Titanium Alloys: Russian Aircraft and Aerospace Applications [1 ed.] 9780849332739, 0849332737

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Titanium Alloys Russian Aircraft and Aerospace Applications

Advances in Metallic Alloys A series edited by J.N. Fridlyander, All-Russian Institute of Aviation Materials, Moscow, Russia and D.G. Eskin, Netherlands Institute for Metals Research, Delft, The Netherlands Volume 1 Liquid Metal Processing: Applications to Aluminum Alloy Production I.G. Brodova, P.S. Popel and G.I. Eskin Volume 2 Iron in Aluminum Alloys: Impurity and Alloying Elment N.W. Belov, A.A. Aksenov and D.G. Eskin Volume 3 Magnesium Alloy Containing Rare Earth Metals: Structure and Properties L.L. Rokhlin Volume 4 Phase Transformations of Elements Under High Pressure E.Yu Tonkov and E.G. Ponyatovsky Volume 5 Titanium Alloys: Russian Aircraft and Aerospace Applications Valentin N. Moiseyev

Titanium Alloys Russian Aircraft and Aerospace Applications

Valentin N. Moiseyev

Boca Raton London New York Singapore

A CRC title, part of the Taylor & Francis imprint, a member of the Taylor & Francis Group, the academic division of T&F Informa plc.

Published in 2006 by CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2006 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number-10: 0-8493-3273-7 (Hardcover) International Standard Book Number-13: 978-0-8493-3273-9 (Hardcover) Library of Congress Card Number 2005041889 This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. No part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC) 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe.

Library of Congress Cataloging-in-Publication Data Moiseyev, Valentin N. Titanium alloys in Russia / Valentin N. Moiseyev. p. cm. -- (Advances in metallic alloys ; v. 5) Includes bibliographical references and index. ISBN 0-8493-3273-7 (alk. paper) 1. Titanium alloys--Russia (Federation) I. Title. II. Series. TN693.T5M58 2005 620.1'89322--dc22

2005041889

Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com Taylor & Francis Group is the Academic Division of T&F Informa plc.

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Contents Introduction

1

Chapter 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

Structure of Titanium Alloys Effect of impurities on the properties of titanium Interaction of titanium with other elements Solid α- and β-solutions in titanium alloys Chemical compounds in titanium alloys Stability of solid α- and β-solutions Phase transformations in titanium alloys Enhancement of mechanical strength of titanium alloys Enhancement of high-temperature strength of titanium alloys References

5 5 9 13 17 22 25 29 35 42

Chapter 2 2.1 2.2 2.3 2.4

Structural Titanium Alloys General characteristics High-ductility low-strength alloys Medium-strength alloys High-strength alloys References

47 47 51 67 84 116

Chapter 3 3.1 3.2 3.3

High-Temperature Titanium Alloys General characteristics Martensite-type high-temperature alloys High-temperature pseudo-α-alloys References

119 119 121 141 147

Chapter 4 4.1 4.2 4.3

Functional-Purpose Titanium Alloys Titanium alloys operated at low temperatures Special-purpose corrosion-resistant titanium alloys Intermetallics-based titanium alloys 4.3.1 Titanium aluminides 4.3.2 Titanium nickelide 4.3.3 Eutectoid-based alloys 4.4 Titanium alloys for production of cast shapes References

149 149 150 156 157 159 161 162 167

Chapter 5 Technological Properties of Titanium Alloys 5.1 Formation of the structure and properties in hot deformation of titanium alloys 5.2 Heat treatment of titanium alloys

169 169 174

vi

THE RISE OF THE SUPERCONDUCTORS

5.2.1 Annealing 5.2.2 Hardening heat treatment (quenching and aging) 5.3 Welding of titanium alloys 5.4 Surface engineering of titanium alloys 5.4.1 Interaction of titanium with atmospheric gases during heating 5.4.2 Effect of mechanical treatment on titanium alloy castings 5.4.3 Surface hardening of titanium alloy items References Chapter 6 6.1 6.2 6.3 6.4 6.5

Subject Index

Applications of Titanium and Titanium Alloys Titanium alloys in the aircraft industry Titanium in engine manufacturing Titanium in rocket manufacturing Titanium in shipbuilding Titanium in the chemical engineering industry and other fields References

176 180 181 186 186 188 189 191 195 196 199 201 202 202 205 207

Introduction

Titanium is a rather new metal and is, probably, the last addition to the comparatively small group of structural materials for large-capacity constructions. Along with iron, aluminum, magnesium, copper, and nickel, it becomes one of the essential metal materials for modern machine building, as its reserves in the Earth’s crust are rather big. The advantages of titanium as a structural material are well known. The major stimulus for titanium to be used in various engineering fields are its high specific strength and high-temperature strength within a broad temperature range, and also a high corrosion resistance in most aggressive media. Titanium used in industries is usually not technical-grade titanium but its alloys, which exceed the nonalloyed metal by mechanical strength, high-temperature strength and other useful properties. The temperature range of the best application of titanium alloys is from the deep-freeze temperatures (cryogenic alloys) up to 500–600°C (high-temperature alloys). A characteristic feature of titanium and its alloys is high sensitivity to impurities, especially atmospheric oxygen and nitrogen. Oxygen, nitrogen and other impurities form alloys of the type of interstitial solid solutions or metallide phases with titanium and significantly affect the properties of metal when present even in minor amounts (decimal and sometimes even hundredth fractions of a percent). This explains the large number of grades of initial spongy titanium cast to produce semiproducts. Russian state standard 17303-72 establishes the following grades and chemical compositions for initial spongy titanium (Table 1). Comparison of titanium with other structural metals shows it to be the most refractory and to have lower values of thermal conductivity, electrical resistance and ooooooooooo Table 1 Grades and chemical compositions of spongy titanium. Grade

TG-90 TG-100 TG-110 TG-120 TG-130 TG-150

Maximal hardness, HB (10/1500/30)

