to special property combinations. The most important of the borides is B4C, which is difficult to consolidate so it is often mixed with other materials to assist with consolidation. It can reach a consolidated strength near 680 MPa and fracture toughness up to 16 MPa(m)½ depending on the amount of binder phase. The high hardness and resistance to high temperature exposure results in applications such as cutting tool and zirconium boride and titanium boride are used as an electrical conductor in electrical discharge machining. Common borides and their densities are as follows: chromium boride CrB (6.14 g/cm3), hafnium boride HfB2 (11.19 g/cm3), tantalum boride TaB2 (12.54 g/cm3), titanium boride TiB2 (4.52 g/cm3), vanadium boride VB (5.60 g/cm3), and zirconium boride ZrB2 (6.10 g/cm3).

boring A metal cutting process that relies on a single point rotating tool and stationary work. Boring is used to cut large holes or large diameter circular contours.

boron (B) Element number 5 with an atomic weight of 10.811 g/mol, boron is a covalently bonded ceramic that exhibits a metallic luster. It has a high melting temperature (variously reported in the 2100EC range), density of 2.4 g/cm3 and rhombohedral crystal structure. Its chief use in powder metallurgy is for melting point depression, since many of the transition metals (iron, chromium, nickel) have a low solubility for boron, yet form deep eutectics that are useful in supersolidus liquid phase sintering; for example, large stainless steel powders sinter to full density at 1250EC if they are doped with 0.3 to 0.5 wt. % boron. To prevent oxidation during handling, it is common to use boron

Section B

Page 11

compounds, such as iron boride or nickel boride.

boron carbide (B4C) A high melting point (2450EC) compound of boron and carbon with exceptional hardness, good strength (typical bend strength exceeds 500 MPa), low density (2.52 g/cm3), and excellent resistance to chemical agents. It has desirable properties with respect to nuclear energy. Hot pressing or hot isostatic pressing require temperatures in excess of 2000EC and pressures in the 20 to 40 MPa range. When silicon carbide is added, the mixture can be hot pressed to full density at modest temperatures, giving a strength of 350 MPa and elastic modulus of 372 GPa. For temperatures over 1300EC, boron carbide is harder than diamond. The combination of high hardness, strength, and low density is useful in ceramic armor. In sintering metals, boron carbide is useful as a substrate if no liquid metal is formed in the cycle.

boron nitride (BN) A high melting temperature compound of boron and nitrogen BN with good chemical resistance up to high temperatures, making it a good choice for atomization components, crucibles, sintering substrates, high temperature lubricants, and furnace hardware. There are several polymorphs, so there is the opportunity to change properties with crystal structure. Hot pressed boron nitride has excellent thermal shock resistance, making it ideal for liquid metal atomization applications. Hexagonal boron nitride is soft and easily machined (Mohs hardness of 2) and has a density of 2.25 g/cm3, thermal conductivity similar to stainless steel, low elastic modulus (75 GPa), and low thermal expansion coefficient. On the other hand cubic boron nitride is second in hardness to diamond and is used for cutting tools and in a composite (metal matrix) is often the preferred machining tool for ferrous powder metallurgy components.

bottom punch Another name for the lower punch. [see lower punch]

Boudouard뭩 reaction The chemical oxidation of carbon, usually in the form of graphite, with oxygen, such as from air, to form carbon monoxide; 2C (s) + O2 (g) W 2CO (g) There are several related reactions, such as graphite reacting with water vapor to form carbon monoxide and hydrogen, or hydrocarbons such as methane reacting with oxygen to form carbon monoxide and hydrogen.

Bragg뭩 law In a diffraction pattern, such as in x-ray diffraction, this law relates to the conditions that

Page 12

Section B

lead to the formation of high intensity diffraction peaks based on the spacing between parallel atomic planes. When the crystal is small, destructive interference is not efficient at angles slightly off the Bragg condition given as,

where λ is the X-ray wavelength, dhkl is the interplanar spacing, and θ is the diffraction angle. Bragg뭩 law tells the angle at which a diffraction peak will occur, but the width of that diffraction peak depends on the crystal size. It provides information on the atomic size, crystal type, and residual strains. William Henry Bragg (1862 to 1942) and William Lawrence Bragg (1890 to 1971) were English physicists that formed a father-son team. They demonstrated this law for x-ray diffraction for alkali halide crystals (such as NaCl) in 1912. Their contribution was recognized by the 1915 Nobel Prize in Physics.

brass Alloys of copper that usually include zinc as a main ingredient. The abbreviation CZ is sometimes used to designate a brass alloy, with C representing copper and Z representing zinc. This might be followed by a numerical composition designation. At full density and a content of 30 wt. % zinc, brass has the following properties: brass

Cu-30Zn

density

8.5

g/cm3

melting temperature

965

EC

heat capacity

377

J/(kg EC)

thermal expansion coefficient

19.9

ppm/EC

thermal conductivity

121

W/(m EC)

electrical resistivity

6.2

µΩ-cm

elastic modulus

101

GPa

Poisson뭩 ratio

0.33

as-sintered yield strength

350

MPa

23

%

as-sintered elongation to fracture

as-sintered ultimate tensile strength 427 MPa With porosity all of these properties fall. When the zinc content increases to 40 wt. %, the strength decreases and ductility increases. Zinc evaporation during sintering is a persistent difficulty in the fabrication of powder metallurgy brass components. In recent years that evaporation has been used to create nanoscale pores in thin sheets,

Section B

Page 13

providing molecular scale filtration.

Brazilian test A strength test that is best applied to brittle materials, especially green compacts. The test set up is shown in Figure B.10. The compact is a simple disk shape. This disk is loaded between two parallel platens and compressed to rupture, leading to a calculated strength σB based on the peak breaking load FB, disk thickness T, and disk diameter D as follows:

The samples tend to be 5 mm to 7 mm thick and the diameter tends to be two to four times larger than the thickness (nominally 25 mm). In contrast, the radial crush test relies on a hollow, long cylinder, while the Brazilian test requires a short, solid cylinder. Strengths by the Brazilian test are lower than obtained using a simple flat-ended cylindrical compressive test.

brazing Joining of two or more components using a low melting filler metal that melts at the braze temperature. The molten braze flows, wets, and solidifies to bond the contacting components; the liquid braze wicks into a gap of approximately 0.5 mm width to create the metallurgical bond. Most powder metallurgy brazing occurs at far higher temperatures than soldering, well over the 450EC (the typical nominal temperature used to separate soldering from brazing). Typical braze materials are based on copper, nickel, and silver, with additives to depress the melting point.

bridging The formation of arched cavities in a powder mass due to poor packing. This tends to be most prevalent near container or die walls. Figure B.11 shows a schematic of particle bridging.

Brinell hardness (BHN) A measure of the resistance to indentation based on forcing a hardened steel ball indenter into a material, where the BHN is the Brinell hardness number given as follows:

where F is the test force in N, D is the ball diameter in mm, and d is the mean diameter of the impression on the material surface in mm. The typical ball is 10 mm in diameter. The test was developed by a Swedish metallurgist, Johan August Brinell (1849 to 1925)

Page 14

Section B who first displayed his test for steels in the 1900 Paris Exposition.

briquet Another term for a compact to indicate a powder that is compressed in a die to form a green body, usually assisted by mixing a binder with the powder.

brittle A material that fails without bending or significant deformation under load. Usually a brittle material is characterized by a low ductility, low toughness, and poor resistance to cracking. Most ceramics and glasses are brittle, while most metals are ductile.

broaching A metal cutting process that relies on a bar with aligned multiple tool tips of increasing size. The broach tool is moved back and forth in a reciprocating push-pull motion while the part is held stationary. It is used to cut grooves, holes, or other features.

bronze Alloys of copper that often contain tin or phosphorous. In some powder metallurgy situations these are given the CT designation (C is copper and T is tin) followed by a designation that gives the approximate composition. Additionally for bearings the bronze might include graphite leading to a CTG designation, where G represents graphite. Porosity degrades the properties, yet for oil-less bearing applications pores are a necessary attribute. The most popular compositions are near 10 wt. % tin. The simple Cu-10Sn bronze has excellent properties when consolidated to full-density as follows: bronze

Cu-10Sn

density

8.88

g/cm3

solidus temperature

840

EC

liquidus temperature

1020

EC

heat capacity

377

J/(kg EC)

thermal expansion coefficient

18.5

ppm/EC

thermal conductivity

55

W/(m EC)

electrical resistivity

18

µΩ-cm

elastic modulus

117

GPa

Poisson뭩 ratio

0.33

Page 15

Section B hardness

85

BNH

yield strength

83

MPa

elongation to fracture

50

%

ultimate tensile strength 262 MPa When fabricated into a porous bearing, the density is much lower (in the 5.8 to 6.8 g/cm3 range) with concomitant decrements in the properties. For bronze bearings formed using the press-sinter route, the strength tends to be measured using the radial crush test, giving K-factors in the range from 100 MPa to 180 MPa, but the tensile strength is significantly lower and might range near 35 MPa.

brown state A term originally used to describe a ferrous component following debinding by slow heating in air to remove the polymer and oxidize the iron. Heating in the presence of oxygen causes the iron to grow a thick oxide that bonds the particles, but also gives a rust-colored component which is the source of the name. Today the brown condition is generally associated with any component that has been debound but not sintered, independent of the debinding process.

Brownian motion Small particles will vibrate and move in a random manner due to unbalanced molecular collisions from the surrounding fluid (gas or liquid); also know as thermally induced motion. As the particle size and mass decrease there is greater opportunity for air or other molecules to strike the particle in an imbalanced mode that will cause it to move. The theory of Brownian motion was developed by Albert Einstein (Investigations on Theory of Brownian Motion published in 1926, five years after his Nobel Prize in Physics) and largely says the average velocity is proportion to the inverse of the particle size. Large particles do not show this vibration or jitter, but nanoscale powders are always in motion and prove difficult to image due to this motion. Some particle size analyzers designed for nanoscale particles measure the Doppler shift in a laser beam due to the mean particle motion to calculate the average particle size.

brushes Sliding contacts for electric motors, usually formed by compacting mixtures of metal powders and graphite. Where the metal provides electrical conductivity and the graphite provides sliding wear resistance.

bubble point The pressure to force gas through an open pore structure, that was previously saturated

Section B

Page 16

with a wetting liquid, gives a rough measure of the pore size. Figure B.12 shows a schematic of the bubble point test. It relies on the Washburn equation to estimate the pore size. In the field of filtration the pressure required to form the first bubble is related to the debris capture characteristics of the filter. For the bubble point test, alcohol is the typical liquid because it has a near-zero contact angle, so θ = 0? and the alcohol spontaneously flows into the pores. Then a counter-pressure is required to displace the alcohol from the pores, and that pressure is higher with smaller pores. After first bubbling, it is common to continue pressurization and to report the pore size of uniform bubbling across the surface. Further, some tests combine the bubble point with simultaneous measurement of the gas flow rate versus applied pressure to characterize both the permeability and pore size. The bubble point test has merit as a quality control tool, but is limited for quantitative pore structure analysis. [see Washburn equation]

bulk density The density a powder assumes in a shipping container and in laboratory tests. This should be close to the measured apparent density for the powder. [see apparent density]

bulk transport controlled sintering Mass flow in sintering that induces densification because atomic motion annihilates vacancies. Typically the atoms move from the grain boundary between contacting grains and deposit into the pores. The pores represent a collection of vacancies, so atomic filling of the pores is equivalent to annihilation of vacancies. The net result is center-to-center motion of contacting particles and grains with densification and the elimination of pores. This can be seen by a bulk dimensional shrinkage during sintering. Operative mechanisms include volume diffusion through the atomic lattice, plastic flow, dislocation climb, and grain boundary diffusion. In contrast, surface transport processes lead to increased strength, but no densification. Grain-boundary diffusion is usually the dominant mechanism for the densification of most metals. While both surface- and bulktransport processes give neck growth, the main difference is in density (or shrinkage) during sintering. Generally, bulk-transport processes dominate at higher temperatures. [see volume diffusion, grain boundary diffusion, dislocation climb, plastic flow]

bulkiness A descriptor for particle shape based on the particle projected area in a microscopic image divided by the particle length times the particle width (essentially the area of a box holding the particle image). For a sphere it is 0.78 (π/4) while for nonspherical particles it tends to range lower.

burnishing

Page 17

Section B

A means to slightly deform and smooth a surface based on a reciprocating round hard tool and stationary component. The motion of the tool over the surface induces friction that causes shear displacement to harden, smooth, or polish the surface. Burnishing involves the application of a high shear rolling force. It is useful for correcting the diameter of internal bores. Burnishing is also used to improve the rolling contact fatigue life of gears, with up to a 40 % increase in allowed load. Burnishing is most effective when applied with several small deformation cycles rather than one large cycle.

burnoff The removal of the polymer binder or lubricant via preheating prior to sintering. The polymer burnout is more specifically termed dewaxing in the cemented carbide field, thermal debinding in the metal powder injection molding field, and delubrication in the traditional die compaction field. All are variants of burnout processes. [see burnout, delubrication, thermal debinding, dewaxing]

burnout A stage during heating to the sintering temperature where polymer burnoff occurs. Depending on the field, burnout and burnoff processes are given terms such as dewaxing, delubrication, and debinding, yet they are similar events designed to remove polymers used in powder shaping or compaction. A first function of the sintering furnace is to remove the polymer additions used for shaping, pressing, or bonding the particles. The process is properly termed pyrolysis, meaning the burning out of the polymer, but is often just termed the burnout stage. Usually this means the polymer is heated to a temperature where it becomes unstable and evaporates. Heat first melts the polymer and then breaks down its molecular bonds, forming small molecules that evaporate out of the compact. Most polymers used in powder metallurgy contain the same basic carbon-carbon, carbon-oxygen, and carbon-hydrogen bonds. Thus, they burnout over the same temperature ranges, in spite of apparent differences in bulk chemistry and lubrication properties. Table B.1 summarizes some of the burnout temperatures in nitrogen. Table B.1. Polymer Decomposition Temperatures (temperatures for 50 % polymer evaporation during heating in nitrogen) polymer

temperature, EC

paraffin wax

295

polyethylene

414

poly(ethylene oxide)

345

polypropylene

387

polystyrene

364

Page 18

Section B poly(methyl methacrylate)

237

poly(vinyl acetate)

269

poly(vinyl alcohol) 274 The evaporating molecules are mixtures of methane CH4, carbon dioxide CO2, carbon monoxide CO, water H2O, and other combustion byproducts. Often polymer burnout is assisted by adding active agents to the atmosphere. A common procedure is to add a small amount of water, oxygen, or carbon dioxide to ensure removal of residual carbon. If the atmosphere is neutral, for example argon, then it is only possible to form volatile molecules via polymer cleavage. Unfortunately, this leads to a deficiency of hydrogen or oxygen, with the possible formation of soot. This is a black dust on the compacts that is clear evidence of rapid heating and low oxidation potential in the delube atmosphere. For very high green densities, the lack of open porosity inhibits polymer evaporation and slower heating rates are required. In advanced sintering furnaces, the atmosphere and gas flow are designed to help lubricant removal without slowing production. This requires a gently increasing temperature and often a long heating zone. Special gas inlets in the burnoff zone of a continuous sintering furnace allow the addition of oxidizing species, such as water, to help in polymer burnout without sooting.

burrs Small asperities located on the edge or corner of a component that usually reflect smeared or pinched metal from the forming operation.

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A to Z of Powder Metallurgy

C C An abbreviation used in some powder metallurgy efforts to designate essentially pure copper (minimum 99.8% copper). Also the chemical symbol for carbon.

CAD An abbreviation for computer-aided design.

CAE An abbreviation for computer-aided engineering.

cake A coalesced mass of unpressed metal powder. For example the sponge-like agglomerated powder formed by passing an oxide powder through a reduction step. To deagglomerate the cake, it is typical to mill the sponge into particles prior to packaging.

calcination Heating in air or oxygen to ensure full and stable oxides are formed. Often used in ceramic powder preparation to ensure no change in chemistry during sintering. Likewise, a step applied to various metal powder production processes where an intermediate chemistry is stabilized prior to hydrogen reduction, for example salts or compounds of tungsten are calcined into WO3 prior to hydrogen reduction into tungsten.

calibration The act or process used to determine the output from a device and the possible bias or error in that output. For example, in microscopy a standard reference material is used to properly adjust the magnification prior to analysis of an unknown sample.

calorimetry A general term describing a measurement in which heat flow is measured as a chemical reaction process occurs, such as during the oxidation of a metal.

CAM

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Page 2

An abbreviation for computer-aided manufacturing.

Cam-Clay model A constitutive model used in computer simulations to treat the compaction of metal powders. The name comes from the original development at Cambridge University of a soil and clay compaction model.

CAP The abbreviation for consolidation at atmospheric pressure.

Cap model A constitutive model based on the Drucker-Prager concept used in computer simulations of die compaction for metal powders. It assumes the powder exhibits work hardening as it approaches a perfectly plastic state during deformation.

capillarity A general description of the pressure difference caused by a curved surface. When a liquid encounters a curved surface the liquid will either be absorbed due to wetting or repulsed due to nonwetting. Capillary events are involved in several important powder metallurgy processes or tests, such as pore characterization using the bubble point test, pore size distribution measurement using mercury porosimetry, and pore filing by polymer impregnation or liquid metal infiltration. Capillarity results in a strong surface densification force on the solid when a liquid forms, causing rearrangement, densification, and contact flattening. As illustrated by the pendular bond in Figure C.1, the capillary force F between two spheres with a pendular bond of wetting liquid is attractive and can be approximated by 5 γLV D within a factor of two for most cases, where D is the particle diameter and γLV is the liquid-vapor surface energy. The attractive force works to achieve zero separation between the particles and is the reason for a sudden dimensional change on liquid formation in liquid phase sintering. Alternatively, for a non-wetting liquid, the liquid causes separation of the particles, leading to swelling. [see wicking]

capillary force The attractive force acting at contacting surfaces, such as particles, from a wetting liquid in the form of pendular bonds. [see capillarity, pendular bond]

capillary rheometer A device for measuring viscosity by applying pressure on molten feedstock (a mixture of powder and polymeric binder) and determining the flow rate dependence on applied

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Page 3

stress as the feedstock is pushed through a small capillary tube. The conceptual layout of the test is shown in Figure C.2. It is useful in determining feedstock flow and homogeneity. Viscosity is determined by the pressure drop and flow rate, using the Poiseuille equation, when the capillary diameter and length are known. Homogeneity is measured by the chatter in that viscosity as regions of excess binder (low viscosity) and excess powder (high viscosity) pass through the capillary.

capital costs Major equipment is termed capital equipment and the cost of this equipment is a burden on manufacturing costs for several years in the future. Thus, capital equipment costs are a factor in determining the cost of production and the final component price. The purchase price to qualify as capital costs vary by region, but presses, molders, and furnaces are common capital costs in powder metallurgy.

carbides Compound phases consisting of a metal and carbon, with common examples involving iron (Fe3C), vanadium (VC), silicon (SiC), chromium (Cr3C2), hafnium (HfC), molybdenum (Mo2C), tungsten (WC), vanadium (VC), titanium (TiC), tantalum (TaC), or other metals. Carbides tend to be hard, narrow in stoichiometry, and are generally thermodynamically stable to high temperatures in inert atmospheres, but might react with oxygen. The most important and technologically useful carbides are compounds with refractory metals, where the metallic atom size is sufficiently large that the carbon acts as an interstitial. These are widely used in powder metallurgy cermets and compounds and tool steels, including titanium carbide (TiC), zirconium carbide (ZrC), hafnium carbide (HfC), niobium carbide (NbC), tantalum carbide (TaC), molybdenum carbide (MoC), and tungsten carbide (WC). These are all hard, stable, and high melting temperature compounds useful in the formulation of wear and cutting tools.

carbo-nitride A surface treatment for sintered powder metallurgy steels, where they are heated after sintering in an atmosphere doped with ammonia to induce both carbon and nitrogen hardening. The carbo-nitrides form on the compact surface where they induce a higher strength and hardness. In some cases sintered tool steels have been produced with tensile strengths over 3 GPa. The typical treatment temperature for a powder metallurgy steel is in the 800EC to 850EC range for 30 min to 60 min, giving a typical nitride depth of 0.5 mm. A related technique is termed nitro-carburization, where nitrogen and carbon are reacted and deposited at the component surface. This technique is only useful for high density steel components, typically more than 7.0 g/cm3. Processing temperatures are between 570EC and 600EC in an atmosphere of 50% ammonia and 50% endothermic gas (an air-methane mixture). The surface reaction slightly expands the component, but the greatly improved wear resistance is critical for sliding wear

Section C

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applications. A typical surface hardness of 60 HRC is measured after nitrocarburization.

carbon-carbon composite A carbon fiber-reinforced composite with graphite or carbon as the matrix or filler phase. The carbon matrix is typically formed by infiltration of the pores between the fibers with an organic material that is thermally converted to form graphite. These composites are very durable at high temperatures and are used for substrates, trays, and furnace hardware, as well as high performance aerospace bodies.

carbon control A measure of the sophistication of a production operation, based on the ability to remove a processing polymer and to manage oxygen-carbon levels in the powder during its consolidation to attain the specified final carbon level. Carbon control is an important aspect of successful heat treatment. Carbon control is quantified by the final carbon level and the uniformity of that level between parts, over several days, possibly using different furnaces and material batches. For some materials the desire is no final carbon (for example in austenitic stainless steels), while others require a precise final carbon level (for example in carbon steels and tool steels). In most structural steels the carbon level is adjusted in the initial powder mixture (where the initial oxygen level is known) to obtain a target value after sintering to meet specifications for hardness, strength, and ductility in an application. Proper control requires anticipation of decarburization from any oxygen in the powder or process atmosphere. In all heat treatments, it is important to minimize furnace overloading and to avoid overlapping of parts so that uniform heat transfer occurs to give the same carbon level and hardness to all regions.

carbon potential An atmosphere parameter that indicates the final carbon level expected for a steel processed in that atmosphere at the sintering temperature. In a carburization treatment, the atmosphere carbon potential establishes the constant surface source carbon level and the mobility of the carbon in the lattice then determines the hardness versus depth profile.

carbon steel Alloys generally consisting of just iron and carbon with minor alloying additions. In powder metallurgy they provide moderate strength except when forged to full density. Without other alloying ingredients, carbon steels are inexpensive compositions, but relatively weak. However, they do find applications because of the soft magnetic properties at low carbon levels. The most popular carbon steel alloys range from 0.2 to 0.5 wt.% C. These alloys are sometimes abbreviated as F for iron, followed by

Section C

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numerical indices to show the target carbon range; for example the designation F-0005 indicates Fe with 0.5 wt.% C. Alternatively, the AISI designation is simply 1050 for the 0.5 wt. % C composition.

carbonitriding Introduction of carbon and nitrogen into a ferrous alloy by holding above the temperature at which austenite forms in an environment containing the two species.

carbonyl powder A metal powder prepared by the thermal decomposition of a metal carbonyl molecule; such as iron carbonyl Fe(CO)5 or nickel carbonyl Ni(CO)4 precursors. The resulting particle size is typically in the range from 1 탆 to 10 탆 . The particle shape can be spherical, agglomerated, or highly angular. Figure C.3 is a scanning electron micrograph of a carbonyl nickel powder, showing the outer structure. A cross-section would show an inner onion-like layered structure. Carbonyl powders are formed by vapor decomposition and condensation of the metal. For example, wrought nickel is reacted with carbon monoxide to form nickel carbonyl using simultaneous heating and pressurization. The resulting carbonyl molecule is cooled to a liquid at 43EC and fractional distillation is used for purification. Reheating the liquid in the presence of a catalyst leads to vapor decomposition into a powder. Besides iron and nickel, other metals can be formed, but the high energy utilization and health hazards of carbonyl molecules discourage widespread use beyond nickel and iron.

carbothermic process A means to remove oxygen from a metal oxide using carbon (usually derived from a carbon source such as graphite) via the formation of carbon monoxide or carbon dioxide at high temperatures. The most famous version is in the formation of aluminum from alumina, with the release of substantial vapor.

carburization Heating a component in contact with a carbon source, typically in an atmosphere containing methane (CH4), to provide additional carbon at the surface and in the surface connected pores to harden the surface. It is possible to carburize a material if heated quickly, where any polymer in the pores will decompose into graphite which increases the carbon content of the sintered material.

carburization-decarburization Two events critically related to carbon control, since the addition of carbon to a material occurs by carburization from a decomposing binder, graphite addition, or atmosphere source, while decarburization occurs by reacting carbon in the material with hydrogen, oxygen, or carbon monoxide. These are reversible reactions that are controlled by the

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process atmosphere.

carburize The introduction of carbon into a solid ferrous alloy by holding above the temperature at which austenite forms in the presence of a carbon source, such as graphite or methane.

Carney funnel A funnel used in the Hall Flowmeter apparatus test but with a 5 mm diameter opening in the funnel (versus 2.5 mm for the Hall funnel). It is used to test flow time and apparent density of powders that tend to be uneven in flow or will not flow in the Hall funnel.

case carburize A post-sintering heat treatment aimed at diffusion of carbon into the surface of a compact to increase the surface strength and hardness as desired for improved wear or fatigue resistance. The process occurs by the penetration of carbon into the surface from an atmosphere source - in a solid metal this is purely by diffusion, but in a porous metal there is often faster penetration by permeation through the open pores. The depth of carburization X depends on the square-root of hold time t for a given situation,

The factor K is a rate constant that depends on the temperature, material, and open porosity. Usually the case depth is measured by the distance into the compact that achieves a hardness of at least 50 HRC. As an example of the carburization rates in vacuum, a depth of 0.5 mm is formed for a 7.0 g/cm3 compact heated at 840EC for one hour. Alternatively, the same compact density gives a depth of 0.6 mm after one hour at 925EC. Carburization is slower if the pores are closed. Closed pores cannot contribute to vapor permeation to transfer carbon into the interior regions. For steels, case carburization is not recommended when the porosity exceeds 15% (about 6.7 g/cm3) or the base carbon level exceeds about 0.4%. Most carburization treatments are done in a protective atmosphere, but it is possible to use a vacuum as well.

case depth A measure of the penetration of hardening carbon and transformation of a steel component into a high hardness surface layer from a case carburization treatment. Usually this depth is intentionally small, less than 1 mm.

case harden Use of a packed powder or atmosphere gas to diffuse carbon into the exterior surface of the component to increase surface hardness. [see case carburize]

Section C

Page 7

casting An object obtained by solidification of a substance in a mold. For metals this is the oldest fabrication route, with the cavity first formed in a sand cavity and the molten metal poured into that cavity where it solidifies to take on the cavity shape. In powder metallurgy, slurry casting is a similar process, but only the wax or polymer containing the metal powders is melted.

catalytic debinding The partial removal of a polymer used as a binder in powder injection molding via heating in an atmosphere containing an agent that induces depolymerization. The most common form involves extraction of polyacetal using a nitrogen atmosphere doped with nitric acid. For this reaction the byproduct is formaldehyde that must be treated prior to atmosphere release.

cathode The negative electrode in an electrolytic cell at which reduction is the principal reaction. Electrons flow to the cathode through an external circuit. Typical cathodic processes allow for the combining of cations (ionized metals) with electrons to deposit solid metal coatings or powder. Electrolytic powders are harvested at the cathode.

CCT An abbreviation for continuous cooling transformation.

cellulose A natural powder derived from wood or cotton that can be mixed with water to form a reverse thermoplastic binder for extrusion, die compaction, injection molding, or other powder forming process. Natural cellulose powders are slightly soluble in water. The powder-cellulose-water mixture has its lowest viscosity when cold and increases viscosity with heating. Unlike traditional thermoplastic or wax binders, which are soft when hot, this system has the opposite behavior. Unfortunately most of the common cellulose binders contaminate the powder and furnace during burnout, so their use is discouraged except where firing in oxygen or air is not harmful to the component. Thus, cellulose binders are more commonly used in shaping oxide ceramics. [see methyl cellulose]

cemented carbide A solid composite consisting of a metal carbide and a matrix phase; also termed hardmetal and in some cases cermets. Usually the carbide is based on tungsten (WC) and the matrix is cobalt, but in some instances the matrix might contain nickel. A typical microstructure is shown in Figure C.4. The composite is formed by liquid phase sintering a milled mixture of carbide and matrix metal powders. The properties are very

Page 8

Section C

dependent on grain size, composition, alloying, and porosity. If sintered to full-density, the straight carbides (WC-Co) compositions are hard and strong. For example, a straight carbide consisting of WC-10Co has the following properties: cemented carbide

WC-10Co

density

14.6

g/cm3

heat capacity

209

J/(kg EC)

thermal expansion coefficient

4.3

ppm/EC

thermal conductivity

100

W/(m EC)

electrical resistivity

16

µΩ-cm

elastic modulus

640

GPa

Poisson뭩 ratio

0.2

hardness

90

HRA

3000

MPa

as-sintered transverse strength

as-sintered compressive strength 5100 MPa Considerable variation is possible based on the composition and microstructure, with common alloying additions being VC, TaC, and TiC to control grain growth and hardness. In these mixed carbides, the strength and hardness can vary considerably depending on the desired durability. [see tungsten carbide]

cementing A term for the bonding of particles such as is possible by heat or chemical reactions or solidified metallurgical phases.

cementite The hard carbide of iron (Fe3C) where there are three iron atoms for every carbon atom. The crystal structure is orthorhombic. It is a hard (800 VHN) and brittle phase that provides good wear resistance to a steel. The hardness and stoichiometry vary with alloying additions. Although 25% of the atoms are carbon, the carbide only contains 6.67% carbon by weight. Cementite formation is avoided by rapid cooling from austenite, resulting in a body-centered tetragonal phase known as martensite.

centrifugal atomization The formation of spherical particles by combining a melt with a centrifugal force such that the melt is disintegrated into high velocity droplets which spheroidize prior to

Section C

Page 9

solidification. A sketch of one variant is given in Figure C.5, where molten metal is dropped into a spinning cup and the melt is thrown over the lip of the cup as droplets that solidify into powder. Centrifugal atomization is useful for high-temperature and reactive metals since long-term contact with a melting crucible can be avoided. The key role of the centrifugal force is to create thin, long ligaments that pinch into spherical droplets prior to solidifying into powder. Unfortunately it proves difficult to sustain contact between the melt and centrifuge to accelerate the melt into a very thin ligament, so centrifugal atomized powders tend to be larger in size when compared with other technologies. Several variants exist based on spinning cups, disks, or wheels, but the most popular has been the rotating electrode process. [see rotating electrode process]

centrifugal casting A novel means to generate high compaction pressures based on centrifugal forces that happen in molds that are rotated at very high velocities, generating upwards of 10,000times gravity. When a slurry is introduced into the tooling, the centrifugally cast deposit reaches very high densities. The approach is especially effective for hard particles that normally will not deform in compaction. Unlike die compaction where the total body is fed into the die at one time, centrifugal casting attains a high packing density since the particles are fed in sequentially, allowing for more efficient packing and rearrangement.

centrifugal compaction Powder consolidation from a powder-liquid slurry based on centrifugal settling of the heavier particles into a mold with separation of the lighter fluid phase. Revolution rates can reach 50,000 RPM and stresses can exceed those typical to most powder shaping options. The approach is mostly used for hard particles that are difficult to deform in compaction, such as alumina and tungsten carbide.

centrifugal sedimentation The determination of particle size based on accelerated sedimentation via centrifugal force, whereby the gravitational acceleration in Stokes? law is supplemented by a centrifugal forces. Often the particle size analysis is simply based on measuring the cake height at the bottom of a glass tube versus the time of centrifuging. The technique can also be used to separate small particles from a solution or to classify powders.

ceramic injection molding (CIM) A subset of powder injection molding with the limitation that it is applied to ceramic powders. [see powder injection molding]

ceramic-matrix composite

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Page 10

A composite of two or more distinct phases where the continuous matrix phase is a ceramic.

cermet A particulate composite consisting of ceramic particles bonded with a metal matrix. The original term was coined by joining ceramic and metal into a single word to describe a composite.

changeover The point in a molding cycle where the operation shifts from speed control to pressure control

char A term from analytical chemistry that denotes the residue after combustion tests, and in powder metallurgy this is normally used to describe the noncombustible residue after subjecting a binder or lubricant to a burnout cycle. Clean burning polymers will have less than 0.1 % char, but some polymers such as polystyrene and various cellulose binders might have up to 15 % residue.

Charpy impact energy The fracture energy, in J, measured by breaking a rectangular sample (10 mm by 10 mm in cross-section by 55 mm long) in the Charpy test. The test is named after a French metallurgist, Georges Albert Charpy (1865 to 1945), who was trained as a chemist, but pioneered several developments including standardizing the impact fracture test that bears his name. The typical samples are shown in Figure C.6. The traditional test requires a notch; however, in some aspects of powder metallurgy the data are collected without a notch so they are not comparable with handbook data. Further caution is required since some portions of powder metallurgy use subsize samples that are invalid, and even subsize samples lacking a notch, making the reported impact energies meaningless.

Charpy test An impact test for measuring the toughness or Charpy impact energy. It involves a notched specimen which is struck behind the notch by a hammer or striker mounted on the lower end of a pendulum. The energy consumed in fracturing the specimen is reported as the impact energy, usually in J, or J/cm2 since the cross-sectional area is 1 cm2. Figure C.7 provides a diagram of the test. In standard metallurgy the specimen contains a V-notch, U-notch, or keyhole-notch, with the V-notch being most common. Because many powder metallurgy materials are porous and low in toughness, some of the tests are applied to unnotched bars, resulting in higher values that can not be directly compared with the notched tests.

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[see Charpy impact energy]

check ring A ring at the end of the screw in powder injection molding that operates as a one way valve to avoid hot feedstock from passing backward during the plunging stage of mold filling. The ring seats against the screw tip when pressurized. Figure C.8 shows its location on the screw near the nozzle. When pressed against the screw, the check ring stops backward motion of molten feedstock during mold filling.

chemical characterization Standard tests used to measure the composition, usually to ensure the powder or sintered product is in conformance with specification.

chemical deposition The precipitation of one metal from a solution of its salts by the addition of another metal or reagent to the solution. For example, copper and nickel are commonly plated onto other metals using deposition techniques that are also termed electroless deposition.

chemical vapor deposition (CVD) A coating process in which a reactive atmosphere is fed into a process chamber where it decomposes onto the surface of the workpiece. The decomposition event liberates one material for adsorption or accumulation or reaction on the surface. Byproducts from the reaction are carried away with the excess process atmosphere. The coatings formed by the decomposition process are used on tooling for wear resistance or on components requiring corrosion resistance. Besides uses in cutting tools chemical vapor deposition is used to form encapsulated particles.

chemical vapor infiltration (CVI) The filling of open pores in a sintered body using chemical vapor deposition to effectively close the pores, similar to infiltration only the mass used to fill the pores is added via a vapor phase.

chemically precipitated powder Powder produced as a precipitate by chemical displacement from a solution, typically with a very small particle size. Often a sacrificial material is used to nucleate or reduce the solution to induce precipitation.

chromium (Cr) Chemical element 24 on the periodic chart. It is a body-centered cubic metal used for

Page 12

Section C

alloying, for example for corrosion resistance in stainless steels or for arc erosion resistance in copper. One of the main sources of chromium for melting and other metallurgical processes is via electrolytic powder that is roll compacted into briquettes that are sintered to give a shape that looks like charcoal briquettes. Little chromium is used directly as a powder, although powder metallurgy is the primary means to form the raw material. The properties of full-density chromium are summarized as follows: chromium

Cr

atomic number

24

atomic weight

51.996

g/mol

density

7.23

g/cm3

melting temperature

1875

EC

boiling temperature

2665

EC

heat of fusion

15

kJ/mol

heat capacity

461

J/(kg EC)

thermal expansion coefficient

6.2

ppm/EC

thermal conductivity

91

W/(m EC)

electrical resistivity

13

µΩ-cm

elastic modulus

279

GPa

Poisson뭩 ratio

0.21

hardness

110

VHN

as-sintered yield strength

362

MPa

as-sintered elongation to fracture

44

%

as-sintered ultimate tensile strength

413

MPa

chromium equivalent In ferrous alloys chromium has the tendency to stabilize the body-centered cubic phase as do other alloying additions. A simple linear equation allows for the combination of the alpha-phase stabilizers into a single composition expressed as the equivalent amount of chromium. Usually molybdenum is considered equivalent to chromium, silicon 1.5-times as potent, and niobium is half the potency.

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CIM An abbreviation for ceramic injection molding, where the inorganic phase in the polymer binders is predominantly a ceramic or mixture of ceramics; a subset of powder injection molding (PIM).

CIP The abbreviation for cold isostatic pressing.

cladding Layered materials that are metallurgically bonded onto a substrate. Usually cladding is used to form an outer layer of 1 mm thickness on a thick body for corrosion or wear resistance. The outer clad layer might be formed from powder and sintered, brazed, or hot isostatically pressed onto the body.

clamping force One of several measures of the capabilities of a molding machine. In this case the available force for holding the mold together while pressurized feedstock is filling the cavity. If the applied pressure times the projected part area exceeds the clamping force, then the cavity will open or flash during molding.

class A categorization used to indicate the difficulty of die compaction, with higher numbers requiring more complicated tooling and more sophisticated compaction presses. Class 1 parts are simple single-level shapes with a small height-to-diameter ratio that are compacted using single-acting dies. Class 2 parts are also single level, but with compaction pressure applied from both top and bottom because of a larger height-todiameter ratio. Class 3 parts have two levels and pressure is applied from both the top and bottom. Finally, Class 4 parts represent shapes that are the most difficult to press. These are multiple-level components which are pressurized from both the upper and lower punches.

classification Separation of a powder into fractions according to particle size. This can be achieved by screening if the particles are large or air classification using centrifugal forces for particles with sizes down to nearly 1 탆 . Centrifugal sedimentation is applicable to very small particles.

closed-loop control Use of in situ sensors in a fabrication process that constantly monitors the state of affairs to make decisions on component quality, essentially capable of making every

Section C

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component the same. In powder injection molding a pressure sensor is located inside the molding cavity to monitor filling and the pressure signal is used to adjust the molding machine operation to ensure repeatable filling, weight, and final dimensions. Both weight and dimensional scatter are greatly reduced by using this control logic as compared to open-loop or adaptive controls.

closed pore An isolated pore not linked to the external surface. For example, in a bearing, a closed pore cannot provide a storage site for lubricating oil. Closed pores tend to be spherical and small, and are more prevalent as the fractional density increases. The optical micrograph in Figure C.9 shows a microstructure that contains closed pores, in many cases the pores are spheres that reflect light back from the bottom, giving the bright spot in the center. As a simple guide, few pores are closed at 85% density, usually about half of the pores are closed at 92% density, and all of the pores are usually closed at 96% density.

CN A shorthand designation for copper alloys that contain nickel.

CNC An abbreviation for computer numerically controlled.

Co The chemical symbol for cobalt.

coalescence The merging of two grains to form one larger grain. It is observed in particle fabrication and during sintering. Grain coalescence is promoted during sintering by the rotation of contacting grains into low misorientation angles that progressively merge into a single grain. An alternative mechanism of coalescence involves diffusion induced grain boundary migration, where one grain grows at the expense of its neighbor, leading to a single grain formed from the merger of two initial grains. This grain boundary migration process is sketched in Figure C.10. The driving force can be a grain size difference, where the larger grain consumes the smaller grain, or a chemistry difference that drives diffusion. Frequently, grain coalescence is observed in liquid phase sintering systems. The probability of coalescence is highest in systems with small quantities of liquid where small grains merge into larger grains, which have higher coordination numbers. Since the larger grains constitute a small fraction of the grain size distribution, coalescence has a low probability. Early in sintering about 5% of the grain-grain contacts are undergoing coalescence and that fraction progressively declines over time.

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Section C

coarse fraction That portion of the powder that will not pass through a given screen. Usually the larger particle sizes in a lot of powder.

coarsening The progressive enlargement of the grain size or pore size during sintering due to diffusion, coalescence, or solution-reprecipitation processes. Grain growth or pore growth are common coarsening events, and the general phenomenon is often termed Ostwald ripening. [see solution-reprecipitation, Ostwald ripening]

coated powder Powder treated with a polymer coating or conformal coating by physical vapor deposition, chemical vapor deposition, or other chemical routes, largely to passivate or change the surface properties. A novel conformal coating is formed using parylene (poly para-xylylene) which naturally forms a tenacious protective coating. Coatings can be used to adhere the small particles to the larger particles, thereby eliminating the dusting condition.

cobalt (Co) Element 27 on the periodic chart of the elements. It is a magnetic, polymorphic element (hexagonal close-packed at room temperature and face-centered cubic at temperatures over 422EC). Cobalt is used in powder metallurgy in three forms hot isostatically pressed superalloys matrix phase for cemented carbides and diamond composites liquid phase sintered cobalt-chromium wear alloys. The properties of pure, full-density cobalt are summarized as follows: cobalt

Co

atomic number

27

atomic weight

58.93

g/mol

density

8.92

g/cm3

melting temperature

1495

EC

boiling temperature

2870

EC

heat of fusion

15

kJ/mol

heat capacity

423

J/(kg EC)

thermal expansion coefficient

13.4

ppm/EC

Page 16

Section C thermal conductivity

96

W/(m EC)

electrical resistivity

6

µΩ-cm

elastic modulus

211

GPa

Poisson뭩 ratio

0.32

yield strength

540

MPa

elongation to fracture 6 % Common cobalt powders are very small. Consolidated cobalt alloys contain 15 to 30 wt. % of both nickel and chromium, and alloying additions of iron, tungsten, tantalum, molybdenum, and other reactive or refractory metals. Probably the most famous of the alloys is designated ASTM F-75, which is hot isostatically pressed for use in medical implants.

Coble creep Named after Robert Coble who isolated the grain boundary diffusion role in sintering and high temperature time-dependent deformation. In Coble creep, the grain boundary provides a high mobility path for atoms that move to accommodate an external stress or an internal stress, such as the sintering stress. Coble is also credited with the critical patent for sintering alumina to optical translucency for use in high intensity lighting using a small concentration of magnesia that segregates to the grain boundaries. The length change d(∆L) per unit time dt is divided by the initial length to give the creep rate. For diffusion along grain boundaries the creep strain rate is given as follows:

where T is the absolute temperature, k is Boltzmann's constant, Ω is the atomic volume, G is the grain size, PE is the effective pressure, δ is the grain boundary width (about five atoms wide), and DB is the boundary diffusivity. As long as there is porosity, densification is directly proportional to the creep-strain rate.

coefficient of variation The standard deviation divided by the mean value, giving a nondimensional measure of uniformity or dispersion of an attribute such as a component size, part weight, or mechanical property. The coefficient of variation CV allows normalization of size variation data,

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where σ is the standard deviation and XM is the mean value. This provides a size independent measure of process dispersion or uniformity. Often the value is given as a percent of the mean by multiplying by 100.

coercive (field) force The magnetizing force that must be applied in the direction opposite to that of the previous magnetizing force in order to reduce the net induced magnetization to zero. It also provides a measure of the resistance to demagnetization. It is measured in A/m, formerly measured in Oe.

co-firing When a composite is sintered from mixed phases to form a multiple-material structure, the process is termed cofiring. The approach is used in the fabrication of capacitors, wear coatings, electrical packages, as well as glass-to-metal, glass-to-ceramic, or ceramic-to-metal seals. The initial structure consists of two or more distinct powders, but unlike other mixed phase systems, the powders are geometrically segregated in the green body. The objective is to achieve sintering in both materials simultaneously without distortion or the formation of defects. Co-firing requires the two materials follow the same shrinkage pathway, even though they may exhibit differences in basic properties.

cohesion The state in which particles are held together by weak forces, wetting liquids, or chemical bonds.

coincidence A condition that occurs in particle size measurements when two particles are detected at the same time, leading to an error of counting the two particles as one larger particle.

coining A closed-die squeezing operation, usually performed at room temperature but after sintering, in which all surfaces of the component are confined or restrained to imprint the coining die features on the component. It is especially useful for controlling final flatness, straightness, roughness, or dimensions. The material must be ductile, the tool lubricated and precisely aligned, and the component must be porous. For example, the lettering shown in Figure C.11 was generated by coining. The term comes from a historic use in the fabrication of coins.

cold forging Very high compaction pressures have been used in pressing sintered powders to near full density at room temperature. Cold forging implies the pressures are far in excess of

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the normal pressing ranges and the rates of pressurization are usually high, like forging. Pressures can exceed 1 GPa. Special tooling is required to withstand these pressures. Die wall lubricants are used and the compact is intentionally in a soft condition.

cold isostatic pressing (CIP) A room temperature forming technique in which fluid pressure is used to compress a metal powder inside a flexible die. The pressure is uniform in all directions so the compact is homogeneous in green density. The molds can be fabricated from any of several flexible polymers or rubbers selected because they can be dipped or cast around a master to form a complex shape. Core rods or other inserts are possible to form holes. Figure C.12 is a schematic of a cold isostatic press for the consolidation of a powder billet. For hard particles a polymer is added to increase green strength. Compaction pressures up to 1400 MPa are possible; however, most cold isostatic pressing is usually performed at pressures below 420 MPa. An external container helps hold the bag shape during powder loading and in some situations the bag is evacuated to remove air prior to compaction. The wet bag technique is applied to low production quantities. With a wet bag, the filled and sealed mold is immersed in a fluid chamber which is pressurized. After pressing, the wet mold (or bag) is removed from the chamber and the compact extracted from the mold and pushed off the core. The dry bag approach is favored in high-volume production, because the bag is built directly into the pressure cavity. The flexible bag deforms but is not ejected. End plugs allow powder loading and component unloading without removing the bag from the press.

cold pressing Forming a compact at a temperature low enough to avoid sintering -- usually at room temperature. [see die compaction]

cold sintering A term used to indicate densification in the compaction stage via ultrahigh pressures, thereby substantially reducing the required temperature to sinter the powders to target property levels. The concept was introduced to powder metallurgy by Professor E. Y. Gutmanas.

cold spraying The acceleration of particles in very high velocity gas jets so that the particles heat and stick when impacted on a target. The process is sketched in Figure C.13. In spite of the name, actually the particles must be preheated since the kinetic energy is not sufficient to melt the particles on deposition. For example, a typical particle is 50 탆 in size and accelerated to a velocity near 1500 m/s using gas heated to 800EC. In this combination the kinetic energy is sufficient to cause the particles to melt at the contact points as they

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are deposited. The temperature-velocity combination required to form a semisolid deposit depends on the material. Usually the deposition efficiency is under 70%. Because of a high gas consumption, cold spray technologies are costly. Also, the high particle velocities tend to rapidly erode the nozzle, requiring replacement every few days of operation.

cold welding Cohesion between two surfaces of metals, generally under the influence of externally applied shear stress at room temperature.

cold working Deformation at low temperatures compared to the melting temperature such that the metal undergoes plastic flow to increase its yield strength at the expense of ductility.

coldstream process A means for attritioning powder by chilling the feed material prior to impinging it at high velocities against a cold, hard target. The acceleration of the input powder is achieved using cold pressurized gas at approximately 7 MPa. The product is generally above 10 탆 in size with a rounded but irregular shape. Attritioning is improved by cooling the particles to lower their ductility. The technique is used in fabricating flame spray powders (powders for protective coatings) and stainless steel powders for filters.

combustion synthesis Fabrication of a compound from mixed powders using a self-propagating reaction. The concept is similar to the mixing of hydrogen and oxygen which form water upon ignition. Like hydrogen and oxygen, many mixed powders have a substantial enthalpy release (exothermic reaction) when reacted to form an intermetallic compound. An example would be possible by mixing nickel powder with 31.5 wt.% aluminum powder to form the equiatomic stoichiometric compound NiAl. When the mixed powders are heated to about 600EC the reaction begins and without further heat propagates through the mixture with substantial self-heating, reaching temperatures over 1200EC.

comminution Pulverization, milling, grinding, or otherwise breaking apart a brittle material to reduce the particle size via mechanical energy. The most common form is jar milling with hard balls or grinding media that generate smaller particles via impaction, attritioning, shearing, and compression. Impaction involves the rapid, instantaneous delivery of a blow to a material, cracking the material into smaller pieces, such as by hammering. Attritioning applies to the reduction in particle size by a rubbing motion usually caused by grinding media rubbing against each other. During shearing the material fractures by a cutting process, such as occurs in machining. Indeed, many early metallic powders

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such as silver for dental amalgams (fillings) were created using lathe turning. Powders formed by shearing tend to be large.

communicating pores Pores that are connected to one another. The resulting network allows for the passage of fluids through the body. These open pore networks are necessary for success in various applications ranging from filtration to oil storage in bearings.

comolding Also known as two-color molding because of its common use in plastics to form a single component from two different colors. In powder metallurgy, two different powders are formed into feedstock and molded into the same cavity from different injector units. The sequence of tool motions and mold filling are used to form layers, interconnections, or other compositional differences with position in the cavity.

compact An object produced by the compression of metal powders in a die; similar to a tablet or pill in origin.

compactability The ability to be pressed into a desired shape and to maintain integrity in subsequent handling. Compactability is a function of both the compressibility and green strength.

compacted angle of repose An angle of repose test that is performed by tilting a powder after it is vibrated to the tap density. This provides a measure of the ability to hold shape after a binder is extracted in powder injection molding.

compaction The shaping, deformation, and densification of a powder by the application of pressure through a tool material. Lubricants are an important aspect of compaction. Some variants rely on the application of heat. [see die compaction]

compaction curve A plot of density versus pressure such as illustrated in Figure C.14 for four different ironcopper-carbon mixtures, formed using different purity iron powders. This plot shows how each powder has a characteristic density-pressure response. Such curves allow for selection of the compaction pressures required to reach a density goal.