N2

C

Cl

Fe

90 100 110 120 130 150

0.02 0.02 0.02 0.03 0.03 0.04

0.02 0.03 0.03 0.04 0.04 0.05

0.08 0.08 0.08 0.08 0.10 0.12

0.06 0.07 0.09 0.01 0.13 0.20

Chemical composition, %, no more than Si

Ni

0.01 0.02 0.03 0.03 0.04 0.04

0.05 0.05 0.03 0.05 0.05 0.05

O2 0.04 0.04 0.05 0.06 0.08 0.10

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VALENTIN N. MOISEYEV

Table 2 Physical properties of titanium and other metals. Properties Melting temperature, ° C Density, g/cm3 Thermal conductivity, Cal/cm s deg Electrical resistance, µΩ cm Heat capacity, Cal/g deg Coefficient of thermal expansion, ×106, deg–1 Young’s modulus, kgf/mm2

Ti

Mg

1665 650 4.51 1.74 0.0407 0.35 55.4 4.40 0.126 0.245 8.9 25.7 11200 4500

Al 660 2.70 0.57 2.68 0.211 24.0 7250

Fe

Cu

1535 1083 7.86 8.94 0.17 0.92 10.0 1.72 0.109 0.093 11.9 16.4 20000 12250

and thermal expansion. By density, titanium is attributed to light metals and occupies an intermediate position between aluminum and iron (Table 2). Similar to iron, titanium is a polymorphic metal and has a phase transformation at a temperature of 882°C. Below this temperature, the hexagonal close-packed lattice of α-titanium is stable; above it, the body-centered cubic lattice of β-titanium. There is a great variety of commercial titanium alloys which differ by their structures, physical, chemical, and mechanical properties, and applications. All titanium alloys are divided into three groups by the type of structure: (i) alloys based on solid α- and β-solutions, (ii) alloys based on solid solutions with some amount of a chemical compound and (iii) alloys based on a chemical compound. The most numerous and traditional group are titanium alloys which represent solid solutions. As a rule, they are structural alloys with a high ratio of strength and ductility, satisfactory fusion weldability, capability of hardening heat treatment, good thermal stability, and other properties required for modern structural materials. Solid-solution titanium alloys retain high strength at temperatures up to 350–450°C. Solid-solution alloys with chemical compounds are also widespread. This is a class of titanium alloys based on α-, (α+β) , and β-solid solutions with an amount of disperse formations of a chemical compound, which ensures a significant enhancement of strength and high-temperature strength. Commercial titanium alloys contain a minor amount of a chemical compound or the initial stage of its formation in the α- or (α+β) -matrix. Aluminum (Ti3Al), silicon (Ti5Si3), carbon (TiC), boron (TiB), etc., are used as alloying elements forming chemical compounds in titanium. Other, more complex chemical compounds can be formed in multicomponent alloys. Development of solid solution-based alloys with a chemical compound made it possible to increase the operational temperatures up to 500–600°C. Titanium alloys based on chemical compounds have been developed comparatively recently and quickly became the focus of attention owing to their properties, unique in some cases. There are at least three types of such materials, which are of commercial interest. These are heat-resistant alloys based on titanium aluminides Ti3Al (α2-phase), TiAl (γ -phase); shape memory alloys based on titanium nickelides (TiNi) and fire-safe alloys based on the eutectoid (α+Ti2Cu and α+TiCr). The latter type can only conditionally be attributed to alloys based on a chemical compound; however, their functional properties are determined by eutectoid which includes the compound.

TITANIUM ALLOYS: RUSSIAN AIRCRAFT AND AEROSPACE APPLICATIONS

3

Alloys based on aluminides can be operated at temperatures up to 700–800°C. In Russian practice, there are over 30 commercial titanium alloys that are divided into the following groups by their preferable applications: structural alloys, high-strength thermally hardened alloys, high-temperature alloys, alloys for fabrication of cast shapes, alloys with special properties (for cryogenic temperatures, for very aggressive media, shape memory alloys, etc.). The monograph considers titanium alloys with respect to these groups and applications. Along with the description of the physical and mechanical properties of various-purpose alloys, we discuss the general regularities in the change of alloys’ structure and properties depending on their chemical composition and heat treatment. The data in Introduction make it possible to predict the behavior of various alloys at all stages of production, and also in operation of fabricated products at elevated temperatures and stresses. We present the major parameters of titanium alloys’ treatment, taking into consideration their structural features and the requirements to their physical and mechanical properties. The chapter on special-purpose titanium alloys discusses (besides alloys for cryogenic temperatures, with decreased tendency to oxidation, corrosion-resistant and shape memory alloys) a new direction in developing and using titanium alloys based on chemical compounds and solid solution-based alloys with intermetallic compounds. In conclusion, we look at the experience and efficiency of using titanium alloys in various fields of machine building. The monograph is intended for metallurgists, physical metallurgists, design and process engineers, and metallurgical students.

4

VALENTIN N. MOISEYEV

1

Structure of Titanium Alloys

1.1 EFFECT OF IMPURITIES ON THE PROPERTIES OF TITANIUM Titanium exists in two allotropic modifications: high-temperature (with the bodycentered crystalline (bcc) lattice) and low-temperature (with the hexagonal closepacked (hcp) lattice). α-Titanium exists at temperatures below 882°C, and β-titanium at higher temperatures up to the melting point. The lattice parameters of α-titanium are as follows: a = 2.9504 Å, c = 4.683 Å; at 25°C (as obtained by extrapolation) a = 3.282 Å. The properties of titanium and its alloys depend to a great extent on the content of inevitable gas impurities, which get into the metal from the initial raw material, spongy titanium. Impurities, mainly oxygen, determine the useful property of titanium – its high mechanical strength. Indeed, titanium free of impurities is of no interest as a structural material. For instance, titanium refined using the iodide process has the following mechanical properties: ultimate strength, 250 MPa; yield strength, 106 MPa; relative elongation, 72%; transverse contraction, 86%; Vickers hardness, 83.4 kgf/mm2. In this metal the content of oxygen is less than 0.01%; of nitrogen, less than 0.008%; other impurities are mainly traces. For comparison, technical-grade titanium VT1-00 has ultimate strength of 300–450 MPa, and VT1-0 of 400–550 MPa. The strength of technicalgrade titanium is enhanced mainly due to oxygen and, to a lower extent, due to nitrogen, carbon, silicon and iron. Thus, technical-grade titanium is, in fact, a complex alloy containing elements, which have their own effects on the temperature of polymorphic transformations of titanium. The essence of these effects is that technical-grade titanium has no definite transformation point, but has a temperature range of transformation. In pure titanium, the α-modification transforms into the β-modification at a temperature of 882.5°C. In technical-grade metal, transformation begins at a lower temperature and ends at a higher temperature than in pure titanium. The range of transformation for technicalgrade titanium is 865 to 920°C (at the combined oxygen and nitrogen contents no higher than 0.15%). The difference in microstructure between pure and technical-grade titanium is mainly determined by oxygen impurity.

6

VALENTIN N. MOISEYEV

O, at.% 0

0.50

1.50

1.00

2.00

σ0.2

σB 45

σ0.2

350 300

HV 15

250 200

HV

40

150

δ,%

δ (l = 12.7 mm)

HV, kgf/mm2

σB, σ0.2, kgf/mm2

σB 75

100 20 δ (l = 25.4 mm) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 O, wt.%

Figure 1 Effect of oxygen impurity on the mechanical properties of titanium (from various sources). N2, at.% 0.75

σB, σ0.2, kgf/mm2

75

1.75

1.25

2.25 400

σB HV

σ0.2

45

350 300 250

15

200 HV

150

40

HV, kgf/mm2

0.25

100

δ,%

δ (l = 12.7 mm) 20 δ (l = 25.4 mm)

0

0.1

0.2 0.3

0.4

0.5 0.6

0.7

N2, wt.%

Figure 2 Effect of nitrogen impurity on the mechanical properties of titanium (from various sources).

TITANIUM ALLOYS: RUSSIAN AIRCRAFT AND AEROSPACE APPLICATIONS

7

C, at.% 0

0.5

1.0

1.5

2.5

2.0

3.0

3.5

56

σ0.2 28

200

HV 150

14

100

50

HV, kgf/mm2

σB, σ0.2, kgf/mm2

σB 42

δ (l = 12.7 mm)

δ,%

40 30 δ (l = 25.4 mm) 20 10 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

C, wt.%

Figure 3 Effect of carbon impurity on the mechanical properties of titanium (from various sources).

The high-temperature β-modification in nonalloyed titanium could not be fixed by quenching even at the highest rates of cooling. During quenching, the β-modification passes into the α-modification by a martensite-type instantaneous transformation. The structure formed is designated α′ and is a common polyhedral structure of pure metal. It is approximately the same both for pure and technical-grade titanium. In quenching from the β-region, the microstructural changes of pure titanium are insignificant and are characterized by the emergence of jagged grain boundaries. In the case of technical-grade titanium, the structure after quenching sharply changes from polyhedral to needle-like. If quenching is performed from temperatures inside the transformation range, i.e., from the two-phase region, then various ratios between the equilibrium (primary) and martensite (secondary) α-structures can be obtained. Effects of oxygen, nitrogen and carbon impurities on the mechanical properties of titanium are shown in Figs. 1–3. Graphical interpolation can establish the following approximate coefficients of hardening caused by impurities within the ranges of their content in technical-grade titanium: 0.05% oxygen increases ultimate strength by 60 MPa; 0.05% nitrogen, by 125 MPa, and 0.05% carbon, by 35 MPa. For instance, 0.05% iron increases ultimate strength of titanium by 10 MPa. Interestingly, iron forms substitution solid solutions with titanium, not interstitial solid solutions as oxygen, nitrogen and carbon do.

8

VALENTIN N. MOISEYEV

The combined effect of the four impurities on the properties of titanium can be estimated to accuracy sufficient for practice. Brown proposed the following formula to calculate the hardness: HB = 196 %N 2 – 158 %O 2 – 45 %C – 20 %Fe – 57 , where 57 is the value of hardness of purest titanium and %%N2, O2 are the percentages of these elements. The effect of silicon impurities on the properties of pure titanium should also be considered. According to Goldhoff and coworkers, Vickers hardness of alloys increases almost linearly from 150 up to 645 kgf/mm2 as the content of silicon rises from zero up to 16%. If one takes the coefficient of 0.28 to convert from Vickers hardness to ultimate strength of technical-grade titanium, then 1% silicon should increase the ultimate strength by about 100 MPa; and 0.05%, by 5 MPa. However, it was found that 0.5% silicon increased the ultimate strength of titanium by 130 MPa and, therefore, the increment of strength at 0.05% silicon would be 12 MPa. Therefore, the combined effect of the impurities on the mechanical properties of titanium are rather significant even at their comparatively small amounts given by specifications. Comparison of the hardening effect of various impurities on titanium shows that oxygen can be considered not only a harmful impurity but also a useful alloying additive. Nitrogen, though a more potent strengthener than oxygen, evokes embrittlement of titanium, so alloys with more than 0.05% nitrogen are of no practical significance. Carbon is a comparatively poor strengthener, but at over 0.2% in the alloy it causes the emergence of a brittle and solid carbide phase. Yet, alloying with oxygen up to 0.5% preserves a satisfactory ductility (δ5 > 15%) at a significant increment of strength (σB = 300–800 MPa). Studies of the effect of oxygen impurities on the cold brittleness of titanium showed that titanium with an oxygen content of 0.13–0.15% (σB = 500 MPa) was virtually noncold brittle even at liquid-nitrogen temperature (–196°C), i.e., impact strength decreased by only 15% as compared with the value at 20°C (from 164 down to 140 MPa, respectively). Titanium with an oxygen content of 0.3% (σB = 670 MPa) showed under the same conditions a decrease of impact viscosity by 55%. Specimens used for the experiments were annealed in vacuum and the content of hydrogen in them did not exceed 0.005%. Hydrogen was not taken into account in the estimation of the hardening effect of impurities, because its influence as a strengthener is negligible within the limits of its content in technical-grade titanium. However, being insignificant as a strengthener, hydrogen causes a brittle failure of titanium due to the formation of a hydride phase. Because of this, hydrogen is considered to be one of the most undesirable impurities in titanium. The presence of oxygen enhances the harmful effect of other impurities. There is an opinion that the purity of initial titanium should be constantly increased. If there is a necessity to increase the strength of technical-grade titanium, it is much better to add a strictly definite amount of oxygen than to use low-grade titanium with a large scatter of properties. This is the way it is done, for instance, in the U.K. and the U.S. where three out of four grades of technical-grade titanium contain a deliberately introduced addition of oxygen to ensure the strength required.