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compaction mechanics The study of deformation of powders under the action of stress. It involves fundamental laws of stress, strain, strain rate, thermal softening, strain hardening, strain rate hardening for both elastic and plastic behavior. Constitutive equations emerge from the study of compaction mechanics that are employed in finite element analysis to predict density, strength, friction, cracking, and sintering warpage.

compaction pressure The peak pressure applied to a powder compact during the densification portion of the compaction stroke.

complexity A means to rank fabrication problems and expenses for various components based on the number of features and the tolerances associated with those features. It provides a dimensionless means to assess component design compatibility with various production techniques and company capabilities. Complexity ψ is defined as follows:

where n is the number of specified dimensions and CVi is the coefficient of variation corresponding to the individual tolerances (standard deviation for the tolerance divided by the mean size, as a fraction not as a percentage). Thus, complexity is a nondimensional reflection of the information needed to specify the component. For a given production process and component mass, the general concept is that cost increases linearly with complexity.

component An individual, functional element in an engineering system that cannot be further reduced or subdivided without destroying its stated function. In powder metallurgy these are the discrete objects produced in the compaction step which might be combined to form parts of a system or assembly. Most of powder metallurgy is involved in the production of components that make up larger systems, such as automobiles.

composite material A mixture of two or more powders to form a multiple phase structure, typically designed to deliver properties which are a hybrid of the constituents'. Powder metallurgy generally is restricted to particulate composites, such as aluminum with dispersed silicon carbide particles or whiskers. These mixtures form intermediate property combinations that do not exist from conventional materials. For example steel and titanium diboride composites have an elastic modulus higher than possible with steel and a fracture

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toughness higher than possible with titanium diboride. Likewise, titanium and titanium diboride composites are used in automotive engine applications because of the high hardness, strength, and retention of properties to high temperatures. As another example, die-pressed and sintered alumina has a typical strength of 350 MPa, but hotpressed nanoscale alumina has a strength of 750 MPa, while nanoscale Al2O3 with 5 vol.% SiC has a strength of 1130 MPa.

composite particles Mixed phases at the particle level, where each particle has distinct regions that differ in chemistry and crystal structure.

compressibility Inversely related to the compression ratio. It is a measure of the ease of compaction. For high compressibility powders the green density might be 2 to 3 times the apparent density. To measure compressibility, the compact is made following a procedure in which the die, pressure, and pressing speed are specified. One version reports the pressed density using a preset pressure and another version reports the pressure required to attain a preset density. Particle size is an important factor with respect to compressibility. Smaller particles pack to lower densities, are harder, and strain harden rapidly during compaction, making them more difficult to press. As an illustration, aluminum powder compacted at 175 MPa behaved as follows with a change in median particle size: 3 탆 gave 84% green density 20 탆 gave 92% green density 95 탆 gave 94% green density. Powders designed for die pressing to high green densities tend to be soft and large. Smaller powders resist deformation, so nanoscale powders prove very difficult to compact. Likewise, sponge particles with internal porosity are difficult to compact since the small pores inside the particles resist compaction. For example at 700 MPa, rounded water-atomized iron powder gives 90% green density, but reduced iron powder with internal porosity gives 84% density. Also, small powders and sponge powders have more springback on ejection, making them more susceptible to cracking.

compression The removal of air from molten feedstock by applying heat and pressure. In a reciprocating screw molding operation this is achieved by tapering the screw to reduce the space between the screw and barrel while pressurizing and metering the feedstock toward the screw tip.

compression molding A concept from plastics based on pressing dry, solid feedstock pellets in a closed die

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after metering out the proper mass or volume. In plastics the polymer is usually a thermosetting resin, so heat and pressure are applied to melt, flow, and cure the polymer during the molding cycle. This is similar to die compaction, but in powder metallurgy the resin is mixed with a powder and the heated resin bonds the particles into a final shape, such as magnetic component. [see transfer molding]

compression ratio The ratio of the volume of the loose powder to the volume of the compact made from it. For ductile powders the green density after compaction might be 2 to 3 times the apparent density. The compression ratio CR expresses the volume change or density change with a standardized compaction pressure, say 400 MPa (60 ksi), and can be calculated from the density ratio, height ratio, or volume ratio:

where VL is the volume of the loose powder, VC is the volume of the compacted powder, ρG is the green density, and ρA is the apparent density. For a die with constant cross section, this ratio is simply the fill height divided by the pressed height. A high interparticle friction will give a low apparent density, but other factors such as particle size, hardness, and lubrication affect the compacted density.

compressive strength A crush test typically applied to a right-circular cylinder geometry to measure the failure strength. The top and bottom faces are parallel and the side walls of the cylinder are straight. This cylinder is loaded on the flat faces until it fractures; the compressive strength is the peak load divided by the cross-sectional area. Since flaws in the material are kept in compression, the compressive strength tends to be very high compared to the tensile strength. For some of the low ductility sintered materials the tensile strength is only one-tenth of those measured in compression.

computer-aided manufacturing (CAM) Computer controlled machines used to ensure proper fabrication and resource utilization; included in the broad category of computer integrated manufacturing and computer-aided manufacturing activities might be inventory control, maintenance schedules, production scheduling, tool path analysis, and cost analysis.

computer-assisted design (CAD) Computer tools used during the design process to examine options for the specification of an engineering component, including stress analysis, heat transfer analysis, process specification, material selection, and cost modeling.

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computer-assisted engineering (CAE) A body of techniques that integrate design, material and process selection, economic and functional analysis into computer tools.

computer numerically controlled (CNC) A machine control system that implies the forming process is constantly controlled by sensors using a closed-loop feedback control system operated by a computer. The computer numerically controlled compaction presses used in powder metallurgy are the most sophisticated and most capable in terms of product uniformity and shape complexity. But at the same time they are the most expensive and prove more difficult to initially operate since computer simulations are often required to properly design tooling and operating cycles. The CNC compaction presses are applied to the most complex shapes. CNC technologies are also widely used in machine shops.

computer simulations Techniques for the prediction of changes that might be brought about by variations in the processing parameters. In powder metallurgy several aspects of the process are now treated with simulations, including hot pressing, sintering, die compaction, injection molding, and hot isostatic pressing. These simulations focus on prediction of the product quality while avoiding costly factory-floor experiments. Additionally, computer simulations can predict the effects of various processing parameters, allowing assessment of processing options without the expense of physical trials. Models relevant to the powder metallurgy industry operate over a broad range, from furnace level, down to the component scale, and extend into the particle scale. For example, Figure C.15 shows the predicted green density versus position in a die pressed component. Early progress in computer simulation was restricted to tasks that might only involve the sintering of two particles, because of computer memory and speed limitations. More recently, expanded computational capabilities and lower costs have allowed simulations to penetrate all levels of the industry. In the current concept of multi-scale simulations, each level of computation provides important information to the others, thereby allowing accurate simulations of features such as heat loss, dimensional change, and particle bonding. However, integration of these models into a compatible set of calculations is still a pending problem.

conduction Transport processes associated with heat and electrical flow. The role of conduction in sintering is associated with thermal diffusion in the process atmosphere, a process that tends to be inefficient when compared to heat transfer by convection or at higher temperatures by radiation. In conduction, the heat flow to the component Q (units of W/m2) is through thermal conduction through the gas, which can be described in terms

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of the temperature difference ∆T between the component and furnace,

where y is the separation distance and Κ is the gas thermal conductivity. Heating and cooling are much faster in hydrogen and helium, especially at lower temperatures, because of their high thermal conductivities when compared to other process atmospheres.

conductivity Both thermal and electrical passage in a material are designated as conductivity, and the key determination of how resistant a material is to conducting heat or electricity lies in the composition, microstructure, porosity, and impurities. In both cases, the conductivity κ depends on porosity as expressed by the following correlation,

where κo is the conductivity of the fully densified material and ε is the fractional porosity. The coefficient χ expresses the sensitivity to pores. This equation lacks internal structure-dependent parameters, but analysis of several sintered metal powder compacts, representing a variety of pore sizes and shapes, gives a best fit value of 11 for χ. In the low porosity region, the relative conductivity follows a linear behavior with fractional porosity ε; thus,

where ω is between 1 and 2. This second model is most appropriate at porosities less than 30% for either electrical or thermal conductivity.

cone and quartering A simple sampling procedure applied to a powder that pours the powder into a pile (the cone), and divides that pile along its vertical axis into four separate piles (quartering). The procedure can be repeated using one of the quarters, which is poured into a new pile. It can be quartered and the process repeated until a satisfactory small sample is attained. The final sample is assumed to be representative of the original powder lot.

confidence interval An interval around a test result in which there is a high probability the true value is within the sated range. It is an interval estimate obtained from repeat tests that provides

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the range over which the estimate is certain.

Conformaclad? A sinter-brazing technology for cladding or coating steel bodies with a hard layer of tungsten carbide in a lower melting temperature braze matrix. Compositions of WC tend to range in the 45 to 65 wt. % range, with the typical matrix being a braze alloy consisting of nickel, chromium, cobalt, molybdenum, and boron. The composite hardness tends to range near 55 to 70 HRC, being mostly dependent on the tungsten carbide content. The name is trademarked.

confounding A statistical concept that implies the test results do not have a distinguishable single cause; two or more possible effects or interactions could account for the findings.

confusion principle A concept applied to amorphous metals to indicate the difficulty in forming a crystalline structure due to a slow atomic level sorting process that must take place in freezing a highly alloyed system. Multiple component alloys are easier to freeze as amorphous structures since the sorting out of many different atoms into specific phases and atomic positions takes time. When heat is extracted quickly, the mixture is not able to crystallize. Further, the atomic sorting is slower if the atoms are very different in size, crystal structure, and electronegativity.

congruent transition A transition, such as melting, that exhibits a two-phase equilibrium (solid and liquid) at one temperature where both phases have the same composition. For example a pure metal exhibits congruent melting, since the solid and liquid co-exist at only one temperature and have the same composition. On the other hand, most alloys do not exhibit congruent melting, since they melt over a range of temperatures and the solid and liquid phases are different in composition. Many intermetallic compounds exhibit congruent melting transitions, such as NiTi and FeSi.

connectivity A numerical value that gives the number of grain-grain connections observed on a twodimensional cross-section. In porous materials and in liquid phase sintered composites, the grain connectivity can be low and if too low the material will lack strength. Connectivity is calculated by selecting a grain and tracing its perimeter, counting the number of similar phases with which it makes contact. Usually connectivity is only measured for the major phase that forms discrete grains and not the matrix. It is directly related to the underlying three-dimensional grain coordination number. In a typical liquid phase sintered microstructure the connectivity per grain Cg from two-dimensional

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sectioning relates to the dihedral angle φ and the three-dimensional grain coordination number NC as follows,

This equation assumes a typical grain size distribution and a random section plane through the underlying three-dimensional structure. An upper limit is 14 contacts per grain (three-dimensional) at a volume fraction of solid equal to 100%, and 4 to 6 contacts is expected at near 55 to 60 vol. % solid.

consolidation at atmospheric pressure (CAP) Powder densification based on heating a vacuum-sealed container that softens to transmit stress onto the powder to induce pressure-assisted densification. The consolidation is similar to hot isostatic pressing, but the pressure is just 0.1 MPa or one atmosphere. Such a low pressure can still be sufficient to accelerate densification. One trick is to include additives that form a liquid phase at the particle contacts to soften the structure, so boron or boric acid might be added to the powder to enable densification.

constitutive equation A mathematical relation between properties for a material. Such linkages are necessary in computer modeling of powder metallurgy processes. One simple relation applied to materials at low stresses gives the strain (change in length divided by the initial length) from the applied stress (load divided by cross-sectional area) in terms of a material property known as the elastic modulus. Constitutive equations involve state variables (time, temperature, stress, shear rate) and properties based on a collection of materialspecific parameters.

contact angle The wetting angle formed at the intersection of liquid, solid, and vapor phases. For a horizontal solid, Figure C.16 illustrates the balance of surface tensions that determines the contact angle. In this case a drop of liquid is resting on a solid surface, and the contact angle is given by the angle θ. This is also known as the wetting angle. A wetting liquid has a small contact angle θ, defined by the equilibrium of surface energies,

where γSV is the solid-vapor surface energy, γSL is the solid-liquid surface energy, and γLV is the liquid-vapor surface energy. A wetting liquid provides the surface attraction that aids densification during liquid phase sintering, termed a capillary force. Usually, wetting occurs when the solid is soluble in the liquid. The contact angle will differ when the liquid is advancing versus receding.

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[see Young equation, wetting angle]

containerless hot isostatic pressing The use of a hot isostatic press without the necessity for encapsulation of the powder. Containerless HIP is achieved by first sintering the component to a closed pore condition, which typically occurs near 95 % density. From that point the application of gas pressure works to collapse any internal pores, but the gas cannot penetrate into the component. It is used for cemented carbide densification and in final densification of high performance silicon nitride and other technical ceramics. The technique is also applied to the densification of some tool steels, cobalt alloys, and higher alloy steels. [see hot isostatic pressing, pressure-assisted sintering]

contiguity A microstructural measure of the relative interface area of solid-solid bonds versus the total perimeter for each grain. Assume the grain perimeter is measured, then the contiguity CSS is the portion of the perimeter in contact with similar composition grains. The total grain perimeter might consist of regions in contact with similar grains, pores, and other matrix phases. It is obtained from two-dimensional quantitative microscopy using contact counting. Test lines are randomly overlaid on the microstructure and the number of same grain NS and different grain NX contacts are counted, but only for the phase of interest. Then the contiguity is determined as follows:

The factor of 2 arises since each same-grain contact is only counted once, yet is shared by two grains and so should be counted twice ! once for the left grain and once for the right grain. Contiguity depends on the dihedral angle and three-dimensional grain coordination number. For a typical grain size distribution contiguity increases with increasing volume fraction of solid VS and dihedral angle φ as follows:

assuming no grain shape accommodation, so the relations are not accurate at the highest solid volume fractions. A change in the solid-solid grain boundary or the solid-liquid surface energy (γSS and γSL) results in a change in the dihedral angle and contiguity. Since the surface energy varies during the initial portion of liquid phase sintering, a corresponding contiguity variation also occurs. Figure C.17 shows two schematic microstructures representing a high and low contiguity with similar solid contents.

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continuous cooling diagram A plot of phase transformations for a steel during cooling from austenite that indicates the phases that will form versus time and temperature for various constant cooling rates.

continuous cooling transformation (CCT) A set of curves drawn for a steel showing time and temperature as the two axes. The diagram plots the phase transformations for various cooling rates to show the predicted onset and completion of metallurgical phase transformation associated with different cooling conditions. Figure C.18 is an example diagram for an alloy steel.

continuous furnace A pusher, belt, or walking beam furnace where components are always entering and exiting, so the temperature remains constant at each location in the furnace while the components pass through a sequence of temperature zones on a conveyor system. Figure C.19 sketches how a pusher furnace conveys trays filled with components through a continuous furnace.

continuous sintering High volume production sintering where green parts enter into a preset sequence of zones to provide the desired time-temperature combination. The furnace is always hot and the components move through the furnace in a sequence that might involve many small steps - termed stoking.

continuum mechanics The use of mathematical models that treat the powder body as a homogeneous structure to approximate its response to heat, stress, or other parameters. This lumped treatment ignores the effects from different particle sizes, pores size, or other microstructure parameters.

controlled porosity A natural application for powder metallurgy where the pore space between particles are engineered to deliver target filtration, lubrication, flow restriction, or air distribution functions. The devices end up in applications such as bearings, filters, sound absorbers, heat pipes, and medical implants. Material selection depends on the use environment. For filtration applications, the corrosion-resistant metals such as stainless steels, titanium, and nickel-base alloys are preferred. In bearings, bronze or bronze-steel compositions are desired because of their wear properties. For biomedical applications, the dictates of biocompatibility lead to noble metals and stable oxide formers such as titanium, tantalum, or cobalt-chromium alloys. Smaller components are fabricated by die compaction, while cold isostatic pressing is used for larger items.

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convection The transfer of heat from a gas to or from a powder metal component based on the relative velocity of the gas with respect to the metal surface, such as in a sintering furnace. Turbulence greatly increases the rate of convective heat transfer. Under conditions of a stirred or moving atmosphere, convection is an important contributor to heating. Heat transport by convection varies with the gas-component temperature difference ∆T as follows:

where h is the heat transfer coefficient, which depends on the gas and its velocity, and even varies with temperature. Natural convection and forced convection give widely differing heat transfer coefficients, roughly 20 W/(m2 EC) versus 100 W/(m2 EC). There is an optimal gas flow rate to deliver the best convective heat transfer. Too high a velocity creates cold spots and too low a velocity is inefficient in exchanging heat. Best heating occurs if the atmosphere is intentionally stirred at lower temperatures.

Coolidge process The original route for the fabrication of ductile tungsten as used in lighting applications, named for William D. Coolidge who patented the process in 1910. It involves doping the grain boundaries with dispersoids that inhibit grain growth, leading to a pinned microstructure and a fibrous microstructure.

cooling unit That portion of a sintering furnace that provides a pure atmosphere around the sintered material while heat is extracted. These units are usually one of the longer portions of the sintering furnace.

coordination number The number of touching nearest neighbors for a given particle or grain in a powder compact. Figure C.20 shows sphere packings with coordination numbers of 6 and 12. Generally the coordination number increases with the packing density, being as low as 3 or 4 for loose packed powders and as high as 14 for dense bodies. Small particles with a low coordination number have more interparticle friction and a low apparent density. For large spherical particles, the apparent density is usually near 60% of theoretical and the tap density is near 64% of theoretical with a mean coordination number near 7. The shape of randomly packed spheres compacted to full density approaches a structure know as a tetrakaidecahedron with a coordination number of 14. This 14-sided polyhedron is composed of a mixture of faces with four and six edges (squares and hexagons). One impact of applying pressure to a powder is the particle

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Section C

coordination number increases as the load collapses the pores and flattens particle contacts. During compaction the coordination number Nc can be approximated from the fractional density ρ as follows:

Coordination number is difficult to measure, so it is usually estimated from the twodimensional connectivity Cg if the dihedral angle φ is known,

For example if φ = 45? and Cg = 3, then the three-dimensional coordination number NC is near 12. Properties such as conductivity, strength, and fracture toughness depend on the level of connection in three dimensions. [see contiguity, connectivity, tetrakaidecahedron]

copper (Cu) Element 29 on the periodic table, it is a face-centered cubic metal that is ideally suited for powder metallurgy processing and is most often used in bearings, hardware, and is a common addition to ferrous alloys. It has the following properties: copper

Cu

atomic number

29

atomic weight

63.54

g/mol

density

8.96

g/cm3

melting temperature

1083

EC

boiling temperature

2578

EC

heat of fusion

13

kJ/mol

heat capacity

386

J/(kg EC)

thermal expansion coefficient

16.6

ppm/EC

thermal conductivity

403

W/(m EC)

electrical resistivity

1.6

µΩ-cm

elastic modulus

145

GPa

Poisson뭩 ratio

0.34

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Section C as-sintered yield strength

120

MPa

as-sintered elongation to fracture

45

%

as-sintered ultimate tensile strength 220 MPa Pure copper is not very useful, so most of the applications rely on alloying with tin to form bronze, zinc to form brass, or alumina or chromium to form a hardening dispersion.

copper alloy Alloys that have more copper than any other ingredient. In powder metallurgy the most common are copper, brass, and bronze, with copper contents ranging up from 62 wt.% with zinc, tin, nickel, and lead as alloying additions.

copper-beryllium An alloy used in high hardness and high thermal conductivity applications, such as molds for injection molding, the alloy consists of 1.9 wt. % beryllium in a copper matrix. It has the following properties: copper-beryllium

Cu-1.9Be

density

8.26

g/cm3

melting temperature

870

EC

heat capacity

397

J/(kg EC)

thermal expansion coefficient

17.4

ppm/EC

thermal conductivity

108

W/(m EC)

electrical resistivity

7

µΩ-cm

elastic modulus

130

GPa

Poisson뭩 ratio

0.31

as-sintered yield strength

620

MPa

2

%

as-sintered elongation to fracture

as-sintered ultimate tensile strength 690 MPa Formerly the alloy was fabricated using powder metallurgy techniques, but in recent times that practice has been eliminated in favor of casting techniques.

copper growth A condition that occurs in ferrous alloys when copper is added via alloying or infiltration,

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due to the penetration of molten copper into the grain boundaries, especially when the carbon content is low. It is a swelling phenomena that results in an overall dimensional increase for the component. Prolonged sintering can shrink the structure sufficiently to offset the copper growth.

copper steel Alloys of iron-copper-carbon formed by mixing powders in different ratios to form a steel by melting the copper and diffusing the graphite into the iron during sintering. Various alloys exist with up to 0.8 wt.% carbon and generally a low concentration of copper, often in the 2 wt.% range. In press-sinter processing these are designated as FC alloys (F for iron or Fe and C for copper or Cu). The alloys are also popular for forging to full density, but do not have a strong heat treatment response. A peak strength for Fe-2Cu0.8C (FC-0208) might be 1200 MPa, but lower strengths come with porosity, so most of the production powder metallurgy materials are significantly lower in strength.

copper sulfate test A simple test for corrosion resistance applied to sintered materials such as stainless steels. It consists of a solution containing 1 g cupric sulfate and 2.5 g sulfuric acid in 22.5 ml of water. Test samples are immersed in the solution for 360 s at temperatures near 18EC. If there is no visible sign of copper plating on the sample the material is classified as passing.

cordierite A compound of magnesia, alumina, and silica with a stoichiometry of 2MgO-2Al2O35SiO2 that is often used for sintering substrates because of its low density and excellent thermal resistance and low cost. It has a density of 2.58 g/cm3 and useful strength at room temperature of 110 MPa.

core A magnetic core is comprised of small particles of electrically insulated ferromagnetic material that provide magnetization but avoid hysteresis eddy current loss. This might be accomplished by coating the ferromagnetic powder with a polymer glue to hold the particles in place, while disrupting electrical conductivity.

core rod The separate member of the compacting tool set which forms a hole in the compact. It usually passes through the lower punch and fits into the upper punch to provide location and guidance from both the top and bottom. Core rods are longer than the punches used to provide guidance. On ejection of the compact from the tooling, the core rod is withdrawn to leave a hole in the powder compact. Thus, core rods see both tensile and compressive stresses, and must also resist wear - they are usually fabricated from

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hardened tool steels..

corrosion An irreversible interfacial reaction between a metal and its environment which results in the consumption of the metal and deterioration of the structure over time. The corrosion process can either form a surface compound, and in cases such as aluminum and chromium that compound might be protective, or might lead to dissolution of the metal into the environment.

corrosion resistance The ability of a material to withstand contact with an environment without degradation. This resistance to attack is usually quantified in terms of a rate of mass loss or gain over time per unit area, or the depth of penetration over time. The ability to resist corrosion is usually very specific to the material, its preparation, the test environment, and details of the test conditions. Two popular metallic materials for corrosive applications are titanium and stainless steel, with stainless steel favored for low-cost applications. Three factors dominate the corrosion response - composition, density, and sintering conditions (or heat treatment - including atmosphere, temperature, time, and cooling rate). For example in stainless steels, a high nickel, chromium, and molybdenum content generally increases corrosion resistance and cost. Density is another factor. Pores accelerate corrosion. Hence, a high density is preferred for service in aggressive environments. Even at full density, corrosion rates can be higher than those associated with wrought materials. The difficulty comes from sintering atmosphere reactions. For stainless steel, nitrogen in the sintering atmosphere is particularly detrimental, while hydrogen sintering gives better corrosion resistance. Hence, hydrogen sintering is specified for the fabrication of dental and medical instruments.

covalent ceramic Hard, refractory, and tough ceramic compounds that exhibit a large percentage of the atomic bonding character due to electron sharing (covalent bonding) versus electron exchange (ionic bonding). Good examples of covalent bonding include boron nitride (BN at 22% ionic) and silicon carbide (SiC at 12% ionic). Due to the bonding, it is difficult to sinter these materials, since they have a high resistance to heating. Some of the covalent ceramics exhibit very nontraditional characteristics, for example zirconium nitride (ZrN) is electrically conductive. Other widely recognized covalent ceramics include titanium nitride (TiN), silicon nitride (Si3N4), boron carbide (B4C), and aluminum nitride (AlN). These ceramics are useful for applications in sintering furniture, durable wear coatings, and high temperature substrates.

Cr The chemical symbol for chromium.

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Section C

creep The time-dependent strain occurring at high temperatures and stresses below the yield strength. Often creep situations are avoided for powder metallurgy products because residual pores severely degrade the component life. Creep is one mechanism for the densification of a metal powder during hot pressing or hot isostatic pressing. Faster densification by creep requires higher temperatures and higher stresses. Volume diffusion controlled creep (Nabarro-Herring creep) occurs when the vacancy flow is directed by the stress gradient between grain boundaries in tension and those in compression. An alternative process involves diffusion along the grain boundaries (Coble creep). When both the stress and temperature are high, the rate of densification depends on the rate of dislocation climb and is know as power law creep. Creep models are useful in explaining hot consolidation. [see Coble creep, Nabarro-Herring creep]

critical cooling rate A minimum cooling rate for the extraction of heat during atomization, sintering, heat treating, or other thermal process. In heat treating steels, slower cooling rates tend to form softer phases, so a critical rate is specified to ensure proper formation of martensite. It also is applied to the formation of metallic glasses, where faster cooling is required to form amorphous alloys from low alloying levels. Most of the metallic glass compositions are complex chemistries, since these alloys have slower critical cooling rates, as demonstrated by the examples in the attached Table C.1. Table C.1. Alloying Effect on Critical Cooling Rates to Form Metallic Glasses composition

critical cooling rate, EC/s

Ni

109

Fe-17B

106

Fe-10Si-11B

3A105

Fe-13P-7C

5A104

Ni-38Nb

2A103

Pd-17Si-6Cu

103

Pd-40Ni-20P

2A102

Zr-23Be-13Ti-13Cu-10Ni Pd-30Cu-10Ni-20P

3 10-3

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critical phenomenon In many aspects of sintering there is the occurrence of a critical phenomenon; the classic 뱒traw that breaks the camel뭩 back? situation. Critical phenomena are associated with percolation theory. [see percolation]

critical pigment test A test developed for paints that involves sequential additions of particles to a liquid where the mixture is mixed with a spatula and observed for its ability to hold a shape. The critical pigment content corresponds to the mixture which has an ability to hold a shape, determined by lifting a portion of the mixture and observing if it holds a peak, similar to the meringue on a pie.

critical solids loading The maximum volume fraction of solid particles which can be incorporated in a polymer binder without forming pores. At this concentration or ratio of powder to binder, the particles are in point contact and packed to approximately the tap density and all void space between the particles is filled with binder. The critical solids loading is usually determined using torque rheometry, density, or viscosity tests. Figure C.21 plots the typical behavior based on different ratios of powder to binder. Here the critical solids loading corresponds to the peak density and the point where interparticle friction causes the mixture to be too viscous for flow. The optimal solids loading reflects a slight excess of binder to lower the mixture viscosity.

cross-flow filter A sintered porous metal filter, usually in the form of a tube, that is internally pressurized with high velocity fluid containing debris. The input flow is tangential to the porous metal walls and only part of the fluid passes through the porous metal tubes, leading to a concentrated fluid-debris output. High velocities concentrate the debris at the center of the tube and minimize the build-up of a filter cake, so the fluid passing through the porous metal walls tends to be pure. The primary fluid flow is tangential to the porous metal, versus barrier filtration which is perpendicular to the porous metal. The technique extends filtration to very small particles, but cannot capture 100 % of the fluid, so it is better applied to situations where particle concentration is desired - such as in the harvest of low concentration pigments, particles, and pharmaceuticals. Contrast this situation with barrier filters. [see barrier filter]

cross linking The formation of bonds between polymer chains to give rigidity and strength to the polymer. Thermosetting polymers that harden on first heating are examples of cross

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linked polymers. If the polymer is removed by a burnout step, then there is a tendency to contaminate the powder. For this reason cross linking thermosetting polymers are avoided in powder metallurgy.

crushing The application of compressive stress on a powder or cake of powder to cause fracture in a pulverization process. Several devices exist to perform this function such as a jaw crusher.

cryogenic heat treatment A heat treatment cycle that involves cooling the steel to liquid nitrogen temperatures (at least to -190EC) as a step between the solutionization anneal and the tempering steps. Some steels have a tendency to retain austenite when subjected to the traditional water quench and over time that austenite will transform to cause a dimensional change, even after tempering. To fully induce the austenite to transform to martensite, the steel is cryogenically chilled to below the martensite finish temperature before it is tempered.

CT A shorthand notation for copper-tin alloys, better known as bronze. Alloys with CTG designations also have graphite.

CTE One of the two abbreviations used for the thermal expansion coefficient (also abbreviated TEC).

Cu The chemical symbol for copper, a colored metallic material commonly used in powder metallurgy.

cumulative particle size distribution A plot usually showing the mass percentage of particles smaller than a given size. Figure C.22 is a typical plot showing the amount of powder versus the particle size on a logarithmic scale for a water atomized iron powder. The curve ranges from 0% smaller to 100% smaller, as specified by the minimum and maximum particle sizes. Most often the basis is the cumulative mass, but other measures are possible such as the cumulative number or cumulative volume of particles at each size. For example, from sieve analysis the cumulative distribution is generated by adding the interval percentages and plotting the cumulative percentage versus the particle size. The information given in a sieve analysis is with respect to the weight of powder larger than the specific screen sizes. A smooth curve is passed through the size data for each interval.

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Curie temperature The temperature marking the transition above which a material is no longer magnetic. Formally it is the transition between ferromagnetism and paramagnetism. It is also termed the Curie point.

cut The fraction of powder above or below a given screen size, designating a selective particle size range. For example a cut might be defined as -270/+325 to signify the powder passed through a 270 mesh screen but collected on a 325 mesh screen.

cutting A machining operation that is performed by a tool that penetrates into the work material, while the work material is moved to generate a chip. Cutting chips can be in the form of a continuous ribbon or in discrete particles. Tools for cutting include tool steels, cemented carbides, and exotic composites and ceramics.

CVD An abbreviation for chemical vapor deposition.

CVI The abbreviation for chemical vapor infiltration.

cycle time A critical measure of molding equipment productivity, it is the time for completion of one forming cycle.

cyclic compaction Green density increases slightly with each compaction event so in some efforts repeated pressure strikes are used to boost green density, and research has taken this to the extreme of 10,000 repeated strikes. One machine uses repeat pressurization from a ringing pressure wave. The upper punch is pressed into the powder and then struck with a hydraulic hammer at 10 m/s. This strike generates several pressure pulses that start at 1500 MPa and decay on each successive pulse. One advantage of cyclic compaction is the production of a more homogeneous green body that undergoes less warpage during sintering.

cyclone separation Air classification of a powder into different size fractions based on a tapered cylinder where the powder in a gas enters at the top and the gas is vented out of the top. The design of the cyclone causes the powder to settle into a detachable container at the

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base while the gas swirls and exits the top. Cyclones for powder separation tend to be high in comparison with the top diameter - for example if the top has a diameter of d then the bottom of the cyclone will have a diameter that is only one-third this size and the overall length will be about three times the diameter. Proper particle separation is achieved through the dimensional ratios and gas flow.

CZ Various abbreviations and shorthand notations start with the CZ designation to indicate alloys based on copper (C for copper or Cu) with substantial zinc (Z for zinc or Zn) alloying. Additional initials indicate P for lead or Pb (CZP) and N for nickel or Ni (CZN), and are followed by numerical values that designate the approximate composition.

Section D

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A to Z of Powder Metallurgy

D D10, D50, D90 Indications for the size of particles on the cumulative particle size distribution corresponding to specific percentages, usually taken from the mass distribution. In this nomenclature D10 indicates the particle size at which 10 % of the particles are smaller, D50 the particle size at which 50 % of the particles are smaller which is the median particle size, and D90 corresponds to the particle size at which 90 % of the particles are smaller.

DA The common abbreviation used to designate a dissociated ammonia sintering atmosphere.

Darcy뭩 Law The flow of a fluid through a porous material provides an index of the pore structure termed the permeability coefficient α that is determined using Darcy뭩 law,

where Q is the flow rate (m3/s), A is the cross-sectional area of the material, L is the length, η is the gas viscosity, and P1 and P2 are the upstream and downstream pressures, respectively. The idea for the test is sketched in Figure D.1. Darcy뭩 law is valid for conditions where the flow is laminar, for example, from a pressure of two atmospheres upstream to one atmosphere downstream. The flow rate Q is the standardized gas volume (at one atmosphere pressure) per time and can be converted into a superficial velocity (not the true velocity in the pores) by dividing Q by the area A.

DBTT An abbreviation for the ductile-brittle transition temperature.

DEA An abbreviation for discrete element analysis.

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deagglomeration The attritioning or milling or chemical dispersion of an agglomerated powder to break it into individual particles. Deagglomeration is usually accomplished by a combination of drying, milling, or surface treatments. A common deagglomeration treatment is via light milling with balls or rods in a dry atmosphere. The desire is to break apart the agglomerates via impaction, but not to fracture the particles. For small particles in weak agglomerates, one option is to create repulsive forces between particles using thin polar molecule coatings. Common detergents are often effective, including dilute dishwashing soaps, as well as common surface active agents such as stearic acid, sodium oleate, glycerine, or oleic acid.

debind (debinding) A step between molding and sintering where the majority of the binder used in molding is extracted by heat, solvent, or other techniques. [see solvent debinding, thermal debinding, wicking, catalytic debinding]

deburring The removal of sharp corners, flash, burrs, or parting line defects by tumbling, abrasion, or filing. Simple deburring techniques are wire brushing, tumbling, or vibration of the sintered components with abrasive media and oil. They are useful in removing surface flash without damaging the component. Further, a corrosion inhibitor may be coated onto the component during deburring. Zinc powder can be mechanically vibrated onto the surface to form a coating resistant to corrosion. After tumbling, surface contaminants should be removed by washing in a solvent.

decarburization The loss of carbon from a material, usually a ferrous alloy, due to reaction with oxygen or other species during heat treatment or sintering. The oxygen might come from oxide films or absorbed oxygen on the powder. Other species besides oxygen can remove carbon at high temperatures, such as carbon dioxide, water, and at very high temperatures hydrogen.

decomposition The breakdown of a single alloy or compound into two or more different chemistries or phases. For example brass (an alloy of zinc and copper) will decompose into porous copper and zinc vapor on heating, a fact that makes sintering brass difficult.

deformation A permanent change in shape of a body due to stress, usually from a stress that exceeds the material뭩 yield strength at some point in its processing - includes thermal stresses.

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degassing A thermal treatment applied to a powder to help evaporate contaminant gases. It might be performed after a powder is loaded into a hot isostatic pressing container to ensure the powder is as clean as possible prior to sealing the container. Usual temperatures are at least 200EC and often reach up to 800EC and times under vacuum degassing might run to 24 h.

delamination The cracking of a green compact during ejection from a tool set due to a high elastic springback and low tensile strength. Figure D.2 shows a delamination crack that was evident after ejection from the die.

delta iron Body-centered cubic iron that is stable at temperatures higher than 1392EC up to the melting temperature. From a crystallographic perspective, delta and alpha iron are both ferrite, but differ in the temperature range where they form. [see alpha iron]

delubrication (delube) A burnout process applied to die compacted powder to remove the lubricant originally added to the powder to reduce ejection forces and tool wear. Often a delubrication step is included in the heating schedule to ensure decomposition products are not present at the highest temperature. [see burnout]

dendrite A crystalline morphology produced by skeletal growth leading to a microstructure that looks like a fern leaf. It is a common artifact in castings.

dendritic powder Particles having a structure composed of primary stems with several side branches, usually formed by electrolytic techniques. At high magnification the particles have a fern leaf or pine tree appearance. Figure D.3 is a scanning electron micrograph of a dendritic copper powder.

dense random packing The packing density for a powder at its tap density, where the particles are in a random but tight structure. Monosized spheres with larger sizes (over about 100 탆 ) subjected to repeated tapping will approach a fractional packing density of 0.637 or about 64 % of theoretical with an average coordination number of 7. For other particle shapes the

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packing density varies with shape, and can be very low for long, fibrous particles. If the gravitational acceleration is changed, such as through centrifugal force, then the dense random packing density also changes. [see tap density]

densification The change in porosity divided by the initial porosity due to pressing or sintering. A term loosely associated with property gains in powder metallurgy. Mathematically densification Ψ is defined as the change in fractional density due to sintering ρS from the starting green fractional density ρG, divided by the density change needed to attain a pore-free solid:

Densification, final density, neck size, surface area, and shrinkage are related measures of the particle bonding and pore elimination during sintering. For example, if a steel compact is at 6.80 g/cm3 green, with a theoretical density of 7.86 g/cm3, and sinters to a density of 7.00 g/cm3, then the densification is 19% and the shrinkage is just under 1% in each dimension, actually about 0.96%.

density The mass divided by the volume, usually expressed in g/cm3 (equivalent to Mg/m3) or sometimes given as a ratio to pycnometer or theoretical density. These latter ratios provide the fractional density and this is often given as a percentage. For an iron powder with a theoretical density of 7.86 g/cm3 that is pressed to 7 g/cm3 the fractional density is 0.89 or 89%. Specific gravity is essentially the same as density, but the density is divided by the density of water, which is almost 1.0 g/cm3 at room temperature (at 20EC pure water has a density of 0.997 g/cm3).

density ratio The ratio of the determined density of a compact to the absolute density of a metal of the same composition in a pore-free condition, expressed as a percentage or fraction.

desintering Most typically seen as the progressive loss of sintered density due to gas trapped in the pores and reactions that produce gas pressure in the final stage of sintering. For example oxygen dissolved in the metal might slowly diffuse to the pore and react with hydrogen from the process atmosphere. If the pores are closed, then the production of water vapor will pressurize the pores, decrease density, degrade properties, and eventually form a blister. A related phenomenon is seen in sintering mixed powders, where the rates of diffusion between species are greatly different, leading to vacancy

Page 5

Section D

accumulation and the formation of large pores; a process generally termed the Kirkendall effect. Another cause of desintering occurs when some of the pores are large with trapped internal gas; solution of the gas into the metal will result in coarsening or Ostwald ripening, leading to an increased pore volume, elimination of the small pores, and enlargement of the large pores. This was originally seen in radioactive materials where it was termed fission gas swelling. In sintering this usually occurs if there is a broad pore size distribution and the process atmosphere has some solubility in the metal at the sintering temperature. If the initial packing of particles is very homogeneous, then this problem is avoided. [see swelling, Kirkendall effect]

dew point A measure of atmosphere purity based on water content, it is the temperature where moisture condenses out of a process atmosphere. In sintering furnaces, the dew point is a simple measure of the hydrogen reduction potential. A dew point of 7EC corresponds to 1 vol. % water content in the atmosphere, while !42EC corresponds to 0.01 vol. % water vapor. The following equation allows calculation of the volume percent of water in an atmosphere VH2O if the dew point is known as TD (in EC): log10(VH2O) = - 0.237 + 0.0336 TD - 1.74@10-4 TD2 + 5.05@10-7 TD3 Generally a low dew point (low condensation temperature) corresponds to a cleaner atmosphere and easier oxide reduction during sintering. The dew point requirements to reduce metal oxides of gold, silver, cobalt, nickel, and copper are easily met by most sintering furnaces, but the reduction of oxides on zirconium, beryllium, calcium, and thorium are nearly impossible to attain.

dewaxing Extraction of wax binders from a powder, usually WC-Co, prior to heating. A binder burnout process very similar to thermal debinding and delubrication. It is performed in a separate furnace or at a low temperature to avoid contamination of the material at the sintering temperature. [see burnout]

dezincification Loss of zinc from a material, a problem most commonly seen in sintering brass. During sintering the high vapor pressure of zinc, compared to copper, leads to its preferential evaporation with a progressive increase in the copper content, especially on the outside surfaces. In some cases surface pores are formed on the component due to zinc loss. Under controlled conditions these pores can be quite small and even range to the nanoscale. During service, zinc loss can also occur due to selective corrosive leaching

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from surfaces exposed to an aggressive environment, such as seawater.

diamond An allotropic form of elemental carbon with a cubic structure. Diamond is stable at pressures over 6 GPa at room temperature and is metastable at atmospheric pressure. On heating diamond converts to graphite. Diamond powder is used in the production of abrasive tools, grinding wheels, and surface finishing tools, it is formed from carbon at high temperature and high pressure. Diamond is essentially pure carbon with a tetragonal bonding arrangement known as the diamond crystal structure. The density of diamond is 3.512 g/cm3 with an elastic modulus of 900 GPa. Diamond decomposes on heating, so diamond tool fabrication with mixtures of diamond and cobalt, copper, iron, or other powders, is limited to 1000 to 1200EC peak temperatures for short times. It is one of the hardest materials known to mankind (10 on the Mohs hardness scale and technical diamond reaches 4000 to 5000 VHN), diamond is also tops in thermal conductivity and in the pure form can reach 2100 W/(m EC).

diamond pyramid hardness This is another name for the Vickers hardness test.

diamond tool A combination of metal powder and diamond powder in the form of a hot pressed or sintered composite for cutting rocks or other hard substances. Figure D.4 shows the surface of a diamond tool where the large diamond particles are protruding from the surface. Generally, the diamond particles tend to be large and the matrix is a tough metal such as bronze, copper-titanium, tungsten-copper, cemented carbide (WC-Co), or cobalt. Diamond tools are formed by either infiltration, sintering, or hot pressing.

die A tool usually containing a cavity that is used to compress powders. It is the part that confines the powder during uniaxial compaction. A die has matching punches for applying pressure, and might also include matching core rods and adapters to properly align the punches and die in the press. Compaction dies are formed from steels, tool steels, and sometimes have cemented carbide liners. Dimensional tolerances are often specified in the 5 탆 range. Figure D.5 is a picture of a compaction tool set containing the die, removed from the compaction press.

die body The stationary or fixed part of a die supporting the die insert or liner.

die compaction The consolidation of powder through the application of uniaxial stress while the powder

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is constrained in rigid tooling. A simplified view of the process is sketched in Figure D.6. At the start of the die compaction cycle, the powder from the feed hopper is placed into the die cavity by the feed shoe at the apparent density. At this point there is no bonding between the particles. As pressure is applied by the punches, the particles rearrange, deform, and bond. Progressively more pressure is required to continue compaction since deformation hardens the particles. Eventually the material hardens to a point of diminishing return and nearly no density change occurs with progressive increases in the compaction pressure. Depending on the powder and tool material, the peak compaction pressure ranges up to 1000 MPa and in a few instances up to 4500 MPa. Very hard powders and very soft powders tend to be pressed at the lower pressures. For example, aluminum powders might be pressed at just 150 MPa and reach over 90 % density, but hard particles such as waxed cemented carbide powders (WC-Co) might be pressed at 70 to 175 MPa and only reach 60 % density.

die insert A removable liner for a die body that might be fabricated from a hard material such as cemented carbide. A hard die insert is used to minimize tool wear. The bulk of the tooling would be fabricated from lower cost material, but the die insert would be in contact with the component and fabricated from a hard material. A die liner is similar in concept, but is often not fabricated as a separate component, but is formed as a coating or hard electroplated layer.

die liner A thin, usually hard and wear resistant lining for the die cavity that is custom fabricated for the part such as by electroplating onto the machined tool steel.

die lubricant A lubricant mixed with the powder or applied to the die walls to facilitate the pressing and ejection of compacted powder while minimizing tool wear. [see lubricant, die wall lubrication]

die set The parts of a tool set that drop into a press where the package is assembled for proper positioning of the whole tool set prior to assembly in a press.

die wall friction In die compaction the applied, uniaxial load causes the powder to deform and spread laterally. That lateral pressure works against the tooling to create a wall friction that resists powder flow and sliding during compaction. One consequence of die wall friction is a continual loss of pressure with distance from the punch as the powder bleeds off the applied pressure in the form of die wall friction. Punch motion against the powder is

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similar to plowing snow: close to the punch the packing is dense, but far removed from the punch, the powder is unaffected. This pressure decay with distance is because the powder spreads load to the die wall, in the form of friction. Since pressure determines green density, pressure gradients become density gradients. Lubricants are often added to counteract this friction. Another factor is related to compact ejection from the tooling at the end of the compaction cycle.

die wall lubrication Uniaxial powder compaction where the die cavity is opened after ejection and an external spray of lubricant is dispersed on the die wall, after which powder fill occurs. The process avoids the need for direct lubricant additions to the powder, which is beneficial since the burnout of these lubricants usually slows heating to the sintering temperature. However, the compaction cycle is slower because of the additional die opening and spraying steps prior to filling the cavity with powder. Further, die wall lubrication is not successful on multiple level parts, since the individual fill and compaction positions will differ between levels. The first implementation of die wall lubrication was for the production of uranium dioxide nuclear fuel elements, and in that situation the lubricant was introduced via a seal ring built into the lower punch. As the punch moved to the fill position, it left a lubricant film on the die wall. Today, electrostatic sprays are used with an external dispersion unit to coat the die wall prior to filling with powder.

dielectric constant A material parameter associated with ceramics that measures the increase in charge at the surface due to an electric field. The application of a voltage to a typical insulator results in accumulation of a charge density on the insulator that is proportional to the electric field strength. Conceptually the charge density between two electrodes separated by vacuum is altered if there is a ceramic material inserted between the two electrodes to form a capacitor. This amplification of the relative electric permittivity over vacuum is called the dielectric constant. It varies between materials, for example alumina is near 10, beryllia is near 7, and cordierite is near 5, and polymers such as nylon and polyester are under 4.

dielectric strength For a dielectric material this is the maximum voltage gradient beyond which the dielectric material allows appreciable current flow. In other words, this is the limiting voltage over which the dielectric material fails and no longer operates as an insulator or capacitor. For alumina, this is in excess of 9000 V/mm, and common polymers can be twice this value.

differential scanning calorimetry (DSC)

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A technique used to determine the heat flow (differential calorimetry aspect) into or out of a sample during constant heating (scanning aspect). Usually it is applied to lower temperatures where a polymer melts, crystallizes, or evaporates.

differential thermal analysis (DTA) The careful measure of temperature and temperature difference during heating. As illustrated in Figure D.7, the technique relies on measuring the temperature difference between a sample and a reference material as a function of temperature, while both the sample and reference are subjected to a controlled temperature program. This is achieved by heating both in the same furnace. Usually the temperature is ramped linearly from room temperature to a maximum temperature, say at 5EC/min or 10EC/min. Thermocouples located below the test sample and standard are used to measure the temperature and temperature difference during heating. A plot of temperature difference as a function of temperature shows when heat is liberated (the test sample heats with a positive temperature difference) or when heat is absorbed (the test sample lags with a negative temperature difference). Whenever a polymer melts or a phase transformation occurs there is heat required for that phase transformation, so the test sample lags while the standard heats. The data shown in Figure D.8 are an example of the temperature lag associated with heating a stainless steel doped with boron to induce lower temperature melting at about 1225EC.

diffusion The progressive motion of atoms via single jumps or steps. When added up over a length of time, diffusion leads to measurable component shape and strength changes. Mostly used to describe sintering. There are several forms of atomic motion that are variants of diffusion - lattice or volume diffusion, grain boundary diffusion, and surface diffusion. Each involves the jumping of an atom to a new location under the stimulation of heat. [see lattice diffusion, grain boundary diffusion, surface diffusion]

diffusion alloyed steel Compositions of metal powders formed by intentional diffusion of nickel, copper, molybdenum, or graphite into iron powder during the powder fabrication process. This process has the advantage of locating the alloying elements on the surface of the iron powder to ensure a homogeneous final product (no powder separation in handling), but avoids the difficulty associated with compaction of hard, alloyed powders. Diffusion alloyed steels are typically used for medium strength applications that require heat treatment for strength or wear properties. In some nomenclature these alloys are denoted FD followed by numerical values that specify the nominal composition.

diffusion bonding

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A joining process where contacting powder metallurgy parts are heated to a high temperature under low loads, such that atomic diffusion forms a metallurgical bond; the process is also known as sinter bonding. A braze material or other bonding agent might be added prior to the firing cycle. In some cases, even a small powder of the same composition is placed between the two components, and during heating the small powder ensures good diffusion bonding.

diffusion induced grain boundary migration (DIGM) Enhanced grain growth or grain boundary motion due to a small concentration gradient across the grain boundary. This might occur if one grain was a pure metal (say molybdenum) and the other was slightly alloyed (such as molybdenum-tungsten), creating a steep tungsten composition gradient at the grain boundary. The gradient due to a segregated species in one grain leads to grain boundary motion into the pure material and precipitation of an alloy on the other side of the boundary. In some extreme cases, the grain boundary might contain a thin liquid or activator film that accelerates diffusion across the grain boundary. Grain coalescence is a related process where one grain adsorbs another due to the migration of the grain boundary through one grain with the precipitation of the mass onto the growing grain.

diffusional flow A gas passing through a porous metal at low flow rates and low pressures has a mean free path between molecular collisions that is larger than the pore size. This is termed diffusional flow and the rate of flow is controlled by the gas-pore interaction; the flow is independent of the gas viscosity. For example, this is how a gas flows out of the powder compact during heating in vacuum.