TITANIUM ALLOYS: RUSSIAN AIRCRAFT AND AEROSPACE APPLICATIONS

9

One of the tendencies in the development of modern titanium alloys is to increase the extent of alloying. In all alloys with the ultimate strength of about 1000 MPa the sum total of alloying elements is 7– 10% (alloys VT6, VT14, VT3-1, VT22, etc.). Alloys are developed in which the sum total of alloying components reaches 18–40% (alloys VT32, VT35). Alloying increases strength but reduces ductility of alloys. Therefore, the initial material, in this case titanium, should have maximum ductility. The more alloying is to be done, the more ductile and, thus, more free of impurities, the initial titanium should be. Usually, it is not advantageous to increase strength by using impurities, as it is accompanied by a considerable loss of ductility, because the major impurities – oxygen, nitrogen and carbon form interstitial solid solutions with titanium. Besides, these impurities have an adverse effect on other important characteristics of titanium alloys – thermal stability, creep resistance and notch sensitivity. Notch sensitivity is characterized by the ratio of ultimate strength of a notched specimen to ultimate strength of a smooth specimen. For brittle materials this value is less than unity; for ductile materials, more than unity; i.e., in the latter case the notch acts to strengthen the specimen. The effect of an oxygen impurity on the notch sensitivity of an alloy VT6 for round specimens with the radius of 0.1 mm at the basis of the notch and the notch angle of 60° gave the following results: the oxygen contents of 0.10%, 0.22% and 0.40% yield the ratio σBH/σB of 1.71, 1.63 and 1.47, respectively. This dependence of notch sensitivity on the oxygen content creates the danger of premature breakdown, in particular, of bolted joints. An increase of the purity of the initial titanium would make it possible not only to improve the quality of the existing alloys, but also to develop new alloys with good ductile properties along with high strength.

1.2 INTERACTION OF TITANIUM WITH OTHER ELEMENTS Research and development of commercial metal alloys, including titanium alloys, is determined by the character of interaction of the base with elements of the Periodic Table and also by the regularities of the change of physical, chemical, mechanical and other properties in various systems. The character of interaction of titanium with alloying elements and impurities depends to a significant extent on the atomic radii, position of metals in the electromotive series, valence, ionization potential and some other characteristics. Consideration of these factors enables a prediction of the results of interaction of various elements. Based on the size and structure of titanium and alloyed elements, the phase diagrams of alloys can be represented as shown in Fig. 4. Elements with the difference in atomic diameters of no more than 15%, i.e., with the ratio of atomic diameters of the alloying elements to the atomic diameter of titanium from 0.88 to 1.15, can form (based on the Hume-Rothery rule) substitution solid solutions.

10

VALENTIN N. MOISEYEV

1.7

5

1.6 1.5 1.4 4 1.2 1.1 1.0

3 Substitution solid solutions No solid solutions

0.9 0.8 0.7

2

Ratio of diameters

Goldschmidt atomic diameters, kX

1.3 No solid solutions

0.6 0.5 0.4 1

Interstitial solid solutions

0.3 0.2 0.1

0

0

Figure 4 Position of the atomic diameter of titanium with respect to the atomic diameters of alloying elements.

Based on the Hegg rule, formation of interstitial solid solutions is feasible when the ratio of atomic diameters of the interacting elements is less than 0.59. The elements forming chemical compounds with titanium are between the elements forming substitution and interstitial solutions. Most alloying elements are capable of forming, within a broad concentration range, substitution and interstitial solutions with one of the two or with both modifications of titanium. Interstitial solid solutions are formed with elements with the smallest ratio of their atomic diameter to that of titanium. Among other factors affecting the formation of solid solution, of great importance is the valence of solvent metal and dissolved metal. Titanium is capable of forming solid solutions with transition metals of similar electronic configuration, which contain unpaired α-electrons. These metals are zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten. Within a smaller range of concentrations, titanium alloys form solid solutions with other transition metals, which contain paired α-electrons. They include such metals as manganese, iron, cobalt, nickel, palladium and platinum.

TITANIUM ALLOYS: RUSSIAN AIRCRAFT AND AEROSPACE APPLICATIONS

11

The type of lattice of the metal dissolved is of great significance. Transition elements with a body-centered lattice stabilize the high-temperature body-centered cubic modification of β-titanium. Titanium-based interstitial solid solutions form alloys with hydrogen, oxygen, nitrogen and carbon – the elements with small atomic diameters with respect to titanium. The size and structure factors are determined to a considerable degree by the position of alloying elements in Mendeleyev’s Periodic Table. The structure of electronic shells defines the character of interatomic bonds and specifies the chemical essence of the alloying processes. The basis for considering of the structure of titanium alloys with groups of elements can be the character of the phase diagrams. The binary phase diagrams can be classified into groups by the character of the liquidus lines near the titanium ordinate, and the shape of a segment characterizing the secondary transformation in the systems can be used to divide the groups into subgroups. With more complex phase diagrams, this classification has the form presented in Fig. 5. This classification of the phase diagrams is most convenient for analyzing the formation of solid α- and β-solutions of titanium with various alloying elements and impurities. Consideration of the phase diagrams of titanium with Periodic Table elements shows that at room temperature only some of them form rather broad regions of solid α-, α+β- or β-solutions. Table 3 shows some characteristics of low-temperature segments of the phase diagrams of titanium with elements, which are of greatest interest as alloying elements in the development of commercial alloys based on solid α- and β-solutions or are impurities in titanium. Analysis of the phase diagrams in the region of the existence of solid α- and β-solutions of titanium with Periodic Table elements shows that the choice of elements suitable for developing commercial alloys is limited. Zirconium gives a continuous series of solid solutions with titanium. Molybdenum and vanadium form continuous solid solutions with β-titanium and are restrictedly dissolved in α-titanium. Niobium and tantalum form similar phase diagrams with titanium. Chemical compounds were found in the region of high concentrations in some alloys of titanium with this group of elements. Aluminum and tin are distinguished by a significant solubility in α-titanium, which makes it possible to consider them as important alloying elements for commercial titanium alloys. Aluminum, being a rather potent strengthener, is of greatest interest in this respect. Chromium, manganese, iron, cobalt, nickel, copper and tungsten, which dissolve little in α-titanium are restrictedly used as alloying elements due to the possibility of eutectoid transformation. The use of these elements in commercial titanium alloys is determined by the rate of eutectoid transformation and the effect of eutectoid formed on the physical and mechanical properties. The value of an alloying element in the development of commercial titanium alloys is determined by such factors as the properties of solid α- and β-solutions of

12

VALENTIN N. MOISEYEV

Ia

Ib

Ic

l

l

l l+β

l+β

I

l+β

β

β

β

α+β

α+β α

α+β

α Zr, Hf

V, Nb, Ta, Mo, Re

IIa

β-Ti + α-Th

α+β β + Tin Xm

α

α + Tin Xm

l

l + Tin Xm

l+W

l l+β

β + Tin Xm α+β

β β+W α+β

α α + Tin Xm

O, N, Sn

IIIc

l+β

β

α+β α

Th

IIIb

l+α l+β

α-Ti + α-Th

B, Ce, La, Ge

IIIa

III

l

β + Tin Xm

α α + Tin Xm H, Si, Mn, Fe, Co, Ni, Cu, Ag, Au

β

IIc

l+β

β

β α+β

l

Cr, U

l

l+β

l+β

α + Tin Xm

IIb

l

II

α

β + Tin Xm

C, Al

α

α+β W, Pb

Figure 5 Phase diagrams of titanium binary alloys with various elements.

elements with titanium, by the ultimate solubility of elements in α- and β-titanium and by the effect of an intermetallic component on the properties of the alloy. A predominant number of titanium-alloying element phase diagram has been plotted for an equilibrium achieved at a given temperature within several hundred or, at best, several thousand hours, whereas under real conditions titanium alloys are at operational temperatures for tens of thousands of hours. Besides, under real operational conditions the metal experiences stresses which can exert a significant effect on the character of phase transformations in the metal. The regions of the existence of solid-solution phases in various titanium alloys are given in Table 3.

TITANIUM ALLOYS: RUSSIAN AIRCRAFT AND AEROSPACE APPLICATIONS

13

Table 3 Regions of existence of solid-solution phases in various titanium alloys. Alloys Type of phase diagram

Alloying element, wt. % α

α+β

β

At temperature, °C

Zr Hf

Ia Ia

0–100 0–100

– –

– –

20 20

V Nb Ta Mo

Ib Ib Ib Ib

0–2 0–4 0–9 0–0.5

2–30 4–50 9–70 0.5–30

30–100 50–100 70–100 30–100

20 20 20 20

Cr

Ic

0–0.5

–

–

670

H Si Mn Fe Co Ni Cu Sn O N

IIa IIa IIa IIa IIa IIa IIa IIIa IIIa IIIa

0–0.2 0–0.3 0–0.5 0–0.5 0–1.0 0–0.2 0–2.1 0–18.6 0–3.2 0–2.5

– – – – – – – – – –

– – – – – – – – – –

320 600 550 615 685 770 798 865 400 600

C Al

IIIb IIIb

0–0.5 0–7

– –

– –

920 20

W

IIIc

0–0.8

–

–

715

1.3 SOLID α- AND β-SOLUTIONS IN TITANIUM ALLOYS A feature of the interaction of titanium with other alloying elements is their influence on the temperature of the allotropic transformation of titanium. This factor determines to a considerable degree the diversity of properties of titanium alloys and the possibility of changing them by heat treatment. Alloying elements increasing the statistical significance of atomic stable configurations d5 decrease the temperature of polymorphic transformation. Alloying elements in titanium dissolve in those phases, in which they increase forces of atom–atom interaction and enhance their relative stability. Thus, aluminum increases the elastic constants of solid α-solution and expands the temperature interval of the existence of the α-phase. Molybdenum and vanadium, which stabilize the β-phase, decrease the elastic constants of α-solution, thus increasing these characteristics for β-solution.

14

VALENTIN N. MOISEYEV

°C 940 Al

β

920

α

900 880

+

α-Al β-Sn β-Mo

β

α+β

860

Sn α-Al α-Sn β-Mo

840 α

α+β

820

Μο

800 0

2 4 6 8 10 Percentage of alloying addition

Figure 6 Three types of titanium phase diagrams. T, °C

2

Tn2 Tn1

1

T1 α Ti

β

α+β C1α

C1

C1β C2β

β-Stabilizing element, %

Figure 7 A scheme of “titanium–β-stabilizing element” phase diagram illustrating the effect of the α-stabilizing element on the boundaries of phase regions in binary (1) and ternary (2) alloys.

With respect to titanium, alloying elements are divided into elements stabilizing the α-phase and those stabilizing the β-phase in titanium. Elements having little effect on the temperature of allotropic transformation in titanium (tin, zirconium) are singled out into a group of neutral strengtheners (Fig. 6). Elements of this type have an effect on the structure and properties of alloys, which differs from that of typical α- or β-stabilizing elements.