DIGM The abbreviation for diffusion induced grain boundary migration.

dihedral angle The angle formed by a grain boundary with a solid, pore, or liquid during sintering. It is a measure of the relative interfacial energies. As illustrated for the solid-liquid case in Figure D.9, the dihedral angle φ is determined by the surface energy balance. For the case of a grain boundary in contact with a liquid (during liquid phase sintering) the vector balance gives,

with γSS being the solid뻮olid interfacial energy (grain-boundary energy) and γSL being the solid-liquid interfacial energy. In the case of a grain boundary in contact with the free surface, the dihedral angle is determined not by the γSL surface energy, but by the solid-

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vapor surface energy γSV. Dihedral angle grooves are observed where grain boundaries emerge at free surfaces with the solid-vapor surface energy determining the dihedral angle. Figure D.10 shows the grooving of a grain boundary in a liquid phase sintered tungsten heavy alloy, corresponding to approximately a 60E groove. In the case of three solid grains in contact, the dihedral angle should be 120E if the grains are isotropic (surface energy the same in all crystal directions) except if the composition differs between grains. If one of the solid grains has a different composition, then the dihedral angle will depend on the relative interfacial energies.

dilatant flow Viscosity that changes with flow conditions where the mixture dilates (changes volume) under stress. Newtonian flow says the shear stress and shear strain rate are proportional to each other, and the proportionality coefficient is the viscosity. In dilatant flow, the shear stress τ increases faster with the shear strain rate dγ/dt and is characterized as follows:

where K is a material viscosity parameter and m is greater than 1 for dilatant flow (m = 1 for Newtonian flow). This form of flow is characterized by an increase in viscosity as the shear rate increases and is generally characterized as shear thickening. It is the opposite from pseudoplastic or shear thinning behavior. Dilatant flow is generally undesirable in powder injection molding feedstock since it induces powder-binder separation during molding.

dilatometer A device for measuring dimensional change during sintering, heat treatment, or for measuring thermal expansion induced dimensional change versus temperature. Figure D.11 is a sketch of a vertical dilatometer that operates with a single push-rod. The push rod rests on the sample and transmits dimensional change information to the computer. Thermal expansion causes the sample to enlarge, while sintering casues the sample to shrink, and chemical reactions often cause the sample to swell. Dimensional change signals are collected by the computer versus temperature to isolate processing temperature ranges for sintering. Vertical dilatometers have the benefit of minimal contact with the powder, a condition needed to minimize changes in the sintering signature due to the measurement process. On the other hand, horizontal dilatometers tend to be spring loaded to ensure contact with the sample, leading to a change in stress with extension or shrinkage. Additionally, dilatometers come in noncontact forms that rely on lasers, but with a sacrifice in signal fidelity. Both vertical and horizontal versions are in use and some models use two push rods to include a parallel calibration sample for comparative dimensional change data during heating.

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[see video imaging]

dilatometry Measurement of dimensional change during thermal processing to determine the sources of sintering densification, phase transformation, or other causes of dimensional control problems. Usually dilatometry is conducted with a laser or a pushrod dilatometer that tracks the specimen size in a furnace. Sintering experiments designed to find the densification temperature for a powder are conducted using constant heating rate dilatometry. A record of shrinkage or swelling versus temperature provides guidance on sintering cycles. Figure D.12 is a plot of shrinkage for a mixture of carbonyl iron with 2 wt. % nickel, heated at a rate of 10 EC/min, showing the onset of sintering shrinkage at about 600EC and a maximum shrinkage rate near 820EC. Also, the phase transformation in iron from alpha to gamma (body-centered cubic to face-centered cubic) is indicated to show how sintering is slower after the phase transformation. Dilatometry is a sensitive means to follow size changes and to assess the effects of differences in powder, mixing, compaction, alloying, heating rate, peak temperature, hold time, or atmosphere. A related tool for following dimensional change in sintering is high temperature video imaging. Here the aperture on a video camera is coordinated with a flash to illuminate the compact inside a furnace, allowing digitization of all dimensions and determination of sources of warpage. Such advanced tools are currently only available in research settings. In some thermal analysis laboratories dilatometry is known as thermomechanical analysis.

dimensional change The component size variation from the die or mold size to the size of the final sintered and heat treated state. The most common protocol is to rationalize the dimensional change to the tool dimensions, which ignore springback or expansion from ejection. Since height variations and green density variations are a normal part of shaping and compaction, it is sometimes best to use only radial dimensions. It is also possible to rely on the green dimension, which is usually slightly different from the molded dimension due to springback. This latter means of tracking dimensional change then ignores springback and is less useful for tool design purposes. Most materials shrink, but in some mixed powder systems the chemical reactions during sintering produce swelling.

dimensional control The repeatability of final dimensions as measured by the scatter part-to-part, tool-totool, day-to-day, and batch-to-batch. Usually quantified by the standard deviation (or a multiple of standard deviations, perhaps as large as six) observed in a dimension as normalized by that dimension. One parameter is the coefficient of variation (standard deviation divided by mean size) and is usually expressed as a percent. Dimensional variation is an inherent aspect of all powder metallurgy operations, hence machining

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and grinding are often used to improve dimensional control on critical surfaces. Various efforts have examined the sources of dimensional variation, and those data suggest that essentially everything is important. In die compaction and sintering, an audit of production operations showed a capability to hold dimensions to ? 40 탆 in the height direction and ? 5 탆 in the radial direction (one standard deviation). In that situation, dimensional variation was linked to raw powder changes (powder lot effect), powder mixture homogeneity (mixing), feedshoe operation, lower punch motion during filling, stiffness of the compaction press, and even position in the sintering furnace. Most important, in traditional die compaction the mass variation in pressing must be minimized. Data show that green mass variation is a key predictor of sintered dimensional variation, where green mass variation accounts for about 80% to the final sintered size variation. Distortion is an unfortunate byproduct of heat treatment that hinders precise dimensional control. Larger components and higher carbon levels and thicker sections result in more dimensional variation. Rapid heating and rapid cooling (quenching) exacerbate dimensional control problems. Slow heating helps avoid distortion, but often results in lower mechanical properties. Thus, one justification for higher alloying additions in ferrous powder metallurgy is to slow the necessary cooling rate to help reduce distortion. Cost is the offsetting factor, since high hardenability compositions involve costly alloying additions.

direct sintering Electrical discharge through the green or presintered compact where electrical resistance causes heating to the sintering temperature without externally supplied heat. Direct sintering is usually applied to long, cylindrical compacts that are clamped at each end for electrical contact. The process is related to spark sintering. It is common to surround the material with a reducing species such as hydrogen when sintering tungsten and vacuum when sintering tantalum and other hydride formers. [see spark sintering]

directed metal oxidation Processes that convert a liquid metal into an oxide based on using selected oxidation conditions. The process is applied to the generation of aluminum oxide reinforcements in an aluminum matrix as a three-dimensional composite. One example alloy ideal for directed metal oxidation is Al - 7 wt. % Mg which is oxidized to a maximum of 50 vol. % alumina. The composite has a density of 3.3 g/cm3, yield strength of 127 MPa, and ultimate tensile strength of 317 MPa, with an elastic modulus of 159 GPa. The thermal expansion coefficient is 14 ppm/EC and the thermal conductivity is 66 W/(m EC).

discontinuous grain growth Another name for exaggerated grain growth.

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discrete element analysis (DEA) A computer routine applied to powders where each particle is a single calculation cell. During a simulation the motion and bonding of each particle is continuously updated to predict the properties or behavior of the particle assembly, such as flow, die filling, compaction, or sintering. Discrete element analysis contrasts with finite element analysis since DEA calculates the behavior at the particle level, while FEA ignores the particles and calculates the behavior for the body based on a mesh of calculation cells. For example, Figure D.13 shows a discrete element analysis for the packing of a random bimodal mixture of spheres. Such an analysis is useful in extracting the pore size distribution for different mixtures of solid particles.

disintegration The reduction of a massive solid material into powder. This can be accomplished by milling, grinding, atomization, or other routes that create particles out of a bulk material.

dislocation climb The motion of a dislocation to a new slip plane due to vacancy motion. Dislocation climb is a sintering densification process that couples the dislocation population in a powder compact with the vacancy emission from a pore. The dislocations annihilate the vacancies and this effectively removes a line of atoms from the dislocation, resulting in dislocation climb to a parallel slip plane. It is effectively a short-circuit means to densify a powder compact during heating, since the vacancy diffusion distance can be relatively short in contrast with the diffusion distance from a pore to a grain boundary. However, dislocation climb, and in general sintering, result in a declining population of dislocations over time. Hence, dislocation climb is only active during initial heating and is sensitive to the plastic deformation of the metal powder in compaction - the greater the compaction deformation then the higher the dislocation density and more contribution to dimensional change in heating from dislocation climb. Most of the insight into this mechanism came from positron annihilation studies performed by Werner Schatt in the early 1980s.

dislocations A linear crystal defect that moves by slip in response to a shear stress when the resolved stress on the slip plane exceeds the critical resolved shear stress, leading to plastic flow. As shown in Figure D.14, there are pure screw and pure edge dislocations, but in most materials the dislocations exist as mixtures of the two forms. During sintering the dislocation population generated by plastic deformation in compaction provides a means for rapid early sintering. The dislocations interact with vacancies and undergo dislocation climb, improving sintering densification by a bulk transport process. Schatt has analyzed dislocation climb during heating and concludes that the early densification rate during sintering is related to dislocation climb, where the dislocations

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collect vacancy fluxes being emitted by the pores. The effect appears to be active during initial heating while the dislocation density is high, over about 2@1012 m-2. That can contribute between a 10- and 100-fold increase in sintering rate over normal diffusion effects during heating. Indeed, most sintering studies show the most intense sintering action occurs during heating, not during the isothermal period.

dispersant A surface active agent added to a powder in small concentrations to induce particle separation via electrostatic repulsion. Often the dispersant is a polyelectrolyte or detergent type molecule; small polar molecules with charged anionic or cationic terminal groups. The terminal groups might be hydroxyl (OH-), sulfonate (SO3-), sulfate (OSO3-), or ammonia (NH4+) complexes. Shear forces help in dispersing a powder, and intense mechanical stirring or ultrasonic agitation are often used during agglomerate disintegration. One of the easiest means to ensure dispersion is to add a polymer coating to the particles when it is formed. The polymer prevents agglomeration and the need for a dispersion step after storage. This can be performed with volatile molecules that spontaneously polymerize on the fresh powder surface.

dispersion A two-phase system consisting of particles in a fluid. Often chemical surface treatments are required for the powders to induce particle-particle separation in liquids such as water. Dispersants or surface active agents (surfactants) are selected that attach to the particle surface to repel the particles from each other. The opposite behavior is seen in agglomeration, granulation, and flocculation.

dispersion strengthen The incorporation of small, inert second-phase regions dispersed throughout the microstructure to provide high temperature strength and creep resistance. Usually an oxide is used in strengthened metals, where the oxide is selected for its resistance to dissolution in the metal at high temperatures. When the dispersoids are small, on the order of 10 nm, and closely spaced, then the impediment to dislocation motion is large and significant strengthening occurs. If the dispersoids are inert and insoluble in the matrix, then dispersion strengthened materials retain strength nearly to the matrix melting temperature.

dispersoid A small particle of insoluble material, usually a hard oxide ceramic, that is placed in the metal matrix to provide strengthening. Favorite dispersoids are alumina, used in aluminum, copper, and nickel alloys. [see dispersion strengthen]

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dissociated ammonia (DA) A reducing atmosphere produced by the thermal decomposition of anhydrous ammonia over a catalyst, resulting in a gas of 75 % hydrogen and 25 % nitrogen; it is also termed cracked ammonia. Dissociated ammonia is sometimes used as a substitute for pure hydrogen. It is delivered as a liquid under pressure. Prior to entering the furnace the ammonia molecule is broken into hydrogen and nitrogen, 2 NH3 (g) 6 3 H2 (g) + N2 (g) using a catalyst heated to about 1000EC. Residual ammonia in the dissociated ammonia is typically below 250 ppm. As long as the moisture content is low the dissociated ammonia atmosphere is nearly neutral with respect to carbon, yet the atmosphere is useful for reducing oxides during sintering for several metals. However, in some instances the nitrogen reacts to form nitrides and harm the product.

distortion Nonuniform dimensional change measured versus the average sintered dimensions, for example using the standard deviation in dimensions point-to-point over the component exterior as referenced to the mean dimensions. Most sintering distortion has its origin in the forming or compaction step, and a significant amount of sintering distortion is traced to the green mass variation. Density gradients in the green body are another cause of distortion. Higher sintering temperatures tend to increase the net dimensional change in a compact while at the same time making the material weaker. Both factors amplify the green density gradients. For this reason, many ferrous powder metallurgy compacts are sintered at lower temperatures to minimize the net dimensional change in sintering. If a component is located in a thermal gradient, where heat is applied from only one direction, then there is a tendency for more shrinkage to occur on the hot face. The consequence is illustrated in Figure D.15, where the component has distorted toward the hot source. Distortion tends to increase as with the component size since due to nonuniform heat transfer in sintering or heat treating. [see dimensional control]

Dorn technique In this approach experiments are conducted during dilatometry using changes in the rate of heating to identify the activation energy for mass transport. The change in response with the instantaneous change in heating rate allows determination of the activation energy. It can also be used during isothermal sintering by changing the temperature slightly. The sintering rate is noted just before and just after the change. The apparent process activation energy Q is calculated from the ratio of shrinkage rates,

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where R is the gas constant, T1 and T2 are the two temperatures, and v1 and v2 are the instantaneous sintering rates. This technique provides insight to the fundamental sintering mechanism by identification of the activation energy for shrinkage from rapid, but small temperature changes using in situ measurements. No corrections are required for changes in density, grain size, or other geometric features since the comparison is made with only a change in temperature.

double-action pressing A method by which a powder is pressed between opposing punches where the combined motion of all punches is toward the die center. Although this is one of the better ways to reduce green density gradients, more typical in production is to rely on a lower-cost variant termed the floating die. A simple sketch of double-action pressing is shown in Figure D.16. [see floating die]

double-cone mixer A mixer consisting of two cones attached to each other at their open bases, which provides a means to dry mix powders when it is rotated. When one chamber tip reaches its peak, the opposite tip reaches the bottom and the powder falls between the two chambers. Figure D.17 shows a characteristic design. The interior design of the mixer determines mixing efficiency. Baffles and high intensity spinning blades enhance intermixing, but might attrition the powder. The volume of powder in the mixer determines the mixing efficiency. As the chamber becomes filled with powder, the relative motion of the powder is inhibited. A powder volume between 20 and 40% of the chamber capacity is usually optimal. The rotation speed also has an effect on mixing efficiency. A slow rotation speed will prolong the time necessary to obtain adequate mixing, and extensive free-fall of the powder in the mixer will cause preferential size settling and impact grinding, but rapid rotation will impart a centrifugal force to the powder which interferes with mixing. As with most mixers, the highest efficience often occurs when they make the greatest amount of noise.

double-press and double-sinter (DPDS) A variant on traditional die compaction powder metallurgy where the pressed and sintered compact is passed through compaction and sintering a second time to increase density and precision. In doing this, the first sintering temperature is usually low; for steels in the alpha iron region to avoid carbon dissolution into the iron. That keeps the

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compact soft for more densification on the second pressing. The second sintering cycle reaches the austenite temperature range to allow carbon to dissolve into the iron for strengthening. Double-press and double-sinter components show a higher density when compared with single press-sinter components, with improved precision. The density gain is about 2 to 3% but the strength gain might be 20% with less dimensional variation.

DPDS The common abbreviation for double-press and double-sinter processing.

draft angle A slight taper on the tooling that is primarily used to assist in ejection. In injection molding, the taper or draft is perpendicular to the parting line. For thin or squat components, a taper is not needed, but as the ejection length increases sticking in the tooling raises the ejection stress, thereby increasing the probability of component damage. Tooling can be designed to reduce the ejection problems. In powder injection molding just a 0.5E positive taper is sufficient, and in the worst case no more than 2E is needed. If no draft angle is included, then there is the possibility of component damage during ejection.

drainage In dealing with porous materials there is a hysteresis between the pressure-volume relation seen on pore filling and that seen on pore emptying. If the liquid is extracted without conversion to vapor (burnout), then the extraction process is termed drainage. This can be performed by simply allowing the liquid to wick out of the body. When a liquid is removed via drainage, a certain portion remains behind in the pores. That is termed the irreducible saturation. [see irreducible saturation]

draw A term used for tempering to indicate the shift to lower hardness and higher toughness by reheating to a moderate temperature after quenching.

drilling The creating of a hole using a rotating fluted tool that operates on a stationary component to cut or remove mass while generating a hole.

Drucker-Prager model A constitutive model used in computer simulations to treat the plasticity of the powder during compaction. The model assumes the powder behaves elastically up to some state of stress where yielding occurs. It is a modified treatment of die compaction that

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relies on the von Mises yield criterion. [see cap model]

dry bag cold isostatic pressing Similar to traditional cold isostatic pressing, but the rubber container that holds the powder remains in the die cavity and only the compact is ejected. Figure D.18 illustrates one example for the fabrication of a cylindrical compact, showing both the initial powder fill and the pressed size. Note friction on the top and bottom punch leads to pinched corners.

drying The removal of a binder or liquid from inside a powder structure via moving gas. Certain powder injection molding binders must be dried prior to sintering. If the drying process is too rapid, then the component will crack, so modest drying rates enable constant flow of the liquid to the surface.

DSC The abbreviation for differential scanning calorimetry.

DTA The abbreviation for differential thermal analysis.

dual action pressing Compaction of a powder with both the upper and lower punches moving simultaneously toward the horizontal center of the die. It is more common in powder metallurgy to approximate dual action pressing using a floating die. More commonly termed doubleaction pressing. [see double-action pressing]

dual level part A powder metallurgy component with an intermediate plane (besides the top and bottom) perpendicular to the pressing direction. Figure D.19 shows an example of a dual level part, looking at both the top and bottom.

ductile A material with considerable ability to stretch or bend prior to fracture. For example gold can be deformed to a great extent without fracture, while most ceramics are not ductile. Ductility is usually measured in a slow strain rate tensile test, with the elongation to fracture being several percent in a ductile metal. Unfortunately, sintered steels tend to be low in ductility if they are porous and heat treated for a high hardness. As a rule of

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thumb, materials with more than 3 % elongation are generally considered ductile. The fracture surface shows evidence of material pulling and flow prior to failure.

ductile-brittle transition temperature Materials often change from ductile failure to brittle failure as the test or use temperature decreases, and the temperature where this shift occurs is denoted as the ductile-brittle transition temperature or DBTT. Materials such as tungsten have a ductilebrittle transition temperature that depends on both the grain size and impurity level, and for a high purity material with a small grain size the DBTT can be near 150EC. But with a low purity and large grain size the DBTT increases to 450EC. Many of the refractory metals and steels exhibit similar sensitivities that change the mechanical properties in almost unexplained behavior patterns. Some sintered steels exhibit a DBTT very close to room temperature and are not satisfactory for cold applications.

ductility A measure of the permanent stretching or permanent deformation a material can take prior to failure. Brittle materials have no ductility, while materials such as stainless steel exhibit large ductilities. Figure D.20 shows the ductility associated with fracture of a metallic tensile sample, evident as both the stretching and reduction in area at the central region. There are two common measures of ductility, one being the elongation to fracture and the other being the reduction in area. Both are measured with a tensile test. For a ductile material such as an austenitic stainless steels sintered to full density, the elongation to fracture is as high as 60 %. A few powder metallurgy materials with small grain sizes can exceed 100 % elongation, leading to the concept of superplasticity. Generally, a sintered steel should be treated as a brittle material when it has a ductility below 3 %. [see elongation, reduction in area]

dusting The tendency for a wide particle size distribution to separate the small particles from the large particles during discharge, loading, or other forms of transfer. Larger powders follow the planned trajectory, but the small particles separate, become caught up in convection currents, and form a plume of dust. Polymer additions are commonly employed to adhere the small particles to the large particles, thereby avoiding dusting. For example a polymer is commonly used to adhere small graphite particles to iron powder during transport and compaction. During sintering the polymer burns out and the graphite diffuses into the iron to form steel. The dust might cause health problems, fire hazards, or product quality changes over time.

dwell The time period during which maximum pressure is applied to a compact during

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compaction or shaping. Usually this is a split second, but in a few components there is an observed benefit from a prolonged pressure dwell since bonding polymers can move inside the compact.

dynamic compaction Explosive or gas gun compaction of powder at shockwave velocities. Dynamic compaction applies to several techniques that rely on a traveling pressure wave to compress the particles and to induce frictional heating at the particle contacts. With a high input energy, frictional heating will melt the particles at the contact points, but the bulk temperature rise is small. Peak wave velocities up to 10 km/s have been reported with pressure pulses over 300,000 atmospheres. Gas gun concepts rely on impacting a high-velocity mass against the powder compact using a projectile launcher. For example, fully dense iron powder is formed using 500 m/s impact velocities and peak pressures of 2 GPa. With aluminum a peak pressure of 1.2 GPa gives full density. These techniques break up the surface oxides, giving good particle bonding, especially if the shock wave produces melting at the particle contacts. Even though the heating might form a thin liquid layer at the particle contacts, the liquid duration time is very short. Consequently, amorphous powders are densified and bonded with preservation of the amorphous state. [see explosive compaction]

dynamic light scattering A technique used to measure the average particle size for very small powders in the nanoscale range. It relies on the Doppler shift in the wavelength of a laser beam that occurs due to natural Brownian motion. This size analysis technique is also termed photocorrelation spectroscopy. A sketch of the process is given in Figure D.21. The approach uses laser light and an internal beam splitter for calibration. The laser is monochromatic, so any shift in wavelength provides information on the Doppler frequency shift due to Brownian motion of the particle in the fluid. Over the particle size range from approximately 5 nm to 5 탆 , the frequency shifts are from 1000 to 1 Hz, inversely varying with the particle size. To measure the size distribution, intensity versus frequency data are collected over a few minutes. Subsequently those data are analyzed to deconvolute the particle size distribution. This approach requires knowledge of the optical properties of the fluid and particles, fluid viscosity, and temperature. It is best applied to nonconductive particles that do not agglomerate.

dynamic properties Material properties that rely on fast or cyclic loading, such as fracture, impact, and fatigue. Often the term is applied to properties associated with strain rates that exceed 0.1 per second. These tend to be problem areas for sintered metals. On the other hand, strength and hardness which are measured at low strain rates, are less sensitive to

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residual porosity and are called quasi-static properties. [see fracture toughness, Charpy impact energy, fatigue strength]

dynamic ratio In automated particle size analysis, this is the ratio of the largest to smallest particles that can be measured simultaneously without resetting the instrument뭩 operating parameters. In laser instruments that rely on multiple detectors, the dynamic ratio can be as large as 7000 to 8000. In techniques such as electrical zone sensing, the dynamic ratio is less than 30.

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A to Z of Powder Metallurgy

E EBS The common abbreviation for ethylene-bis-stearamid, a lubricant used in powder metallurgy.

eccentric press A mechanical press in which an offset or eccentric is used to move a slide that drives punch motion. In a simple sense the up and down motion of the punch is derived from the rotation of a crank, where the punch is pivoted as illustrated in Figure E.1. On each revolution of the crank the punch goes through a compaction stroke.

ECM An abbreviation for electrochemical machining.

economic batch size A typical lower limit for the application of a powder metallurgy production process, expressed in terms of number of components per year. At lower levels of production, the time and cost to engineer, layout the plant, order, inspect, and install the tooling makes a powder route noncompetitive. For example, powder injection molding has historically best matched with industrial needs at production quantities more than 5,000 per year and has reached more than 100,000,000 per year. Each technology has its lower production quantity where it is no longer economically competitive; this lower limit is termed the economic batch size.

eddy current An electrical current induced due to a shifting local magnetic field. The eddy currents reduce electrical machine efficiency and are avoided whenever possible. Eddy currents can be stimulated by an oscillating magnetic field and that proves useful in performing nondestructive inspection, since cracks and other defects resist current flow.

EDM An abbreviation for electrical discharge machining.

effective pressure

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In full density processing the applied pressure is amplified at the particle contacts. The local pressure acting at the particle contacts is much higher than the applied pressure. A relation exists to estimate how the applied pressure PA is amplified into a higher effective pressure PE at the particle contacts based on the fractional density ρ and fractional green density ρG as follows:

Figure E.2 plots the ratio of PE/PA versus fractional density for an initial fractional green density of 0.6; only at full density are the two equal. The concept is important to modeling hot densification processes such as hot isostatic pressing, where the effective pressure at the particle contact determines the densification rate. As the density decreases the local pressure in the porous body is large.

effluent Any waste liquid or gas from a process that must be properly discharged or disposed, such as from electroplating baths, cooling baths, or furnace.

ejection The final stage of die compaction where the pressed or molded powder compact is forced out of the die. Usually the compact is mechanically locked into the tooling because of relaxing strains after compaction. The same problem exists in many forming technologies. Lubricants are either placed on the tooling or added to the powder to help with compact ejection. In die compaction, the initial force needed to initiate ejection is termed the stripping force or pressure, and the pressure needed to sustain pushing of the compaction out of the tooling is termed the sliding pressure. A high lubricant concentration lowers the peak ejection pressure. Since the compaction tools are strong and hard, but the compact is weak, cracking can occur during ejection. Thus, conditions must exist to keep the maximum ejection stress below the green strength. Then lubricants are important in reducing the ejection force and further it is sometimes necessary to add a binder to improve green strength.

ejector pin In injection molding, an ejector pin pushes a powder injection molded compact out of the mold cavity after cooling. Ejector pins are attached to an ejector bar what actuates the ejection motion from an auxiliary hydraulic cylinder. The pressure of ejection often leaves a slight impression on the compact surface. The ejection force and number of ejector pins depend on the contact area between the tooling and component, tool surface finish, coefficient of friction, and thermal contraction in the cavity. Prior to ejection, inserts, internal cores, or threads must be retracted. Figure E.3 is a picture that shows ejector pin marks. Normally, the pins are placed to impress on noncritical

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locations and constitute more than 10% of the projected compact area.

elastic limit The maximum stress that a material is capable of sustaining without any permanent deformation remaining when the stress is removed. A material that takes on a new size or shape after stress would have been loaded past its elastic limit. [see yield strength, proportional limit]

elastic modulus Also known as Young뭩 modulus or stiffness. A material constant that measures the proportionality between stress and strain during the initial, elastic portion of the tensile test. It can be estimated from the slope of the linear portion of the stress-strain curve, as illustrated in Figure E.4, or measured using ultrasonic wave propagation velocities. The linear relation between stress and strain is not valid for strains that induce permanent deformation, which occurs at approximately 0.4 % to 0.5 % strain for many sintered metals. Like other mechanical properties for sintered metals, the elastic modulus E varies with fractional density ρ in a power law relation,

where Eo is the full-density elastic modulus and the exponent Y varies from 0.3 to 4, depending on the pore structure. For example, for a sintered steel the elastic modulus at 100% density was measured at 207 GPa, but at 90% density the elastic modulus was 147 GPa, and at 80% density it was 89 GPa, less than half of the full density value. In that case, the exponent Y = 3.4.

elastic strain Nondimensional stretch, or length change, divided by the original length, fully recoverable strain when the stress is removed. It is associated with atomic bond stretching.

elasticity A material property associated with stress, related to the return to an original shape when a stress is removed. A perfectly elastic body will totally recover its original shape and size after release of a stress.

electrical conductivity The inverse of resistivity. A measure of the ability to allow electron or ion passage through the material. Most testing for electrical conductivity is performed using a fourcontact technique, with two outer contacts providing current flow and two inner contacts measuring voltage. Resistance is simply voltage divided by current, but resistivity (a material property) depends on the sample area and length. Resistivity and its inverse,

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conductivity, are geometry independent. At low porosities, the electrical conductivity parallels the thermal conductivity. Often the electrical conductivity is expressed as a ratio to copper. [see International Annealed Copper Standard]

electrical discharge machining (EDM) Metal removal using a rapid spark discharge between different polarity electrodes. One of the electrodes is the workpiece and the other is the forming electrode. A machining technique based on electrical spark machining across a small gap (smaller than 1 mm) filled with a dielectric fluid. The sparks evaporate and expel material to form a tool profile in the work. It is a standard means to create hard tooling with complicated shapes. Variations include plunger EDM and wire EDM.

electrical materials Products for electrical applications are characterized by conductivity properties. Copper and silver are fabricated into composite electrical contacts, using a variety of dispersed phases ! chromium, polymers, tungsten, and cadmium oxide as examples. Both residual porosity and impurities lower conductivity. Other speciality materials include tantalum for capacitors and refractory metals for electrodes so many other metals are fabricated using powder metallurgy for speciality electrical applications. The high melting temperatures make the refractory metals the common choice for welding electrodes and lightbulb filaments. The high hardness and wear resistance coupled to arcing resistance leads to the use of composite tungsten-silver and tungsten-copper liquid phase sintered products in circuit breaker contacts.

electrical properties Properties associated with the conduction of electrical current. [see electrical materials, electrical conductivity]

electrical zone sensing A means to measure particle size by dispersing the particles in a saltwater or other electrically conductive solution. As shown in Figure E.5, the solution is pumped through a small orifice in a glass tube that has an induced current from inside and outside electrodes. As particles are captured in the fluid, they disrupt the electrical signal, providing a pulse that is proportional to the particle size. Continued pumping of the electrolyte and powder mixture allows for determination of the particle size distribution using the pulse heights in the electrical zone. The technique was originally developed for blood cell analysis, but has been extended to many powders. Through calibration procedures and selection of the appropriate aperture diameter (about 1.6 times the largest particle size) the technique can be applied over several size ranges, each with a dynamic ratio of approximately 30. The lower particle size capability is 0.5 탆 .

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electrochemical machining (ECM) Controlled metal removal in a machining situation by anodic dissolution. Direct current passes through a flowing film of conductive solution which separates the workpiece from the electrode. The workpiece is the anode and the forming electrode is the cathode.

electroconsolidation A novel powder densification process that is a hybrid between spark sintering and hot pressing. The powder component is packed in a graphite particle matrix and graphite die. To induce heating an electrical current is passed through the powder compact and the graphite packing from opposing rams. To induce densification those rams push toward the powder. Figure E.6 is a sketch of the process.

electroless plating A process in which metal ions in a dilute solution are plated onto a substrate by means of a spontaneous plating event. No external voltage is required.

electrolytic powder Powder produced by electrolytic deposition and subsequent pulverization. The resulting powder has a dendritic or sponge shape as evident by the copper powder shown in Figure E.7. It is mostly used for elemental powders such as palladium, chromium, copper, iron, zinc, manganese, and silver. The main benefit of an electrolytic method is the high product purity. The cycle begins with dissolution of the metal into the bath at the anode under an applied voltage with deposition at the cathode. Operating conditions are adjusted to ensure the deposit is fragile and easily converted into powder. Periodically the porous cathode deposit is removed, washed, dried, milled into powder, and annealed to remove any strains or volatile impurities.

electro-magnetic field compaction A novel technique based on rapid electrical discharge to create a collapsing magnetic field that can be used to compact powder.

electron beam welding Joining of materials based on local melting at points of contact by use of a high energy electron beam.

electron microprobe analysis A technique developed in the 1950s to perform chemical analysis on micrometer sized regions in a material. The instrument uses a focused beam of electrons to excite the

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production of X-rays from a small spot on the test material. The distribution in wavelengths of the X-rays provides information on the composition being stimulated by the beam. The quantitative analysis is much more accurate than possible using energy dispersive analysis of X-rays.

electron spectroscopy for chemical analysis (ESCA) The same thing as X-ray photoelectron spectroscopy. It is a technique for determining the elemental and chemical composition of a surface. The material is bombarded by soft X-rays and the electron photoemissions are analyzed for their source and its chemistry.

electrophoretic deposition (EPD) A forming process for depositing small particles into thin layers. The process is sketched in Figure E.8. It relies on particles suspended in an electrolyte solution. An applied voltage packs the dispersed particles into the desired layer using a permeable membrane as the mold. The initial particle suspension is in the range of 10 vol. % powder. Small particles dispersed in an electrolyte become charged and in the presence of an electric field can be packed into a shape. The electrolytes are solvents with a small concentration of polymer. In EPD the green body density is independent of the particle size, so it is attractive for very small particles which are difficult to compact. Also, since the forming rates are low the particle composition in the electrolyte can be changed over time, leading to gradient microstructures - for example the bottom surface might be pure ceramic and the top surface might be pure metal. The approach was initially perfected for painting using solid paint particles, but is now applied to a variety of small particles. Since there is no stress, the tooling is fabricated from soft or easily formed materials such as polymers or graphite backed by a metallic electrode.

electroplating A secondary operation designed to coat an object with a material that protects the surface from corrosion, oxidation, or other forms of attack. Electroplating is a surface chemical deposition or coating process that is driven by an applied voltage. It is usually applied to components over approximately 90 % density. For lower densities, it is appropriate to fill the pores with a resin to avoid plating chemicals from becoming trapped in the pores. Burnishing or other surface deformation techniques such as shot peening or buffing are successful in sealing pores as well. Electroplated coatings for corrosion resistance are based on chromium, nickel, copper, zinc, or cadmium. Subsequent corrosion problems may occur if the electroplating solution is trapped in the surface connected pores. Thus, thorough cleaning after coating is crucial. Plating with copper is usually followed by nickel electroplating or coating with a protective lacquer, since copper will naturally corrode. Nickel plating is used for wear resistance or corrosion resistance, and might provide a needed base for chromium electroplating. Chromium is selected for corrosion and wear resistance.

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electrostrictive A property of some materials that they change size or shape when activated by an electrical field.

elemental powder Powder of a single chemical species like iron; a powder with no alloying ingredients.

elongation A measure of ductility. It is the amount of permanent plastic stretch a material undergoes prior to failure in a tension test. Elongation is the most common measure of ductility and is calculated as the change in length divided by the initial length. Usually in tensile testing that initial length is standardized to a gage length near 25 mm located near the center of the sample. [see ductility]

elutriation Classification or separation of powder particles by size using a rising stream of gas or liquid acting against the settling due to gravity, similar to air classification. The larger the particle the faster the gas or liquid flow needed to carry the particle upwards. Elutriation is essentially sedimentation where the fluid flow is adjusted to balance gravitational settling. It provides a means to separate or classify a powder into a sized fraction, since only certain sized particles can be carried away by the gas flow.

emulsion A stable dispersion of one liquid as colloidal droplets in another liquid, often facilitated by an emulsifying agent that has affinity for both the matrix and dispersed liquids. Milk is an emulsion.

endogas Another name for an endothermic atmosphere.

endothermic atmosphere A reducing gas atmosphere used for sintering. It is produced by the reaction of a hydrocarbon fuel gas and air over a catalyst with the aid of an external heat source. The resulting atmosphere is low in carbon dioxide and water vapor with relatively large percentages of hydrogen and carbon monoxide. It relies on a ratio of about 60 % fuel. A typical composition in the sintering furnace consists of 39 % nitrogen, 39 % hydrogen, 21 % carbon monoxide, and less than 1 % of water, carbon dioxide, oxygen, and methane.

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endothermic reaction A reaction that absorbs heat, such as the melting of a metal.

endurance limit or endurance strength The maximum stress that a material can withstand for an infinitely large number of loading cycles. It is a characteristic material property measured by fatigue testing and for practical purposes is the stress for which a material survives 10 million loading cycles. A typical plot of failure stress S versus the number of cycles to failure N is shown in Figure E.9. This is sometimes called an S-N curve. The stress corresponding to 107 cycles is the most common value cited for the endurance limit. It might be given in terms of the 50 % survival stress, 90 % survival stress, or 99 % survival stress, with the latter being the more conservative.

engineering material Mostly manmade materials that can be tailored to the strength, corrosion, electrical, thermal, magnetic, or other dominant concerns. For example, stainless steel and polyethylene are manmade materials that do not occur in nature and are common engineering materials. Wood, leather, stone, and other natural materials tend to be more costly and more variable.

engineering strain It is the change in component length divided by the initial length for a structure with an applied load. There are no units to strain, but in some instances it is given as a percentage of the initial length. The true strain will differ from the engineering strain at larger strains, such as encountered in deformation processing.

engineering stress The load divided by the initial cross-sectional area of the material supporting that load, where the area is measured perpendicular to the load. Usually this load over area is close to the true stress, except during plastic deformation when the true stress (load divided by actual area) will be much higher.

enhanced sintering Methods that increase either the kinetics or driving force associated with mass flow during heating. Most techniques adjust the rate of sintering through the activation energy for atomic motion. The typical desire is to reduce the sintering time, temperature, or to improve the degree of sintering. Many examples exist, ranging from use of nuclear radiation to generate lattice defects to cyclic heating through a phase transformation to generate internal stresses to improve sintering. Other routes use magnetic or electrical forces to assist in atomic motion. Mixed powder effects are most common, such as the addition of boron to stainless steel.

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Section E [see liquid phase sintering, activated sintering]

environmental properties Attributes that monitor the deterioration of a sintered material due to interactions with its ambient environment. [see biocompatibility, oxidation resistance, wear resistance, corrosion resistance]

EPD The abbreviation for electrophoretic deposition.

epitaxial coating Atomic spacing in a coating that perfectly matches and maps the atomic spacing in the substrate material.

EPMA The abbreviation for the European Powder Metallurgy Association.

equiaxed powder Particles with approximately the same size in all three (perpendicular) dimensions.

equilibrium constant Reactions such as oxide reduction occur during sintering as controlled by thermochemical reactions that balance forward and reverse events. One of the most important during sintering is oxide reduction. The ease of oxide reduction is measured by the equilibrium constant. It is a quantity that characterizes the equilibrium state based on the amount of reactants and products. Assume the solids are of fixed composition, such that a metal M is in equilibrium with oxygen gas O2 and the oxide MxO2, where the subscript represents the stoichiometry of the oxide (s = solid, g = gas), x M (s) + O2 (g) X MxO2 (s) For this reaction, the equilibrium constant K is defined as follows:

where a designates a thermodynamic quantity known as the activity. For the solid phase, the activity is unity (meaning there is plenty of each solid available for reaction) and the PO2 oxygen partial pressure is the only factor that determines which way the reaction progresses. Thus, the equilibrium constant for the oxidation-reduction reaction

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depends only on the inverse partial pressure of oxygen, and lower partial pressures favor oxide reduction. In turn, the equilibrium constant reflects the standard free energy ∆G for the reaction,

where R is the gas constant and T is the absolute temperature. The free energy for such reactions can be determined from tables or charts available in standard metallurgical reference books or via computer routines.

equivalent spherical diameter The particle size calculated from a measure of the particle surface area, volume, projected area or settling rate, in each case assuming the particle is a sphere. For example, if a particle has a projected area A, then the equivalent spherical diameter based on projected area DA is calculated by setting the projected area equal to the equivalent area of a circle, giving,

Alternatively, if the particle volume V is measured, then by a similar manipulation the equivalent spherical volume diameter DV is given as,

If the external surface area S is measured, then the equivalent spherical surface diameter DS is given as,

In all these cases the equivalent spherical diameter is derived from some particle metric.

erosion The dissolution of the metal compact at the surface where a liquid infiltrant flows into the part. Erosion is mostly from dissolution of the solid into the liquid. One means to minimize erosion is to presaturate the liquid with the key constituents in the solid, such that the newly formed infiltration liquid will not dissolve solid at the surface where it first forms.

error budget

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A manufacturing concept that realizes that process difficulty increases and process yield decreases as the number of tightly toleranced features increase. The error budget is similar to the monthly household budget in that it partitions the dimensional tolerance allowance to various features. If very tight tolerances are required on one dimension, then loose tolerances must be used elsewhere to offset the cost implications of the tight tolerances.

ESCA The abbreviation for electron spectroscopy for chemical analysis.

etchant A chemical solution used to attack a metal to reveal microstructure information, such as the grain size.

etching Subjecting a polished metal surface to a corrosive solution that provides sufficient attack to observe microstructure details such as grain size.

ethylene-bis-stearamid (EBS) A lubricant powder commonly mixed with metal powders to reduce stress during ejection after pressing. It has a density of 1.05 g/cm3 and melts at 144EC. The polymer chain consists of the following structure: CH3-(CH2)16-CONH(CH2)2HNOC-(CH2)16-CH3

Euler relation As sintering progresses the individual particles meld to form a connected grain structure and the initial powder characteristics are blurred as the grain structure emerges; the relation between the grain microstructure then becomes

which is known as the Euler relation. It links the number of grain faces f, grain corners c, and grain edges e. Because there is a grain size distribution, not all grains are the same size nor shape.

European Powder Metallurgy Association (EPMA) The industrial trade association located in Europe, involved in technology promotion, conferences, and education while promoting the technology and its use.

eutectic

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An alloy having the composition indicated by the eutectic point on an equilibrium phase diagram, meaning two solid phases crystallize from one homogeneous liquid at a single temperature. Figure E.10 shows the eutectic composition and temperature for a simple binary system. On cooling the liquid through the eutectic point, the single phase of liquid decomposes into two solids of compositions S1 and S2, as marked on the diagram.

eutectoid A reversible reaction in which a solid solution is converted into two phases on cooling and reverts back to a single phase on heating through the same eutectoid transition temperature. In powder metallurgy the most important eutectoid occurs in steels with about 0.77 wt. % carbon. The corresponding critical temperature is 727EC, termed the eutectoid temperature, as shown in Figure E.11. For steels at the eutectoid composition and temperature, austenite (γ) directly transforms into a lamellar mixture of ferrite (α) and cementite Fe3C giving a microstructure called pearlite. There are then three types of steels based on the carbon level - hypoeutectoid with less than 0.77% carbon, eutectoid steels with about 0.77% carbon, and hypereutectoid steels with higher carbon levels. Only at the eutectoid composition and temperature is it possible to just form ferrite and carbide on cooling. During slow cooling a hypoeutectoid steel first forms ferrite on cooling, while a hypereutectoid steel first forms iron carbide. Any phase to form before the eutectoid transformation is designated as a proeutectoid phase. In highly alloyed steels, the carbon level corresponding to the eutectoid and the eutectoid transformation temperature are shifted.

eutectoid steel Plain carbon steels that contain approximately 0.8 % carbon, or similar values that depend on the alloying level, such that on slow cooling austenite transforms directly into ferrite and cementite.

evaporation-condensation A sintering mechanism where the material evaporates from convex surfaces and recondenses on concave surfaces. The vapor pressure of the material must be sufficiently high to sustain high rates of mass transport. Although the process results in strengthening due to sinter bonding, there is no net dimensional change; the sintered density and green density are the same. It is favored in systems with inherently high vapor pressures at the sintering temperature, but can be induced artificially in metals by doping the sintering atmosphere with halides, such as chlorine and fluorine. During evaporation-condensation, surface atoms are repositioned. For any material, the equilibrium vapor pressure P depends on absolute temperature T with an Arrhenius dependence (thermally activated),

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where Po is a pre-exponential material constant, Q is the activation energy for evaporation, and k is Boltzmann's constant. Higher temperatures give a higher vapor pressure and more vapor phase transport, since the flux depends on the evaporation rate. Consequently, evaporation occurs preferentially from convex surfaces. Preferential deposition occurs at concave necks between particles where the vapor pressure is slightly below equilibrium. Most of the materials that exhibit this process are ceramics, such as NaCl, PbO, TiO2, Si3N4, BN, and ZrO2. As a practical guide, materials that exhibit weight loss during sintering (beyond adsorbed impurities) should be suspected of vapor transport processes.

evolved gas analysis (EGA) A thermal analysis technique used to monitor the outgassing or chemical reactions during heating. At atmospheric pressure the chemical vapor species can be collected and analyzed using gas chromatography and during heating in a vacuum the analysis relies on residual gas analysis. A few vacuum furnaces have attempted to use the outgassing signal for control of the heating rate - the furnace heating is slowed when there is a high rate of outgassing.

exaggerated grain growth A condition during sintering where one grain suddenly grows much faster than any of its neighbors, effectively becoming a cancer in the microstructure. Most frequently exaggerated grain growth arises from impurities on one grain or crystal face that induce rapid diffusion across the grain boundary, leading to a chemical gradient and rapid transport medium that promotes one grain consuming its neighbors. It is a negative attribute in most situations, since exaggerated grain growth often slows sintering densification and lowers the sintered properties. In some ceramics an anisotropic crystal can be grown to provide improved toughness.

exogas An abbreviation for an exothermic atmosphere.

exothermic atmosphere A gas atmosphere used in sintering, produced by partial combustion of a hydrocarbon fuel gas and air using a catalyst or external heat source. The maximum combustible content is 25 %. A typical composition in the sintering furnace consists of 67 % nitrogen, 13 % hydrogen, 19 % carbon monoxide, and less than 1 % of water (steam), carbon dioxide, oxygen, and methane.

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exothermic reaction A reaction that liberates heat and usually becomes faster and faster as it progresses, and in the extreme can become explosive. A simple example is the ignition of a mixture of oxygen and hydrogen, which forms water vapor with an explosive release of excess energy.

expansion The increase in dimensions of a compact due to unbalanced chemical reactions, pore formation, or pore growth during sintering.

explosive compaction Shock wave compaction of a powder based on ignition of an explosive near the loose powder. If the shock wave velocity is sufficiently high, then there is melting at the particle-particle contacts that induces excellent bonding, but the bulk temperature rise is small. Figure E.12 is a conceptual illustration of explosive compaction for a flat plate powder geometry. Once detonated, a traveling wave pressurizes the powder and the frictional heating at the particle contacts causes sinter bonding. The shock wave propagation occurs in the first 0.1 탎, but the frictional heating is delayed, so densification occurs prior to local melting between the particles. The ratio of explosive mass to powder mass mainly determines the final density. The required ratio is higher for materials that are inherently stronger. Also, small particles are difficult to compact and brittle materials must be heated before consolidation to minimize fracture. Peak wave velocities well over 8 km/s have been reported with pressure pulses over 35 GPa, but the pulse duration is only microseconds. Product shapes are relatively simple, but often are cracked by the shock waves that echo back through the powder compact. In some instances the shock wave is generated via a projectile, electrical discharge, or collapsing magnetic field, but the underlying events are the same as with an explosive. [see dynamic compaction]

extrusion (high temperature) A high pressure process that pushes a hot powder through a die using a ram to form and consolidate the powder into shapes such as tubes. Figure E.13 shows how the canned powder (usually degassed during canning to remove impurities) is pressurized by a penetrator to densify the powder and force the can through the die. In this high pressure operation, the extrusion constant C provides a measure of the difficulty in achieving deformation and flow of the powder. The extrusion force F and extrusion constant are related as follows:

where A is the cross-sectional area of the feed material and R is the reduction ratio. The reduction ratio equals the cross-sectional area of the billet divided by the cross-sectional

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area of the product. The force to initiate first flow in extrusion is higher than the force needed to maintain flow; both increase with smaller particle sizes. Although the intrinsic material properties have an effect on the ease of extrusion, temperature is the main process control variable. Too high a temperature damages the microstructure and shortens the life of the extrusion tooling. Alternatively, too low a temperature makes extrusion difficult because the powder is stronger and more resistant to deformation. The reduction ratio in hot powder extrusion must exceed 10 for adequate densification. Indeed, some products are extruded at reduction ratios as high as 25. The extrusion constant declines as temperature increases, but increases as the extrusion strain rate increases. Low melting temperature alloys have lower extrusion constants. For example, aluminum (660EC melting temperature) has an extrusion constant of 180 MPa at 200EC, while stainless steel (1400EC melting range) has a value of 350 MPa at 1000EC, and molybdenum (melting temperature of 2610EC) has an extrusion constant of 480 MPa at 1400EC.

extrusion (low temperature) The conversion powder-binder feedstock into uniform cross-section lengths by forcing the mixture through an orifice of desired cross-section. The idea is illustrated schematically in Figure E.14. Low temperature extrusion relies on a polymer-powder feedstock and relatively low pressures to shape, but not deform the particles. It is used to form long, thin rods, tubes, honeycombs, and twist drills based on the die at the barrel exit. Usually the feedstock is evacuated during mixing to avoid bubbles in the extrudate. After extrusion the binder acts as a glue to hold the particles in place prior to sintering. Applications include forming microelectronic substrates, porous tubes, welding rods, and automobile exhaust catalytic converter substrates. One novel use of polymerpowder extrusion is in tracing out a two-dimensional layer. The motion of the extrusion syringe is controlled to deposit a bead that is grown into a three-dimensional object. A sacrificial support material is added to temporarily fill any hollow spaces. This is necessary to allow cantilevers, overhangs, and other features. The extrusion nozzle path is controlled by a computer to generate a solid object that looks like the stored image. The feedstock is nearly the same as that used in injection molding, so the debinding and sintering cycles are the same as those seen in powder injection molding.

exude The expulsion of a low melting temperature alloying ingredient from the pore structure due to poor wetting. Figure E.15 shows the surface drops of liquid after being exuded during sintering. Also, infiltrated materials will exude the infiltrant if held at high temperatures, since the porous solid skeleton will sinter and squeeze the liquid out of the shrinking pores. Exudation is also observed if a polymer decomposes to contaminate a component, a problem common with polymers such as polystyrene and many cellulose binders.

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Section F

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A to Z of Powder Metallurgy

F F An abbreviation used to designate pure iron (F) or a simple steel based on iron and carbon. Typically the F is followed by four digits that indicate the carbon content. As an example, F-0008 indicates iron alloyed with 0.8 wt. % carbon, while F-0000 indicates essentially pure iron.

F15 Although widely known as Kovar (trademarked name), there is a use of the designation from ASTM International for this alloy that consists of 29 wt. % nickel, 17 wt. % cobalt, the balance being iron. It is a favorite for electronic applications that require a low thermal expansion coefficient to match with glass seals. After injection molding and sintering to a density of 7.85 g/cm3 the yield strength is 280 MPa, ultimate tensile strength is 420 MPa, and the elongation to fracture is 30 %. The thermal expansion coefficient is slightly more than 5 ppm/EC.

F75 A cobalt alloy consisting of 27 to 30 wt. % chromium, 5 to 7 wt. % molybdenum, 1 to 2 wt. % nickel, and traces of other metals and a low carbon level (below 0.3 wt. %). The designation comes from the ASTM International specification. It is typically consolidated by hot isostatic pressing to full density, but can also be consolidated using powder injection molding. It is a favorite in medical implants and exhibits a wide range of mechanical properties depending on the processing cycle and heat treatment. Hot isostatically pressed F75 has a yield strength of 840 MPa, ultimate tensile strength of 1275 MPa, 16 % elongation to fracture, and 765 MPa fatigue endurance limit.

fashion materials A design concept that captures the sense of creating the functional properties for the application, often by mixing powders to synthesize the desired properties. These compositions often are short lived as applications, properties, and performance requirements change.

fatigue limit The maximum stress that a material can withstand under cyclic loading for a specified

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number of stress cycles; if that is 10 million cycles then this is equivalent to the fatigue strength. [see S-N curve]

fatigue strength The maximum cyclic stress a material can withstand for a given number of cycles before failure occurs, usually 10 million cycles. This resistance to repeated loading without failure tells the stress at which say 50 % of the samples will fail in 10 million cycles. [see rotating beam fatigue test]

fatigue striations These are stair-step looking bumps on the fracture surface associated with failure by fatigue and are easily resolved using scanning electron microscopy as illustrated in Figure F.1. These steps represent the microscopic crack advance events that take place on every loading cycle up to catastrophic failure when the remaining ligament of metal can no longer support the intense load. Typical step sizes are on the order of a micrometer.

FC An abbreviation used to designate an iron alloy (F) containing copper (C) as the main metallic alloying addition. The FC is followed by four digits that indicate the copper and carbon content. As an example, FC-0208 indicates iron alloyed with 2 wt. % copper and 0.8 wt. % carbon, while FC-0200 indicates 2 wt. % copper and no carbon and FC-0505 indicates 5 wt. % copper and 0.5 wt. % carbon. Typical densities are in the range from 6.7 to 7.3 g/cm3 after sintering, leading to good strength (400 to 700 MPa) and a small level of ductility.

FD A special abbreviation used to indicate a diffusion alloyed steel. Most of these alloys contain 1.3 to 1.7 wt. % copper, 0.4 wt. % to 0.5 wt. % molybdenum and various carbon and nickel levels. The FD is followed by four digits that indicate the nickel and carbon content. As an example, FD-0208 indicates the iron-copper-molybdenum is further alloyed with 2 wt. % nickel and 0.8 wt. % carbon.

Fe The chemical symbol for iron.

feed shoe The powder conveyance (usually a flat hopper) that places loose powder at the apparent density into the die cavity between compaction strokes. The feed shoe fills the die and might also displace the previously pressed and ejected compact. Most are

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simple devices that sweep over the die cavity as the lower punch drops to the fill position, but in recent years there has been effort to include fluidization to better lift the powder prior to filling the cavity. Studies on particle segregation in the die cavity show the momentum of the moving feed shoe causes particle segregation by density and this then results in nonuniform sintered dimensions.

feedback control Control systems that employ embedded sensors to make continuous corrections to the operation to ensure proper quality.

feedstock The mixture of powder and binder that is used in injection molding or other binderassisted shaping processes such as extrusion, slurry casting, slip casting, or tape casting. The feedstock formulation involves several decisions, including the powder to binder ratio, mixing temperature and sequence, and polymeric binder formulation. Often the binder is a mixture of several polymers to tailor the melting, strength, flow, and powder adhesion characteristics. A typical binder is composed of mostly paraffin wax or other hard waxes and about 30 % polypropylene, with appropriate lubricants or wetting agents to provide binder adhesion to the powder. That binder is fully molten at about 150EC. The amount of binder tends to range near 40 vol. % of the mixture, depending on the powder packing characteristics; the weight fraction of binder might range from 2 % to 15 %, depending on the powder density. In molding, the feedstock flows from the nozzle at the end of the barrel through the sprue, runner, and gate prior to filling the cavity.

Feret뭩 diameter The mean value of the distance between pairs of parallel tangents to the projected image of a particle, perpendicular to some reference line, typically generated using transmitted light microscopy. Figure F.2 illustrates this particle size measure.

ferrite The body-centered cubic phase of iron and its alloys, usually stable at room temperature, also known as alpha-iron. The high temperature delta-ferrite is essentially the same phase, except it forms at temperatures near the melting point. Also, the term ferrite is used to describe various compositions based on iron oxide that are used to fabricate common ceramic magnets, usually containing other metal oxides such as zinc, nickel, copper, cobalt, or manganese. Magnetic ferrites are formed from powders for uses in loudspeakers, television picture tubes, power supplies, radar systems, magnetic recording devices, telecommunication filters, microwave systems, and small motors.

ferritic steel

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An alloy based on iron that consists of a body-centered cubic crystal structure. It is the most common form of low alloy steel when slow cooled, but is usually heat treated to form martensite.

ferroelectric A material that exhibits electrical polarization in the absence of an applied electric field such as seen in piezoelectric, pyroelectric, and high permittivity devices. Ferroelectric materials are used in acoustic and pressure transducers, actuators, and infrared detectors, as well as thermistors and acoustic wave generators. Generally the ferroelectric materials derive their unique properties from crystal structures that are not symmetric. The most famous ferroelectrics are used to generate stress from an applied voltage, such as in an ultrasonic transducer or ultrasonic cleaner.

ferromagnetism A property exhibited by certain metals, alloys, and compounds of a few transition metals (iron, nickel, and cobalt), and a few of the rare earth elements. Below the Curie temperature the ferromagnetic metals have an ability to align internal atomic magnetic moments into a single direction that provides a strong net magnetism. Ferromagnetic materials exhibit a strong attraction or repulsion to a magnetic field.

ferrotic or Ferro-TiC A composite or cermet consisting of a ferrous alloy matrix, often a tool steel, and a dispersed titanium carbide (TiC) hard phase. The titanium carbide is usually less than 50 vol. % of the cermet. Originally developed in the late 1940s the cermet was initially fabricated using a sintered titanium carbide preform that was infiltrated with tool steel via a process called sinter-casting. More recently the fabrication has shifted to hot isostatic pressing of mixed powders. A typical density is near 6.5 g/cm3 with a tensile strength of 1200 MPa (no ductility) and heat treated hardness of 69 HRC.

ferrous A material based on iron, including all steels, tool steels, stainless steels, and similar high iron content alloys. Probably more than 10,000 ferrous alloys are in common use, and about 1 % of those are routinely available via powder metallurgy techniques.

FFC process An electrochemical oxide reduction process named after the three inventors (FrayFarthing-Chen) from Cambridge University. In the presence of hot calcium chloride, an oxide such as titania (TiO2) is reduced to form titanium powder. Several variants make the technique conceptually useful for a variety of reactive and refractory metals. To induce the desired reduction at the cathode, the cell operates at temperatures between 500 to 800EC. Calcium chloride acts as the electrolyte and the byproduct of the cell is

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carbon dioxide. As this powder process is developed, there is opportunity to form many materials, with demonstrations to date of Nb3Sn, NbTi, NiTi, Al, Ti, Be, Ni, Co, Ta, Pt, and Pd.

Fick뭩 first law A relation between the flow of atoms and the change in atomic energy over distance, effectively providing a mathematical model that says the equivalent of 뱖 ater flows downhill? Fick's first law is as follows:

where J is the flux in terms of atoms or vacancies per unit area per unit time, DV is the diffusivity, dC is the vacancy concentration change over a distance dx. The diffusivity depends on the number of vacancies and the mobility of the vacancies, both being thermally activated parameters that change with composition and temperature. Thus, the diffusivity has an exponential temperature dependence described as an Arrhenius temperature dependence. Fick뭩 first law is used to explain the rate of neck growth or pore elimination during sintering. Aldolf Eugen Fick (1829 to 1901) was a German physiologist, but his work on the mathematics and physics of blood flow resulted in diffusion equations known as Fick뭩 first and second laws. [see Arrhenius relation, volume diffusion]

Fick뭩 second law A differential equation that relates the change in concentration to the geometric concentration gradient, where the material diffusivity is involved in determining the rate of change. His second law is as follows:

Actually the full three-dimensional form involving the gradients in x, y, and z directions is formally correct, and further the concentration gradient really should be the chemical potential gradient. In spite of the mathematics, standard solutions are available in handbooks. Solution to this equation requires knowledge on the diffusivity, DV and the geometric changes during sintering.

field activated sintering The use of an applied electric field during sintering to promote excess atomic defects that induce more rapid sintering. The concept was developed by Zuhair Munir at the

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University of California - Davis, to explain the anomalous increase in sintering densification for many materials that otherwise prove difficult to sinter. Several related technologies correspond to the pressures, electric fields, and direct current and alternating current pulses used to manipulate the defect structure, atomic mobility, and densification. [see spark sintering]

fill height The distance between the lower punch and the top plane of the die body when the tooling is in the powder fill position. The die provides the cavity into which the powder is filled and the upper punch is retracted during powder filling. The lower punch position during powder entry then determines the fill height and determines the amount of powder placed in the die.

fill position The position of the lower punch with respect to the die cavity when powder is filled. The fill position and apparent density determine the mass of the final component.

fill ratio The height of powder placed in a die cavity divided by the final compact height; it is related to the compression ratio.

filling After the feedstock is metered and plasticated in the barrel, this is the first phase of the molding cycle where feedstock flows into the mold cavity under pressure from the screw. In the same manner, it is the first portion of a die compaction process where the feed shoe is discharging powder into the die cavity based on lower punch suction or gravity.

filtration rating A metric for the nominal pore size associated with a porous material intended for applications in filters. The rating is not uniform across the industry, but provides a nominal measure of the particle size where 98 % capture occurs in the porous materials. For example, a filtration rating of 20 탆 indicates that the filter will capture 98 % of the particles sized 20 탆 or larger. An ideal filter will remove all of the contaminants. This is not possible in reality, so an efficiency rating is given to express the percent removal of a certain size contaminant. A log-6 rating at a given contaminant size corresponds to allowing 1 part per million to pass (captures 99.9999%). The HEPA (high efficiency particulate air) filter rating is one based on allowing only 1 part per million (ppm) to pass. To achieve this rating requires a low flow velocity and sufficient thickness to capture a significant amount of contamination. Deepak Kapoor generated a

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useful empirical correlation between the filter rating and pore characteristics given as follows: α = B (ε F)N where F is the filtration rating and is usually given in terms of the nominal particle that will not pass through the filter, ε is the porosity, and α is the permeability coefficient. Both B and N are empirical constants which depend on the processing cycle. Irregular pores prove most useful in industrial filters.

final stage sintering Final stage sintering occurs when the pores spontaneously close and the compact is approaching full density. During pore shrinkage, eventually the pores become long and thin cylinders. The surface energy of a long pore is higher than a collection of spherical pores, so a cylindrical pore of length L and diameter dP will close into spherical pores when L $ π dP, corresponding to the Rayleigh instability criterion. For cylindrical pores occupying grain edges this instability occurs at approximately 8 % porosity or 92 % density. The pores become spheres with a final diameter 1.88 times the cylinder diameter, resulting in a pore size increase as final-stage sintering occurs. For a pore sitting on a grain boundary, the equilibrium between the grain boundary energy and the solid-vapor surface energy will lead to a dihedral angle groove, giving lens-shaped pores. During final stage sintering it is possible for the pores to coarsen, leading to an increase in the mean pore size while the number of pores decreases. Differences in the pore curvature and vacancy concentrations lead to growth of the larger pores at the expense of the smaller, less stable pores - a process known as Ostwald ripening. If the pore has a trapped gas, then the gas slows or even prevents pore elimination. The rate of pore elimination in the final stage depends on the balance between the surface energy γ and pore gas pressure PG. The densification rate equation is given as,

where ρ is the fractional density, t is the hold time, Ω is the atomic volume, Dv is the volume diffusivity, k is Boltzmann뭩 constant, T is the absolute temperature, G is the grain size, γ is the solid-vapor surface energy, dP is the pore radius, and PG is the gas pressure in the pore. Trapped gas in the pores causes densification to stop before all porosity is eliminated. Thus, full densification is not possible in most cases unless sintering is conducted in a vacuum or an atmosphere that dissolves into the solid. A distribution in sintering stages between neighboring regions avoids a sharp transition between intermediate and final sintering stages.

fines In general the portion of a screened powder that passes through the smallest opening

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screen during a sieve analysis or size classification. Usually reserved to designate that portion of a powder composed of particles which are smaller than 45 μm or 325 mesh; also known as the subsieve size fraction.

finishing operations The steps applied to a sintered material after removal from the sintering furnace. These steps tailor the dimensions, properties, or attributes to the intended application; examples include machining, heat treatment, and electroplating steps.

finite element analysis An engineering analysis approach based on subdividing a component into many calculation cells. Stress, temperature, or other inputs are applied to the component and the calculation transfers the heat or force to each neighboring cell based on material constitutive equations for the applied input and component shape. Decisions are then made based on where the maximum stress occurs or how heat is distributed in the component.

Fisher Subsieve Sizer (FSSS) A device for measuring the particle size based on surface area as calculated from gas permeability through a packed bed of powder. Darcy뭩 equation for flow in a packed bed of particles says the flow rate Q in m3/s depends on the pressure drop ∆P = PU - PL and the gas viscosity η as follows:

where the sample length is L and cross-sectional area is A. The sample is packed into a right circular cylinder using a mass equal to the theoretical density for the powder - for molybdenum the theoretical density is 10.2 g/cm3, so 10.2 g of powder are packed into the cylinder. The parameter κ is known the permeability coefficient. At a constant and small pressure differential, the gas velocity passing through the powder is given as,

with V equal to the flow rate per unit area (Q/A). Based on an analysis by Kozeny and Carman, the surface area of the compact is related to the permeability through the fractional porosity ε as,

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with ρM equal to the theoretical density of the material. The Fisher Subsieve Sizer converts the surface area into an equivalent spherical diameter D as follows:

The technique is applied to particles between 0.5 탆 and 50 탆 . This is only an approximate measurement tool; however, the measurement is simple and the devices are widely available.

fixed costs Those costs that are independent of the batch size are known as fixed costs. In production cost estimating there are variable costs that scale directly with the order size; for example, if more parts that are ordered this increases the amount of powder used, so powder cost is variable. On the other hand, the golf club membership dues for the president of the firm do not scale with the order size, so this is a fixed cost. In powder metallurgy, many items are treated as fixed costs and simply pro rated over the annual production, including advertising, research, trade association dues, and new computer systems.

flake powder A flat or scale-like powder which is relatively thin with a large aspect ratio.

flame spray A thermal coating process that uses a gas torch with injected metal powder to create a spray of molten droplets. The deposition of a coating from these droplets is the basis for corrosion, oxidation, or wear coatings.

flash The thin, extra leaf of material on a component that forms because of feedstock penetration along the mold parting line, leaving a surface blemish on the compact. In powder injection molding this lip of material can be binder rich. Figure F.3 is an extreme case of flashing.

FLC An abbreviation used to designate some sinter hardened steels. These contain iron (F), carbon, and copper (C) with around 1 wt. % molybdenum. Typically the FLC is followed by four digits that help categorize the composition, but the nomenclature is not logical so composition translation tables are required to specify the alloy.

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flexible tooling Soft tooling made from rubber, polyurethane, or other polymers for early trials. Rubber tooling is backed by steel plates to provide stiffness. In powder injection molding such flexible tools have been used for up to 300 shots during early stage development projects.

flexural strength A mechanical property of a solid material that indicates its ability to withstand a bending or transverse loading.

FLN An abbreviation for sinter hardened steels. These are typically alloys of iron (F), carbon, and nickel (N) with up to 1 wt. % molybdenum. Typically the FLN is followed by four or five digits that help categorize the nickel and carbon contents, but the nomenclature is not logical and requires consultation with a composition translation table. If FLNC is used this additionally indicates the alloy contains about 2 wt. % copper.

floating die A tool set designed to allow transmission of force from the upper punch through the powder into the die wall, thereby producing an effective double-action compaction but with only upper punch motion. As sketched in Figure F.4, the die is mounted on heavy springs that allow vertical motion during compaction. As the compaction pressure increases the die moves in proportion to the upper punch motion to effectively place the lower punch near the compact center line.

flocculate The agglomeration of wet particles to form clusters in a suspension or dispersed system, often based on polymer additions, such as used in ceramics and mineral processing. Dispersed particles, such as in paint, are treated to prevent flocculation. On the other hand, in mineral processing flocculation is desired to assist in harvesting target minerals. The flocculation or aggregation of particles dispersed in a liquid is manipulated by polymers or electrolytes. The strength of the flocculated powder is weak. The resulting structure is called a floc and the polymer additive is termed a flocculent. In powder metallurgy, flocculation is generally avoided by the addition of a dispersant to keep the particles separated, especially in particle size analysis. [see agglomeration]

flow analysis Computer simulation of the molding process to assess the location of the gate, runner, vent, and cooling passages and other important aspects of tool design and molding to

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minimize errors in production.

flow line A mark on a green molded part that is made by the meeting of two flow fronts during molding, also termed a weld line.

flow time The time required for a powder sample to flow through an orifice in a standardized test. The flow time gives a measure of the interparticle friction. The most common test measures the time in seconds for 50 g of powder to flow through the Hall flow meter. Short flow times indicate free flowing powders while long times indicate high interparticle friction. The test usually repeats within ? to 3 s giving a typical 5% error. The apparent density is often measured at the same time.

flowability The ease with which a powder flows on a relative basis.

fluid bed A technique for powder levitation in an upward flowing gas stream for coating or reacting the powder with a fluid. The upward gas motion keeps the particles in motion to behave as a fluid. Usually the gas is heated and the powder is either sprayed with a polymer-solvent solution or other agents are added to the gas to treat the powder surface. Figure F.5 illustrates the concept of a fluid bed, where the powder is levitated and stirred by flowing hot gas containing the coating vapor species. The critical Reynolds number (RN = D ρf V / η where D is the particle size, ρf is the fluid density, V is the fluid velocity, and η is the fluid viscosity) for achieving fluidization of the powder is given as,

where εf is the fractional porosity of the powder bed at the onset of fluidization (typically near 0.44), and A is given as

with D being the particle size, ρf being the fluid density, ρm being the powder material density, g being the gravitational acceleration, and η being the fluid viscosity. One problem with fluid beds is elutriation, the preferential removal of smaller particles from the powder bed. This happens when the gas velocity exceeds the terminal velocity for the smaller particles. A variant that eliminates some of the elutriation problem is the

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spouted bed. In this approach the particles circulate from the top to bottom of the reactor with a fluid fountain passing up the center.

fluorescent penetrant inspection Inspection for surface defects based on application of a fluorescent liquid that penetrates surface cracks. After the liquid is wiped clean, the location of surface flaws is revealed by fluorescence using ultraviolet light because the liquid preferentially wets cracks.

FN An abbreviation used to designate an iron alloy (F) containing nickel (N) as the main metallic alloying addition. Typically the FN is followed by four digits that indicate the nickel and carbon content. As an example, FN-0208 indicates iron alloyed with 2 wt. % nickel and 0.8 wt. % carbon, while FN-0405 indicates 4 wt. % nickel and 0.5 wt. % carbon.

foamed metal A class of very high porosity materials fabricated using powders that use a decomposition reaction during sintering to swell the pores. In one variant powders are consolidated with an additive that induces gas generation inside the compact, causing severe swelling. Titanum hydride is a favorite additive that can be mixed and consolidated with aluminum powder. During heating the hydride decomposes to produce large hydrogen gas pockets. Densities as low as 0.18 g/cm3 are generated in aluminum with crush strengths in the range of 1 to 10 MPa. Although the strength is low, on a density-normalized basis these are high specific strengths. Similar products can be formed using plastic pore forming agents mixed with ceramic or metal powders. One approach uses plastic balls and pours the powders as a slurry into the spaces between the balls, and another approach infiltrates a powder slurry into a polymer foam. Heating then drives out the polymer and sintering creates the porous structure. These plasticbased foamed materials provide considerable latitude in the amount and size of the pores, so are more widely employed than the titanium hydride route.

forging The plastic deformation of a metal into a desired shape using a hammer or punch impact. The deformation is usually performed hot at high strain rates in constraining dies. For a porous powder metal preform, there is simultaneous densification as well as reshaping and work hardening.

fountain flow The preferred mode of mold filling in powder injection molding, where the molten feedstock sticks and freezes to the walls of the mold cavity while fresh and molten

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feedstock flows through the center of the component to progressively fill toward the vent. This is the opposite from jetting, where the feedstock shoots across the mold and captures an air pocket inside the component.

Fourier transform spectrometer A scanning interferometer that splits a beam into two or more components and then recombines those components with a phase difference. The spectrum is obtained by a Fourier transformation of the interferometer output. Organic species are identified by the vibrational spectra and a common form used in polymer burnout studies in powder metallurgy is the infrared version, often designated Fourier transform infrared spectroscopy or FTIR.

fraction That portion of a powder sample which lies between two stated particle sizes or mesh sizes.

fractional density The relative measure of the porosity in a powder compact expressed as a ratio of the pore-free density. Often expressed as a percentage of full density or theoretical density, such as 98 % dense, implying 2 % porosity.

fracture strength The final failure stress of a tensile sample, giving a measure of the strength and resistance to deformation.

fracture toughness A generic term for measures of the resistance to crack propagation in a material. In fracture mechanics the term is related to the applied stress when a known crack starts moving. High fracture toughness materials, such as stainless steels, provide more safety when compared with low fracture toughness materials such as glass or ceramics. Failure is measured by increasing the stress on a cracked structure until the crack grows. That stress depends on the square-root of the crack size, leading to units of MPa%m or ksi%in. Most sintered ceramics are low in this regard, typically under 10 to 15 MPa%m, but a few full-density powder metallurgy alloys can be taken to values more than 100 MPa%m. The common measure of fracture toughness is a parameter known as KIc which stands for the critical stress intensity for crack growth in tension. Since porosity is essentially a small crack, the presence of residual pores greatly lowers the fracture toughness. In situations where catastrophic failure must be avoided, say in a landing gear of an airplane, the data show that only full density powder metallurgy products are competitive.

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fragmentation In liquid phase sintering, once a wetting liquid forms it penetrates through the pore and grain structure by a combination of reaction and capillarity, leading to disintegration of the solid structure. In most systems, newly formed liquid is far from an equilibrium composition. This spreading liquid penetrates the solid-solid interfaces within a few seconds after liquid formation, giving a large dimensional change. Swelling depends on the penetration rate, which is estimated as,

where x is the depth of liquid penetration, dP is the pore size, γLV is the liquid-vapor surface energy, θ is the contact angle, t is the time, and η is the liquid viscosity. A small dihedral angle is needed for a liquid to remain connected along grain edges. Penetration of the grain boundaries disintegrates the solid and allows semisolid forming.

Fraunhofer diffraction Low-angle light scattering using monochromatic laser light and dispersed particles to monitor particle size. Typically the particles are dispersed in a fluid stream which is passed in front of a detector system. When a particle, larger then the wavelength of the laser light, passes through the beam it creates a scattering signature of maxima and minima that are characteristic of the particle size. These signatures are collected and analyzed to extract the equivalent spherical particle size. The scattering angle varies inversely with the particle diameter, while the intensity of the scattered signal varies with the square of the particle diameter. The refractive index is not a factor as long as the particles are larger than the wavelength of the laser light. Computer analysis of the intensity and angle data then gives the particle size. Fraunhofer scattering is typically applied to particles from 1 탆 to 200 탆 . The technique is in widespread use because data collection is easy. Typically the particles are subjected to ultrasonic dispersion prior to analysis to eliminate agglomeration.

freeform fabrication Powder technologies that do not require tooling to form the component, usually restricted to small production quantities. Several options exist in the freeform fabrication routes, including the following: subtractive processes such as green machining an oversized powder compact prior to sintering; additive processes where a computer controls the build process, such as the gluing of powders into the desired green shape followed by sintering; high-temperature direct-build processes where a computercontrolled laser or other energy source directly forms the shape; spray fabrication processes where molten droplets or high-velocity particles are used to build the component. Underpinning these freeform fabrication routes is the ability to couple the

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forming process to a computer image of the component. In most cases the computer design is digitally imported into the fabrication machine, leading to truly tool-free and even paperless construction.

freeze casting A novel shaping technique that relies on a water-powder slurry and freezing to form a shape. The water contains polyvinyl alcohol or latex. After casting into hard tooling, the powder-binder mixture is frozen in the tool set. Subsequently, the ice is extracted by vacuum evaporation (freeze-drying or sublimation) from the compact, leaving behind a polymer component to hold the particles in place for subsequent handling. Finally, the powder is sintered to a high density. There are several similar processes known by names such as adiabatic forming, freeze-firing, and quick set.

freeze firing A concept developed in the 1960s using water as the binder for a powder where the mixture is frozen in a tool cavity. Freezing is based on most of the fluid being water. Molding is typically performed at low pressures using a slurry. After the mold is filled, the mold and mixture is frozen in the tool set. Aluminum tooling is preferred because of its low cost, ease of machining, and high thermal conductivity. A role of the polymer is to compensate for the volume expansion normally seen when water freezes; certain polymers and starches work to give no volume change on freezing. In one variant vacuum freeze-drying is used to remove the water from the frozen body. Freeze-drying holds the compact at a temperature and pressure below the triple point for water. That allows direct sublimation (evaporation without forming a liquid) of the water to avoid cracking. Usually a polymer phase is added that remains after the water is evaporated to provide handling strength. Rapid temperature changes cause cracks, so freezing processes tend to be slow. Another problem is the sublimation times tend to be long. One novel variant uses frozen powder and water, and when subjected to high pressure the frozen mixture melts and flows into a die cavity. The water instantly freezes when pressure is released.

Frenkel relation The first quantitative model for sintering of two spheres, in this case by viscous flow. The energy dissipation in growing the bond between contacting amorphous spheres was used to estimate the time dependence of neck growth. This first formal theory of sintering was published just after World War II, and it was followed over the next few years by the more useful diffusion model of Kuczynski that proves more relevant to metals.

frequency distribution A plot of number or mass of objects versus sizes such as generated by a sieve analysis

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that shows the weight of powder retained on various sized screens. It gives the relative probability of each size range.

friction products These are largely brake pads and clutches for automobiles, trucks, and industrial machinery that are formed from compacted and sintered powders. Figure F.6 is a photograph of a sintered metallic clutch and aircraft brake pad that convert mechanical energy into heat. A common material is a mixture of constituents, including iron, graphite, copper, zinc, silica, and tin; for example 60 % Fe, 20 % Cu, 5 % Sn, with about 15 % carbon (in the form of coke and graphite). Additives are used to increase friction, including SiO2 or Al2O3-MgO phases, or to lubricate the material, including MoS2 and graphite.

friction welding The joining of two metals using inertial energy from a moving or spinning object is converted into heat at a frictional contact to cause local melting and bonding.

frit A porous glass structure and generally synonymous with sintered glass particles. Frits are used for gas bubbling in a liquid, such as a home aquarium. Often the term is generic to sintered porous structures, leading to the mistaken application to sintered metallic filters.

FSSS The abbreviation for the Fisher Subsieve Sizer.

fugitive material A polymer or other additive that is mixed with a metal powder to form a pore. During heating to the sintering temperature the fugitive material evaporates to create an artificial pore, allowing control of both the porosity and pore size by proper selection of the polymer size. Early variants were ammonium carbonate, camphor, or other easily evaporated compound, but more recent variants are polyethylene and ethylene bis stearamid.

full-density processing Means to consolidate a powder to a pore-free condition that broadly include various compaction, sintering, or simultaneous heating and pressurization technologies. The approaches fall into a few clusters: - low-stress routes that operate at high temperatures, such as sintering, that are dominated by diffusion controlled processes, - intermediate-stress routes that operate at intermediate temperatures via diffusional

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creep processes, such as hot isostatic pressing, - high-stress routes that operate at lower temperatures via plastic flow, such as powder forging, - ultra-high-stress routes that attain full density at room temperature, such as explosive compaction. There are many additional factors, such as particle size, green density, and component size, that separate the processes, but this categorization helps sort out the key approaches to full-density processing. In simplified terms, if the yield strength of the material is known, then the pressures needed for rapid full densification are roughly three times the yield strength. Slower densification occurs as the pressure falls from this value.

full-density sintering Any of several techniques designed to take a powder compact to essentially a pore-free condition during the sintering heating cycle. Most of the successes rely on either a small particle size, high green density, or formation of a liquid phase during sintering. Various combinations exist and sometimes the liquid is formed inside the particles (supersolidus liquid phase sintering) or between the particles (persistent liquid phase sintering), or might rely on a high diffusivity phase.

functional gradient The creation of systematic and continuous property changes across a material based on laminated powder mixtures in the forming process. One example would be a surface that was metallic and progressively is layered with ceramic across the thickness so that the opposite surface is pure ceramic. One favorite material is a nickel alloy on one surface and zirconia on the opposite. Functional gradients allow for performance or property customization, such as low thermal expansion coefficient on one surface and high thermal conductivity on the opposite surface. Applications for powder metallurgy formed gradient microstructures are found in heat engine components, biomedical structures, electronic and optical devices, and metal-to-ceramic bonds or metal-to-glass bonds.

funicular bonds Contrast with saturated pores and pendular bonds; funicular bonds are in between these two extremes. The wetting liquid between particles bridges between contacts to form a continuous film on the particle surface, but the pores are not saturated. Figure F.7 illustrates the funicular bond. [see pendular bonds]

furnace A box or oven that controls the time-temperature path in the sintering cycle while

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protecting the components from adverse atmosphere reactions. The furnace holds the process atmosphere, provides for removal of the lubricants and binders, heats the work to the sintering temperature, and potentially performs a post-sintering heat treatment. Furnaces perform these functions in either a batch or continuous mode. The difference between a batch and continuous furnace depends on control of either the furnace temperature versus time or compact position versus time.

FX An abbreviation used to designate an iron or steel (F) that has been infiltrated (X) with copper. Typically the FX is followed by four digits that indicate the copper and carbon contents. As an example, FX-2000 indicates pure iron infiltrated with 20 wt. % copper, while FX-1005 indicates iron containing 0.5 wt. % carbon infiltrated with 10 wt. % copper.

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A to Z of Powder Metallurgy

G gage length A region in the center of a tensile sample that represents the minimum cross-sectional area that concentrates the stress. Engineering strain during a tensile test is calculated by dividing the change in gage length by the initial gage length.

gamma iron Also known as austenite, it is a high temperature form of pure iron, but many substitution alloys based on iron also have the same form. Gamma iron consists of a face-centered cubic atomic arrangement where for each cube there is one iron atom on the corners and one iron atom in the center of each of the six cube faces. Since each corner is shared by four neighboring cubes and each face is shared by two neighboring cubes, this gives a net of four atoms per cube. The crystal symmetry controls several properties; for example gamma iron alloys are ductile but not magnetic. In pure iron, the gamma phase exists between 910EC and 1392EC. At higher and lower temperatures iron exists as a body-centered cubic crystal structure. Alloying additions can significantly change this stable temperature range, and in some instances it is possible to stabilize the gamma structure to below room temperature.

gas adsorption surface area analysis Best known as the BET surface area analysis technique. [see BET specific surface area]

gas atomization The disintegration of a thin molten metal stream into powder using high pressure gas. The melt is heated a few hundred degrees over the melting range and is either pressure discharged or siphoned into a thin stream. That melt stream is impinged by high pressure gas either as a directed jet, group of jets, or as a surrounding concentric nozzle. One version is sketched in Figure G.1. The expanding gas breaks the melt stream into molten droplets that spheroidize prior to solidification, producing spherical particles. Smaller powders are produced with higher melt temperatures, higher gas pressures, and close distances between the gas stream and the melt. Variants include the use of air, nitrogen, helium, or argon as the gas. Equipment designs vary with respect to how the molten material is fed into the gas nozzle, and the sophistication of

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the melting and collection chambers; however, the main idea is to deliver energy (from a rapidly expanding gas) to the molten stream to form droplets which immediately solidify into particles. Low-temperature atomizers are based on horizontal designs that siphon the melt into the gas expansion zone. For high-temperature and high quality metals, a closed, inert gas-filled chamber is used to prevent contamination.

gas atomized powder A rounded or spherical powder formed by the disintegration of a melt stream by a high pressure gas expansion nozzle. The particles solidify during free-fall after atomization. Figure G.2 is a scanning electron micrograph of a typical spherical gas atomized powder. When performed under inert conditions, the powder has a high purity. The normal particle shape is spherical and the particle size has a wide distribution, with most of the product larger than 10 탆 .

gas chromatography A separation technique used to identify the mobile phase in a gas based on differences in permeation rates with molecular size. It provides a means to identify the chemical species present in a process atmosphere, such as in sintering or heat treatment.

gas classification Another phrase for elutriation or air classification. The separation of a powder into different size fractions by means of a gas stream with controlled velocity flowing countercurrent to gravity. As the gas velocity increases, progressively larger particles are swept away.

gas constant A factor that relates atomic motion to absolute temperature, equal to 8.314510 J/(molAK), it is fundamentally defined as the product of Boltzmann뭩 constant and Avagadro뭩 number.

gas forging High strain rate deformation of a sintered powder compact based on heating the material and exposing it to a gas pressure pulse that exceeds the material strength, effectively forging and densifying the powder without a tool cavity. Most variants rely on a hot isostatic press and fill the heated chamber with liquid argon or liquid nitrogen, which immediately transform into gas and generate a rapid pressure pulse. One variant starts with a sintered compact that is inserted hot into a heated pressure vessel for consolidation. Pressure is created by introduction of liquid nitrogen. Densification is immediate, because the pressure is far greater than the compact뭩 strength. Peak pressures up to 1000 MPa have been recorded, effectively forging the compact to full density, but without the use of a container if the compact is over 92 % dense.

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gas permeability surface area analysis The use of gas flow through a packed bed of powder to measure the laminar flow resistance. The greater the surface area of the powder the more resistance to flow, so a rough correlation exists between the permeability and the surface area. This form of particle size analysis is usually performed in the Fisher Subsieve Sizer (FSSS). Under laminar conditions, the FSSS measured permeability is proportional to the surface area. Darcy뭩 equation for flow in a porous material says the flow rate Q in m3/s depends on the pressure drop ∆P = PU - PL and the gas viscosity η as follows:

where the sample length L and cross-sectional area A are as shown in Figure G.3. The parameter κ is known as the permeability coefficient. The gas velocity exiting from the low pressure side is given as,

with V equal to the flow rate per unit area (Q/A). Based on an analysis by Kozeny and Carman, the surface area of the compact is related to the permeability through the fractional porosity ε as,

with ρM equal to the theoretical density of the material. A preweighed amount of powder is exposed to a known flow rate and the pressure drop is measured to determine the permeability. Knowing the porosity and theoretical density, the specific surface area is calculated. The technique is applied to particles in the 0.5 to 50 탆 range and the measured surface area is converted into an equivalent spherical diameter. This is usually reported as the subsieve size, and generally is accurate to a few percent. [see Darcy뭩 law]

gate The constricted opening for filling the mold cavity in an injection molding tool set. The gate is located at the end of the runner as it enters the mold cavity and is designed to freeze before the part, runner, or sprue. As shown in Figure G.4, the gate leaves a characteristic blemish on the compact that often is removed by a human or robot. Ideally, the solidified gate holds pressure in the cavity to counteract the shrinkage associated with feedstock cooling and binder contraction. Too low a pressure when the

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gate freezes results in voids or sink marks, while excessive pressure causes sticking in the mold. It is desirable to have the gate located on the thick portion of the component. This reduces heat loss and lowers the pressure required for mold filling. Gate size is determined by the filling speed and the section thickness. For the gate to freeze before the compact requires a gate smaller than the component wall thickness. If the gate is too small, then the higher feedstock flow velocity causes tool wear or powder-binder separation. On the other hand, if the gate is too large, then molding is slowed by the need to use the screw position to control pressure on the feedstock during cooling. If the gate is on a thin section, then filling is hindered because of flow resistance and feedstock cooling.

gatorizing A full density process used to produce superalloy turbine components by first canning an inert gas atomized powder, followed by hot isostatic pressing to full density, with subsequent forging steps to create the final shape. The process was developed to ensure all prior particle boundary phases were fully disrupted in the final product. It was developed by a group of engineers in Florida, so the name relates to the University of Florida Gators.

gauge length The initial length of the uniform cross-section on a tensile sample that is used in strain and elongation calculations. Strain is calculated as the change in the gauge length divided by its initial size. For most powder metallurgy tensile testing the gauge length is 25 mm, while for most wrought materials it is 50 mm; the length of the gauge length affects the reported ductility.