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15

A generalized titanium–β-stabilizing element phase diagram, which covers α-, α+β- and β-regions in solid state (at low temperatures) is given in Fig. 7. This diagram is valid for elements isomorphic to β-titanium, such as vanadium, niobium, tantalum and molybdenum. It could also be used in consideration of titanium alloys with eutectoid-forming elements, if solid β-solution does not undergo eutectoid transformation. A generalized diagram consists of two curves plotted from a common origin, which corresponds to the temperature of titanium allotropic transformation. The lower curve restricts the region of the existence of solid α-solution, and the crosssection of this curve with the abscissa axis in point Cα corresponds to the ultimate concentration of solid solution at room temperature. The upper curve determines the boundary between the α+β- and α-regions, and the point of its intersection with the abscissa axis Cβ corresponds to the minimum required concentration of the β-stabilizing component to form solid β-solution stable within the entire temperature range. This diagram is valid both for binary alloys of titanium with a β-stabilizing element and for alloys with several alloying elements stabilizing the β-phase in titanium. At a certain temperature (for instance, T1), irrespective of the content of the β-stabilizing element in the alloy, its content in the α- and β-phases is constant (C1α and C1β, respectively). An increase in the content of the β-stabilizing element in the alloy with titanium at a given temperature in the α+β-region is accompanied by an increase in the amount of the β-phase, without a change of its chemical composition. The physical and mechanical properties of titanium alloys, representing solid αand β-solutions, are determined by the properties of these solutions. Table 4 presents the physical and mechanical properties of a solid α-solution, as determined during the tension of annealed titanium alloys cantoning alloying elements within the range of solubility in α-titanium. The properties of a solid β-solution were determined on alloys quenched from the β-region and having a mechanically stable β-phase, i.e., the β-phase which does not undergo any transformations distorting the true properties of the phase during the tension test. As it follows from Table 4, the ultimate strength of saturated solid α-solutions in all alloys of titanium with β-alloying elements are small as compared with the strength of non-alloyed titanium. It varies within the range of 47 to 59 kgf/mm2. At the same time, solid α-solutions with such elements as Al, Sn, Zr and O have a higher ultimate strength. The ultimate strength of investigated solid β-solutions in binary titanium alloys with various β-stabilizing elements are considerably larger – from 54 up to 108 kgf/mm2 in alloys of titanium with niobium or cobalt. As a rule, segments of multicomponent phase diagrams in the region of solid α-solution are plotted by extrapolation with insufficient significance. Nevertheless, data available suggest that the solubility of β-stabilizing elements in α-titanium should decrease proportionally to the amount of elements occurring in the alloy simultaneously. Therefore, there is no sufficient reason to assume that the strength of solid α-solution could be considerably increased by alloying with several β-stabilizing elements. From this point of view, hardening of solid α-solution by

16

VALENTIN N. MOISEYEV

β- and α-stabilizing elements or by neutral strengtheners simultaneously is of great interest. In this case, as it follows from the ternary “titanium–β-stabilizing element– α-stabilizing element or neutral strengthener” phase diagrams, the solubility of β-stabilizing elements in α-titanium does not decrease; in some cases it even increases. Table 4 Some physical and mechanical properties of solid α- and β-solutions of binary titanium alloys. Element

Content, %

Properties σB, kgf/mm2 σ0.2, kgf/mm2

E, kgf/mm2

δ5, %

42 50 42 46 45 48 43 49 44 72 40 57 38 41 41 45

11000 11050 11000 11000 11100 11050 11050 11050 11070 11200 11300 11250 11050 11050 11050 11070

40 38 45 44 33 27 30 24 25 19 42 36 32 28 35 28

77 91 45 65 98 97 100 101 98

10100 10200 10000 10100 10200 10000 10000 10000 10300

24 18 19 20 13 18 15 14 16

α-alloys

Mo V Nb Ta Fe Cr Mn Co W Al O O Zr Zr Sn Sn

0.5 2.0 4.0 9.0 0.5 0.5 0.7 0.7 0.8 7.5 0.15 0.30 3.0 6.0 2.0 4.0

Mo V Nb Ta Fe Cr Mn Co W

18 20 50 50 9 12 13 9 30

53 58 47 50 59 53 59 57 59 83 48 64 48 58 48 53

87 95 54 72 101 101 106 108 100

β-alloys

As for solid β-solutions, in the interaction of titanium with two or more β-stabilizing elements the phase fields are mainly additive to the binary systems of titanium with each β-stabilizing element. In multicomponent systems of titanium with β- and α-stabilizing elements the region of the existence of α+β-alloys slightly expands, and the β-region becomes

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17

narrower. The character of interaction of α- and β-stabilizing elements in titanium is schematically shown in Fig. 7. Besides boundary 1, which separates the (α+β)- and β-regions, there is boundary 2, which corresponds to alloys additionally containing an α-stabilizing element (for instance, aluminum or oxygen), which increases the → β transformation. As it follows from the diagram, the alloy temperature of α+β ← containing no α-stabilizing element (of concentration C1) has a β-phase with the concentration C1β of the β-stabilizing element at a temperature T1. In the alloy additionally containing an α-stabilizing element, the concentration of the β-stabilizing element in the β-phase would be higher (C2β), although the amount of β-phase in the alloy decreases. All this provided that the solubility of the β-stabilizing element in α-titanium remains the same.

1.4 CHEMICAL COMPOUNDS IN TITANIUM ALLOYS Intermetallic compounds in titanium alloys are numerous and diverse. Intermetallics formed in the regions adjacent to solid α- and β-solutions are of commercial interest for developing new alloys. Of special interest are intermetallic compounds formed in alloys of titanium with aluminum, because aluminum occurs in almost all commercial alloys as an alloying additive. In Ti–Al alloys, solid α-solution is formed at a temperature of 1020–1100°C in the peritectoid reaction. As the temperature decreases, the solubility of aluminum in α-titanium goes down, being about 6.5% at room temperature. Solid α-solution oversaturated with aluminum tends to breakdown to form the α-phase at a slow cooling or isothermal heating. The α-phase has the stoichiometric composition of the type of Ti3Al. Formation of this phase during aging is accompanied by a considerable loss of ductility without any noticeable increase of hardness or strength of the alloy. As in some titanium alloys the content of aluminum reaches 7% or more, the breakdown of the solid α-solution in Ti–Al alloys is the phenomenon to be taken into account. At an aluminum content of up to 7%, annealed alloys (800°C, 1 h, cooling on the air) and after heating up to 500°C for 100 h have a satisfactory ductility. As the content of aluminum is increased further, the ductility characteristics decrease after a prolonged heating, which is due to the formation of the α2-phase in ever-increasing amounts. An alloy containing 9% Al has a low ductility in annealed and aged state (δ5 = 2.5%, ψ = 10%). At the same time, alloys quenched from 800°C in water or air have a good ductility at an aluminum content of up to 10%. In this case, an accelerated cooling makes it possible to suppress α → α2 transformation which decreases the ductility of the alloy (Fig. 8). The alloys containing up to 7% Al, quenched and aged for 100 h at temperatures from 400 to 500°C do not change their mechanical properties. As a result of aging at 450 and 500°C the alloys containing over 7% Al, become embrittled and their