Gaussian distribution Sometimes also called the normal distribution. It is a characteristic distribution for many aspects of life. The size of sintered components tends to follow a Gaussian distribution and often the test scores on standardized tests do the same. Two parameters are required to fully describe the Gaussian distribution - the mean (average) and the standard deviation. The mean is the center of the distribution and the standard deviation describes the width of the distribution. This mathematical description of measurement errors was originally developed by Johann Karl Friedrich Gauss (1777 to 1855) for locating astronomical objects. Gauss was a German mathematician who also developed the least squares means for data fitting.

gel A colloidal state consisting of interspersed solid and liquid, in which the solid particles are interconnected to form a soft three-dimensional network.

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gelation A binder setting process where a macromolecule grows in a solution to form a highly interlinked structure with most of the solution trapped in cells formed by the long-range molecule. Most typically the solution is water. Brightly colored and fruit flavored desserts are formed from gelation systems. In powder metallurgy, slurry casting with a gelation binder is one means for forming low production quantity shapes with little equipment. [see agar]

gelcasting A variant of low pressure injection molding and slurry casting, where the binder consists of a monomer that polymerizes in the die cavity, forming a rigid polymer to hold the particles in place, often supported by a catalyst addition just as the feedstock is molded. [see slurry casting]

getter A sacrificial material used to remove impurities, usually from the processing atmosphere during sintering. For example, heated titanium chips are very effective in removing oxygen, moisture, and other contaminants from argon. Other example getters include copper and zirconium. Getters are also used in inert glove boxes, lamps, and debinding furnaces to remove trace impurities.

Gibbs energy A measure of the thermodynamic potential, calculated as the enthalpy minus the product of the absolute temperature times the entropy. It is also called the free energy. The lower the Gibbs free energy, the more stable the material or reaction production.

glass formation temperature During cooling of an amorphous metal, there is a progressive reduction in the atomic motion to a temperature where effectively atomic motion is too slow to induce a phase transformation. At temperatures below this glass formation point (also called the glass transition temperature) the amorphous structure is frozen in place. It is often defined as the temperature where the viscosity is 1013 PaAs or higher. Effectively the temperature where the liquid becomes stable as an amorphous solid.

globar A common name given to rod shaped silicon carbide heating elements.

glove box An enclosure in which material may be manipulated in isolation from the operator and ambient environment. The operator stands outside the box and works with the powder

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Section G

through rubber gloves in the walls of the box. Glove boxes are often filled with inert atmospheres to allow manipulation of pyrophoric, high purity, or toxic powders. Figure G.5 is an example of a standard inert atmosphere glove box.

gold (Au) Element 79 and one of the most famous pure elements due to its high intrinsic value. Gold is a face-centered cubic metal. In powder metallurgy it is alloyed with copper and other lower cost elements to form various jewelry alloys, and in a few instances sintered porous biomedical devices or dental restorations. As a pure element gold is too soft for most applications, but its properties are well known as shown below: gold

Au

atomic number

79

atomic weight

197

g/mol

density

19.32

g/cm3

melting temperature

1063

EC

boiling temperature

2807

EC

heat of fusion

13

kJ/mol

heat capacity

130

J/(kg EC)

thermal expansion coefficient

14.3

ppm/EC

thermal conductivity

316

W/(m EC)

electrical resistivity

2.4

µΩ-cm

elastic modulus

83

GPa

Poisson뭩 ratio

0.42

hardness

25

VHN

as-sintered yield strength

7

MPa

as-sintered elongation to fracture

45

%

as-sintered ultimate tensile strength

16

MPa

grain A single crystal of material in a polycrystalline body. Particles might be composed of many grains, but once consolidated and bonded the particle becomes a grain in a

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polycrystalline structure. When a metal is etched with acid, the boundaries between grains are preferentially attacked. As illustrated in Figure G.6 the resulting polished and etched metal surface can be imaged in a reflected light microscope where the grains are delineated by the grain boundary grooves formed by etching.

grain boundary A narrow interface zone between two crystals, where atomic bonding is disrupted in the grain boundary for a few atoms on each side. Grain boundaries are common sites for impurities and are active diffusion paths in sintering.

grain boundary diffusion Atomic motion along the defective region at the grain boundary. Atoms jump from site to site, and because of imperfect atomic bonding atoms favor the grain boundary as a fast pathway for migration or diffusion. Grain boundary diffusion is characterized by a grain boundary width which is very difficult to measure so is normally assumed to be about five atomic sizes wide. The diffusion rate is then defined by a vibrational frequency and an activation energy. The latter is a rough measure of how difficult it is to break an atom free and move it to a new site. As temperature increases the atoms have more vibrational energy so the migration success increases. Thus, grain boundary diffusion is very sensitive to temperature. Of course if the material lacks grain boundaries, such as in an amorphous metal or glass, there is no grain boundary diffusion. Further, if the grain size is large, then grain boundary diffusion is not a significant factor. Grain boundary diffusion is important to the sintering densification of most metals and compounds. During sintering a grain boundary forms in the sinter bond between contacting particles and the defective character of this grain boundary allows mass flow with an activation energy that is usually intermediate between that for surface diffusion and volume diffusion. Grain boundary diffusion controlled sintering is evident in many metals as they undergo densification.

grain boundary doping Selected agents are added to grain boundaries to adjust mobility and thereby influence grain growth and sintering densification. Generally the idea is to keep the densification rate high while sustaining a low rate of grain growth. Sometimes the difference in activation energies between densification and grain growth can be used to suppress grain growth while enhancing densification via selected temperature-time pathways. The final grain size depends on the powder size, the rate of grain growth during sintering, and any pore drag by moving grain boundaries, leading to a complex temperature-time dependence. Indeed, the sintered grain size can vary in a nonsystematic manner with respect to the particle size. The dynamic manipulation of the grain structure during sintering proves a major challenge in solid-state sintering and leads to some interesting proprietary doping secrets to adjust grain boundary motion. The most famous example

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of this sort is the addition of 0.1 wt. % MgO to Al2O3. Magnesia is very effective in changing the relation between grain growth and densification in the final stage. Doping with MgO reacts with the inherent CaO impurities to lower the grain boundary mobility, keeping slower moving pores attached to moving grain boundaries. Alternatively, the addition of FeO to Al2O3 has just the opposite effect, allowing rapid grain growth.

grain boundary mobility A term that describes the rate of motion of a grain boundary under the action of some force during sintering, such as a difference in grain size or grain chemistry. The grain boundary mobility determines the relative rates of densification, grain growth, and pore migration; in turn these attributes determine the final or terminal sintered density. In many materials, densification and grain growth processes have similar activation energies. Thus, separation of the events proves difficult via temperature adjustments. If grain growth is rapid in comparison with the pore mobility, then the pores are isolated away from grain boundaries, resulting in slow densification by long-range volume diffusion. Alternatively, if pore mobility is high, via surface diffusion or evaporationcondensation, then the pores can remain attached to moving grain boundaries and continue to shrink. Usually the grain boundary mobility is much larger than the pore mobility, leading to pore drag that reduces the rate of grain growth during final stage sintering, as long as the pores remain attached to the grain boundaries. Some novel sintering cycles propose a constant temperature reduction during sintering to balance the decreasing pinning from grain boundaries with a lower temperature to reduce grain boundary mobility.

grain growth At high temperatures a material will exhibit a progressive increase in the average grain size and a decrease in the number of grains. The boundary between neighboring crystals is known as a grain boundary and it has high energy. The natural tendency is to reduce energy by the migration of grain boundaries, typically by the large grains consuming the small grains. Grain growth is a general term for this time-dependent change toward a larger grain size that normally occurs during sintering or heat treatment. In the absence of pores, grain growth is rapid at temperatures used for sintering. For dense materials, the mean grain size G increases with time t according to the classic law,

where t is the isothermal time, GO is the initial grain size, and κ is a thermally activated parameter similar to those seen in sintering. When pores are present, then the factor κ is reduced roughly in proportion to the fraction of grain boundaries intersected by pores. Thus, in early sintering when most grain boundaries intersect pores there is much impediment to grain growth. But as densification occurs and pores are eliminated, κ

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increases to allow for larger grains which results in slower sintering.

grain growth inhibitor Typically a chemical species added in low concentrations to slow the mobility of grain boundaries during sintering. An effective inhibitor will do little to the sintering kinetics, but will impede grain growth to produce a smaller sintered grain size. These are very common in ceramics, and 0.1 wt. % magnesia additions to alumina are the most famous, since that allows sintering to optical translucency. Other common examples are found in most sintered tungsten carbide compositions, where vanadium carbide is the most common and potent inhibitor. In tungsten the same role is found with oxides and carbides, such as hafnium carbide. In molybdenum, low concentrations of silica have proven effective.

grain refinement The manipulation of the atomization process to cause a smaller grain size in the particles. For example helium has a much higher thermal conductivity when compared to nitrogen, so finer grain sizes are observed in helium atomized powders.

grain shape accommodation The progressive shape change of neighboring grains during liquid phase sintering such that the grains attain a high packing density, nonspherical shape to release liquid to fill remaining voids. Figure G.7 is an excellent example of a liquid phase sintered material consisting of single crystal grains that have undergone grain shape accommodation to release the matrix phase to fill voids, giving a 100 % dense product. The shape of a solid grain depends on several factors, but is most affected by the dihedral angle, liquid content, and surface energy anisotropy. Nearly flat contacts form between neighboring grains. These contacts allow the grains to change shape to attain better packing. For dihedral angles over 60E and small volume fractions of liquid, the liquid structure is dispersed along grain edges and is not continuous. At large dihedral angles, typically over 90E, the microstructure is unstable for all quantities of liquid. Consequently, the liquid exudes from the compact. Liquid phase sintered microstructures have a connected morphology for all dihedral angles below 60E, independent of the amount of liquid. [see tetrakaidecahedron]

grain size A characteristic measure of the length, area, or volume of grains in a sintered material, usually expressed as an average over a large region, or might be presented as a grain size distribution. Unfortunately grain size is reported in several forms without designation of the actual parameter being measured - number of grains per unit area, linear intercept size, two-dimensional equivalent circle size, or equivalent sphere

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diameter based on the three-dimensional grain volume. A simple measure of grain size is obtained by counting the number of grain boundaries intercepted by a twodimensional test line or circle placed on the magnified image. The mean twodimensional grain intercept size G2D is the total test line length LL divided by the magnification M and number of boundary intercepts NB,

where ρ is the fractional density. Since a pore is empty space that separates two grains, each pore intercept is equivalent to one grain boundary. Since polishing cuts through the grains randomly, the grains appear smaller than their maximum dimension. In that case, the three-dimensional grain size G3D is larger by a factor of 1.5, giving G3D = 1.5G2D.

grain size distribution In a sintered material the grains are not all the same size, so various mathematical distribution functions are used to describe either the two-dimensional (observed by microscopy) size distribution or the three-dimensional (not observable) size distribution. In liquid phase sintered materials, the cumulative two-dimensional distribution is best described by a simple Rayleigh function as follows:

where F(L) is the cumulative fraction of grains of size L = G/G50 where G is the grain size and G50 is the median grain size, so L is a normalized grain size. Since polishing cuts through the grains randomly, the grains appear smaller than their maximum dimension.

granular forging A pseudo-isostatic powder densification process where a heated powder compact is embedded in ceramic or graphite granules, and the granules are then subjected to high pressure compression. The granules transmit the pressure to the compact, allowing for forging without the requirement for a closed pore condition in the forged part. The precision is low and the deformation is anisotropic (the compact shrinks more in the pressing direction) so the technology sees only application in development projects, component bonding, and hardfacing. The key step is embedding the powder compact in heated granules just prior to forging. The original granules were ceramics, leading to the name ceramic consolidation or Ceracon. When graphite pellets are used, they can be heated by direct electrical conduction using a low voltage (10 V) and high current (2000 A) discharge for a few minutes. Such hybrid compaction routes are faster than hot isostatic compaction and can be applied to compacts with open pores. Further, the consolidation resembles forging more than hot pressing, since the pressures often

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exceed the yield strength. However, because the pressure transmission is not uniform in all directions, the height change is about twice that of the perpendicular dimensions. For example, a steel powder compact would be consolidated at temperatures over 1000EC and pressures over 560 MPa.

granular powder Particles having approximately equidimensional, nonspherical shapes and a granule size that is substantially larger than the starting particle size.

granulation A general term for the production of coarse powder agglomerates using binders, heat, moisture, or liquid metals.

granules Agglomerates intentionally formed by the addition of a binder, generally to improve the powder flow into compaction or shaping equipment.

granulometry A term sometimes used to indicate particle size analysis, especially determination of the particle size distribution by routes such as sedimentation. [see particle size analysis, particle size distribution]

graphite A hexagonal crystal structure form of carbon used for alloying iron powder to form steel. It sublimes at about 3500EC but will start to oxidize at 350EC if heated in air, and will rapidly react at 800EC in air. Graphite powders, such as shown in Figure G.8, are mixed with the iron prior to compaction. During sintering the carbon diffuses into gamma iron and that contributes substantial strengthening to the sintered steel on cooling. Graphite is also used to form tooling in hot pressing and various electrical discharge compaction devices and is used to form heating elements for some vacuum furnaces. In the purified condition, graphite has a density of 2.26 g/cm3, room temperature elastic modulus of 14 GPa, no ductility, and strength ranging from 20 to 70 MPa, depending on the grade. It has a lower thermal expansion coefficient of 3.34 ppm/EC. Thermal conductivity depends on the grain orientation; for a random microstructure it is just 45 W/(m EC), but can be ten times higher in oriented structures.

green The term applied to powder compacts at an intermediate manufacturing step after shaping but prior to sintering. Various attributes at this point are known as the green density, green mass, green strength, green size, and so on. The implication is these are not the final properties.

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green assembly Shapes are bonded in the green state to create more complicated structures after sintering, with joining between shapes taking place as part of the sintering cycle. Green assembly can be performed using three approaches: ? molding the first component and using it as an insert for molding the second component, similar to plastic over-molding on a metal insert, ? sequentially molding two feedstocks into a tool cavity, where the tool cavity expands after the first filling to make room for the second feedstock, ? assembling two separately molded components in the green state - after molding but prior to debinding and sintering. The straightforward green assembly approach involves pressing or injection molding two components from the same material. They are glued together, usually using a small amount of binder or solvent, and passed through the sintering cycle. When both components are formed from the same material, a compatible dimensional change in sintering ensures good bonding. When the components are from different materials, then adjustments to particle size, heating rate, and other factors are required to avoid interface defects from differential sintering shrinkage.

green cracks The generation of a laminar defect in a powder compact in the pressing or shaping stage. Most green cracking arises from rough tooling, improper tool motion, or high stresses during ejection, and might be small such as shown in Figure G.9. The compact has friction with the tooling during ejection while the green strength is low. Green cracks form when the ejection stresses exceed the green strength. Delamination is the most common form and they are cured by application of a positive, hold-down pressure on the compact during ejection. Another problem comes from tool motion that improperly compresses each level to a different density or in a wrong sequence, creating a crack at the interface between different heights. Cracks oriented perpendicular to the press axis usually occur during ejection. Another problem is stress relaxation of long tool components. Elastic strains in the tooling are proportional to the tool length. Longer tool components relax more than shorter tool components, leading to differential strains at the end of the compaction cycle, leaving one portion of the compact under stress while a neighboring region is unsupported. The feed shoe can cause cracks if it strikes the ejected compact too aggressively.

green density The powder density after shaping or compaction. Density is mass over volume, and most typically green density is reported as g/cm3, but might also be given as a fraction of theoretical or as a percent of theoretical. For example, iron has a theoretical density of 7.86 g/cm3 so when pressed to 6.8 g/cm3 green density, it has a fractional green density of 0.865 or 86.5 % of theoretical.

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green expansion The increase in component size or dimensions relative to the die size. It is measured after the compact is ejected from the die and is usually different in the pressing direction when compared to perpendicular to the pressing direction. This expansion is also termed springback.

green machining Mass subtraction from a powder compact that is performed prior to sintering. Often used in a rapid prototyping process, but also used to drill a hole or add an undercut that is difficult to create in the compaction step. The process is widely employed to improve sintered dimensions on cold isostatically pressed compacts. In cases where only a few early trial components are required, green machining avoids tooling costs. Green machining works best with high-speed cutting tools and small cuts. Some applications do most of the shaping via green machining, such as in the production of custom cemented carbide cutters, forming tools, and drilling tools, some of which range up to 100 kg in mass. As green strength has increased with new polymers, green machining has grown popular since often it is more cost-effective to add features, such as undercuts, in the green state.

green state The condition of the molded component prior to debinding or firing. The term comes from the ceramic concept of green-ware in reference to formed bodies that are not sintered.

green strength The strength of the as-pressed or molded powder compact. A low green strength implies much difficulty in handling and potential production yield problems. A typical green strength in powder metallurgy is 20 MPa and usually the lowest acceptable in a production situation is 3 MPa. Green strength comes from either a binder mixed with the powder or deformation and cold welding at the particle contacts. If the particles are soft, they will smear and bond at the deformed contacts. Powder with contaminated surfaces requires a high compaction pressure to disrupt the surface films. When irregular particles are pressed to high green densities, the particles cold-weld together and mechanically interlock. Smearing and interlocking both contribute to a higher green strength. For comparison, a 50 탆 copper powder pressed at 400 MPa gave a green strength of 5 MPa for spherical particles but 35 MPa for irregular particles. Polymeric binders are often the source of green strength with small and hard particles.

grindability Like the term machinability, grindability measures the volume of material removed during grinding divided by the surface distance and normal force; thus, for grindability

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the units are volume (cm3) removed per unit applied force (N) per unit sliding distance (m).

grinding Mass removal from a component using a moving abrasive wheel and either stationary or rotating work. It is used to generate a smooth surface (similar operations are honing and lapping). Grinding is appropriate for small dimensional corrections and attainment of tight tolerances on hard or brittle materials; for example cemented carbides. The cost of grinding is high. Indeed, for many of the liquid phase sintered WC-Co compositions, up to 40 % of the total manufacturing cost occurs in grinding after sintering.

grit Angular, crushed particles used in grinding and polishing and water jet cutting, typically from an abrasive material such as silicon carbide or alumina. Also, the term grit is used to designate the size of abrasives and the size of sand blasting media. A similar concept to mesh size.

growth Although the economic community refers to growth in terms of financial metrics, in powder metallurgy at the shop level it refers to the increase in dimensions of a powder compact. Some growth occurs on ejection from the forming tool, better termed springback, and during sintering for porous powders and in some mixed powder systems. It is the opposite dimensional change from shrinkage, and mostly comes from chemical reactions during the sintering cycle. [see springback, swelling]

guar gum Also called guaran, it is a thickener used in powder suspensions, foods such as ice cream, and water-containing feedstocks. It is a complex natural polymer consisting of nonionic rod shaped molecules that are economical when used to increase viscosity or to stabilize a suspension. It hydrates fairly rapidly in cold water to produce a pseudoplastic suspension, and at concentrations of 1 wt. % produces a thixotropic suspension.

Page 1

Section H

A to Z of Powder Metallurgy

H hafnium carbide The equiatomic compound of hafnium and carbon HfC, it is the one of the highest melting temperature materials known to mankind with estimated melting at 4160EC. It has a hardness of 2000 VHN, density of 12.6 g/cm3, strength of 240 MPa, elastic modulus of 460 GPa, and thermal conductivity of 29 W/(m EC). Used as a reinforcement phase for high temperature refractory alloys, such as tungsten, rhenium, and molybdenum, but difficult to fabricate directly. A candidate material for the combustion zone in high performance rockets.

halide sintering atmosphere A sintering atmosphere that contains a volatile halide gas, such as chlorine or fluorine, typically in low concentrations and typically based on hydrogen. The halide acts to induce faster surface transport sintering to provide higher sintered strengths with low dimensional change. The most common instance involves adding 1 % hydrogen chloride (HCl) to a hydrogen atmosphere. The typical result is a higher strength.

Hall flowmeter A device for measuring the apparent density and flow time for a free flowing powder. Figure H.1 shows the device. The 25 cm3 cup collects the powder after passing through the 60E funnel. The time for a 50 g sample to flow through the funnel is designated as the flow time. A powder is designated as non-free flowing if it will not flow in the funnel. A modified Carney funnel with twice the opening diameter (5 mm diameter hole) is used for some smaller or slower flowing powders.

Hall-Petch relation Generally the strength of polycrystalline materials follows an inverse dependence on the grain size, since plastic flow is limited by the distance a dislocation can travel prior to an entanglement or grain boundary. The Hall-Petch relation, σy = σo + Γ G -1/2 relates σy the yield strength, G the grain size, with σo and Γ as material constants. Initially it was hoped that nanoscale materials with small grains would give further

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strength gains. As a demonstration of the strengthening, below are data on the yield strength for a simple steel (Fe-0.2C), showing significant strength gains with a small grain size: grain size, 탆 yield strength, MPa 30 290 5 430 1 700

hard magnets Permanent magnets that retain their magnetism after being removed from a magnetic field. In a direct current motor, such as the starter motor in an automobile, a permanent magnet with a large remnant magnetization is required. In the one case, the composition might be nearly pure iron or iron-nickel and in the other, compounds as Fe14Nb2B and SmCo5 are used.

hard material A group of compounds that are typically metal carbides, borides, oxides, or nitrides that exhibit a high hardness. To fabricate components, various combinations of the hard materials are mixed with cementing metallic phases during liquid phase sintering - for example TiC is sintered with Ni or Fe to form a cermet.

hard metal (hardmetal) A collective term that designates a sintered material with high hardness, usually accompanied by high wear resistance. Like hard materials, but usually reserved for the WC-Co family of cemented carbides. Sintered borides and nitrides with a metal matrix are generally termed cermets. The hardmetal name is more popular in Europe.

hardenability The relative ability of a ferrous alloy to form martensite when quenched from a temperature where austenite is the stable phase, known as the upper critical temperature. Hardenability is most often measured as the distance below a quenched surface at which the steel exhibits a specific hardness or a specific percentage of martensite in the microstructure. Several factors affect hardenability, including grain size and carbon content, but alloying effects are dominant. Pores are very detrimental, since they lower thermal conductivity and effectively lower hardenability. The most potent additives for steels are chromium, molybdenum, and manganese. Unfortunately these also require high sintering temperatures. Various indices are available to decide equivalent hardenability levels. For example, at an addition level of 0.5%, manganese is twice as effective as silicon, and chromium is twice as effective as nickel. A predictive technique uses multiplying factors to calculate the diameter of a bar that will produce 50% martensite at its center for a given alloy. The procedure requires the density and

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alloying composition be known. For full density, hardenability factor tables are consulted and a bar diameter that will produce 50% martensite at the core is calculated as follows,

where DC is the core diameter which depends on the grain size and carbon level, and the F factors are determined from available tables, but largely depend on the alloying level. For example,

where X indicates the concentration in wt.%. Such factors clearly show the potent effects from chromium, manganese, and molybdenum. Hardenability is also used as a measure of heat treatment ease in forming martensite via the Jominy test where large depths for martensite formation are favorable. [see Jominy test]

hardfacing The application of a hard, wear-resistant material to the surface of a component by thermal spraying or sinter bonding an abrasion resistant material. [see thermal spraying]

hardness A test of a material뭩 resistance to surface penetration by a pointed or rounded indentor under a given load. Usually the pressure and impression area are related to the hardness; hard materials produce smaller impressions. There are many useful hardness tests where the area, depth, or width of the penetration is measured to determine a relative resistance to penetration. Often this relates to other material properties such as strength. Hardness scales used in powder metallurgy include Rockwell tests (HRA, HRB, and HRC), Knoop (KHN), Vickers (VHN), and Brinell (BHN). Figure H.2 contrasts the various tests. Other scales exist, but these cover most all sintered metal powders. [see Vickers hardness, Rockwell hardness, Knoop hardness, Brinell hardness]

Hastelloy X A lower grade superalloy that is responsive to powder metallurgy processing, especially via powder injection molding. The alloy is based on nickel and contains nominally 22 wt. % chromium, 18.5 wt. % iron, 9 wt. % molybdenum, 1.5 wt. % cobalt, and 0.6 wt. % tungsten. When sintered to 98% density and heat treated to a hardness of 30 HRC, it has a yield strength of 303 MPa, ultimate tensile strength of 675 MPa, and 74 % elongation to fracture.

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Hausner ratio The ratio of tap density to apparent density. This ratio is close to unity for spherical powders and increases as the particle shape becomes irregular.

heat capacity A material property that expresses the energy required to increase a material뭩 temperature usually measured on a per unit mol or per mass basis, thus the units are J/(EC mol) or J/(EC kg). Also known as the specific heat if reported on a per mass basis.

heat treatment Heating and cooling cycles applied to a material after sintering to adjust the phases, microstructure, or to adjust properties. Many materials exhibit some changes in a postsintering heat treatment, ranging from softening to hardening, as well as various changes that impact on performance. The specific cycle depends on the alloy and the desired properties.

heating elements The electrical conductors in a furnace that convert electrical current into heat. Several materials operate as heating elements, including graphite, molybdenum, molybdenum silicide, and silicon carbide.

heavy alloy (heavy metal) A class of high density alloys based on tungsten with small concentrations of alloying additions such as nickel, iron, or copper. These alloys are liquid phase sintered from mixed elemental powders to create a composite material. Most of the applications are for weights, radiation shields, thermal management heat sinks, or projectiles. [see tungsten heavy alloy]

hematite The common name for the Fe2O3 compound of iron and oxygen, it is a phase observed in the oxidation of iron. It has a density of 5.24 g/cm3 and melts at 1570EC. It can easily be reduced in hydrogen to form iron powder.

HERF The abbreviation for high-energy-rate forming.

Herring scaling law A fundamental relation between the effects of a change in particle size and the required change in sintering time to produce an equivalent degree of sintering. The formulation requires identification of the mass transport mechanism to calculate the time change for

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any change in particle size. It was first proposed in 1950 by Herring assuming knowledge on the time t1 required to sinter a particle of diameter D1 to achieve a sintered neck size of X1, then the effect of a change in particle size can be predicted. The sintering time t2 for a particle of size D2 to reach the same neck size ratio (X1/D1 = X2/D2) is,

where m varies with the sintering mechanism. Since many materials densify by grainboundary-diffusion controlled sintering (m=4), the Herring scaling law says a twofold increase in particle size requires a 16-fold increase in sintering time to achieve the same relative degree of sintering (same strength). This demonstrates the significant role of particle size in sintering.

Hertz (Hz) The SI unit for inverse seconds, used to describe a frequency. Electric current for example is 50 to 60 Hz in most countries. Light has a frequency in the 1014 Hz range.

Hertzian stress The intense stress at a point contact, for example at the spot where a ball bearing is in contact with the bearing race. This stress is very large, even when the general stress is low. The intense local contact stress can exceed the yield point of the material and in applications such as bearings the repeated loading and unloading with a high stress leads to fatigue of the material.

heterodiffusion Atomic motion from one material to another, where the rate of atomic motion has a chemical composition effect. For example, in the activated sintering of refractory metals, the segregated grain boundary layer of transition metal (nickel, iron, cobalt, palladium as examples) provides a fast diffusion path for the refractory metal that induces very large reductions in sintering temperature. The fast diffusion path is a heterodiffusion situation for the refractory metal, since it has fast diffusion due to the differing chemical environment in the segregated grain boundary layer.

heterogeneous nucleation Phase change, such as a liquid forming a solid, based on an interface that provides a growth site to reduce the barrier to nucleation, enabling faster transformations than might occur with homogeneous nucleation. Essentially the external dirt enables the transformation. In homogeneous nucleation, the small crystal nucleus must spontaneously form in the melt to initiate the transformation. That nucleus then grows to

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produce the solid crystal.

high-cycle fatigue Progressive material degradation due to cyclic stressing where failure is not expected to occur for at least 10,000 loading cycles. In practice, the number of cycles to failure depends on the loading cycle and the metal properties.

high-energy-rate forming (HERF) A forging process that occurs with a rapid release of energy, involving higher power than most conventional deformation processes, resulting in higher velocities with velocities in the 10 m/s range. Deformation is accomplished by release of stored energy from a high pressure gas or from the explosion of a combustible fuel-air mixture. The impact velocities of the punches are above those characteristic of traditional forging (4 m/s), but below the velocities encountered in explosive forming (200 m/s).

high pressure compaction Generally any technique the induces a peak pressure during the compaction stroke that exceeds the typical range for powder metallurgy, that being in the 700 MPa range. Various techniques with carbide tooling have generated peak pressures of near 5 GPa, and in the presses constructed for synthesis of diamond it is possible to reach 50 GPa peak pressure. The ultrahigh compaction pressures are expensive to achieve since they require special presses and have slow cycles.

high speed steel A group of ferrous tool steel alloys with refractory carbides that provide adequate strength and hardness to operate as cutting tools.

high temperature sintering Generally a misnomer, since sintering is often carried out over a broad range of temperatures up to 3000EC, but in the ferrous powder metallurgy field the typical furnace with a stainless steel belt cannot operate over about 1150EC; thus, any sintering performed at temperatures over about 1150EC is termed high temperature sintering. Outside the ferrous powder metallurgy field the concept is meaningless; but for ferrous alloys a high sintering temperature induces pore shape rounding and contributes to improved resistance to fatigue, impact, or corrosion situations. Other justifications for high temperature sintering emerge when new alloying options are considered. Many additions do not diffuse into iron at the common 1120EC sintering temperature. By going to higher sintering temperatures, alloying additions such as manganese, chromium, titanium, and aluminum are viable. Another advantage is the easier reduction of oxides, making it possible to use alloying additions such as chromium. For example, just 2% chromium doubles the hardness of a steel containing

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0.35% C.

high-velocity compaction Compaction in rigid tooling where a punch impacts on the powder at a velocity measured in the 10 m/s range, but below the range of shock wave compaction.

HIP The abbreviation for hot isostatic pressing.

HIP diagram (or map) Computer simulations of densification in terms of the time-temperature, mechanism, and grain growth behavior. The initial model was provided by Professor Mike Ashby of University of Cambridge to predict processing effects on final density based on selected time, temperature, particle size, green density, and other parameters. The basic creep, plastic flow, and diffusion events are combined to compute these maps. From these maps it is possible to assess the interplay between pressure and temperature for promoting full density. An example map is shown in Figure H.3, in this case for the hot isostatic pressing for 100 탆 low carbon steel showing the combinations of time (1/4, 1, 4, and 16 h), temperature, and pressure required to attain full density. Such diagrams help isolate processing combinations that provide a densified product.

histogram particle size distribution A presentation of particle size data by showing the percentage or mass or number of particles in a size interval. Often the size intervals are defined by mesh sizes as illustrated in Figure H.4. Generally the cumulative particle size distribution is preferred since that distribution shows a better resolution of the distribution characteristics, especially if the size intervals are not evenly spaced on the histogram distribution.

hobbing A metal forming process based on a rotating cutting tool with teeth arranged along a helical thread for use in cutting gear teeth by synchronized rotation of both the hob tool and sintered powder metallurgy preform.

homogeneous nucleation The formation of a new phase, such as the creation of solid from cooled liquid, based on the random atomic motion sometimes creating a stable cluster during cooling. If that cluster is large enough, then it will continue to grow, but most of the nuclei are too small and redissolve. This makes the transformation difficult and only favorable with large undercoolings.

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homogenization During sintering mixed powders of differing compositions interdiffuse to form an alloy; the process is an alternative to forming compacts from prealloyed powder. For homogenization to proceed without pore generation requires minimal chemical reaction between species, effectively requiring a solid solution. In cases where strong chemical reactions occur, the chemical potential gradients overshadow sintering, possibly leading to pore formation. Homogenization is best performed in systems with little or weak chemical interactions. It occurs by diffusion of each species into the other. Classic examples of sintering homogenization to form solid solutions are W-Mo, Ni-Cu, and FeNi-C where sintering occurs simultaneously with interdiffusion. Even for weakly interacting systems the chemical potential gradients are large in comparison with sintering stress; hence, homogenization often dominates the early portion of the sintering cycle, and in some cases can be much slower than densification. Compositional gradients enhance the overall diffusional fluxes and the interface between phases aids vacancy creation while retarding grain growth.

homologous temperature A nondimensional temperature scale where all actual temperatures are converted into the absolute scale (kelvin) and expressed as a fraction of the absolute melting temperature for the material. For example, if titanium is sintered at 1400EC (1673 K) this corresponds to a homologous temperature of 0.86, since titanium melts at 1668EC or 1941 K. Often when sintering is studied, the homologous temperature shows similar behavior for materials of differing melting temperatures.

honing An abrasive finishing operation that uses fine grit rotating abrasive wheels to produce accurate dimensions and excellent finishes

Hooke뭩 law The relation between stress and strain in the elastic region for metals, where the proportionality is a material constant termed the elastic modulus. Simply, Hooke뭩 law says the following:

where σ is the stress, ε is the strain, and E is the elastic modulus (also sometimes known as Young뭩 modulus). It is applicable far below the proportional limit before first plastic deformation.

hot densification

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Page 9

The hot pressing or deformation of a powder preform, usually in uniaxial pressing or forging, to densify the preform and to conform it to the shape of a die. Primarily for the elimination of pores in the compact.

hot extrusion The simultaneous densification and forming of constant cross-section structures by pressurizing a powder to force it through a die, useful for forming tubes or other straightwall and long geometries from alloy powders. The powder billet is preheated and additionally heats due to the extensive deformation. Cross-section area reductions in the range of 25 are often required to fully densify the powder (the preform cross-section area is 25-times larger than the final product). Streamline dies help reduce the work required for extrusion and densification. Also, prealloyed powders are used to obtain high densities and mechanical properties. Variants on the extrusion process include starting with loose powder, presintered powder, or powder densified by another process such as HIP. In one variant of powder extrusion, the powder is loaded into a cavity of a large billet, possibly with a complex shape, and the billet is extruded. Later the outer billet material is removed (possibly by chemical dissolution) to expose the inner core of extruded powder. [see extrusion (high temperature)]

hot forging The application of a high strain rate impact to a preform to densify the heated powder preform, usually performed in a closed die. [see powder forging]

hot isostatic pressing (HIP) A process for simultaneously heating and applying gas pressure to a powder compact in a single densification cycle. The powder compact undergoes densification by plastic flow and creep. In one variant the powder is sealed in a container that is flexible at the peak temperature (for example titanium or stainless steel and in some cases glass containers suffice if heated prior to pressurization). In another variant, HIP is applied to powder compacts that are first sintered to a closed pore condition, and since the pores are sealed there is no need for a container. Figure H.5 shows an example HIP sequence for consolidation of powder in a thin walled container. First the powder is encapsulated in a flexible, but gas-tight container made from glass, steel, stainless steel, titanium, or tantalum. The filled container is heated under a vacuum to remove volatile contaminants. After evacuation and degassing, the container is sealed. For densification there is both heating and pressurization, usually with the heating and pressure cycles being independently programmed to soften the container prior to pressurization. Temperatures up to 2200EC and pressures up to 200 MPa are possible using HIP. Chambers come in various sizes up to 1.5 m diameter and 2.5 m high.

Section H

Page 10

Unlike hot pressing and spark sintering, which apply pressure along one axis, hot isostatic pressing applies pressure from all directions simultaneously. This gives less particle-particle shear. One consequence is that surface films on the particles can remain as prior particle boundary decorations which degrade properties, even when full density is achieved. Also, the compact surface is contaminated by the container and needs to be removed after HIP by chemical dissolution, machining, or abrasion. This adds to the cost and makes HIP less of a net-shape technology; thus, it is often referred to as a near-net-shape process.

hot pressing The high-pressure, low-strain-rate forming of a powder or powder compact at a temperature high enough to induce sintering and creep processes. As shown in Figure H.6, it is achieved by simultaneous heating and pressing. Hot pressing is often used for the consolidation of materials that are unstable at high temperatures, such as diamond, or for materials that resist sintering, such as covalent ceramics like boron carbide. Also known as uniaxial hot pressing, and vacuum hot pressing when performed under a vacuum. Graphite dies are used most frequently. Initially the densification is by particle rearrangement and plastic flow at the particle contacts. Once the effective stress falls below the in situ yield strength, then further densification depends on grain boundary and volume diffusion rates. Temperature is a critical factor, and small grain sizes aid densification. Hot pressing cycles are slow, with cycle times up to hours because of the large thermal mass. Maximum temperatures depend on the die material and range up to 2200EC (4000EF) and maximum applied pressures are 50 MPa (7 ksi). A vacuum is often selected for the process environment to minimize contamination of the compact. Consequently, hot pressing equipment can be expensive.

hot repressing Similar to hot forging, where a heated sintered compact is contained in a closed die and pressurized to increase density and possibly to improve surface finish or dimensional precision.

hot runner A mold design where the feedstock flow path is kept hot between shots to eliminate recycle of runners and sprues. The built-in heaters and valves are coordinated with the molding machine operation to ensure no freezing of the feedstock in the flow path.

hot working The plastic deformation of a metal at a temperature and strain rate that minimizes work hardening.

HRA, HRB, and HRC

Section H

Page 11

These designate hardness values determined on the Rockwell A, B or C scales.

humpback furnace A hydrogen furnace that is designed such that the highest temperature portion is above both the entrance and exit. This design helps to stratify pure hydrogen to the high temperature region, since pure hydrogen is lower in density when compared to the reaction products, such as steam, which tends to sink to the colder and lower positions.

hybrid press A powder compaction machine that relies on a blend of hydraulic and mechanical motions or pneumatic and mechanical motions to adapt the advantages of each. For example, a mechanical press might have pneumatic pressure buffers on the punches to control the peak pressure: the core technology is a mechanical press, but the upper punch has a pressure cylinder for controlling the final densification.

hydraulic press A compaction press that drives the tool motions using a hydraulic pressure system, where a sequence of valves determines the events and motions in compaction. Hydraulic presses rely on a central pressure system and servo-valves to proportion hydraulic pressure to the pressing components. They are good at controlling pressure and are best suited to controlled-density pressing such as for pressing filters and other controlled porosity structures.

hydride-dehydride powder For some metals it is possible to form a hydride (hydrogen compound) that is brittle, thereby allowing powder fabrication by first subjecting the material to hydrogen, then milling the brittle hydride, followed by heating in vacuum to extract the hydrogen. The particles are angular, yet ductile, and often contain small pores. The hydride-dehydride powder production route is applied to niobium, titanium, zirconium, uranium, and other hydride forming metals.

hydrogen loss The loss in weight of metal powder caused by heating for a specified time and temperature in a purified hydrogen atmosphere. Weight loss also occurs for a compact that might have polymers or contaminants. It is a rough measure of impurities, such as oxygen and water, and is best applied to materials that contain oxides that can be easily reduced by hydrogen. These would be metals based on iron, copper, nickel, and cobalt. It is not useful for materials that react with hydrogen to form hydrides, such as titanium, zirconium, and niobium

hydrogen reduced powder

Section H

Page 12

Powder produced by the hydrogen reduction of a compound, usually an oxide. The resulting powder often has a high surface area or large amount of internal porosity. Figure H.7 is a micrograph showing the typical porous, agglomerated character of a hydrogen reduced powder.

hydrogen sintering atmosphere Sintering performed in an atmosphere consisting of pure hydrogen. Hydrogen is a reactive, flammable atmosphere, useful for sintering alumina, stainless steels, tungsten alloys, intermetallics, steels, and some refractory metals. Certain materials react with hydrogen to form brittle compounds known as hydrides. For this reason, metals such as titanium, tantalum, niobium, zirconium, and uranium cannot be sintered in hydrogen. It is flammable over the range from 4 to 74 % air. Hydrogen has a high thermal conductivity which improves heating and cooling. A high dew point lowers its ability to reduce oxides and makes the atmosphere decarburizing. If a liquid hydrogen supply is used, then the inlet atmosphere has less than 1 ppm oxygen and 8 ppm water (ppm is the abbreviation for parts per million or 0.0001%). However, it is sometimes useful to intentionally increase the dew point to control coarsening and blistering in some materials, especially tungsten alloys.

hydrothermal synthesis A powder production technique used to form small particles from a solution via precipitation under high pressure hydrogen. For example, CuSO4 is dissolved and reacted in an aqueous solution with hydrogen at 130EC and 3 MPa (30 atmospheres) pressure to produce H2SO4 and copper particles. Additives to the solution control the reaction and the final precipitated copper powder is further heated in hydrogen to give a final 99.8% purity. Particle size is adjustable in a range from approximately 10 nm to 20 탆 . Cobalt and nickel powders may also be formed using similar techniques. One variant of a hydrothermal reaction relies on microwave heating to significantly increase the reaction rates.

hydroxyapatite A compound of calcium, phosphorous, oxygen, and water, with a stoichiometry of Ca5(PO4)3OH, that has a structure similar to human bone and teeth. Accordingly hydroxyapatite is biocompatible and often combined with metallic systems such as titanium to fabricate higher strength but biocompatible composites for implants. One chemistry is Ca10(PO4)6(OH)2 with a hexagonal crystal structure, 3.16 g/cm3 density, melting temperature near 1200EC, strength near 100 MPa, and elastic modulus of 42 GPa.

hygrometer

Section H

Page 13

A device for measuring moisture content.

hypereutectoid steel Usually indicating a steel with more than approximately 0.8 % carbon, over the eutectoid composition. The eutectoid composition varies with alloying, so this value shifts with alloying.

hypoeutectoid steel Usually indicating a steel with less than approximately 0.8 % carbon, although the exact eutectoid composition varies with alloying.

hysteresis The gap between the forward and reverse paths in any process. Magnetic materials working in an oscillating field will experience a difference in magnetic state depending on the direction of applied field. This gap is a measure of energy loss on each magnetization cycle. Likewise in mechanical loading there is often a small hysteresis that leads to heating.