18

VALENTIN N. MOISEYEV

σB, MPa

1000 Annealing

2

900

3

800

1 Quenching

700

Annealing

60 15

2 1

δ, %

10

Quenching

2 3

5

Annealing

0 1 ψ, %

30

Quenching

20 3 10 0

2 5

6

7

8

9

10

Al, %

Figure 8 Mechanical properties of Ti–Al alloys quenched in water from 900°C and aged at 400°C (1), 450°C (2) and 500°C (3) for 100 h as compared with annealed state (800°C, for 1 h; cooling at a rate of 3°C/min). Al, % 8 α + α2 + β 7 1 6

2

5 0

2

4

6

8

10

12 Mn; Mo, %

Figure 9 The interphase boundary between the regions α+β/α+β+α2 in Ti–Al–Mo (1) and Ti–Al–Mn (2) systems.

ductility proves the same as after annealing and slow cooling. The temperature of 400°C is insufficient for intensive breakdown of the oversaturated α-phase and the alloys preserve their properties which are close to the quenched state. It should be noted that an alloy with 10% Al showed a noticeable decrease of ductility after aging at 400°C. Evidently, the more oversaturated the α-phase is by aluminum, the more apt to breakdown it is. Aging of alloys for 1 h shows that a sharp decrease of ductility is observed after heating at temperatures over 480–500°C. Addition of a third element to binary Ti–Al alloys considerably affects the interface of the α/α+α2 phase regions. Contradictory data on the effect of various

TITANIUM ALLOYS: RUSSIAN AIRCRAFT AND AEROSPACE APPLICATIONS

t, °C

α + β + TiMn

3

500 400

19

1

2 α+β

300 500

(a) 1

2

Α

400 3 300 200 0

2

4

6 Mn, %

8

(b) 10

Figure 10 The interphase boundary between the regions α+β/α+β+α2 in Ti–Al–Mn systems.

elements on the position of the interface of the phase fields have been published. Nevertheless, it could be maintained that most elements narrows down the region of the existence of the α-phase. Figures 9 and 10 give as an example the interphase boundary between the regions α+β/α+β+α2 in titanium alloys with an isomorphic β-stabilizing element, molybdenum, and an eutectoid-forming element, manganese, which were annealed at a temperature of 800°C for 1 h and cooled at room temperature at 2–4°C/min. Mechanical properties of alloys – hardness, ultimate strength, reduction of area, impact strength, impact strength of a specimen with a crack – are more sensitive characteristics to fix the initiation of α2-phase formation, which give this phenomenon a greater practical importance. By their effect on the contractions of the region of solid α+β-solutions, β-stabilizing elements can be arranged into a series by the decrease of solubility in α-titanium: zirconium, tin, vanadium, molybdenum, manganese, chromium, iron – the same series as their stabilizing effect on the β-phase. Most transition elements, such as chromium, manganese, iron, cobalt, nickel, copper, silver, silicon, beryllium, bismuth, lead, and others dissolve in α-titanium insignificantly but form solid solutions with β-titanium. These solid solutions decompose by the eutectoid reaction to form solid α-solution and an intermetallic component. In binary alloys of titanium with copper, silver and gold the eutectoid decomposition of the β-phase is intensive. In alloys of titanium with chromium, manganese, iron and cobalt the eutectoid process is slack. Nickel occupies an intermediate position. Depending on the β-stabilizing element, eutectoid transformation proceeds by the reaction β → α + chemical compound or α1 → α + chemical compound. The rate of eutectoid transformation determines an increase of eutectoid transformation temperature (Table 5). Assessment of the effect of eutectoid on the mechanical properties during the heating of an as-quenched alloy up to the eutectoid transformation temperature is

20

VALENTIN N. MOISEYEV

impossible due to the formation of intermediate metastable phases or states – ω-phase and dispersed formations of α- and β-phases. To exclude the side metastable transformations of the β-phase, in assessment of the effect of eutectoid on the mechanical properties of Ti–Fe, Ti–Cr, Ti–Mn, Ti–Co and Ti–W alloys they were subjected before heating to a stabilizing annealing, which included slow cooling from the annealing temperature. Then the alloys were heated near the eutectoid transformation temperature for various times. The phase composition of the alloys after a prolonged heating was determined by the X-ray method under an optical microscope and an electron microscope. Table 5 Comparative rate of the eutectoid reaction in alloys of titanium with various elements. Element

Critical concentration, wt. %

Eutectoid composition, wt. %

Eutectoid formation temperature, °C

Manganese Iron Chromium Cobalt Nickel Copper Iron Silver Silicon

6.5 4.0 8.0 7.0 8.0 13.0 β-phase not fixed β-phase not fixed β-phase not fixed

20 15 15 9 7 7 16 22.8 0.9

550 600 675 685 770 790 830 855 860

Figure 11 shows the phase regions of the existence of the eutectoid component in Ti–Mn and the change of ductility depending on the manganese content, temperature and duration of heating the alloys preannealed at 800°C for 1 h and cooled at a rate of 2–4°C/min. t, °C 1

500

α + β +TiCr2

2 3

400

α+β

300 500

(a) Α

1

400

2 3

300

(b)

200 0

2

4

6 Cr, %

8

10

Figure 11 The change of position of the interphase boundary between the regions α+β/α+β+TiMn and a related decrease of plasticity (reduction of area by 25%) in the Ti–Mn system as a function of temperature and time of heating.