Section I

Page 1

A to Z of Powder Metallurgy

I IACS The abbreviation for International Annealed Copper Standard, which is a measure of electrical conductivity.

ideal gas A gas which obeys the equation of state PV = nRT which is known as the ideal gas law (P is the pressure, V is the volume, n is the amount of molecules, R is the gas constant, and T is the absolute temperature). To a first approximation the inert gases such as argon are ideal gases.

image analysis A processing and data reduction process that yields numerical values that are useful in particle size distribution determinations, particle shape determinations, densification and distortion measurements, microstructure, and even component inspection.

impact energy (strength) The energy consumed to fracture a specimen in the Charpy test, usually given in J or J/cm2; also, know as the impact strength and impact toughness. Impact testing is performed using the standard notched Charpy bar, which is 10 by 10 by 55 mm with a 2 mm deep 45? notch at the center. Some porous sintered materials are tested without a notch and those results are not valid Charpy values. Porous metals are low in impact properties, but efforts to adjust the tests for better apparent properties are very much against the dictates of conservative engineering design. The use of subsize or unnotched samples must be reported with each result to avoid confusion with standard metallurgical Charpy data. [see Charpy impact energy]

impact toughness Usually measured by the energy needed to break a small test sample. Usually called the Charpy test, although some powder metallurgy samples are not notched, unlike that typical to ferrous metallurgy. [see Charpy impact energy, Charpy test]

Section I

Page 2

impregnation A process of filling the pores of a sintered compact with a nonmetallic material such as oil, wax, or polymer resin. Although both impregnation and infiltration are based on capillary action, impregnation is different from infiltration which fills the pores with a liquid metal at high temperatures. Impregnation is widely employed for sealing pores connected to the outer surface. Variants fill the pores with polymers for low friction, epoxy-like fluids for improved machining, or polymers for corrosion resistance. Usually impregnation is performed on heated components under a vacuum to aid fluid penetration of the pores. Oil impregnation might be performed by forcing lubricating oil into evacuated pores, usually at elevated temperatures so the oil viscosity is low. The impregnating fluid spreads to fill the open pores. Large pores have less resistance to filling, so they fill rapidly. Once a resin is in the pores, it is polymerized or cross-linked, but a volume change must be avoided to prevent damage to the porous structure. At 90EC a typical resin cures in about 10 min. One use for impregnation is to eliminate open pores so the component can be used in an application where leaks are not tolerable.

in vitro Formally it means in glass, referring to the study in the laboratory with a biological system, cell, tissue, or implant. Roughly applied to laboratory testing for biocompatibility such as corrosion tests to determine suitability for subsequent in vivo study. [see in vivo]

in vivo Testing performed in the body, referring to a study performed on a living organism. For powder metallurgy materials this implies the testing involves animals or other living systems, for example to measure the tissue growth rate into a porous implant. [see in vitro]

InconelTM A variety of nickel-base alloys invented by International Nickel Company designed for high temperature applications, ranging from furnace components to jet engines. One of the popular alloys for furnace hardware is Inconel 600. It consists of Ni with 16 wt. % chromium, 7 wt. % iron, and traces of manganese, silicon, and copper. This alloy has a density of 8.48 g/cm3, melting temperature near 1350EC, elastic modulus of 214 GPa, yield strength of 205 MPa, tensile strength of 550 MPa with 50 % elongation to fracture. The thermal conductivity is 103 W/(m EC) and thermal expansion coefficient is 10.4 ppm/EC.

induction heating Heating by a combined electrical resistance and hysteresis energy loss induced by

Section I

Page 3

subjecting the metal to a rapidly varying magnetic field, that field being created by a coil carrying alternating electrical current. The surrounding coil carrying the alternating current is water cooled, but the alternating magnetic field induces electrical flow in the material which heats the material. Figure I.1 shows a schematic of one possible induction heating process. In powder metallurgy, induction heating is mostly used for heat treatment, but can be used for sintering and hot pressing. Induction hardening is useful for powder metallurgy steel gears where surface strength, hardness, and wear resistance are critical. The component core is usually a tough, ductile structure such as pearlite or bainite formed during cooling from the sintering temperature. This is because induction heating only works on the surface so the core remains largely unaffected. Rapid induction heating forms austenite at the surface and when the component is quenched, that surface transforms into martensite. However, the core remains ductile and tough, since it never heated significantly. Induction hardening is especially useful for porous sintered steels because the pores reduce heat transport, making it easier to form a hard surface. For very large components, the unheated mass in the core is sufficient to quench the surface once the induction coil is turned off. Carbon can be added via the atmosphere surrounding the compact during induction heating.

inert atmosphere sintering A process gas that does not react with the material being sintered. Inert gases such as argon, helium, and neon always have this behavior, and in some instances nitrogen is inert. The inert gases reduce evaporation and can be purified to high levels. Most oxides have an equilibrium over-pressure of oxygen at high temperatures, so even inert atmospheres result in some oxide reduction. However, this is a weak reduction potential in comparison with carbon monoxide or hydrogen-containing atmospheres. Several metals react with hydrogen to form hydrides that make the metal brittle so inert or vacuum sintering are the only two options. Tantalum, titanium, zirconium, and niobium are susceptible to hydriding reactions (as well as oxidation, carburization, and nitridation reactions) and typically must be sintered in a vacuum or inert gas.

inert gas Generally recognized as the group of gaseous molecules that do not react with a metal, consisting of helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). Sometimes nitrogen is included since it can be nonreactive as a sintering atmosphere, but this is an incorrect designation for nitrogen since it is reactive with many metals, including aluminum, chromium, titanium, and tantalum.

inert gas atomization Gas atomization performed with an inert gas such as argon. The inert gas helps minimize contamination with oxygen being the most common concern. [see gas atomization]

Page 4

Section I

inertial flow At very high flow rates in a porous body, the output quantity of gas eventually becomes independent of the pressure across the porous material since flow is choked. Because of turbulence and other forms of energy loss, inertial flow becomes controlling at high pressures.

inertial materials A term applied to materials with a density higher than lead used for guidance, counterbalance, vibration, and center of gravity control - such as kinetic energy penetrators, golf club weights, and aircraft wing weights. Generally the high inertial alloys are based on tungsten heavy alloys. The tungsten heavy alloys have densities in the 15 to 19 g/cm3 range, which is considerably higher than the density of lead (11.4 g/cm3). The greater the tungsten content, the higher the density. However, ductility and toughness decrease dramatically as the tungsten content increases. The mechanical properties of representative tungsten heavy alloys are summarized in Table I.1. Many compositions are in use, including W-Ni-Fe, W-Ni-Cu, W-Cu, W-Cu-Co, W-Ni-Co, W-NiMn, W-Ni-Sn, and W-Cu-Sn, each with a different property combination. For bird shot, golf club weights, dart shafts, and other lower-performance applications the lower-cost and lower sintering temperature from copper-containing compositions is most beneficial. On the other hand, for the high-performance military applications the compositions with nickel and cobalt tend to be favored. Some new high-density alloys are based on amorphous tungsten-based alloys. Table I.1. Representative High-Inertial Alloys and Properties

composition

density g/cm3

hardness VHN

yield strength MPa

tensile strength MPa

elongation %

97W-2Ni-1Fe

18.6

300

610

900

19

93W-5Ni-2Fe

17.7

280

590

930

30

90W-7Ni-3Fe

17.1

270

530

920

30

97W-2Ni-1Cu

18.6

—

—

660

3

90W-5Mo-3Ni-2Fe

—

280

—

1000

17

86W-4Mo-7Ni-3Fe

16.6

280

625

980

24

82W-8Mo-8Ni-2Fe

16.2

315

690

980

24

74W-16Mo-8Ni-2Fe [see heavy alloy]

15.3

365

850

1150

10

Section I

Page 5

infiltration The process of filling the pores of a compact with a lower melting temperature metal or alloy, typically copper or bronze. Infiltration occurs spontaneously when the component and the infiltrant are heated to a temperature where the infiltrant melts, typically near 1100EC for copper-based infiltrants. When the pores are filled with wax, polymer, or oil at room temperature the process is termed impregnation. Both processes rely on capillarity and wetting to fill the pores. For a wetting liquid, the capillary pressure pulling the infiltrant into the pores ∆P varies with the inverse of the pore size dp as follows:

where γ is the surface energy of the liquid and θ is the contact angle. In the copper infiltration of steel at temperatures near 1100EC, the capillary pressure wicks molten copper into the pores where it remains and solidifies to form a composite. During infiltration, the liquid metal can dissolve solid and erode away the surface. Also, swelling can occur if the solid and liquid react. For that reason, infiltration cycle times are short to keep dimensional changes small. There is preferential dissolution on the surface where the infiltrant forms, so there is either a loss of precision or a sacrificial surface is added. Many components swell during infiltration, by about 0.25%. If an excess of infiltrant is used, then the compact distorts.

infiltration sintering A full density process geared to the fabrication of composites with minimum densification using first sintering followed by infiltration, possibly in the same cycle, also known as sinter-casting. It is possible to sinter a porous preform during heating and to form a liquid to subsequently fill the pores. If the liquid is not wetting, then pressureassisted infiltration or pressure casting are alternatives. Since the infiltration step requires the pore structure be open and interconnected, only certain compositions are possible.

inhibited grain growth Means to reduce the rate of grain growth during sintering to capture improved properties. Several means exist to inhibit grain growth during sintering, such as through the addition of a grain boundary second phase (like a carbide or oxide).

initial stage sintering The first or early portion of sintering in which first bonds or necks growth between contacting particles, but there is no overlap of neighboring necks; the neck size is small and there is no interaction between neighboring necks, as shown in Figure I.2. The initial stage is characterized by a neck size ratio of less than 0.3 and large loss of surface area, but very little shrinkage (less than 3 %). Some strength develops in the

Page 6

Section I

initial stage of sintering. In cases of compaction it is common for sintering to start in the intermediate stage since compression can induce considerable initial neck growth by particle deformation. For most metals initial stage sintering is by surface diffusion, which grows necks and strengthens the compact, but does not produce densification. To predict the rate of sintering requires assembly of several mathematical equations that include the change in vacancy population between convex and concave surfaces. Knowing the volume of the neck as related to neck size and the area over which diffusion occurs provide a means to estimate the neck size as a function of time, temperature, and particle size. The Frenkel model was the first to suggest the relation, and it was followed by the Kuczynski model. Most of the solutions give the neck-size ratio X/D as follows:

where X is the neck diameter, D is the particle diameter, t is the isothermal sintering time, and temperature enters in an exponential form,

where BO is a collection of material, temperature, and geometric constants, R is the gas constant, T is the absolute temperature, and Q is an activation energy associated with the atomic transport process. The values of n, m, and B depend on the mechanism of mass transport as described in Table I.2. Table I.2. Initial-Stage Sintering Model: (X/D)n = B t / Dm mechanism viscous flow plastic flow evaporation뻙ondensation 3 lattice (volume) diffusion grain-boundary diffusion surface diffusion symbols γ = surface energy η = viscosity b = Burgers vector k = Boltzmann뭩 constant T = absolute temperature Θ = theoretical density δ = grain-boundary width

n 2 2 2 5 6 7

m B 1 3 γ / (2 η) 1 9 π γ b Dv / (k T) (3 P γ / Θ2) (π/2)½ (M / (k T))3/2 3 80 Dv γ Ω / (k T) 4 20 δ Db γ Ω / (k T) 4 56 Ds γ Ω4/3 / (k T) Dv = volume diffusivity Ds = surface diffusivity Db = grain-boundary diffusivity P = vapor pressure M = molecular weight Ω = atomic volume

Section I

Page 7

injection molding A hydrostatic forming technique for shaping powders using plastic binders and relatively low temperatures and pressures. The powders tend to be below 20 탆 in particle size and rounded particles are preferred. The binders are usually thermoplastics that melt on heating to temperatures in the 130EC to 190EC range and harden on cooling. The mixture of powder and binder is termed feedstock. The binder imparts viscous flow characteristics to the mixture to allow filling of complex tool geometries. The steps are outlined in Figure I.3, including the mixing of feedstock and the post-molding steps of debinding and sintering. Shaping relies on freezing the binder in the mold, and once the shape is formed and the polymer has frozen the shape is ejected. Next, the binder is removed and the powder structure sintered. The product may then be further densified, heat treated, or machined. The sintered compact has the shape and precision of an injection molded plastic, but is capable of performance levels unattainable with polymers. [see powder injection molding]

injection pressure The pressure placed on molten feedstock during mold filling. It depends on the screw diameter and feedstock viscosity.

injection rate In injection molding this is the maximum volumetric rate for filling the mold and is associated with the specification of the screw forward motion rate in the cylinder. This is effectively a means to control how rapidly the mold is filled.

injection speed profile A preset velocity profile for the ram during the mold filling stage.

inkjet printing A freeform (rapid prototyping) powder shaping approach that relies on binder-solvent droplets precisely sprayed onto a powder bed to form a green body. Inkjet technologies are well known for printing documents at high speeds. In freeform fabrication, the droplets cause some motion of the loose powder, so the jetting precision is lost in the formed body. Once the polymer-solvent droplet splats onto the powder layer, the solvent evaporates to leave the polymer as a glue between the particles, similar to an injection-molded green body. Sufficient binder must be jetted to deliver strength and the layers must be thin enough to ensure bonding between layers. Sintering provides the strength and liquid metal infiltration is used to fill any remaining pores after sintering. Accuracy ranges from ? .4 mm to ? .1 mm. Current machines are capable of generating a green body in a half-day. A few more days of processing are required to burn out the polymer, sinter the powder, and infiltrate the pores. Section thicknesses are

Section I

Page 8

limited to a minimum of 3 mm while maximum dimensions are restricted by the size of the build chamber, and might be 200 mm. One popular system is based on layers of stainless steel powder, debound and sintered, and then infiltrated with bronze. This system delivers up to 25 HRC hardness with yield strengths up to 455 MPa.

inspection Usually the final production stage where component quality is verified based on composition, dimensions, and other properties that typically are measured with nondestructive tests.

integral work of sintering A concept that estimates the net sintering response for materials subjected to heating cycles that might include heats and holds, such that cross-comparisons are possible between different cycles. A mathematical integral of the time-temperature path is used to correlate densification, shrinkage, warpage, grain size, or other sintered properties. The concept arose out of research performed by D. Lynn Johnson at Northwestern University. He has discovered that essentially the same degree of sintering is possible with many different combinations of heating rate, peak temperature, and hold time. Assuming no changes in sintering mechanism or events such as a phase transformation or melting, he showed that the integral work of sintering Θ(T,t) can be represented by the following equation:

where T is the absolute temperature, t is the time, R is the gas constant (8.32 J/mol/K), and Q is the activation energy for sintering. Integration is performed from the start to the end of the sintering cycle. Much of the early analysis was applied to ceramic densification. However, this integral concept is a unifying principle that allows assessment of tradeoffs in cycles or furnaces for equivalent levels of sintering. In this form the units of Θ(T,t) are s/K, reinforcing the interplay between longer times at lower temperatures giving equivalent levels of sintering. The concept is also called the master sintering curve when presented in a graphical form. [see master sintering curve]

interconnected porosity The network of contiguous, open pores connected to the surface of a sintered compact. The term is usually applied to materials where the interconnected porosity is determined by impregnating the specimen with oil. Filtration and lubrication properties rely on the interconnected pores.

Section I

Page 9

interface The surface marking the boundary between two phases, such as a grain boundary. In powder metallurgy the interface is susceptible to impurity segregation and contamination.

interface controlled grain growth During coarsening associated with grain growth or grain coalescence there are two possible rate limiting steps, with most commonly diffusion between grains being the slower step. If the grains are polygonal in shape, then the limited number of sites for dissolution into the liquid or precipitation out of the liquid control coarsening - termed interface controlled grain growth. Such a situation arises in impure systems with limited reaction sites, systems with highly anisotropic surface energy, or systems having multiple diffusing species, where the diffusion rate is fast in comparison to the reaction rate. In these cases, the predicted grain size versus time relation is as follows:

where kr is the interfacial reaction rate constant. For reaction control, the activation energy is usually high, resulting in a high sensitivity to the sintering temperature.

intermediate stage sintering Sintering characterized by open, round pores. The intermediate stage initiates when neighboring sinter necks grow sufficiently large to overlap. It ends when the tubular pores sitting on grain boundaries pinch closed, as illustrated in Figure I.4. This typically corresponds to densities between 75 and 92 % of theoretical. In the intermediate stage the material properties significantly increase and often there is a large dimensional change. The surface area declines and grain growth becomes active, especially as the amount of porosity declines. The rate of density increase dρ/dt is determined by the flux of vacancies and atoms (which depends on the pore size and temperature), the diffusion distance (which depends on the grain size), and the number of pores per volume, generally leading to a density equation of the following form:

where ρS is the fractional sintered density, ρI is the fractional density at the beginning of the intermediate stage, BI has an Arrhenius temperature dependence. In this model tI is the time corresponding to the onset of the intermediate stage, and t is the isothermal sintering time (greater than tI). Typically, BI varies with the inverse cube of the grain size, reflecting the strong role played by grain boundaries in sintering densification. Hence, retarded grain growth and enhanced grain-boundary diffusion greatly improve

Section I

Page 10

intermediate stage sintering densification.

intermetallic A phase that is formed by two metals, typically having a narrow composition range and simple stoichiometric ratio of metals. Some examples include NiTi, FeAl, and Ni3Al. Most intermetallics have novel properties and differ from alloys because of the ordered placement of the atoms. Usually the thermodynamic stability makes the intermetallic melting temperature higher than the rule of mixtures - for example the equiatomic mixture of Ni and Al would have a rule of mixtures melting temperature of about 1050EC, yet the actual melting of NiAl is 1630EC. This excess stability is from the desirable bonding situation that comes with each atom having dissimilar neighbors in the ordered intermetallic.

internal oxidation The formation of isolated oxide dispersoids inside a powder or a powder metallurgy product due to selective reaction with oxygen introduced from a process atmosphere. The surface oxygen dissolves into the solid and progressively reacts to form the oxide. One useful form is in copper reinforced with alumina (Al2O3) formed by internal oxidation. That product has useful high temperature arc erosion resistance useful in welding electrodes.

internal powder porosity A powder might have pores inside the particles that can be open or closed to the particle surface. Figure I.5 shows a cross-section micrograph of a powder with internal pores. Usually these pores prove difficult to remove in compaction and sintering, so powders with internal pores are avoided for sintering structural components. However, such structures are useful in certain applications such as in friction materials. To observe the internal powder structure, the powder is first mixed with an epoxy resin. The particles settle as the resin cures. Subsequent polishing and etching brings out the internal particle structure. For small powders a similar approach uses transmission electron microscopy where the powder is entrained in an electroplating deposit for sectioning via ion milling.

International Annealed Copper Standard (IACS) A standard used in reporting electrical conductivity of a metal, defined as %IACS = 1724.1 divided by the electrical resistivity of the material in nΩ-m (58 m/Ω-mm2). It effectively provides a comparative conductivity with respect to copper.

International Organization for Standardization (ISO) A master standards setting body that spans across individual trade associations, government bodies, or regional bodies to provide global standards for materials,

Page 11

Section I processes, and environmental activities.

International System of Units (SI) A consistent set of measurement units that builds from length (m), mass (kg), temperature (K), time (s), and other similar defined and consistent parameters. It then leads to a simple means to define stress (Pa = N/m2 = kg/(m s)) and other factors via simple conversions; for example the unit of power (W) is a joule per second.

interparticle friction The friction between powders which limits sliding, packing, and densification. This friction manifests itself in inhibited particle packing, poor or uneven flow, and a high angle of repose. [see apparent density, tap density, flow time, angle of repose]

interstitial An alloying element that is small compared to the metal atom, allowing for it to squeeze into the spaces between the metal atoms that must occupy crystal lattice positions. Most important are carbon and nitrogen, especially in steels.

InvarTM A group of trademarked low thermal expansion alloys of iron-nickel or iron-nickel-cobalt where a martensitic phase transformation is balanced against the thermal expansion coefficient to give a near zero thermal expansion coefficient over a range of temperatures. The alloys were invented by Charles-Edouard Guillaume who won the Nobel Prize in Physics in 1920 for his systematic investigation of low thermal expansion and low thermoelastic coefficient alloys for construction of precise measurement tools. Today several powder metallurgy alloys serve the low thermal expansion coefficient field, largely in the form of heat sinks, microelectronic packages, and components for instruments. The classic composition is iron with 36 wt. % nickel, giving the following properties: invar

Fe-36Ni

density

8.05

g/cm3

melting temperature

1425

EC

heat capacity

515

J/(kg EC)

thermal expansion coefficient

0.35

ppm/EC

thermal conductivity

11

W/(m EC)

electrical resistivity

82

µΩ-cm

Page 12

Section I Curie temperature

280

EC

elastic modulus

141

GPa

Poisson뭩 ratio

0.26

hardness

80

HRB

as-sintered yield strength

276

MPa

as-sintered elongation to fracture

30

%

as-sintered ultimate tensile strength 517 MPa If fabricated to less than full density, then these properties are significantly lower. Invar powder is mixed with silver or copper powder and consolidated to form low thermal expansion composites with high thermal conductivities. Most invar produced by powder metallurgy is formed using powder injection molding out of prealloyed powder.

iron (Fe) Chemical element 26, it is the basis for all steels, stainless steels, tool steels, and many specialty powder metallurgy materials. Iron comes in two equilibrium crystal structures, body-centered cubic (ferrite, sometimes called alpha α or delta δ) and face-centered cubic (austenite, sometimes called gamma γ). The body-centered cubic crystal structure is stable at room temperature and changes to the face-centered cubic crystal structure on heating above 910EC. Prior to melting gamma iron reverts back to the body-centered cubic form at 1390EC, which is then called delta (δ) iron. When alloyed, iron can be induced to form a strained body-centered cubic structure, best described as a bodycentered tetragonal crystal that is known as martensite. The latter phase is hard and brittle. The properties of alpha iron at 100% density are given below as a benchmark for understanding the role of alloying and powder metallurgy processing: iron (alpha)

α Fe

atomic number

26

atomic weight

55.87

g/mol

density

7.87

g/cm3

melting temperature

1536

EC

boiling temperature

2750

EC

heat of fusion

15

kJ/mol

heat capacity

456

J/(kg EC)

Page 13

Section I thermal expansion coefficient

12.3

ppm/EC

thermal conductivity

78

W/(m EC)

electrical resistivity

10

µΩ-cm

elastic modulus

210

GPa

Poisson뭩 ratio

0.29

as-sintered yield strength

131

MPa

as-sintered elongation to fracture

45

%

as-sintered ultimate tensile strength

262

MPa

iron-bronze A mixture of iron and bronze powders, used for bearings; for example one common alloy consists of iron with 40 wt. % copper and 4 wt. % tin. It is used in bearings and a related composition is used in laser sintered rapid prototype components (sintered low carbon steel powders infiltrated with bronze). For bearing applications the mixture is sintered to about 20 % open porosity, and those pores are filled with oil to provide an internal lubricant source. Although pores are required for oil retention, they reduce strength.

iron-copper steel Alloys containing a small content of copper, where the copper melts during sintering to provide cohesion to the steel particles. Most of the alloys are formed using mixtures of iron, copper, and graphite powders. If sintered at 1120EC, then the copper melts and the carbon dissolves into the iron to form steel, producing copper bonded steel particles. Iron-copper steels are used to fabricate components for automotive, appliance, and lawn equipment. A favorite alloy consists of 2 wt. % copper and 0.8 wt. % carbon, with the balance being iron. At a density level of 6.8 g/cm3 this alloy has the following properties: iron-copper-steel

Fe-2Cu-0.8C 6.8

g/cm3

melting temperature

1083

EC

heat capacity

460

J/(kg EC)

thermal expansion coefficient

13

ppm/EC

thermal conductivity

45

W/(m EC)

density

Page 14

Section I electrical resistivity

9.2

µΩ-cm

elastic modulus

130

GPa

Poisson뭩 ratio

0.25

hardness

90

HRB

420

MPa

2

%

as-sintered ultimate tensile strength

550

MPa

as-sintered fatigue endurance limit

260

MPa

as-sintered yield strength as-sintered elongation to fracture

unnotched impact energy 11 J 3 When the density is increased, say to 7.3 g/cm , then the tensile strength is improved to about 1230 MPa. Density is a key determinant of properties.

iron-neodymium-boron Several amorphous metal compositions used in magnets are formed near the Fe14Nd2B atomic ratio. The amorphous powders are consolidated by hot pressing, die pressing and sintering, or even injection molding. The compound has the following typical properties: iron-neodymium-boron

Fe14Nd2B

density

7.68

g/cm3

heat capacity

420

J/(kg EC)

thermal expansion coefficient

1.2

ppm/EC

thermal conductivity

7

W/(m EC)

electrical resistivity

143

µΩ-cm

elastic modulus

142

GPa

Poisson뭩 ratio

0.27

hardness

60

HRC

fracture strength 600 MPa Most applications rely on the outstanding magnetic properties, including a high energy product and coercive force.

Page 15

Section I

iron-nickel steel Usually based on mixtures of iron, nickel, and graphite powders, these alloys are used to form structural low alloy steels or magnetic components. The nomenclature changes with the composition and application; for example the iron-nickel steels are sometimes designated FN alloys, with two digits giving the nickel weight percent and two digits designating the carbon content in thousands, so FN-0208 indicates Fe-2Ni-0.8C. The properties depend on the alloying level, density, and post-sintering heat treatment. For example, at full density a simple alloy of iron with 2 wt. % nickel gives the following properties: iron-nickel steel

Fe-2Ni

density

7.93

g/cm3

melting temperature

1535

EC

elastic modulus

196

GPa

as-sintered hardness

50

HRB

149

MPa

30

%

as-sintered yield strength as-sintered elongation to fracture

as-sintered ultimate tensile strength 330 MPa Considerable strengthening comes with carbon additions and post-sintering heat treatments, with sintered strengths exceeding 1200 MPa.

irreducible saturation When a porous body is saturated with a liquid, then on drainage there is a certain portion of the liquid that can only be extracted by evaporation. This small amount of remaining liquid is in the form of isolated pockets or pendular bonds. Such liquid is termed the irreducible saturation since it cannot be removed by surface forces. It is also known as the connate saturation.

irregular powder A powder which lacks shape symmetry in the individual particles. Water atomized powders, such as shown in Figure I.6, are the most common irregular powders encountered in powder metallurgy.

ISO The abbreviation for International Organization for Standardization.

isostatic pressing

Section I

Page 16

The compaction of a powder by subjecting it to equal pressure from all directions; hydrostatic compaction. When the powder is heated in a container the process is termed hot isostatic pressing, and when performed at room temperature in a flexible rubber mold the process is termed cold isostatic pressing. [see cold isostatic pressing, hot isostatic pressing]

Section J

Page 1

A to Z of Powder Metallurgy

J Japan Powder Metallurgy Association (JPMA) Located in Kyoto, Japan, this trade association helps set industry standards, promotes the technology, and organizes conferences and topical meetings for the Japanese community.

jar mill A simple means to grind, attrition, or reshape powder using a cylindrical container that is oriented with the central axis in a horizontal alignment, as shown in Figure J.1. This cylinder with about 50 vol. % fill is sealed on the two ends to form a closed chamber that is rotated on its long axis. The powder in the mill tumbles during rotation to slowly remove asperities. If hard balls are added to the jar, then impact attritioning occurs to reduce the particle size over time. Flakes can be produced out of powders for ductile metals with proper selection of the rotational speed, ball size, and fluid in the mill. Thus, jar milling is used in particle reshaping for improved packing, powder attritioning, and flake production.

jaw crusher A machine for the primary disintegration of metal chunks, such as the sponge emerging from annealing or hydrogen reduction. The crushing action occurs as a jaw opens and closes via a rotation motion, pushing and shearing the feed material against a stationary platen. As fragments form, the gap between the jaw and stationary platen determines when the fragments fall though the mill. The input and output sizes depend on the closest distance between the jaw and stationary platen.

jet mill A self-impact attritioning technique where powder is propelled at high velocities into a chamber where they impact on particles already circulating in the mill. Particle size reduction occurs as the particles impact each other. Such a milling process requires no moving parts and is suitable for reducing the particle size down to the micrometer size range. Over time the small particles exit from the circulating zone using a cyclone, giving an output particle size smaller than the input particles.

jetting

Section J

Page 2

A condition that arises with the rapid filling of an injection mold where the feedstock shoots across the mold and fills toward the gate. Generally this occurs when the initial mold filling is too fast, gate is too small, or the molten feedstock is not properly wetting the mold cavity. Figure J.2 is a picture showing an example of jetting. Jetting is to be avoided, since it results in sealing the vent and the capture of air pockets in the component. [see fountain flow]

joining Bonding objects together to form a single body or assembly. Welding, soldering, brazing, and sinter bonding are examples of joining techniques. The assembly often is formed from different materials that cannot be combined by sinter bonding. All of the common joining processes are applicable to powder metallurgy products. However, bonding porous materials requires special care. Pores reduce thermal conductivity and wick molten metal away from the area being joined. Braze, diffusion, and adhesive bonds are all weaker than other joining techniques, while electron beam and laser welding can cause distortion in porous components. Adhesive bonding is used rarely in powder metallurgy since the strength is low, near 30 MPa. Rivets and other simple fasteners (screws and bolts) can bond components together, assuming the stress concentration is not deleterious to function or performance. At the other extreme, laser bonding gives high quality bonds in thin sections. Section thickness, porosity, and composition factors influence the selection of the joining process. At low porosity levels, below approximately 8 %, no major difference exists between joining procedures for sintered and wrought metals. For porosities between 10 % and 20 %, techniques that form molten metal welds work well, but must be carefully controlled to reduce the melt quantity. With gas tungsten arc welding, a filler metal is required to fill the pores, while the feed wire in the metal-inert gas technique helps fill pores during welding. At higher porosity levels, molten metal joining techniques should not be used because of distortion and residual stress generated by the contracting weld. It is important that the porous steels be free of oils, cooling fluids, or other contaminants before welding. When joining relies on melting there is also a heat affected zone near the weld with a reduced strength. The thermal stresses associated with the bond can cause distortion, cracking, or accelerated corrosion in this zone. Stress induced cracking is a common problem in joining sintered steels. The best practice is to use as high a sintered density component as possible to ensure a strong weld.

Jominy test A metallurgical test to determine hardenability. As illustrated in Figure J.3, one end of an austenitized steel rod is chilled by a water jet. The water chilled end cools quickly and forms martensite, a hard phase. The far end away from the water jet then cools slowly and often forms soft pearlite. After the test the hardness is measured versus distance

Section J

Page 3

from the quench end to determine ease of forming hard martensite. A large depth of hard phase indicates a material that can be easily converted into martensite. Usually, in sintered steels the hardenability falls quickly if there is residual porosity.

Joule (J) A fundamental unit of energy, equal to the one newton of force applied over a distance of one meter. It is also one watt applied for one second or one volt applied to one coulomb of charge.

JPMA The initials for the Japan Powder Metallurgy Association.

Section K

Page 1

A to Z of Powder Metallurgy

K K The standard abbreviation for temperature on the SI kelvin scale. Also, the chemical symbol for potassium.

K-factor A form of strength as determined using a bearing that is crushed radially between two parallel platens. Since this radial crush strength is not the same as the tensile, transverse rupture, or compression strength, it is simply designated as a separate strength factor when determined on a straight-wall, cylindrical bearing. The test was shown earlier as the bearing strength test (Figure B.3) along with the expected fracture condition. The maximum load FB in crushing is recorded and the K-factor is calculated as follows:

where L is the cylinder length, D is the outer diameter, and T is the wall thickness. The cylinder length tends to be 1.5 times the cylinder diameter; a common diameter is 25 mm. For common sintered bearings, such as pure iron and iron-copper-carbon, the Kfactor range from 140 to 280 MPa and densities range from 5.6 to 6.8 g/cm3.

Keltool A popular infiltrated composition fabricated using rapid prototyping ideas to give a final alloy of iron with 35 wt. % cobalt, 21 wt. % chromium, 9 wt. % tungsten, 1.5 wt. % carbon, and 30 % copper. The density is 8.59 g/cm3 and the final hardness is 40 HRC. It has no significant ductility, but does have a reasonably high strength at 880 MPa.

kelvin (K) The SI unit for thermodynamic temperature with the customary abbreviation of K. It is the defined as 1/273.16 of the thermodynamic temperature corresponding to the triple point of water.

kg The common abbreviation for kilogram.

Section K

Page 2

KIc The common abbreviation for the critical stress intensity for crack growth in tension. The K represents the stress intensity (units of MPa(m)½), the I stands for tension (mode I is pulling the crack open which is the worst case), and c stands for critical or the condition that causes catastrophic failure. [see fracture toughness]

kilogram (kg) A basic SI unit of mass defined as 1000 g which equals 2.2 pounds.

kinematic viscosity The dynamic viscosity divided by the density of the fluid.

kinetic energy The energy associated with the motion of a body, defined as one-half of the mass times the square of the velocity, ? m V2 where V is the velocity and m is the mass.

Kirkendall effect Pore growth in a diffusion situation, such as occurs in sintering mixed dissimilar metal powders, where one atomic species moves much faster than the other, resulting in an imbalance in mass flow and the net accumulation of vacancies to form pores. Figure K.1 is an example of such pore formation in the sintering of mixed iron and aluminum powders, where the aluminum diffuses into the iron to leave a pore. In some binary metal systems this can be extreme, leading to formation of a porous body. Mixed powder homogenization works best with small particles which inherently have small diffusion distances. Kirkendall pores are observed in sintering the several common mixed powder systems. Lower temperatures slow homogenization and pore growth, but also restrict sintering densification. An extreme Kirkendall effect is observed in sintering mixed powders with very different melting temperatures, such as copper and nickel or iron and aluminum.

knit line A linear defect occurring where divided feedstock streams merge in an injection molded component. The two streams of hot feedstock come from two gates or from flow around a core or other solid portion of the mold. If mold filling is slow or the feedstock is cold, then the knit line will not seal, leaving a defect such as illustrated in Figure K.2.

knock out A feature that is included in the part during production to stabilize or ease production, but is not included in the final component or design; this feature is removed after

Page 3

Section K sintering, often by hammering it loose. For example, two thin sections might be stabilized with respect to each other by a connecting web that is removed after sintering.

Knoop hardness One of several test scales for determining the resistance to indentation as a rough gauge of mechanical properties. It is a microhardness test that relies on a load P to press a diamond pyramid (length L is 7.11 times the width) and measures the hardness KHN from the length of the impression based on the following formula:

knuckle press A rotating crank is tied to a mechanical slide as diagramed in Figure K.3, such that on each rotation of the crank the punch moves through a compaction cycle. This design provides a favorable combination of speed and force control in die compaction. Also, depressurizations is slower with this type of press versus other designs, resulting in longer pressure dwells during pressing.

Kovar A glass to metal sealing alloy widely employed in microelectronic packaging and other situations where a matched thermal expansion coefficient is required. Kovar is a trademarked name, so sometimes it is specified as F15 to indicate the ASTM International specification. It is almost always fabricated using powder metal injection molding and is sintered to about 98 % density. The nominal composition is Fe with 28 to 31 % Ni, 15 to 18 wt. % Co, and small concentrations of less than 0.5 % Mn and less than 0.2 % of Cr, Mo, Cu, and Si. At full density Kovar has the following properties: Kovar density

8.36

g/cm3

melting temperature

1450

EC

heat capacity

460

J/(kg EC)

thermal expansion coefficient

5.2

ppm/EC

thermal conductivity

17

W/(m EC)

electrical resistivity

49

µΩ-cm

elastic modulus

207

GPa

Page 4

Section K Poisson뭩 ratio

0.3

Curie temperature

435

EC

hardness

80

HRB

as-sintered yield strength

276

MPa

as-sintered elongation to fracture

30

%

as-sintered ultimate tensile strength 517 MPa It is commonly fabricated to near full density using powder injection molding, but is not formed via die compaction.

Kozeny-Carman equation A relation that links the pore structure and its effect on laminar gas flow and the approximate surface area of the packed powder. The Kozeny-Carman equation is applied to loose powder packings to estimate the surface area from the fluid carrying permeability. One form of the equation relates the permeability coefficient α, surface area S, tortuosity τ (the length of the actual flow path as a ratio to the wall thickness), and open porosity εO, as follows:

where C depends on the pore shape and is often near 0.8. It is fundamental to the Fisher Subsieve Sizer and its use for estimating average particle size for small powders. [see Fisher Subsieve Sizer]

Kuczynski neck growth model The fundamental law for initial stage sintering of two contacting spheres, first published by George Kuczynski in the late 1940s. This law says the neck size ratio is proportional to a fractional power of time and varies with an Arrhenius temperature dependence. Figure K.4 shows the predicted progressive growth of the neck between two contacting spheres (only a portion of one sphere is shown since the image is symmetric in all other views). The integral form of the Kuczynski model gives the isothermal neck-size ratio X/D as follows:

where X is the neck diameter, D is the particle diameter, t is the isothermal sintering time, and temperature T enters in an exponential form,

Section K

Page 5

where BO is a collection of material, temperature, and geometric constants, R is the gas constant, T is the absolute temperature, and Q is an activation energy associated with the atomic transport process. The values of n, m, and B depend on the mechanism as treated by several theoretical derivations. The Kuczynski model is valid for a neck-size ratio X/D below 0.3. The mechanism of mass-transport is embedded in the parameter B and always follows an exponential temperature dependence. The final form of this equation depends on the transport mechanism.

Section L

Page 1

A to Z of Powder Metallurgy

L laminar flow Fluid flow that is predominantly controlled by the viscosity of the gas or liquid. In powder metallurgy, the primary concern is flow through the pores where the flow rate is controlled by the pore dimensions and the fluid viscosity. In contrast, at very low pressures, the mean free path between molecular collisions is large compared to the pore dimensions and flow is controlled by diffusion. At very high velocities and pressures the flow is choked. Laminar flow is treated by Darcy뭩 law for porous sintered metals. [see Darcy뭩 law]

laminated object fabrication A rapid prototyping process for generation of three-dimensional green bodies using a computer image that defines the profiles on each slice through the object. The slices are generated by laser cutting from tape-cast powder sheets. The layers are stacked sequentially to form the three-dimensional object corresponding to the computer image. Alternatively, computer controlled robot arms or knives attached to x-y plotters can be used to generate each slice. In the height direction the image resolution is controlled by the stair-step effect from the stacked layers. Smoother surfaces require thinner layers, but thin layers slow the build process. Further, there can be a systematic error in the positioning between layers such that a small error propagates to distort the green body. Thus, laminated object fabrication is forced to balance various takeoffs between accuracy and build rate or cost. Like the other powder-based rapid prototyping technologies, there is a separate firing cycle after the build process. To minimize sintering shrinkage the body might be infiltrated rather than sinter densified. [see rapid prototype]

lamination A crack in a pressed compact resulting from ejection stresses exceeding the green strength. The lamination cracks tend to occur parallel to the punch face. Also, called delamination.

Laplace equation A relation between curvature and stress that is applicable to all surfaces and underpins

Section L

Page 2

the description of sintering events. Formally, the Laplace equation gives the stress σ associated with a curved surface as,

where γ is the surface energy, and R1 and R2 are the principal radii of curvature for the surface. As an illustration, consider a bubble. Small bubbles (small R1 and R2) have high stresses on their surfaces and large bubbles have low stresses; hence, small bubbles tend to merge into larger bubbles to increase the overall radii and lower the stress. In the same manner, curved surfaces generate departures from equilibrium leading to gradients that drive sintering. [see capillarity]

lapping A finishing operation that uses abrasive grit loaded into a flat rotating disk to polish a surface smooth and flat. The component being lapped moves over the rotating disk to attain the desired smooth, flat surface.

laser A source of ultraviolet, visible, or infrared radiation that produces light amplification by stimulated emission of radiation (hence the abbreviation laser). The emitted light from a laser is coherent and monochomatic so it is a single wavelength or color. Lasers are used in powder metallurgy for a wide range of applications, including particle size analysis, dimensional measurements, rapid prototype construction, cutting thin sections, surface hardening, and surface glazing. [see laser glazing, laser hardening, laser light scattering, laser sintering, laser peening, laser welding]

laser glazing A technique for improving corrosion behavior of full-density, high-performance sintered materials based on a laser beam rastering over the component surface to produce a surface liquid. Because the bulk material below the molten surface is not heated, the melt is rapidly quenched into a surface glaze. After melting the dispersed surface particles cool at 106 EC/s. Often this is sufficient to quench the molten particles into a glassy metal that lacks grain boundaries, leading to increased corrosion resistance. Treatments of this sort change the surface state of a component without degrading the bulk properties. Sometimes, the quenching process is rapid enough to produce an amorphous coating that exhibits excellent corrosion resistance. Generally, these treatments affect less than the first 100 탆 of the surface.

Section L

Page 3

laser hardening Laser hardening is a surface treatment based on quick heating or pulsing using a laser to produce a strained surface layer. Heat conduction into the component is slow compared to the rate of surface heating, so the surface goes through a rapid heat and cool cycle, but the bulk material is unaffected. It is most effective in forming hard martensite layers on steel components.

laser light scattering A common term for Fraunhofer diffraction, representing a means to measure particle size of particles dispersed in a fluid, where the angle and intensity of laser scattering are measured to deduce the particle size. [see Fraunhofer diffraction]

laser peening Pulses of laser light are used to create a surface compressive stress on a component to provide extended fatigue resistance. It has the same impact on fatigue life as shot peening. [see shot peening]

laser sintering High-power lasers are capable of directly sintering or melting a powder and are coupled with a computer control to create freeform products without tooling. Lasers only work on the top layer of a powder, so they are not useful for bulk heating. This is an advantage in freeform fabrication, since the laser sintering effectively only works on the top surface where a loose powder is traced out into the desired shape by the laser beam. The powder in the laser beam forms a semisolid pool that quickly solidifies to form the part. A sketch of the process is shown in Figure L.1. Most variants use multiple powder feeders which allow the mixing of composites. Rapid cooling of the deposit reduces the grain size and promotes higher strengths than possible in normal sintering cycles. In molds created using laser sintering, it is possible to include conformal cooling channels that cannot be generated using traditional machining. However, tolerances are not very good, so most products require final finishing by machining, grinding, and polishing. These are slow steps and reduce the time benefit from laser sintering. Other applications are in component prototyping and limited production applications for automotive, defense, and aerospace fields. The fabrication of high strength titanium aerospace components is one of the first applications to reach production status.

laser welding Joining of powder metal components or wrought and sintered components via local melting under the intense heating from a focused laser beam.

Section L

Page 4

lattice diffusion Also known as vacancy diffusion, volume diffusion, and self-diffusion, it is atomic level motion that occurs because there are missing atoms on the crystal lattice. When the material is heated, atomic motion is stimulated by a higher vibrational amplitude. Occasionally the atoms jump into neighboring vacant sites, effectively moving the vacancy in the opposite direction. As the sintering temperature approaches the melting point, this level of atomic motion results in each atom jumping to a vacant site up to a million times per second. Over time the intense atomic motion provides for sintering densification.

levels A simple means to differentiate die compacted components based on the distinct number of horizontal planes that would require separate tool motions during compaction. The simplest component has a single level, while six levels is about the maximum possible with most die compaction technologies. The cross-sections shown in Figure L.2 illustrate the concept of levels.

LIGA A deep lithography process used to carve out microminiature molds for forming very small components. The acronym comes from German and stands for lithographie, galvanoformung, abformung. A photomask is used to mask the substrate while it is exposed to x-rays or synchrotron radiation in the 0.1 to 0.5 nm wavelength region, and through a sequence of steps the structure is exposed and etched until a mold cavity is created. This cavity is then used in powder injection molding for the fabrication of microminiature components. [see microminiature]

light blocking Light blocking is a means to perform particle size analysis based on measuring the shadow size of particles passing through a small illuminated optical window. The light beam is interrupted by the dispersed particles, as illustrated in Figure L.3, where the particle passage partially blocks the light reaching the detector. Assuming a spherical particle shape, the amount of light blockage is equated to an equivalent circular cross-sectional area. The lower particle size is determined by the optical resolution, with 1 탆 being the typical smallest size. As the data are generated, the counts are sorted by number of particles of each size.

light bulb filament One of the first modern products created from powders was the filament for the incandescent light bulb. Edison demonstrated his incandescent filament light bulb in 1879, but early filaments failed quickly. His choice was carbonized natural fiber, building

Section L

Page 5

on earlier efforts that used platinum filaments in evacuated glass containers. By 1905, Whitney had optimized metallized carbon filaments with outputs of 4 lumens per watt. About this time the investigation turned to the refractory metals, first tantalum and then tungsten. The first tungsten filament was produced from powder using binder-assisted extrusion. The binder was burned off and the particles sintered to make a fragile filament. The output was 8 lumens per watt, twice that of the carbon filaments. In 1910, Coolidge developed ductile tungsten lamp filaments using tungsten powder. Tungsten allowed a higher operating temperature and gave more light in contrast to the other options. In 1937, the coiled filament was developed, and in 1959 halogens were added to regenerate the coil during operation. Figure L.4 illustrates a typical coiled tungsten light bulb filament. Today, 1 kg of tungsten powder produces 150,000 filaments for 40 W bulbs. Each filament delivers approximately 1000 h of life with an output approaching 13 lumens per watt. New options in electrodeless lamps with sodium-mercury vapors have efficiencies of 135 lumens per watt and 30,000 hours life. Also, light-emitting diodes are offering a lower operating temperature alternative.

light scattering The redirection of a light beam due to interactions with small particles dispersed in air or a liquid. This is the same concept as Mie scattering, and is used to measure particle size by recording optical information on the angle and intensity of the scattering event. Light scattering is generally assumed to be elastic scattering.

liquid phase sintering Sintering at a temperature, where a liquid and solid coexist due to chemical reactions, partial melting, or eutectic liquid formation. Various forms of liquid phase sintering are employed in powder metallurgy. One common form is persistent liquid phase sintering where the solid and liquid mixture coexist all of the time the compact is at the sintering temperature. Figure L.5 sketches the liquid phase sintering process, with key events being particle rearrangement on liquid formation, solution-reprecipitation as the smaller particles disappear and the larger particles coarsen and shape accommodate, and final stage sintering of the solid skeleton. This is how cemented carbides or hard metals (tungsten carbide and cobalt) sinter. In contrast, transient liquid phase sintering denotes those cases where the liquid is soluble in the solid, so the liquid disappears over time. For example, when pressed tin and copper powders are heated, the tin melts and dissolves into the copper to form solid bronze. Reactive sintering designates a form where a heat generating exothermic reaction occurs during heating. For example, in heating nickel and aluminum powders a reaction starts when the aluminum liquid forms, producing nickel aluminide compounds.

liquidus The lowest temperature at which an alloy is completely liquid and is thereby a

Section L

Page 6

temperature that must be avoided in the sintering furnace. Alternatively, the liquidus is the lowest temperature at which a melt can exist without forming a solid. When a liquidus temperature is quoted, it is the equilibrium temperature observed with very slow temperature changes. In powder processes, especially atomization, rates of temperature change give significant departures from equilibrium. [see solidus]

lithium stearate A stearate-based lubricant that is mixed with metal powders to provide improved powder flow and reduced ejection pressures. The molecule is based on 5 wt. % Li2O attached to a chain of 18 carbon atoms ((CH2)16-CH3). It softens in the 195EC to 212EC range, melts at 220EC, and has a density of 1.01 g/cm3. Lithium stearate is particularly useful in stainless steel applications, due to increased surface activity from the oxygen gettering action of lithium during decomposition.

litre A unit of fluid volume equivalent to 0.001 m3 or 1000 ml.

loading Loading is the filling of a die cavity with loose powder. Also related to the volume percentage of powder in injection molding or slurry casting feedstock, where it is properly termed the solids loading.

log-normal A statistical distribution useful for dealing with particle size data. It has the positive attribute of giving a classic bell curve when the particle size is expressed on a logarithmic basis. For the cumulative distribution, a linear plot results when the standard deviations of the cumulative distribution are plotted against the logarithm of the particle size. Figure L.6 is an example of a cumulative log-normal particle size distribution plot, showing the y-axis with a probability scaling (corresponding to linear standard deviation scaling), and the x-axis with a logarithmic size scaling. Most naturally formed powders exhibit a log-normal particle size distribution. The log-normal particle size distribution is a modified form of the Gaussian distribution. When the particle size is on a linear scale, the distribution is skewed to a high frequency of small powers. Since most metal powders exhibit straight line behavior on the log-normal plot, the particle size distribution can be reduced to just two parameters, the slope and the median size. The slope parameter provides a measure of the particle size distribution width. Since D90 and D10 differ by 2.56 standard deviations and the size is expressed on a logarithmic basis, the slope, better known as the size distribution width SW, is given as follows:

Section L

Page 7

If the particle size distribution is very narrow, then D90 and D10 are close and the cumulative distribution is steep, giving a high SW. On the other hand, if the distribution is very wide, SW is a low number. In practice, most metal powders are naturally formed with a polydisperse particle size distribution and have SW in the range from 4 to 5 (D90/D10 = 3.2 to 4.4). A few intentionally blended powders with a high packing density have mimicked a multimodal distribution with a slope of 2 (D90/D10 = 19). Monosized distributions, where 90 % of the particles are within ? 5 % of the median size, so SW is over 80 and D90/D10 is less than 1.08. The transformation from a population-based particle size distribution to a mass-based particle size distribution, or vice versa, is relatively simple if the powder conforms to a log-normal distribution and this transformation is commonly provided in the software of most particle size analyzers.

loose random packing The lowest packing density attainable through the random filling of a container, corresponding to approximately the apparent density. For monosized spheres filled into a container with minimal vibration, this tends to be near 60 %. As the particle shape departs from spherical, the general tendency is for the loose random packing density to fall. Further, as the particle size becomes smaller, especially below 10 탆 , then the loose random packing density declines. It is common to have nanoscale powders (size below 0.1 탆 ) that exhibit loose random packing densities of just 2 % or 5 % of theoretical. On the other hand, broad particle size distribution can often have better filling of interparticle voids by small particles, resulting in higher loose random packing densities. Values up to 80 % of theoretical are possible with polydisperse large particles. [see apparent density]

loose powder sintering Sintering of uncompacted powder with no external pressure. Used for the fabrication of porous or weak structures where porosity is desirable, such as in filters and air distribution plates. The powder is simply poured and vibrated into a machined cavity and the plate with the cavity is heated to induce particle-particle binding. The loose particles conform to the cavity shape, so no compaction is involved. Since there is no compaction pressure, the green density is between the apparent density and tap density. Loose powder sintering is employed with stainless steel, bronze, and other high alloy powders. The structures are used in filtration applications where a high surface area is the primary concern and the shapes are simple cylinders, cones, or disks - for example, in the fabrication of fuel filters for automobiles. Besides applications as filters,

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loose powder sintering is also used to fabricate sound attenuation surfaces, electrodes, gas distribution plates, bubblers, and polymer extrusion packs. Most typically, sintering temperatures are low and sintering shrinkage is small, so the porous structure is easily removed from the tooling. One recent variant has been to form ceramic cavities, possibly using rapid prototyping devices, and the cavities are filled with powder, sintered, and infiltrated.

loss on reduction A measure of the volatile mass loss when a powder is heated in hydrogen, which is useful for estimating the contamination level and the amount of moisture or oxygen that might be extracted during sintering. The technique is mainly targeted at measuring the quantity of easily reduced oxides on iron, lead, cobalt, nickel, tungsten, or copper powders, so it is not applicable to high stability oxides, such as alumina, titania, silica, magnesia, calcia, or beryllia. Mass is measured prior to the test and after heating to temperatures from 550EC (30 min) for tin to 1120EC (60 min) for iron, steel, nickel, or tungsten. The relative change of weight is reported.

low alloy steel A category of ferrous alloys (mixture of iron and carbon) that exhibit good mechanical properties and often include alloying elements such as nickel, molybdenum, or chromium at concentrations less than 2 wt. %. The alloying provides substantial strength and hardenability gains.

low carbon steel Alloy consisting of largely iron and carbon, with the carbon content below 0.8 %.

low cycle fatigue Failure that occurs under cyclic loading at relatively few stress cycles, generally regarded as fatigue failures occurring at less than 10,000 repeat loadings.

low-density materials Materials selected for weight-sensitive applications, such as in aerospace applications, with titanium and aluminum being the most common powder metallurgy materials. For low-performance applications, die compaction and sintering can be used, such as for noncritical applications in automobiles, sporting devices, and computers. Since aluminum is a relatively weak material, composites with ceramic particles in an aluminum matrix are common ! known as metal matrix composites. The dispersed ceramic phases include alumina (Al2O3) and silicon carbide (SiC). With more than about 20 vol. % ceramic phase the composite has a low ductility.

low pressure molding

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The use of lower pressures and low viscosity binders (largely water-based or waxbased) to fill out a complex tool cavity without packing the shape. This route is successful for components where internal flaws are not a concern, such as nozzles, spray tips, or other geometries where external shape is the key concern.

lower punch The member of the compaction tool set or die which determines the volume of powder fill and forms the bottom of the part being produced. As illustrated in Figure L.7, multiple segment lower punches may be necessary to aid in uniform filling, densification in pressing, and defect-free ejection of multiple level parts.

lower ram The part of a die compaction press that is moving under the action of the lower pressure cylinder to transmit pressure to the lower punch.

LSW model Named after Lifshitz, Slyozov, and Wagner, this is a means for describing Ostwald ripening in dilute solutions. The LSW model has been applied to liquid phase sintering to explain why the number of grains decreases while the mean grain size increases during sintering. By the LSW model, grains with a size smaller than average shrink, while grains larger than average grow. The diffusion controlled growth gives the grain size G cubed coarsening from an initial grain size Go over time t as follows:

where the parameter K is the grain growth rate constant and it relates to the diffusion rate of dissolved solid in the liquid. A prediction from this classic solution is that a maximum grain size in the grain size distribution is 1.5 times the mean grain size, under steady-state conditions. However, data for liquid phase sintering systems generally show this ratio is 3, as illustrated by the results plotted in Figure L.8. By the LSW model, the maximum growth rate declines as coarsening continues since the overall solid-liquid surface area declines. The model is incorrect for liquid phase sintering because of several factors. It assumes a very dilute solid content such that the solid-liquid interactions take on an average behavior that only depends on grain size. In practice liquid phase sintering occurs with a high solid content with significant neighbor-neighbor interactions, where some grains grow yet are predicted by the LSW model to shrink, and vice versa. Also, the model assumes no solid-solid grain contacts (contiguity) and no grain coalescence, but both are observed in liquid phase sintering. Other assumptions include an isotropic surface energy and spherical grain shape, both of which are not valid.

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Section L

lubricant An organic additive which is mixed with a powder to minimize die wear and aid in ejection after compaction. As illustrated in Figure L.9, most powder metallurgy lubricants are fabricated into small powders for mixing with the metal powder prior to compaction. Some favorites in powder metallurgy are lithium stearate, zinc stearate, and ethylenebis-stearamide. At low compaction pressures most of the lubricants reduce die wall friction and improve powder compaction, but at high compaction pressures the lubricant takes up volume and interferes with particle deformation during compaction. In practice, high-viscosity lubricants are best, especially when used in low concentrations. Most common lubricants are based on stearic acid, a short chain molecule consisting of CH3(CH2)16-CO2H. Many stearates have metal oxides in the chain to aid in attachment to the metal powder surface. Common oxides include zinc, lithium, magnesium, and calcium. Because most of these metallic stearates contaminate the sintered product or furnace, it is becoming more common to use a wax that consists of two stearates bonded by nitrogen and carbon: CH3-(CH2)16-CO-NH-CH=CH-NH-CO-(CH2)16-CH3 There are several variants. Such waxes are commonly used in compaction for hard materials that prove difficult to press, especially where residual contamination is a problem. New options are to use iron or copper stearates that function with the desirable metallic oxide, yet avoid furnace contamination.

lubricating Mixing the metal powder with a lubricant polymer to provide reduced tool wear and friction with the tool wall during pressing. Die wall lubrication relies on placing the lubricant directly on the tooling rather than mixing the lubricant with the powder.

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A to Z of Powder Metallurgy

M MA An abbreviation used to designate an alloy or process that involves mechanical alloying. For example, MA 754 is an alloy that consists of nickel with 20 wt. % chromium, 1 wt. % iron, 0.3 wt. % aluminum, 0.5 wt. % titanium, and 0.6 wt. % yttria. It is designed mostly for high temperature properties. [see mechanical alloying]

machinability A term that provides a comparative measure of the ease or difficulty in removing mass from a sintered component, often via cutting, milling, drilling, or similar techniques. The most extensive data are for drilling after sintering, under controlled conditions of drill size, drill design, rotation speed, feed rate, coolant, and failure criteria. Here machinability is measured by the number of holes that can be drilled with less than 0.38 mm wear of the drill. A higher machinability rating is assigned to materials that allow more holes to be formed without damage to the drill. In powder metallurgy steels, both alloying (carbon level) and heat treatment dominate this parameter. Very soft and very hard materials are the most difficult to machine. Pore filling agents (copper or polymers) and machinability additives (manganese sulphide, boron nitride, tellurium, lead, tin, molybdenum compounds, or polymers) improve machinability. Besides the sintered component condition, the hardness of the cutting tool is important. Unfortunately high hardness cutting materials are more costly, so usually a compromise is struck between performance and cost. Sometimes it is possible to rough machine the component in the green or presintered conditions to reduce final material removal.

macrohardness A term applied to hardness tests that bridge over broad regions in the material, such that the hardness reading includes most of the phases and any effects from residual porosity. For example, the Rockwell hardness is a macrohardness test where the readings tend to be different than those obtained from microhardness tests.

macropore A term applied to catalytic materials where the pore diameter is larger than 50 nm

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magnesia The stoichiometric oxide of magnesium, MgO, it is a favorite for forming brick work in sintering furnaces. The compound is often mixed with alumina or silica, but the latter tends to degrade properties. Pure magnesia has a theoretical density of 3.58 g/cm3, melting temperature of 2825EC, hardness of 890 VHN, elastic modulus of 303 GPa, and fracture strength near 200 MPa. Magnesia has an advantage in that its thermal expansion and thermal conductivity match most steels, thereby avoiding thermal stresses or distortion in the sintering equipment.

magnetic coercivity test A standard test for determining the magnetic force needed to demagnetize a material after first imposing magnetization. This test is often applied to cemented carbides to provide a nondestructive evaluation of quality in terms of the carbon content, cobalt content, or grain size. The cobalt is magnetic, so things that impair the magnetization of cobalt are usually also detrimental to the carbide performance.

magnetic domain A region in the crystal grains where the electron spins align to provide matched magnetism. As the applied magnetic field is increased, magnetic domains with a favorable alignment to the applied field grow at the expense of unfavorably aligned domains. At saturation all of the domains have become matched to the applied magnetic field.

magnetic induction A sintered ferromagnetic material will enhance or magnify an applied field to produce a higher and more powerful total magnetic field. The induced magnetization depends on the applied magnetic field and the permeability of the ferromagnetic material. At saturation there is no improved magnetization even when the applied field is increased. [see magnetization curve]

magnetic materials Generally applied to ferrous powder metallurgy products, since they offer the lowest cost options for various soft and hard magnetic property combinations. Offerings from the powder metallurgy community include iron, iron-nickel, iron-nickel-molybdenum, iron-nickel-cobalt, iron-cobalt-vanadium, iron-phosphorous, iron-silicon, nickel-iron, some stainless steels, and related compositions. Soft magnetic materials are magnetic only when subjected to an applied field and are used in alternating current applications. Hard magnetic materials retain their magnetism after removal of the inducing field and are used in direct current applications.

magnetic properties

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Properties of iron, nickel, and cobalt alloys (and a few rare metals) as related to their response in a magnetic field. These properties include magnetic permeability, saturation inductance, coercive force, and remanent magnetization, as well as energy loss and eddy current response parameters. [see magnetic permeability, saturation inductance, coercive force, remanent magnetization]

magnetic susceptibility The magnetic moment is a measure of the magnetic field generated by a test sample. The magnetic susceptibility is expressed in units of AAm2 and the magnetic moment per unit volume is then A/m. The susceptibility then is the magnetization per unit applied field. For many solids, magnetization is anisotropic so the response depends on orientation. Often the magnetic susceptibility is measured with a constant applied magnetic field.

magnetic transformation temperature Generally refers to the change from ferromagnetic state to nonmagnetic state, which depends on purity and other factors, but for example occurs at 771EC for iron. [see Curie temperature]

magnetite A common mineral that is an oxide of iron, in the form of Fe3O4, a compound that is useful as a surface coating formed by steam treating sintered steels near 550EC to close any surface connected pores. The growth of magnetite on the powder metallurgy component surface provides an aesthetic dark surface color that improves corrosion and wear resistance. The compound has a theoretical density of 5.20 g/cm3.

magnetization curve A plot of the induced magnetization on the y-axis as a function of the applied magnetic field. As illustrated in Figure M.1, the test usually starts with a demagnetized material. Via current in an external coil, the applied magnetic field is progressively increased and the degree of magnetization is measured via a second sensing coil. The initial slope is termed the initial permeability, but more important is the tangent to the magnetization curve which is reported as the maximum permeability. As the magnetic domains in the material align, there is a declining rate of magnetization and eventually the material reaches saturation. When the applied field is reduced, the induced magnetization does not follow the same curve, leading to an attribute termed magnetic hysteresis. When the applied field is taken to zero, there is a remanent magnetization or residual magnetization. Finally to drive the magnetic response to zero requires the application of a coercive force. This curve can be generated for both the north and south magnetic orientations using an alternating current.

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[see magnetic permeability, saturation inductance, coercive force, remanent magnetization]

magnetostriction The change in dimensions of a body resulting from a change in magnetization. Usually the change in length is small, on the order of a few parts per million, but some rare earth metals exhibit strains of 2000 ppm on magnetization.

maraging steel A high strength, high toughness class of iron-nickel-molybdenum alloys that lack carbon, so they attain their strength by an age hardening process that precipitates intermetallic compounds (molybdenum-nickel, nickel-titanium, and iron-molybdenum) in a carbon-free martensite. Strength levels of 4 GPa or 600 ksi are possible with these alloys. Most of these alloys are designated by a number that indicates the expected yield strength in ksi - 300 grade would be expected to have a yield strength of 300 ksi (over 2000 MPa). These alloys tend to have theoretical densities over 8 g/cm3 with a fracture toughness approaching 10 MPa(m)½.

martempering Heat treatment of a steel that involves slow cooling through the martensite transformation range to reduce the residual stresses that result from the formation of the strained martensite phase.

martensite A general term for microstructures formed by diffusionless phase transformations in which the parent and product phases have specific crystallographic relationships. The most famous form is in carbon containing steels where rapid cooling produces a distorted ferrite crystal structure (the common variant is a distorted body-centered cubic phase that forms a body-centered tetragonal structure) due to carbon supersaturation in rapidly transformed ferrite. This form of martensite is shown in Figure M.2, and is hard and brittle, hence it is usually tempered to partially relax the hard phase for improved toughness. It is characteristically an acicular microstructure with residual strain. In carbon-free martensite, such as in iron-nickel alloys that form maraging steels, the martensite phase is soft and ductile since the alloy is a substitutional solid solution. Usually martensite has a temperature where the transformation starts (MS for martensite start) and a lower temperature where the transformation is finished (MF for martensite finish), indicating the transformation is dependent on temperature but not time. For powder metallurgy steels, martensite is associated with rapid cooling. Its formation usually requires a quenching medium - oil, water, water-polymer, water-oil, or salt solution. Quench fluid selection depends on the alloying additions and carbon level. In some sintered steels it is possible to form martensite during cooling in the sintering

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furnace, termed sinter-hardening. This is of benefit since it eliminates post-sintering heat treatments. The high hardness and strength of martensite provide major opportunities to strengthen a sintered steel. Unfortunately, most steels that are fully martensitic are low in toughness and can be as brittle as glass. Consequently, many quenched martensitic steels are reheated to improve toughness - called tempering. A sintered part must be cooled below the martensite finish temperature to fully form martensite. This might require cryogenic cooling, since heterogeneous microstructures or high alloying levels suppress the transformation. In powder metallurgy steels this can be a problem since most of the alloys are formed from mixed elemental powders. If the alloying ingredients are not homogeneous after sintering, then retained austenite will cause dimensional control problems.

martensitic transition A diffusionless transition that occurs in many steels, where the composition remains constant while a coordinated atomic displacement occurs in the parent phase via atomic jumps that are smaller than the atomic spacing. The cooperative rearrangement of the crystal structure generally takes place by the motion of a two-dimensional interface through the solid. The most famous is the transformation of austenite (a face-centered cubic structure) into martensite (a body-centered tetragonal crystal structure) that occurs in many ferrous alloys.

Martin뭩 diameter The mean chord length of the projected outline of a particle typically taken from a light microscope image. Figure M.3 illustrates the measurement for a nonspherical particle. For a sphere, Martin뭩 diameter is simply the diameter.

mass distribution The presentation of particle size data based on the mass of particles in a given size range or larger than a given size. Historically the mass (or weight) distribution came from sieve analysis or screening, where the mesh size sets the opening and after screening the mass of particles retained on each sieve is measured to determine the mass versus size. The mass distribution always is skewed toward the larger particle sizes when compared to the number or population distribution. [see particle size distribution, population distribution]

master alloy powder An alloy powder rich in desired alloying additions. The master alloy powder is added to a base, such as pure iron, to create an alloy during sintering. For example, to make a stainless steel product, one of the options is to mix iron powder with a master alloy; the master alloy additive powder is an iron-chromium alloy or iron-chromium-nickel mixture. When properly mixed with the iron, the bulk composition meets the specification for a

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stainless steel.

master sintering curve A mathematical concept that results in a graphical unification of various sintering data taken for different powder characteristics, heating cycles, peak temperatures, and hold times. Effectively, a body of sintering results, such as density, grain size, or strength, are fit to an integral work of sintering term. Various parameter fits are possible, but the structure of the curve is essentially the same. Once constructed, a master sintering curve allows for interpolation of sintered material properties under various proposed heating cycles. The concept was proposed by Professor D. Lynn Johnson at Northwestern University. Assuming no changes in sintering mechanism over the range of processing conditions (no phase transformation or melting), the integral work of sintering Θ(T,t) is represented by the following equation:

where T is the absolute temperature, t is the time, R is the gas constant which equals 8.32 J/(mol@K), and Q is the activation energy for sintering. In this form the units of Θ(T,t) are s/K, reinforcing the interplay between longer times at lower temperatures giving equivalent levels of sintering. Integration is performed from the start to end of the sintering cycle using any desired combination of heating rates, holds, peak temperatures and times, and cooling rates. Figure M.4 is a example master sintering curve for grain growth during heating and sintering of a 17-4 PH stainless steel that started with an initial grain size of 10.2 탆 , showing an activation energy Q of 390 kJ/mol. This is a unifying principle that allows intelligent assessment of takeoffs in cycles or furnaces for equivalent levels of sintering.

matrix metal The continuous phase in a polyphase alloy or mechanical mixture; the physically continuous metallic constituent in which separate grains of another constituent are embedded.

maximum induction The highest value of induction attained in a direct current hysteresis magnetization test. This value depends on the peak magnetization field strength.

maximum permeability In a magnetization test the plot of induced magnetization versus applied magnetic field is analyzed for the slope of a line extending from the origin to a tangent with the magnetization curve. Figure M.5 illustrates the determination of the maximum

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permeability for a sample initially starting in the demagnetized state.

maximum pore size The pore size detected by first gas flow in the bubble point test, corresponding to the largest pore and is the maximum continuous opening in a porous material, such as a filter, through which no larger particle will pass. [see bubble point]

mean pore separation See pore separation.

mean size The average value from the particle size distribution. For histogram particle size data, the approximate arithmetic mean size Da and geometric mean size Dg are calculated as follows:

where Di is the midpoint size for each interval, yi is the frequency of occurrence in each size interval, and N is the total number of occurrences (N is the sum of yi over all size intervals).

mechanical alloying (MA) A high-energy milling process for producing composite metallic powders with a small scale microstructure by milling mixed elemental powders for a prolonged time. Mechanical alloying is carried out in an agitated high intensity stirred ball mill that achieves the alloying by repeated cold welding, work hardening, and fracture events to progressively form composite particles. After prolonged milling, the particles are homogeneous at the atomic level. Particulate composites such as oxide-dispersionstrengthened materials, have been produced this way since the 1960s. The process was discovered by Max Quantinez at NASA as a means to fabricate thermal protection materials with high-temperature creep resistance and developed by John Benjamin for nickel-base superalloys. The process starts with a mixture of balls and elemental powders in a stirred mill. Figure M.6 shows a schematic diagram of an attritor mill, a high intensity ball mill, and the progressive homogenization of ingredients over time. Unlike other milling techniques, the balance between cold welding and fracturing keeps the particle size fairly constant. The technique is not particularly energy efficient; however, the product is a specialty composite powder. Under certain conditions, a

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metastable, nanoscale (grain size below 100 nm), or even amorphous (glass-like without crystallinity) powder is created. Contamination issues are minimized by making the balls, stirring rod, and tank from the same material as the powder. Organic fluids such as heptane or alcohol are important for balancing the milling and welding events. Hot consolidation techniques have been developed to consolidate these powders. One example alloy powder is MA 754, a nickel-base alloy containing 20 wt. % chromium, 1 wt. % iron, 0.3 wt. % aluminum, 0.5 wt. % titanium, and 0.6 wt. % yttria as the dispersoid. The technology is applied to several alloys, often with yttria or other oxides.

mechanical granulation An alternative to spray drying is to prepare a slurry with a binder and solvent, and to stir (and possibly heat) that slurry while the solvent evaporates. Other options include use of pelletizers, extruders, and even fluid beds. One approach uses rapidly spinning blades in a horizontal mixer to generate mechanical fluidization. A mist of polymer and solvent solution is then sprayed onto the moving powder to create agglomerates in the form of polymer-bonded balls as the solvent evaporates.

mechanical press A powder compaction press where motions of the tool set and punches are derived from a flywheel or rotating crank, such that when engaged a powder compact is formed on each flywheel rotation. Mechanical presses are driven by an electric motor. Accordingly, common nomenclature relates the pressing stage to the rotation angle, recognizing that 360 degrees represents a full rotation. To function the press, cams and levers take power from the flywheel. Mechanical presses are best at controlling green dimensions. There are several variants to the mechanical pressing concept, including rotary presses, anvil presses, and hybrids. Rotary presses follow the same tool motions, but use a spinning table with several dies that undergo the compaction cycle on each table rotation. When equipped with 50 tool sets, rotary presses can form 8,000 compacts per minute. Anvil presses form simpler shapes, but provide faster pressing rates since they use a flat plate to cover the die, avoiding an upper punch. For small mechanical presses, electric motors are used to directly move the tool motions. Hybrids exist that might add a pneumatic pressure buffer onto a punch to control the peak pressure: the core technology is a mechanical press, but the upper punch has a pressure cylinder for controlling the final densification. Other designs rely on computer feedback systems to ensure uniform products.

mechanical properties A large group of material properties related to the design of structural, load-bearing components. These properties are measured by applying forces while recording the material response. The main properties are generated via tensile testing and include the elastic modulus, yield strength, ultimate tensile strength, elongation to fracture, and

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possibly the reduction in area. Other properties included in this category are the impact toughness, fracture toughness, fatigue endurance limit, and Poisson뭩 ratio. [see elastic modulus, Poisson뭩 ratio, ultimate tensile strength, ductility, elongation to fracture, fracture toughness, impact energy, fatigue strength]

median size The centroid of the particle size distribution, where half of the particles are larger and half are smaller; it is identified as the 50 % value on the cumulative particle size distribution. It is also known as the D50 particle size. Figure M.7 shows how the median particle size is extracted from the cumulative particle size distribution as the size corresponding to 50 % of the powder being smaller.

melt explosion atomization A means to form a metal powder based on saturating the melt with a gas that is used to internally disintegrate the melt when it is exposed to a vacuum. One variant is diagramed in Figure M.8 based on hydrogen saturation of the liquid metal. Prior to atomization the melt is pressurized with a few atmospheres of hydrogen or nitrogen. A siphon tube exhausts the saturated melt into a large vacuum chamber. When that melt exits into the vacuum it forms a droplet spray; the melt literally explodes into the vacuum chamber. The technique is used for superalloy powders, although variants using hydrided titanium and other metals have been demonstrated. Because solidification is in a vacuum, with no convective heat transfer, the slow cooled particles tend to be dendritic.

melt index A measure of how fast hot feedstock will flow at low shear strain rates based on a capillary tube test. Molten feedstock is extruded through a capillary tube using a dead load. The melt index depends on the capillary tube diameter and applied load, but is always reported as the grams of feedstock collected from the tube in 10 minutes.

melt spinning A rapid solidification technique based on a large, spinning copper disk that is cold and a thin stream of melt injected onto the edge of the copper disk. Amorphous and rapid solidification products are generated by the combination of rapid heat extraction and centrifugal force. Usually the product is a ribbon that is milled into powder, but if the copper disk is textured then different powder shapes are possible. Figure M.9 shows a sketch of the standard technique designed to produce ribbons.

mercury porosimetry A technique for measuring the pore size distribution of open pores using high pressure intrusion of mercury as the test fluid.

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Section M [see porosimetry]

mesh The screen number representing the number of wires per linear inch in a square grid of woven or electroformed openings. The higher the mesh number, the smaller the opening size. Screening is a process for particle size analysis and powder classification that requires separation of the powder into fractions based on progressively smaller mesh openings. Figure M.10 is a picture of a standard woven wire mesh. The convention for specifying mesh size relies on the number of wires per inch. For example, 200 mesh implies 200 wires per linear inch. This mesh gives 127 탆 spacing between wire centers, but the wires are 52 탆 in diameter; thus, the remaining opening size is 75 탆 . Mesh sizes cannot go to very small opening sizes. Consequently, sieve analysis is usually applied to particles larger than about 38 탆 (400 mesh). There are electroformed meshes available down to 5 탆 , but particle agglomeration and particle adhesion to the mesh generally make the smaller electroformed screens of no practical use. The opening sizes for the standard screen series is given in Table M.1. That screen series has openings nominally spaced at a ratio of the fourth-root of two (a size ratio of 1.19, corresponding to a mass ratio of 1.7 between screen openings). [see screen analysis] Table M.1. Standard Sieve Sizes mesh size opening, 탆 18 1000 20 850 25 710 30 600 35 500 40 425 45 355 50 300 60 250 70 212 80 180

mesh size 100 120 140 170 200 230 270 325 400 450 500

opening, 탆 150 125 106 90 75 63 53 45 38 32 25

mesh belt furnace A continuous furnace that relies on a belt as the conveyor mechanism. The most common belts are formed from woven stainless steel wires to form a mesh that is pulled through the high heat zone. The powder compacts rest on the belt or on trays that rest on the belt. Belt widths range from 15 cm to 60 cm and belt velocities range up to 20 cm/min. Belt speed and hot zone length determine the time at temperature; typical combinations give times at the peak temperature between 10 and 40 min. Prior to the

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high temperature region, preheat zones burnout the polymers and bring the work up to the sintering temperature. The last and largest section of the furnace is the cooling zone. Depending on the belt alloy and the peak temperature, the maximum load can range up to about 50 kg per m2 and typical production rates can range up to about 300 kg/h. Lower loads are necessary at higher temperatures to extend the belt life. [see belt furnace]

metal injection molding (MIM) A shortened phrase for metal powder injection molding. [see powder injection molding]

metal matrix composite (MMC) A mixture of two phases, with the metallic phase being the continuous phase with a dispersed reinforcing phase, typically an oxide or carbide or other hard material. Some typical metal matrix composites formed by powder metallurgy are aluminum with whiskers or particles of silicon carbide, boron, or graphite, nickel aluminide with silicon carbide or alumina, and titanium with titanium carbide or titanium diboride. Aluminum reinforced with carbon fibers offers a very high specific elastic modulus (elastic modulus divided by density). However, aluminum reinforced with silicon carbide is more common, giving tensile strengths of 530 MPa with 55 vol. % SiC, but almost no ductility.

metal powder Discrete solid particles of elemental metals (copper, iron, nickel, aluminum, chromium, cobalt, tungsten, and such) or alloys (bronze, monel, stainless steel, tool steel, or sterling silver) that are nominally below 1 mm or 1000 탆 in maximum size.

Metal Powder Industries Federation (MPIF) A trade association of powder producers, parts fabricators, and other companies involved in promoting the use of metal powders and their products, largely for the North American market.

metal powder injection molding Metal powder injection molding, where the inorganic phase suspended in the polymeric binder is predominantly a metal or alloy powder. Common engineering alloys are possible by mixing elemental powders and forming the alloy during sintering (homogenization) or by use of a prealloyed powder where each particle contains all of the elements. MIM is a subset of powder injection molding (PIM). Ferrous alloys are popular by PIM, and nearly half of the commercial activity is in stainless steels for orthodontics, computers, surgical instruments, electronic fixtures, decorative components (cellular telephones, computer logos), and automotive engine components. [see powder injection molding]

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metallic glass A noncrystalline alloy commonly produced by drastic supercooling of a molten alloy, molecular vapor deposition, electrochemical, or vapor deposition, or even by intensive attritor milling. One composition used for the fabrication of golf clubs was based on Zr23Be-13Ti-13Cu-10Ni. This alloy can be cooled at rates faster than 3EC/s to form an amorphous metal.

metallography The study of the structure of a material as observed via optical or electron microscopy. In the context of materials engineering, microstructure dictates properties. Accordingly, analysis of the microstructure via metallographic analysis underpins any efforts to understand the behavior of a powder metallurgy product. Polished and etched crosssections of the sintered metal are viewed to obtain information on the grain and pore structures, defects, and phases, and spacial relations between features. Quantitative measurements of the feature sizes are made from the two-dimensional images. These measures range from simple average sizes or amounts, to detailed distribution information such as the pore size distribution, grain size distribution, or curvature distribution. There is no standard procedure for preparing the microstructure, but handbooks suggest some of the more successful polishing and etching techniques.

metastable A transient stage that is often stable for long periods of time. Diamond is the most famous metastable material, since thermodynamically it is unstable at atmospheric pressure, yet the slow kinetics of transformation inhibits the transition to graphite.

metering In injection molding this refers to the controlled forward extrusion of molten feedstock past the screw tip and check ring to ensure the proper shot volume is ready for the next mold filling event. Metering occurs while the previous shot is cooling in the mold. Proper metering ensures the feedstock is homogeneous, at the proper temperature, and the proper volume to completely fill the mold on the next shot.

methylcellulose A synthetic gum that is effective as a binder for powders in die compaction, extrusion, and injection molding. It also is used to thicken solutions to reduce powder settling. The microcrystalline form is higher in purity and better suited to powder processing applications. The general use of methylcellulose in powder metallurgy is on the decline since the polymer exhibits incomplete burnout and contaminates furnaces. Alternative, clean burning polymers are replacing methylcellulose, including low molecular weight polyethylene, polypropylene, and custom waxes.

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microhardness A test of hardness as determined by forcing an indenter, such as a diamond pyramid, into the surface of a material under a controlled load. Usually the indentations are small and the impression requires a microscope for making the measurement. The harder the metal, the smaller the indent. Because of the small indent size, microhardness tests are very useful for determining the hardness of individual phases in a heterogeneous microstructure. The most common test gives the Vickers hardness number. If the material is porous, efforts should be made to avoid porous regions to measure the material strength. Microhardness traces have been used to map green density, by correlating hardness to density, and this proves one of the few accurate techniques for determining green density gradients from compaction.

micrometer (탆 ) A common size measure for powders and microstructure features. Formally, a micro (? is one in a million and meter is a fundamental unit of length that is near 39 inches or 1000 millimeter. The abbreviation for a micrometer is 탆 which indicates one millionth of a meter, or about 0.00004 inches, which is about 4000-times larger than an atom.

microminiature Component dimensions measured in the micrometer size range, requiring microscopes for evaluation. A very high value and high growth area for powder metallurgy, with applications in microelectronics (for example capacitors), minimally invasive medical devices (surgical manipulators), mechanical devices (microminiature bearings), diagnostics (chemical and blood analysis), and various industrial (wire bonding tools) and automotive areas (crash sensors for airbags). [see micromolding]

micromolding A new class of technologies geared to the production of components in the millimeter and micrometer size range. Opportunities in small components come from shrinking microelectronic and biomedical systems and the ability to fabricate tooling with dimensions on the order of micrometers. Most of the ideas are extensions of powder injection molding that rely on new plastic micromolding machines, silicon lithography tooling, and powders sized below 0.1 탆 . The small powders are needed to form the small objects, while new molders are needed to control die cavity filling. Tooling is fabricated using ultraviolet lithography techniques developed for semiconductor fabrication. Part handling is automated and built into the molding machines. Debinding and sintering are rapid because of the small dimensions, so the barriers are largely from handling the small components and fabrication of the molds. Many uses are envisioned for micromolded metals, including actuators, sensors, portable consumer products, military projectiles, electronic assembly tools, oxygen analyzers, filters, and health care

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instruments.

micropore A term applied to catalytic materials where the pore diameter is smaller than 2 nm

microscopy The observation of microstructure via optical or electron techniques. [see metallography, quantitative microscopy]

microscopy particle size analysis A universal way to measure particle size is to capture an image of dispersed particles which is subsequently quantified. A simple approach is to overlay circles of various sizes on the particle images to match the circle area to that of the particle, counting the number times each circle encounters a particle. This is now best performed via automated image analysis systems that couple computers directly to the microscopes. The image for analysis is generated by optical reflected light, transmitted light, scanning electron, or transmission electron microscopes. The instrument choice depends on the particle size, so microscopy can be applied to particles from a few nm to mm in size; however, the larger depth of field in the scanning electron microscopy is a distinct advantage. Microscopy particle size analysis produces a population or number distribution showing the relative frequency of each size. This is not directly comparable to the size distribution measured using sieving, which generates a mass distribution.

microstructure Information or features in a material that are resolved by examinations using a microscope, especially the spatial arrangement of the phases including pores and boundaries between phases. Microstructure analysis might be involved in capturing detailed information on the phases, pores, grains, defects, heterogeneities, and other property controlling features, such as the percentage of each, their size and spacing, and the proximity of one feature to another. [see grain size, grain size distribution, porosity, pore size, pore shape, pore separation]

microwave heating (sintering) The use of electromagnetic energy to heat a powder compact where the frequency is in the microwave range, generally at the same frequency as a home microwave (which is 2.45 GHz). Microwave heat is only absorbed at the surface of a dense metal, but surprisingly metal powders are excellent absorbers and can be heated to the sintering temperature in a few minutes. Often the component is packed in a microwave absorbing ceramic, such as silicon carbide, to reduce radiant cooling and to improve the coupling efficiency. Microwaves are most commonly used at lower temperatures to improve normally slow heating rates. At high temperatures there is difficulty with temperature

Section M

Page 15

control, requiring a sophisticated system, that often is more economically derived using conventional resistance heating. The depth of microwave penetration X varies with the inverse square-root of the microwave frequency ν,

where σ is the conductivity and μ is the magnetic permittivity. The number of modes in the microwave cavity determines the uniformity of heating. Most efforts rely on multiple mode cavities to avoid hot spots, meaning the cavity size is large compared to the wavelength of the electromagnetic field. Even so, the sample size and cavity size must be matched to the source frequency to ensure uniform heating, since the powder metal component shifts the field. Accordingly large components exhibit uneven heating due to nonuniform microwaves and because internal heating depends on conduction from the surface. As heating occurs, there is an adsorption shift that affects heating, leading to a potential runaway event. Thermal gradients up to 20EC/mm can be induced in the component, leading to cracking in some cases, especially during intense heating. Although improved sintering has been claimed in microwave heating, it is unclear if there is any new or different sintering behavior. One thought is that the microwaves activate grain boundary phases and enhance grain boundary diffusion, especially since the apparent activation energy for sintering is lower in microwave heating. Another thought is that the large thermal gradients from microwave heating induce vacancy concentration gradients inside the compact that drive more rapid diffusion fluxes.

Mie scattering Small particles dispersed in a fluid will diffuse a portion of the incident light in all directions. Mie scattering theory applies to electromagnetic radiation interacting with particles that are approximately of the same size as the wavelength of the radiation. Particle size analysis based on Mie scattering relies on determining the scattering angle and intensity for particles with sizes on the order of the wavelength of the incident light. The mathematical solution for the scattered light intensity and angle as a function of the particle size and refractive index was solved by Gustave Mie for spheres, so automated particle size analysis based on this model is termed Mie scattering. Accurate size determination requires precise knowledge of the refractive index real and imaginary components. Most typically the Mie and Fraunhofer techniques are combined into a single optical system to provide analysis capabilities over a wide particle size range.

milling Mechanical agitation used to break particles or agglomerates into smaller particles. Commonly performed in a ball or jar mill, but might be achieved in other high intensity

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Page 16

mechanical devices. The particle size is reduced by milling and sometimes the particle shape is rounded for a ductile metal used to produce flakes. Milling also refers to a surface contouring or cutting process used to adjust final dimensions on sintered components. It is especially useful for the generation of flat surfaces. Milling of a component after sintering relies on a rotating tool with multiple tips and stationary work or reciprocating work to generate a flat surface

MIM The abbreviation for metal powder injection molding. [see metal powder injection molding, powder injection molding]

minus sieve The portion of a powder sample which passes through a sieve of a specified mesh. The standard designation is to place a minus sign in front of the mesh size to indicate the powder passed through that mesh; for example -325 mesh indicates all of the particles passed through a screen with 45 탆 openings. This might also be termed minus mesh.

mixed powder sintering Sintering where two powders of differing chemistry are driven toward homogenization using the compositional gradients in the mixed powders to drive early sintering response. Often elemental powders are mixed to form an alloy before compaction and sintering, largely to improve compaction since alloys are harder and more resistant to compaction. Mixed elemental powders are softer and more easily compressed. For example iron is often mixed with graphite, phosphorus, nickel, copper, or molybdenum. The initial microstructure is inhomogeneous. At one location the microstructure is pure iron, while in a neighboring spot it might be pure nickel or another alloying ingredient particle. Diffusion between particles is required to form a homogeneous alloy, so sintering must provide sufficient temperature and time to homogenize the mixture. Thus, the sintering cycle plays a dual role in forming an alloy via homogenization while bonding particles. Usually the homogenization process is slow compared with bonding. This dictates higher temperatures or longer times to achieve uniform microstructures and properties. Mixed powders are easier to compact as compared with alloyed powders, and usually they are lower in cost as compared with prealloyed powders. However, high temperatures are needed to fully homogenize the powders.

mixed powders Two or more materials combined via mechanical agitation to form a new chemistry intermediate between that of the ingredients. In powder metallurgy, the elemental powders such as iron, copper, and nickel are much softer and easier to press when compared to alloy powders such as steel. Thus, mixed powders with different chemical compositions are combined to form an alloy, but homogenization is delayed until the

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sintering step.

mixer A device for combining powders or powders and fluids into a homogeneous state. Mixing is achieved in a mechanical device that tumbles, shifts, shakes, stirs, shears, and diffuses the two or more materials in a container. A few simple designs are illustrated in Figure M.11. Each revolves around some central axis and in many cases the mixer might also have a central intensifier bar with blades that spins with a high velocity to disrupt the particles during a tumbling motion. Most mixers split and recombine the powders to create new interfaces and to remove segregation and stratification. Unfortunately, these same devices also can induce segregation of powders based on differences in particle size, particle shape, or density. Thus, mixing reaches an asymptotic level of homogeneity that is not perfectly homogeneous. [see double-cone mixer, twin shell mixer, twin screw mixer]

mixing The thorough intermingling of powders of two or more different compositions. Mixing is sometimes confused with blending, which is much the same technology but for blending the particles are the same composition, but differ in particle size or production lot. In mixing the powders being combined are different in composition.

mixture homogeneity A measure of the compositional fluctuation from point-to-point in a powder mixture based on tests such as apparent density, heat capacity, electrical conductivity, viscosity, or color. Homogeneity is determined by statistical analysis of many samples taken from widely spaced areas. It is quantified using the variance in powder concentration between samples S2, the variance anticipated for perfectly mixed but random samples Sr2, and the variance for the initial segregated mixture So2:

The mixture homogeneity M varies from 0 to 1, with unity representing an ideal mixture. Based on several repeat samples, the mixture variance is calculated. The precision varies with the square root of the number of samples taken to compute the variance. The powder-binder mixture starts as a totally segregated system with an initial variance given as follows:

where Xp is the concentration of the major powder component. The final variance for a fully-mixed, randomly sampled system should approach zero, or Sr2 = 0 in the ideal.

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Page 18

This gives a simplified form of M = 1 - S2/So2 for the general case. Usually the homogeneity improves rapidly with initial mixing, but over time a steady-state homogeneity is attained that is less than unity; the maximum mixture homogeneity occurs when the rate of mixing equals the rate of segregation.

MMC The abbreviation for metal matrix composite.

Mo The chemical symbol for molybdenum.

mobile binder In powder shaping processes that rely on a thermoplastic binder, part of the binder is considered to be a pore filler that does not lubricate powder sliding or flow, but the excess binder over that at the critical solids loading provides lubrication and is termed the mobile binder. Figure M.12 shows an exaggerated dilation of the powder and immobile binder to illustrate how the mobile binder provides a lubricating layer. [see critical solids loading]

mode size The most common particle size. The mode corresponds to the size at the peak in the frequency or interval histogram particle size distribution. It often differs from the mean size for skewed distributions and tends to be closer to the median size for many powders.

modulus of elasticity This is the same as Young뭩 modulus and most commonly known as the elastic modulus. [see elastic modulus]

modulus of rupture A phrase for the transverse rupture test when it is used by the ceramics community to measure the strength of brittle ceramics. It is a misnomer, since the parameter is a strength, not a modulus, and is properly termed a transverse rupture strength. [see transverse rupture strength]

Mohs hardness A scratch test used to derive a relative hardness ranking as commonly performed in the field by geologist or mineralogist. There are ten reference minerals, and the Mohs hardness is based on which materials are soft enough to be scratched - diamond is the

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top at 10, conundum is next at 9, topaz is 8, quartz is 7, feldspar is 6, apatite is 5, fluorite is 4, calcite is 3, gypsum is 2, and talc is 1.

mold A tool geometry that provides a negative of the desired powder component shape. Usually in uniaxial die compaction this cavity is called a die and in powder injection molding it is termed a mold, but they are similar in function.

mold flow simulation The use of computer simulations for analysis of the flow, packing, venting, sizing, cooling, and other events and tool or machine parameters. These are three-dimensional simulations performed for a specific tool, feedstock, and molding cycle. The output parameters range from flow visualization, showing the progressive flow from the gate to the vent, as well as shear rates, velocity contours, stresses, temperatures, and other data required to minimize defects or processing difficulties. Figure M.13 is an example of the filling sequence for a surgical hand tool geometry.

mold release A spray or coating that reduces component sticking to the die cavity, aiding ejection without defects. They are often prepared as suspensions of lubricants, such as Teflon or zinc stearate, in a solvent and delivered using a spray.

moldability A relative measure of the ease of filling out a tool cavity during injection molding. It can be determined by the length of filling for a long, narrow passage. In plastics, the classic test is a flow spiral, while in metal powder injection molding a zig-zag test is preferred to add a component of powder-binder separation. [see zig-zag test]

molding pressure The pressure applied to the feedstock to compress the molten material in the cavity prior to cooling.

mole (mol) A measure of the amount of substance, equal to 6.02A1023 atoms or molecules. The definition is based on 12 g (0.012 kg) of the carbon-12 isotope (graphite has an atomic mass of 12.011 g per mole since not all of the atoms have 6 neutrons, some have 8 neutrons giving carbon-14 which is used for radioactive dating). When the term mole is used, the elementary entities must be specified as atoms, ions, molecules, or other complexes.

Page 20

Section M

molybdenum (Mo) Element 42 on the periodic chart. Molybdenum, is a body-centered cubic crystal structure that is generated via powder metallurgy techniques. The powders are formed by reduction of the oxide and those powders are consolidated to form mill shapes or die pressed into compacts. Molybdenum is widely used in furnace hardware and microelectronics. The former due to its combination of lower cost and higher melting temperature; at 1000EC molybdenum has a tensile strength over 100 MPa. Molybdenum뭩 use in microelectronics is largely due to cost, low thermal expansion coefficient, and high thermal conductivity. Indeed one of the highest production rates in powder metallurgy is in the production of 100 million heat sinks fabricated out of molybdenum every day. Laminates exist of molybdenum and copper for anisotropic heat sinks for high performance electronics. Also, molybdenum-copper powders are sintered to form composites for heat dissipation in microelectronics. One of the favorite high temperature molybdenum alloys is TZM (titanium zirconium molybdenum) and newer versions are alloyed with oxide dispersions such as lanthanum oxide. In the fulldensity condition sintered molybdenum has the following properties: molybdenum

Mo

atomic number

42

atomic weight

95.94

g/mol

density

10.22

g/cm3

melting temperature

2610

EC

boiling temperature

4612

EC

heat of fusion

28

kJ/mol

heat capacity

251

J/(kg EC)

thermal expansion coefficient

5.4

ppm/EC

thermal conductivity

138

W/(m EC)

electrical resistivity

5.2

µΩ-cm

elastic modulus

328

GPa

Poisson뭩 ratio

0.293

hardness

200

VHN

as-sintered yield strength

225

MPa

3

%

as-sintered elongation to fracture

Page 21

Section M

as-sintered ultimate tensile strength 300 MPa If molybdenum is cold worked, then it can have a room temperature strength near 620 MPa. If fabricated as a thin diameter wire, then the tensile strength reaches 2100 MPa. At 1095EC molybdenum has a stress rupture life of 100 h at a stress of 85 MPa.

molybdenum disilicide An intermetallic compound with the stoichiometry of MoSi2 that is used to form sintering furnace heating elements for operation in air. It has a theoretical density of 6.26 g/cm3, elastic modulus of 160 GPa, and room temperature strength of 450 MPa. Depending on the alloying and ceramic phases, the material can operate up to 1600EC or higher, and reportedly the compound will not decompose or melt up to 3700EC. Most commercial grades are not pure stoichiometric compounds, but might include oxygen, iron, carbon, and other intentional and inadvertent ingredients.

monel A nickel-copper powder metallurgy alloy used for corrosion applications. It is used in filtration situations. Various compositions are in use with roughly 40 wt. % copper, and small contents of iron and manganese. For example, at full density an alloy of nickel with 32 wt. % copper, 1.4 wt. % iron, and 0.9 wt. % manganese has the following properties: monel (Ni-32Cu-1.4Fe-0.9Mn) density

8.84

g/cm3

melting temperature

1300

EC

heat capacity

430

J/(kg EC)

thermal expansion coefficient

14

ppm/EC

thermal conductivity

23

W/(m EC)

electrical resistivity

55

µΩ-cm

elastic modulus

179

GPa

Poisson뭩 ratio

0.36

hardness

137

VHN

as-sintered yield strength

240

MPa

as-sintered elongation to fracture

40

%

as-sintered ultimate tensile strength

520

MPa

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It retains some strength up to 600EC. When fabricated as a filter, the pores significantly lower these properties. The Curie temperature (roughly 20EC) is too low for use in magnets.

monodisperse A powder system where the dispersion in particle size is quite small; all of the particles have essentially the same size, forming a narrow size distribution. Formally, a particle size distribution where 90 % of the particles are within a tolerance band of ? 5 % of the median particle size. It is the same idea as monosize powder.

monolayer A single, closely packed layer of atoms of molecules covering a surface, such as the outside surface of a powder during gas absorption surface area analysis.

monosize A powder sample where all of the particles have very nearly the same size. Mathematically this occurs when the D90 and D10 on the cumulative particle size distribution are close. By convention, assuming a log-normal particle size distribution, monosized powders correspond to 90 % of the particles being within ? 5 % of the median size; thus, D90/D10 is less than 1.08 and SW is over about 80. [see log-normal, size distribution width]

Monte Carlo simulation A computer technique designed to simulate time-dependent microstructure evolution by allowing for many random atomic events to occur with a set of governing rules to energy minimization. The selection of an atom to move is done randomly and the decision on the success of motion and the change in configurational energy is based on simple rules, usually pinned by a goal to reduce energy. The output provides a sense on the types of microstructures expected to form. It is used for sintering simulations, grain growth, and pore-boundary interaction studies, but most of the applications are not realistic since they are restrained to the simple two-dimensional problem, which cannot be directly scaled to the reality of three-dimensional problems. The intent is to identify possible outcomes from random, iterative process.

MPIF The abbreviation for the Metal Powder Industries Federation.

muffle furnace A sintering furnace for continuous sintering that relies on a ceramic or metallic lining on the inside of the furnace and a gas-tight outer jacket. The muffle supports the heating elements and provides containment for the process atmosphere to avoid intersections

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between the heating elements and atmosphere. Additionally, the muffle provides support to the trays or racks or belts that convey the sintered compacts. Transport of the trays or racks through the muffle might be via a mesh belt or pusher mechanism.

mullite A ceramic compound of alumina and silica, with a stoichiometry of 3Al2O3-2SiO2, that provides a lower-temperature substrate for sintering trays and furnace hardware. It can be stabilized up to 1800EC, but often decomposes at lower temperatures. The theoretical density is 3.17 g/cm3 and room temperature strength is 270 MPa. Use in sintering furniture is favored by the 6.2 ppm/EC thermal expansion coefficient.

multimodal A powder size distribution which exhibits several modes, possibly generated by blending several monosized powders. These can be as simple as trimodal, which uses three different powders and in practice have ranged up to mixtures of five different powders. Multimodal mixtures are not useful with respect to packing density unless the powders are significantly different in size. With a mixture of 10-fold differing particle sizes (11 % small, 22.5 % intermediate size, and 66.3 % large) the fractional packing density approaches 90 % of theoretical.

multiple cavity mold Powder injection molding often relies on multiple cavities in the mold to obtain more parts per molding machine per unit time. Figure M.14 is an example of a central cluster four-cavity design. Up to 40-cavity molds have been used in cemented carbide production, and trials have reached 320-cavity molds for tungsten alloys. Multiple cavity molding presents some control problems; hence, the balance between the molding problems, tooling cost, and machine productivity often results in a compromise of about four-cavity molds.

multiple level part A level in a press-sinter powder metallurgy part is a step in the pressing direction that requires a separate tool motion and segmentation of the punches. Multiple level parts require differences in punch positioning and travel during compaction to ensure uniform density. The more levels that exist in the component, then the more complicated the tooling and the more sophisticated the press and its control system.

multiple mechanism sintering Mass transport from several sources during sintering, leading to accelerated and cooperative bonding and densification. The models assume the instantaneous mass flux arriving at the bond between the particles is the linear sum of the contributions from each mechanism at any instant,

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where X is the neck size, t is the time, and dX/dt is the rate of neck growth. This concept assumes that each of the individual mechanisms, represented by the subscript i, provides an instantaneous contribution to the total neck growth. Accordingly, computer simulations are employed to continuously calculate the contributions from each transport mechanism at a given time. Once the rates of mass flow are determined, the net rate is used to enlarge the neck by determining its new volume and shape for the next instant of time.

multiple platen press A means in die compaction to form complicated shapes by filing and transferring the powder prior to pressurization via selective motion of tool parts to locate the powder properly (after fill) using independent and often complicated tool motions as now possible in computer controlled and other advanced presses. The moveable platens support various tool components to reposition the powder and to press the powder after it has been filled into the die cavity.

multiple pressing A method of compacting a powder whereby two or more compacts are formed simultaneously. In most cases this is achieved by aligning parallel die cavities and aligning multiple punches to press in each cavity at the same time; however, in hot pressing it is common to segment the punch and to form a stack of powder compacts with a single top and bottom punch.

Section N

Page 1

A to Z of Powder Metallurgy

N Nabarro-Herring creep Creep is a time dependent plastic deformation that occurs below the yield strength of a material due to the combined effects of temperature induced atomic diffusion and stress. When creep occurs by vacancy diffusion through the crystal lattice, it is termed Nabarro-Herring creep. It is one means for powder compacts to densify in processes such as hot pressing and hot isostatic pressing. A high temperature induces atomic motion via atom-vacancy exchanges while the stress creates a bias for vacancy migration away from pores to annihilation at grain boundaries, via atomic diffusion. The result is progressive elimination of pores to eventually form a dense compact. Fundamentally, Nabarro-Herring creep occurs by atomic motion from points in the porous microstructure under compressive stress to points under tensile stress. The volume diffusion controlled shrinkage rate - the change in length, normalized to the original length, over the change in time - is given as follows:

where T is the absolute temperature, k is Boltzmann's constant, Ω is the atomic volume, Dv is the lattice or volume diffusivity, G is the grain size, and PE is the effective pressure. As long as there is porosity, densification is directly proportional to the creep-strain rate. Diffusivity is very sensitive to temperature; thus, it provides an important controlling influence on densification. [see volume diffusion, creep, Coble creep, Arrhenius relation]

nanometer (nm) Formally the word nano means 10-9 and a meter is a fundamental length unit, so a nanometer is 10-9 m or 10 Angstrom. It corresponds to 0.001 탆 . Like micrometer (10-6 m), it is widely employed to indicate very small items. Often the prefix nano is employed to indicate a small scale powder, composite, phase, or other feature. Unfortunately, it is so overused that everything small is now termed nanoscale - nanosecond, nanocomposite, nanowhisker, and nanofabrication as examples.

nanoscale Powders or microstructures with sizes that are measured in nanometers. Typically the

Section N

Page 2

powders are less than 100 nm in size, or less than 0.1 탆 . Several natural materials, such as clay, form at this size scale, and metal powders have been available since the 1960s in the 20 nm to 50 nm size range. However, handling, contamination, and preservation of the nanoscale microstructures during consolidation is a significant barrier. Various mechanical properties, such as hardness and strength, scale with grain size, so there is much speculation on property gains with a consolidated nanoscale powder; however, some properties are degraded, such as electrical and thermal conductivity. Data from a few materials show the property gains with decreasing crystal size eventually breaks down and that some of the promises of the nanoscale domain are not realistic below say 10 nm.

Nb The chemical symbol for niobium.

NDE The abbreviation for nondestructive evaluation.

NDT The abbreviation for nondestructive testing.

near-net shape A compact that has the general shape of the final product, but is oversized and requires machining to reach the final specified size. This contrasts with net-shaping, which generates the final size directly in the powder consolidation step to avoid final machining. For example, most hot isostatically pressed compacts are fabricated to a near-net shape.

neck The bond between contacting particles developed during sintering. Figure N.1 is an example of a neck formed by sintering spherical particles. The bond size is a measure of the degree of sintering. Necks form during sintering and represent saddle surfaces with both a positive and negative curvature, which drives mass transport and the further growth of the neck at high temperatures.

neck curvature stress The sintering stress associated with the saddle point geometry of the sintering neck between two particles. At the sintering neck the saddle surface shape gives a mixture of convex and concave curvatures. For equal sized spheres, the stress depends on the neck size X, particle size D, and surface energy γ (assumed isotropic) as follows:

Section N

Page 3

This is also called the sintering stress. For example, if the material has a surface energy of 1 J/m2 and the powder is 10 탆 in diameter with a neck of 2 탆 , then the stress is compressive at 9 MPa (1300 psi). Most important, the particle contacts are under compression without an external applied pressure. Thus, there is a natural compression from the neck curvature stress during sintering.

neck growth The bond between contacting particles grows progressively over time at high temperatures and the growth rate provides a means to identify the sintering mechanism. Neck growth scales with time, temperature, and particle size, especially temperature since the rate of atomic motion accelerates at higher temperatures. During sintering the bond enlarges until the neck size X encounters the limit dictated by the dihedral angle φ and particle or grain size G, where

As a consequence, once sintering achieves a bond size ratio X/G as defined by the equilibrium dihedral angle, then any further neck growth depends on the rate of grain growth. Early in sintering the grain size is smaller than the particle size, but as the sinter bond grows there is simultaneous grain growth to the point where the grain size exceeds the initial particle size. The above equation relies on the later of those two microstructure parameters. [see neck size ratio, dihedral angle]

neck size Formally the size of the sinter bond between contacting particles. The neck size X is measured as the diameter of the bond at the saddle point in Figure N.2. Most typically the ratio of neck to particle size X/D is used in sintering theory. Most properties of a sintered body relate to the neck size ratio.

neck size ratio The neck size X divided by the particle size D is known as the neck size ratio. This dimensionless parameter, X/D, provides a monitor for the degree of sintering. Many properties of the sintered compact scale with the neck size ratio, including strength, electrical conductivity, and thermal conductivity. After initial bond growth, a stabilized geometry arises as determined by the dihedral angle. For example, after exposure to a liquid, a stabilized solid뻦iquid structure emerges based on the two-grain configuration

Section N

Page 4

shown in Figure N.3; note the saddle point associated with initial sintering is now lost as the neck size is determined by the dihedral angle. That equilibrium condition halts neck growth to the point where the neck size X depends on the grain size G and dihedral angle φ,

Since grain growth continues during sintering, usually with G - t1/3 then the rate of neck growth will eventually converge to X - t1/3. Like the contact angle, the dihedral angle φ is determined by the surface energy balance. For solid-liquid systems encountered in liquid phase sintering the equilibrium is given as follows:

with γSS being the solid-solid interfacial energy (grain-boundary energy) and γSL being the solid-liquid interfacial energy. An analogous version applies to the situation where the grain surface is in contact with a vapor phase:

with γSV being the solid-vapor surface energy.

necking A ductile metal will show exaggerated cross-sectional area reduction during a tensile test and the peak engineering stress will decline once strain becomes localized. The onset of localized deformation occurs at the ultimate tensile strength and necking is a term used to describe the locally reduced cross-sectional area.

necklace microstructure A two-phase morphology that forms when a transient reactive liquid forms and wets grain boundaries, the wetting is induced by solid dissolution into the newly formed liquid. On cooling, the liquid films on the grain boundaries retract and solidify to form discrete lens-shaped islands along the grain boundaries. In an optical micrograph the grain boundary looks like a string with attached stones (the lenticular phase sitting on the grain boundary), which appears as a necklace as illustrated in Figure N.4. This microstructure is found mostly in reactive systems after sintering (such as titanium doped iron). It can also be induced in a two-phase system where solubility changes with temperature, by cyclic temperature changes to dissolve and then precipitate the second phase on grain boundaries (such as in tungsten heavy alloys).

needles

Section N

Page 5

Elongated, rod-like particles.

neodymium iron boron magnets A group of amorphous metal magnets based on the general Nd2Fe14B composition. They are produced by rapid solidification. The magnetization-demagnetization is dependent on the domain wall mobility and microstructure. Two processing techniques are applied to this compound - one involves magnetic alignment in compaction and sintering (anisotropic) while the second (isotropic) approach involves plastic forming techniques (injection molding, compression molding) with a final heat treatment. The remanence magnetization is on the order of 1.16 T and the coercivity is 848,000 A/m, but the Curie temperature is a low 310EC.

net shape A compact manufactured to final density and dimensions without the need for machining. Although very desirable, in reality most components require some straightening, flattening, machining, or polishing after sintering since contemporary tolerances are often hard to hold in mass production. Also, certain features are difficult to build into the forming tooling, and are best added after sintering - such as undercuts and threaded holes.

neutral zone The region at the approximate center of a compact where the particle motions from the upper and lower punches are balanced; the motion from upper punch and lower punch (or die float) gives near zero relative motion at the neutral zone. In computer numerically controlled compaction it is possible to insert features perpendicular to the compression axis by controlling the tool forces to ensure the neutral zone is located on the perpendicular tool components.

Newton (N) A fundamental unit of force, equal to 1 kg at an acceleration of 1 m/s2. A mass of 1 kg on earth then corresponds to a force of 9.8 N (acceleration of gravity is 9.8 m/s2).

Newtonian cooling approximation Applied to powder atomization and the cooling rate of liquid droplets during free-fall, this approximation says the cooling rate dT/dt can be estimated as follows:

where T is the temperature of the particle, A is its surface area, To is the ambient temperature, and κ represents the rate of heat transfer per unit area. During atomization the particle temperature drops quickly at first, while T - To is large, but the cooling rate

Section N

Page 6

decreases as the particle temperature approaches ambient temperature. This approximation fails to include the liquid to solid phase transition and the associated enthalpy and heat capacity terms needed to accurately predict the cooling path. It is better applied to situations where there is no phase change on cooling.

Newtonian flow An idealized viscosity situation applicable to fluids that are insensitive to the shear strain rate, although they will be sensitive to temperature. In the Newtonian flow model the stress is proportional to the shear strain rate. Shear stress τ is the force per unit area that causes the fluid (or powder-binder mixture) to flow in a die. The shear strain γ is the relative motion of the fluid over the surface. The shear strain rate dγ/dt, also known as the shear rate, is the change in shear strain divided by time. Effectively, it is how fast the fluid moves. Fluid resistance to shearing is termed viscosity η, and it links the shear strain rate to the shear stress,

where m =1 for Newtonian fluids. For water the viscosity depends on temperature and pressure, but not on the shear rate, so it is Newtonian. Powder-binder mixtures exhibit more complex behavior, usually with a viscosity that depends on the shear rate and temperature, but m is not unity. [see Bingham flow, pseudoplastic flow]

Ni The chemical symbol for nickel.

nib A pressed and sintered cemented carbide compact that is used in wire drawing.

nichrome An alloy of nickel and chromium that is used for hot applications in air, such as heating elements and conveyor trays in lower temperature sintering furnaces, and fixtures in heat treatment furnaces. A typical alloy contains nickel with 20 wt. % chromium, 1 wt. % silicon, and 0.4 wt. % titanium. Although not an important product via powder metallurgy, it is an important material to the thermal processing of powder metallurgy compacts. It has a theoretical density of 8.37 g/cm3, room temperature properties of elastic modulus equal to 220 GPa, yield strength of 414 MPa, tensile strength of 690 MPa, and 30 % elongation to fracture. The electrical resistivity is 109 ?Ω-cm, but the alloy has a thermal conductivity of just 13 W/(m EC).

Page 7

Section N

nickel Element 28 on the periodic chart, it is a common alloying addition in ferrous powder metallurgy and is used as a pure sintered metal for batteries and filters. Small powders are formed using the carbonyl decomposition route. Larger nickel powders are formed by atomization. Other than electronic, filter, and battery applications, nickel is mostly used as an alloying addition in low concentrations. When consolidated to full density, nickel has the following properties. nickel

Ni

atomic number

28

atomic weight

59.69

g/mol

density

8.902

g/cm3

melting temperature

1453

EC

boiling temperature

2732

EC

heat of fusion

18

kJ/mol

heat capacity

452

J/(kg EC)

thermal expansion coefficient

13.3

ppm/EC

thermal conductivity

91

W/(m EC)

Curie temperature

360

EC

electrical resistivity

9.6

µΩ-cm

elastic modulus

214

GPa

Poisson뭩 ratio

0.31

hardness

35

BHN

as-sintered yield strength

148

MPa

as-sintered elongation to fracture

47

%

as-sintered ultimate tensile strength 462 MPa Much of the powder is small to enable easy sintering and alloying.

nickel aluminide An intermetallic compound of aluminum and nickel, usually Ni3Al but the name is also applied to the compound NiAl. The NiAl compound has a theoretical density of 5.86

Page 8

Section N

g/cm3 and room temperature elastic modulus of 290 GPa and strength up to 900 MPa (with little ductility); alloying is used to modify the properties. In contrast, the Ni3Al compound has attracted more interest from powder metallurgy because of its ductility and anomalous increase in strength when heated to temperatures up to 800EC. It has the following properties: nickel aluminide

NiAl

Ni3Al

86

203

g/mol

density

6.05

7.25

g/cm3

melting temperature

1640

1380

EC

heat capacity

843

123

J/(kg EC)

thermal expansion coefficient

11.9

12.5

ppm/EC

thermal conductivity

92

29.5

W/(m EC)

electrical resistivity

25

33

µΩ-cm

elastic modulus

290

179

GPa

Poisson뭩 ratio

0.31

0.35

as-sintered yield strength

540

450

MPa

3

50

%

atomic weight

as-sintered elongation to fracture

as-sintered ultimate tensile strength 900 640 MPa The thermal hardening character of Ni3Al (increasing strength as temperature is raised) gives a peak strength at temperatures in the range from 600EC to 800EC. However, the properties listed above require hot isostatic pressing or hot pressing. Both nickel aluminide compounds are modified with chromium for improved high-temperature oxidation resistance. Main uses are in thermal barrier coatings deposited by flame or plasma spray and as bonds for ceramics or carbides for high temperature composites.

nickel equivalent Nickel is a common stabilizer for the face-centered cubic crystal structure in ferrous alloys, known as gamma-iron or austenite. Other alloying additions are also used to stabilize austenite, but often with less potency. The nickel equivalent is a simple means to calculate the austenite stabilization effect from various combinations of gammaphase stabilizers. The calculation gives the equivalent amount of nickel and usually includes at a minimum carbon and manganese (the former is much more potent - 30fold, and the latter is only about half the potency of nickel).

Page 9

Section N

nickel silver Quite surprising this alloy does not contain silver, although its appearance is silver-like; the nominal composition is copper with 18 wt. % zinc and 18 wt. % nickel. The alloy is soft and has a low strength in the 150 MPa to 250 MPa range, depending on sintered density. Its use is limited, but the appearance makes it a favorite for coinage or decorative shapes.

nickel steel A group of powder metallurgy alloys that can be fabricated by several routes, but all are based on nickel and carbon alloying in iron. High nickel contents without carbon are used for magnets, but the nickel steels used for structural applications have from 2 to 6 wt. % nickel. The most popular alloy is iron with 2 wt. % Ni and up to 0.8 wt. % C. At full density these alloys have a density of 7.83 g/cm3, hardness typically in the 50 HRB range (but the hardness is significantly higher if a post-sintering heat treatment is used). The elastic modulus is 196 GPa, and in the as-sintered condition the yield strength is 149 MPa with a tensile strength of 330 MPa and 30 % elongation to fracture. Heat treatments are commonly used to promote much higher strengths with a corresponding loss of ductility, and strengths over 1200 MPa are possible with about 3 % ductility; however, residual porosity significantly degrades these properties.

niobium (Nb) Element 41 on the periodic chart, it is a body-centered cubic refractory metal. Most niobium is initially fabricated as powder, and subsequently consolidated into sheet, wire, or other mill shapes; the largest uses are for alloying steels to remove oxygen prior to solidification. Various applications rely on niobium as a high-temperature replacement for nickel-based superalloys, largely because of the high operating temperatures and low density, but oxidation problems are a barrier. In the full-density condition, the properties of niobium are as follows: niobium

Nb

atomic number

41

atomic weight

92.9

g/mol

density

8.57

g/cm3

melting temperature

2468

EC

boiling temperature

4927

EC

heat of fusion

27.2

kJ/mol

heat capacity

268

J/(kg EC)

Page 10

Section N thermal expansion coefficient

7.2

ppm/EC

thermal conductivity

53.7

W/(m EC)

electrical resistivity

12.5

µΩ-cm

elastic modulus

113

GPa

Poisson뭩 ratio

0.38

hardness as-sintered yield strength as-sintered elongation to fracture

80

VHN

255

MPa

26

%

as-sintered ultimate tensile strength 365 MPa One form under intense investigation for use in high temperature applications relies on directional solidification of a composite consisting of a niobium alloy reinforced with a niobium silicide.

nitinol A shape memory alloy based on the equiatomic intermetallic compound NiTi, named for its two elements Ni and Ti and the site of first discovery - the Naval Ordnance Laboratory. It is usually fabricated from mixed powders or prealloyed powder mixed with excess nickel or titanium to adjust the transformation temperatures. After some plastic deformation (usually less than 8 % strain), the alloy will return to its original, trained shape with modest heating. There are wide differences in commercial alloys, but for the standard NiTi composition at full density, the properties are as follows: nitinol

NiTi

density

6.5

g/cm3

1310

EC

thermal expansion coefficient

11

ppm/EC

electrical resistivity

82

µΩ-cm

elastic modulus

75

GPa

elongation to fracture

10

%

melting temperature

ultimate tensile strength 1150 MPa It has superplastic attributes and can be trained to transform shape at temperatures near room temperature. As a shape memory alloy, nitinol can be trained to two shapes

Section N

Page 11

and will change back and forth between those two shapes during thermal cycling.

nitrides Compound phases that form between a metal and nitrogen, usually resulting in a hard phase that lacks ductility. Nitrides often form inadvertently in sintering due to reactions with nitrogen from the sintering atmosphere. The most stable nitrides occur via reaction with refractory metals, such as chromium and tantalum, but there are technically important boron and silicon nitrides. The common nitrides include aluminum nitride (AlN), boron nitride (BN), chromium nitride (CrN), hafnium nitride (HfN), niobium nitride (NbN), silicon nitride (Si3N4), tantalum nitride (TaN), titanium nitride (TiN), vanadium nitride (VN), and zirconium nitride (ZrN). A popular nitride is TiN, which has a gold color and excellent wear resistance, so it is often deposited on sintered materials to form a hard and protective coating with a desirable color. This is commonly seen in cutting tools as well as decorative jewelry. Aluminum nitride is an electrical insulator that is recognized for its exceptionally high thermal conductivity. Boron nitride alloyed with carbon can by stabilized into the cubic crystal structure to have a hardness similar to diamond. Silicon nitride has excellent wear resistance and is often employed to high temperatures in air, such as in turbochargers.

nitrocarburizing Any of several processes in which both nitrogen and carbon are absorbed into the surface layers of a ferrous alloy to provide surface hardening. The prime gains are in wear and fatigue properties. The treatment can also be performed by immersing the component in a hot bath of an appropriate salt.

nitrogen sintering atmosphere A common atmosphere that is used in ferrous powder metallurgy for sintering. A few instances rely on pure nitrogen, but it is more typical to introduce some hydrogen into the nitrogen. Forming gas is a common commercial formulation that is nonexplosive (mixtures with less than 5% hydrogen are not flammable). At low hydrogen concentrations the mixture provides oxide reduction, at a low cost. Nitrogen atmospheres might also have additions of hydrocarbons, alcohol, moisture, hydrogen, carbon monoxide, or methane to obtain the desired thermochemical reactions. The nitrogen gas is usually taken from liquid nitrogen or from a generation site located near the sintering furnace. Since 78% of all air is nitrogen, it is possible to construct on-site generators that collect nitrogen from air via cryogenic or molecular sieve techniques. In spite of being labeled a neutral atmosphere, there are situations where nitrogen changes the material, usually by forming a hard nitride - for example in stainless steels. Nitrides also form with alloys containing titanium, aluminum, tantalum, niobium, zirconium, or silicon. Also, nitrogen increases strength and hardness in ferrous alloys, but degrades magnetic response, corrosion resistance, and ductility.

Section N

Page 12

noble metal Any member of a group of metals that resists corrosion in most acids at room temperature. On the electromotive series the noble metals are highly positive. Gold and platinum are two of the most popular, but the group includes ruthenium, rhodium, palladium, osmium, and iridium. This list is slightly smaller than the precious metals which contain silver; however, silver lacks the oxidation, corrosion, and acid resistance of the noble metals.

nodular powder Irregular particles with knotted, rounded shapes. Figure N.5 illustrates a nodular stainless steel powder that is used to form filters.

noncrystalline A material lacking in ordered atomic packing and long-range order; glasses, many polymers, and amorphous metals fall into this category. Most powder metallurgy compositions are crystalline in atomic structure.

nondestructive evaluation (NDE) Broadly the NDE and NDT terms are considered synonymous and stand for nondestructive testing and nondestructive evaluation. These are quantitative analysis approaches to determining if a device or material will be acceptable for a function by examining it for defects without experiencing any damage from the test. Common tests included in the NDE array include the following: ultrasonics, radiography, eddy current, visual inspection, liquid penetrant, and magnetic particle. Using these tests, a discontinuity is classified by size, shape, type, location and that is compared to acceptable limits. Designs are limited by the ability to inspect and ensure the device will not contain life-limiting defects. In powder metallurgy the effects of pores must be subtracted from the overall evaluation signal to separate out cracks, voids, inclusions, or other larger defects.

nondestructive testing (NDT) Effectively the same as nondestructive evaluation.

nonferrous Alloys that are not based on steel or iron, such as copper, aluminum, and zinc.

nonoxide ceramic A ceramic compound that is an electrical insulator, based on a metal and a species such as nitrogen, boron, carbon, but excludes oxygen. Such ceramics include aluminum nitride, silicon carbide, molybdenum carbide, and titanium diboride as examples.

Section N

Page 13

normal curve Also known as the bell curve or Gaussian distribution. A normal curve is used to represent the distribution in an attribute, such as component size, mass, or strength. These attributes show a typical measurement scatter characterized by the mean (average) and standard deviation. Like grades in school, the center of the distribution is the most typical value around which there is a measured dispersion. For a normal distribution, the mean (average), mode (most common), and median (half are smaller and half are larger) are equal. The standard deviation indicates the spread of the distribution around the central or average value. It is not a good model for particle size or mechanical properties, but is a good model for dimensional variation.

normalize A heat treatment that takes a steel component to a temperature where no phase transformations occur, typically into the austenite range, followed by slow cooling in air to a temperature below which phase transformations occur.

nucleation The formation of a new phase within a matrix. One example would be the formation of the first solid grains in a liquid during solidification. Nucleation is followed by growth of the new phase to complete the phase transformation. Most of the theories assume homogeneous nucleation, where the new phase is assumed to form by random atomic events. This occurs infrequently and requires considerable undercooling. In reality, most phase transformations occur on heterogeneous sites, such as debris or inclusions, and do not require much undercooling.

number distribution The same as the population distribution, giving the percentage or fraction of particles at a certain size or in a certain size range based on the total number of particles. [see particle size distribution]

Section O

Page 1

A to Z of Powder Metallurgy

O ODS The abbreviation for oxide dispersion strengthen.

offset yield strength The stress during a tensile test where the cumulative plastic strain is 0.2 %, determined by the intersection of the experimental stress-strain curve with a line starting at 0.2 % strain and zero stress that is parallel to the linear elastic portion of the stress-strain curve. Most frequently just called the yield strength. [see yield strength]

oil atomization A powder production process similar to water atomization, except the fluid impacting on the molten metal stream is pressurized oil. This removes an oxygen source that can contaminate the powder, but provides a potential carbon contamination source. Oil atomized powders are not as angular as water atomized powders, but are more expensive and the technology has largely fallen out of favor except for some high alloy systems that contain easily oxidized metals.

oil content The measured amount of oil contained in the open pores of an oil-impregnated object, most typically applied to a self-lubricating bearing after impregnation. When properly performed the measurement provides information on the interconnected or open porosity since that pore structure is accessible for oil impregnation. The measure is best performed using a sequence of mass determinations based on weighing the sample free of oil, weighing it after impregnation with oil, and weighing the oil impregnated sample and support wire immersed in water, and weighing the support wire immersed in water. This Archimedes density concept of water displacement allows determination of the sample density and the amount of oil filling the pores. [see open pores]

oil impregnation Filling of open pores with oil to provide lubricity during function. [see impregnation]

Section O

Page 2

open-loop control Processing without any monitoring of the product quality, simply using approximate machine settings to produce components that might be sorted for quality after production.

open pore A pore connected to the compact surface. Often open pores extend completely through a compact from one surface to another. Open pores are sometimes referred to as communicating pores with respect to the component surface, to indicate the ability to pass fluids through these pores. The open pores are useful for lubrication, permeation, or filtration. In the sintered microstructure, large and angular pores are usually open while closed pores are usually circular and small. In many instances it is important to distinguish between these two pore types. As a simple guide, few pores are closed at 85% density, usually about half of the pores are closed at 92% density, and usually all of the pores are closed at 96% density. An estimate of the total porosity helps decide if the pores are open or closed. The Archimedes approach can be used to determine the open porosity. This requires weighing the sample dry (W1), after oil impregnation (W2), and immersed in water (W3). The fractional open porosity is given as follows:

where εO is the open porosity, ΘW is the density of water, and ΘO is the oil density.

opposed ram pressing A die compaction operation where both the upper and lower rams move toward the die center during the compaction stroke. Figure O.1 illustrates the basic structure. Precise positioning and pressurization are possible, but a computer numerical control system is required to fully exploit the technology.

optical properties These are the attributes related to photon interactions with a sintered metal, and include color, reflectivity, surface roughness, and resistance to tarnishing. Much attention is directed to gold-containing alloys used in jewelry. Various materials have inherent colors such as gold, silver, bronze, and alumina. Dopants and surface treatments are used to adjust color, and this is popular in sintered ceramics such as zirconia and alumina. Color measurements are performed using integrating spheres that analyze the optical spectrum and quantify color in terms of three coordinates. In color space, the L value corresponds to the luminosity or lightness, while a reflects the red-green intensity, and b reflects the yellow-blue intensity.

Section O

Page 3

[see surface roughness, color]

optional mixer rotational speed Best mixing in a double-cone or twin-shell mixer occurs when the centrifugal forces are small, but fast enough to ensure some turbulence. A desirable rotational speed is one which partially balances gravitational and centrifugal forces. The optimal rotational speed No (in revolutions per minute or RPM) for a mixer is calculated as follows:

where d is the outer container arc diameter in meters. According to this equation, a cylinder with a 1 m diameter will have an optimal rotational speed of 32 RPM. Smaller diameters require faster speeds to achieve equivalent optimization. The rate of mixing varies with the initial inhomogeneity of the powder. Initially, rapid intermixing is observed, but the mixing rate diminishes asymptotically with time as the rate of mixing approaches the rate of size segregation; thus, mixing is not improved with long times, especially if the powder segregates.

ordered packing A collection of powder particles in a regular pattern, similar to atomic structures or brick walls. Here each particle is in a precise and repetitive position with the same number of neighbors and same spacing between neighbors. The highest density ordered packing of monosized spheres occurs with a 74 % density and 12 contacting neighbors on each particle; this is the close-packed structure. Figure O.2 contrasts an ordered packing with a disordered packing, the latter is random with no repetitive pattern.

ordering A heat treatment applied to intermetallic compounds after sintering to induce proper atomic structure, with each atomic species taking up precise positioning on the crystal lattice. Sintering often leads to disorder as seen by a random atomic placement, but an ordering heat treatment after sintering provides atomic motion (diffusion) that will allow the atoms to sort out proper spatial positions. For example, Figure O.3 illustrates the ordered packing associated with the shape memory intermetallic NiTi.

Osprey process A spray forming technique that combines atomization and freeform spray forming into a single operation. The Osprey process positions a substrate in the path of an atomization plume so the semisolid droplets from the atomizer can be deposited directly on the substrate. [see spray deposition]

Page 4

Section O

Ostwald ripening The progressive coarsening or growth of a microstructure at high temperatures, leading to a larger average size scale with fewer objects per unit volume; the larger crystals grow at the expense of the smaller crystals which have a higher solubility. During sintering Ostwald ripening applies to grain coarsening, but in some cases pores undergo Ostwald ripening as well, often with concomitant swelling. Fundamentally, Ostwald ripening says the volume of the mean grain increases linearly with time. Thus, the grain size cubed (grain volume) enlarges at a rate given as, G 3 = Go3 + K t where G is a size measure, such as the grain size or secondary dendrite arm spacing, and Go is the initial size. The parameter K represents the kinetic rate and is often in the range of 1 탆 3/s, and t is the time. With small-scale microstructures at elevated temperatures there is considerable opportunity for coarsening. This is even true during powder atomization. If cooling from the molten state occurs in 1 s, then the microstructure scale increases to approximately 1 탆 . This implies very rapid cooling rates are needed to form nanoscale structures, such as by chilling a vapor directly into a solid. These coarsening phenomena are named after the first observer, Wilhelm Ostwald, born in 1853 in Latvia. He went on to great fame as a professor, including supervision of three students who won Nobel prizes (Van뭪 Hoff in 1901, Arrhenius in 1903, and Nernst in 1920). He won the Nobel prize in 1909 in Chemistry and founded many scientific journals. His first academic appointment in 1877 at Dorpat University was unpaid.

outgassing A form of powder pretreatment in which the powder or green compact is heated in a vacuum to remove adsorbed or dissolved gasses via evaporation. It is a common treatment for hot isostatic pressing, since during consolidation any remaining contaminants will be entrained in the powder compact with possible embrittlement.

over-sinter A condition where the compact has been heated too long or heated at too high a temperature, usually resulting in distortion, blisters, swelling, or loss of physical or mechanical properties. Figure O.4 shows two example microstructures from an oversintered tool steel. One shows grain coarsening and the formation of a continuous brittle grain boundary phase while the other shows extensive pore generation from a high temperature gas reaction. Both the grains and pores are enlarged, resulting in increasing residual porosity. Most of the damage associated with over-sintering traces to microstructure coarsening.

Section O

Page 5

overaging Aging or precipitation hardening of an alloy after sintering, where the time is too long or the temperature is too high such that peak properties are lost; aging heat treatments that pass through the maximum in strength and hardness.

overfill The placement of an excessive amount of powder in the die cavity during the filling stage, some of which is ejected prior to compaction. It is one means to better control die fill for poorly flowing powders or to improve density or mass homogeneity in die compacted parts.

oversize powder Particles that are larger than the maximum permitted by a particle size specification. One man뭩 waste product might be another뭩 target size, so there is no formal size that is designated as oversize power.

oxidation-reduction A combination of atmosphere and powder reactions that can extract oxygen (reduction) or deposit oxygen (oxidation) in a powder compact during heating, especially during debinding and sintering. Reduction conditions are usually required to properly sinter metallic materials.

oxidation resistance A measure of the ability to withstand exposure to high temperatures and oxidizing atmospheres, often determined using either prolonged exposure to an oxidizing environment (air or moist air). Also oxidation resistance can be tested by cyclic heating in an oxidizing environment, where the weight change is determined versus the number of cycles. Best oxidation resistance is associated with well-sintered, pore-free materials. Selected chemical additions can be made to a sintered material to improve oxidation resistance, for example yttrium is a common alloy addition used for this purpose. In general oxide ceramics are very resistant to oxidation. In metals, alloying with chromium, aluminum, titanium, or other strong oxide formers can provide a tenacious oxide film that protects against oxidation.

oxide ceramic Compounds of metals and oxygen, formed into stoichiometric combinations (say 2 metal atoms to 3 oxygen atoms) that are typically stable to high temperatures and often are hard, but brittle. Oxide ceramics are widely used in sintering for substrates, fixtures, insulators, and spacers. Alumina (Al2O3) and silica (SiO2) are the most commonly used oxides, but others encountered in powder metallurgy include magnesia (MgO), spinel

Section O

Page 6

(MgAl2O4), zirconia (ZrO2), beryllia (BeO), cordierite (2MgO-2Al2O3-5SiO2), and mullite (3Al2O3-2SiO2).

oxide dispersion strengthen (ODS) Alloys that rely on included small oxide particles or dispersoids, often alumina (Al2O3) but formerly thoria (ThO2), that provide high temperature strength. While precipitates often dissolve into the matrix at high temperatures or coarsen via Ostwald ripening, these inert oxide particles resist dissolution and block dislocation motion to high temperatures, thereby providing strength and creep resistance. Usually the most effective dispersoids are nanometer size particles, produced by internal oxidation or mechanical alloying. The original alloy based on oxide dispersion strengthening was called TD Ni to indicate a thoria dispersed in a nickel matrix. Now days ODS alloys are used to fabricate copper electrical contacts and nickel superalloy jet engine components.

oxide network Continuous or discontinuous oxides that follow the prior particle boundaries in a powder product after consolidation.

oxide reduction The use of a species, such as hydrogen, to remove oxygen from a powder or compact during heating. Reduction is possible using hydrogen, carbon, or carbon monoxide. For metals, oxide reduction is easier at higher temperatures and the process is routinely used to produce iron, copper, molybdenum, or tungsten powders from their oxides. For a given atmosphere, it is possible to estimate the minimum temperature needed to ensure oxide reduction based on Ellingham or Richardson diagrams, both of which effectively plot free energy versus temperature for metal-oxide combinations. Figure O.5 is a diagram showing the temperature effect on the oxide decomposition partial pressure for pure iron. The behavior during oxide reduction depends on temperature due to both thermodynamic and kinetic considerations. Thermodynamic concerns relate how stable the oxides and metals are with respect to the reducing gas. The kinetic factors related to the rate at which the reacting gas penetrates into the particle and reaction products diffuse back out. Often the latter is the slow step, and it is common in some forms of powder production to see cycles taking days.

oxygen affinity Certain elements have a strong attraction for oxygen and will preferentially oxidize over other elements in an alloy. A high oxygen affinity indicates a high attraction for oxygen, such as evident with titanium, zirconium, and aluminum, while a low oxygen affinity is associated with elements that easily give up any oxygen, such as copper, iron, and nickel. At various temperatures it is possible to rank the oxygen affinity from least active

Section O

Page 7

to most active. During sintering it is common to see oxygen transferred from the low oxygen affinity metal to the high oxygen affinity metal.

oxygen analyzer The monitoring of impurity levels in a process atmosphere as performed with a sensor that changes conductivity in proportion to the oxygen concentration in the atmosphere. Usually these are calibrated to provide a readout of the oxygen content in parts per million.

oxygen level A quantitative measure of the oxygen contamination or content in a powder or sintered powder metallurgy material, usually given as a percent of total mass or parts per million (ppm). Common water atomized powders will range from 0.3 % or less, corresponding to 3000 ppm (ppm stands for parts per million) or less.

Section P

Page 1

A to Z of Powder Metallurgy

P pack carburization The use of a granular packing to diffuse carbon into a component during heat treatment.

packing A semi-cohesive collection of particles in point contact which exhibits resistance to flow, typically formed by filling a container under gravitational forces. There are several packing structures - random and ordered, and loose and dense. For example a dense random packing of monosized spheres has a fractional density of 0.64. On the other had a dense ordered packing of monosized spheres has a fractional density of 0.74. [see loose random packing, dense random packing, ordered packing]

packing pressure The peak pressure encountered in a powder injection molding operation once the cavity is filled, prior to freezing of the gate. It is precisely controlled to ensure weight uniformity of the molded part, a precursor to optimized final dimensional control.

PANACEA An austenitic stainless steel that contains no nickel, designed for use in medical and dental applications where human contact might induce a nickel allergic response. The alloy was developed at the Swiss Federal Institute. It is iron-based and nominally contains 17 wt. % chromium, 10 wt. % manganese, 3 wt. % molybdenum, and 1 wt. % nitrogen. The nitrogen is typically added via the sintering atmosphere. When sintered to nearly full density and a hardness of 25 HRC, the yield strength is 670 MPa, ultimate tensile strength is 960 MPa, and elongation to fracture is 35 %.

paraffin wax (PW) A simple molecule consisting of a carbon chain with attached hydrogens - CH2, often derived from the higher molecular weight species found in oil. Except for the terminal groups, it has two hydrogens for each carbon. The melting temperature increases as the molecules become longer, but typically ranges near 45EC. The density is 0.9 g/cm3 and the material is essentially a thermal and electrical insulator. It is a favorite agglomeration agent, lubricant, spray drying binder, and second phase ingredient in powder extrusion and powder injection molding.

Section P

Page 2

part levels A means to assess the difficulty in compacting a component, determined by the number of different heights in the pressing direction, termed part levels. Figure P.1 sketches the concept. As the number of levels grows, tool complexity increases rapidly. Class 1 parts are simple single-level shapes with a small height-to-diameter ratio that are compacted using single-acting dies. The Class 2 parts are also single level, but with compaction pressure applied from both top and bottom because of a larger height-to-diameter ratio. Class 3 parts have two levels and pressure is applied from both the top and bottom. Class 4 parts represent shapes that are the most difficult to press. These are multiplelevel components which are pressurized from both the upper and lower punches.

partially prealloyed powder Precursor alloy powders formed using mixed elemental powders that are heated to a temperature where incipient sinter bonds form, but the alloying ingredients have not interdiffused. This procedure is used to minimize segregation or separation of alloying ingredients prior to compaction while sustaining the high compressibility associated with elemental powders. Also known as diffusionally bonded powders.

partially stabilized zirconia A compound of zirconium and oxygen, ZrO2, with a modest content of a second component to produce a two-phase microstructure. The zirconia-rich regions in the microstructure allow for transformation toughening during loading.

particle A discrete solid with a size less than 1 mm. Particles come in many sizes, ranging from the size of a virus to a grain of sand. Engineering particles are measured on a micrometer (탆 ) scale, which is 10-6 m. Many engineering particles range from 0.1 탆 to 200 탆 in size, with ceramic particles tending toward the smaller sizes and plastic particles tending toward the larger sizes. For reference, human hair is typically in the 100 탆 range and the pigment in paint is typically in the 1 탆 range.

particle coordination number The number of touching neighbors for a particle, usually given as an average over the compact. For loose larger particles this tends to average near seven, but for nanoscale particles can drop to as low as two contacts per particle. At full density, the particle coordination number approaches 14. Except in ordered packings, the coordination number is a distributed parameter, with some particles having more and others having less than the average.

particle hardness Hardness of the particle as measured by scratch, indentation, microhardness, on

Section P

Page 3

nanohardness tests. This hardness is the true material hardness as used to explain wear resistance, as compared to the bulk aggregate hardness (apparent hardness) which is often lower because of porosity.

particle packing The density for a powder when filled into a container under normal gravity conditions. Among other things, the particle packing density determines the die fill, binder content, and shrinkage in sintering. Random packing structures are typical of powders. For larger monosized spheres the fractional density is near 60 % of theoretical, and reaches 64 % of theoretical density if the container is vibrated during filling. These values correspond to about six to seven contacts per particle - the coordination number. The actual packing density for a powder depends on its characteristics, namely its size, shape, and surface chemistry. For irregular particles, the packing density often is near 30 % of theoretical; and is even lower for very small, irregular, sponge, or nanoscale powders. In the latter case, particles of 20 nm particle size typically pack to 4 % to 6 % density. At this lower packing density, the coordination number drops to as low as 2 to 4 contacts per particle.

particle purity A measure of the total contamination level for a powder. Purity is usually given in terms of a percentage, such as 99.95 % pure. Impurities are then given in parts per million or ppm. Occasionally in very high purity materials the purity will be cited based on the number of 9's required to express the purity - for example 4 9's (four nines) implies 99.99 % pure. [see parts per million]

particle shape The appearance of a particle, such as spherical, rounded, angular, dendritic, irregular, porous, blocky, flake, or ligamental. Particle shape can be given as a descriptive term, such as shown in Figure P.2, or as a quantitative term. The most common quantitative metric for particle shape is the aspect ratio, which is the largest particle dimension divided by the smallest thickness. Other quantitative measures of particle shape include the ratio of specific surface area divided by the median particle size. Involved profile analysis is another possible means to mathematically express particle shape, but that is not often used in powder metallurgy. [see aspect ratio]

particle size The controlling linear dimension of an individual particle, as determined by analysis with screens, lasers, sedimentation, or other sensing techniques. Most typically the particle size is characterized using the median size from the cumulative mass distribution, which

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is the D50 particle size. [see mean size, median size]

particle size analyzer An automated device for determination of the particle size distribution. A very large array of devices exist, with the laser-based approaches being the most popular for automated analysis. [see Fraunhofer diffraction, laser light scattering, electrical zone sensing, light blocking, sedimentation]

particle size distribution A description of the size variation in a powder lot. The particle size distribution is often expressed as the cumulative percent smaller than a given size as plotted in Figure P.3 for a common metal powder. The percentage can be based on mass of particles or number of particles, and these will give different distributions. An alternative is to capture the mass of particles between two screens and to plot a histogram showing the weight of powder in each screen size interval. This latter method of expressing the particle size distribution is common, but fails to provide insight with respect to the tails of the distribution - since the sizes of the largest and smallest powders might be unspecified. Contemporary reporting is to provide the D10, D50, and D90 sizes corresponding to the particle sizes at the 10 %, 50 %, and 90 % points on the cumulative distribution. [see population distribution, mass distribution, cumulative particle size distribution]

particle technology A branch of science and engineering, much larger than just powder metallurgy, that deals with the fabrication of particles, their characterization, handling, mixing and blending, transport, packing and storage, testing, and the practices required in working with particulate materials. The particles might be minerals, foods, ceramics, paint pigments, plastics, or other forms of particles. Generally the largest dimension is below a centimeter and can range down to the nanoscale. The field loosely encompasses powder metallurgy.

particulate composite Composite material with dispersed hard particles, where the hard particles are easily resolved using optical microscopy. Most particulate composites have ceramic phases in a metal matrix with a ceramic concentration more than 25 vol. %. They are characterized by the composition, size, and shape of the dispersed phase. Two key forms are resin bonded composites and metal-matrix bonded composites. Most particle composites are isotropic since the short particles do not provide a preferred orientation. Examples and applications include cemented carbides (WC-Co) for cutting tools,

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tungsten-copper (W-Cu) electrical contacts, bronze-graphite for electrical contacts, copper-chromium (Cu-Cr) for high voltage circuit breakers, aluminum reinforced with silicon carbide (Al-SiC) for high specific stiffness structures.

particulate materials Small solid particles have fluid-like properties that allow forming via a variety of technologies, leading to unique products often not available via alternative approaches. For large-volume production, particulate processing relies on a die cavity which is replicated over and over by pressing or molding a powder-polymer mixture into that cavity. Freeform fabrication extends the technology to the fabrication of single objects. Cost is always an important factor, so the ability to use particulate materials to fabricate complex shapes to final size has significant economic benefit. Some of the material classes processed by particulate materials techniques include common oxide ceramics (alumina, silica, and zirconia), ferrous alloys (steels, tool steels, stainless steels), cemented carbides (hardmetals), cermets (titanium carbide in tool steel), and refractory metals (tungsten, molybdenum, and heavy alloys), nonferrous alloys (based on nickel, cobalt, or copper), reactive metals (titanium, tantalum, niobium, rhenium), aerospace alloys (superalloys, beryllium), electronic and magnetic compositions, and composites formed by mixing these powders. Specialty materials include a range of intermetallics and compounds such as the aluminides, borides, nitrides, and carbides.

parting line A linear blemish on a compact where two separate tool or die pieces mated during shaping. In injection molding, it is where the two mold halves join together. The tool set slightly deflects during mold filling, in spite of close alignment and high clamping forces. The deflection varies directly with the injection pressure and part size, allowing feedstock intrusion between the moveable parts of the tooling. This intruded feedstock becomes a flash or surface blemish such as shown in Figure P.4. The location of the parting line is a decision made during tool design. Cosmetic and functional requirements may force parting line locations in less than optimal positions.

parts per million (ppm) A metric based on a million objects, similar to how a percentage is based on a hundred objects. Used mostly in chemical analysis, it refers to the concentration of species based on a million point basis. Similar in concept to percentage, which is based on a hundred point basis, but more applicable to low concentration species such as impurities. For example, a common water atomized stainless steel powder might have a chemical specification for carbon at