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T, °C α'(α'')

882 T1

α'(α'') + β β+ω

Tcr T2

α+β

α Ti

β

Mo

Me

C1

Cα

β Ccr C2

Cβ

β-Stabilizing element, %

Figure 12 The change of position of the interphase boundary between the regions α+β/α+β+TiCr and a related decrease of plasticity (reduction of area by 25%) in the Ti–Cr system as a function of temperature and time of heating.

Figure 12 shows the same for Ti–Cr alloys. In binary alloys of titanium with iron and tungsten, an increase of the duration of heating at certain temperatures leads to the coagulation of the intermetallic component, to a decrease of strength and a rise of ductility. This phenomenon is rather pronounced in Ti–Fe alloys (Table 6) and, to a smaller degree, in Ti–W alloys. In such alloys, the formation of eutectoid is not accompanied by any significant deterioration of mechanical properties. Table 6 Change of the mechanical properties of annealed Ti–Fe alloys depending on the duration of heating at 500°C (rod, 12-mm dia). 500°C, h

100 1000 10000

2% Fe

4% Fe

6% Fe

σB, kgf/mm2

δ5, %

ψ, %

σB, kgf/mm2

δ5, %

ψ, %

σB, kgf/mm2

δ5, %

ψ, %

60 60 59 52

20 20 20 21

56 55 55 53

73 70 66 52

20 19 19 23

45 42 40 42

81 80 76 58

17 16 15 19

30 26 24 33

Addition of a β-titanium isomorphic element into alloys of titanium with eutectoid elements expands the region of the existence of solid α+β-solutions. Addition of a β-titanium isomorphic element (Mo) into alloys of titanium with an eutectoid element (Cr) stabilizes the β-phase and the eutectoid transformation is considerably slowed down. Still, it is not suppressed and completes given enough time for the stabilization (Table 7). Table 7 gives the phase composition and the ratio of the transverse contraction of a specimen, subjected to isothermal heating, to the transverse contraction of the initial specimen, depending on the time of heating the alloys.

22

VALENTIN N. MOISEYEV

Table 7 Phase composition and ductility (% of the initial value) of Ti–4% Cr–Mo alloys depending on the Mo content and duration of heating at 500°C (rod, 12 mm dia; annealing at 800°C for 1 h, cooling at 2–4°C/min). Holding time at 500°C, h Initial 100 1000 10000

Content, wt. % 0

2

4

8

α+β (100%) α + β + TiCr2 (80%) α + β + TiCr2 (65%) α + β + TiCr2 (35%)

α+β (100%) α+β (80%) α + β + TiCr2 (70%) α + β + TiCr2 (32%)

α+β (100%) α+β (85%) α + β + TiCr2 (70%) α + β + TiCr2 (33%)

α+β (100%) α+β (87%) α+β (75%) α + β + TiCr2 (31%)

Heating at a temperature of 500°C for 1000 h, irrespective of the Mo content in the alloys, leads to a ductility decrease due to eutectoid transformation. Addition of up to 6% Al, which is a stabilizer, or of neutral strengtheners (up to 6% Zr or 4% Sn) have no effect on the character of eutectoid transformation. Its kinetics is significantly affected by α-stabilizing interstitial elements, oxygen and nitrogen. A higher oxygen content contributes to a decrease in the temperature of the onset of eutectoid transformation and makes the process more complete (Table 8). Table 8 Phase composition of a Ti–4% Cr alloy depending on the oxygen content and the temperature of isothermal heating for 1000 h. Oxygen content, wt. %

350°C

400°C

0.005 0.010 0.015 0.020 0.025

α+β α+β α+β α+β α + β + (TiCr2)*

α+β α+β α+β α+β α + β + TiCr2

Phase composition after heating for 1000 h at temperature 450°C

500°C

α+β α+β α+β α + β + (TiCr2)* α + β + (TiCr2)* α + β + (TiCr2)* α + β + TiCr2 α + β + TiCr2 α + β + TiCr2 α + β + TiCr2

* traces

1.5 STABILITY OF SOLID α- AND β-SOLUTIONS This section considers the stability of solid α- and β-solutions in multicomponent systems depending on temperature, stress and duration of these factors by example of various commercial titanium alloys. Of greatest interest in this respect are those concentration regions of commercial alloys, which are in the vicinity of the phase fields. These are the α/α+α2

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23

interface in alloys rich in aluminum and the α/α+TixMy interface in alloys of titanium with eutectoid-forming elements. Most commercial pseudo-α-Ti alloys use eutectoid-forming elements as a β-stabilizing element and aluminum as an α-stabilizing element. Alloy OT4 containing 4.2% Al and 1.8% Mn (plate specimens, 2.5 mm thick) was heated at a temperature of 400°C for 30 000 h without stress and under stress. As the duration of heating was increased, the β-phase was stabilized, its quantity decreased and it enriched with manganese. No intermetallic compound was found. Further increase in the duration of heating led to a minor increase of strength and a decrease of ductility of the alloy. Application of stress did not result in any significant changes of the properties and structure of the alloy. An experimental alloy containing 7.3% Al and 1.2% Mn (plate specimens, 3 mm thick) was heated at a temperature of 500°C for up to 10 000 h without stress and under stress. The structure and properties of the alloy after these treatments are given in Table 9. Table 9 Mechanical properties of experimental alloy Ti–7.3Al–1.2 Mn. State

Initial (annealed) 500°C, 100 h

500°C, 1000 h

500°C, 10000 h

Steady-state phase composition α+β

α + (β) ----------------α + (β)

α + (β) ------------------------------------------α + ( ( β ) ) + ( ( α2 ) ) α +((β))

Mechanical properties σB, MPa

σ0.2, MPa

δ5, %

a, kgf/cm2

1040

960

18

4.2

1080 -----------1110

990 -----------1030

18 -----15

4.0 ------3.0

1100 -----------1170

1030 -----------1120

16 -----7

3.2 ------1.8

1150

1070

12

2.6

Note: ( ) very small, (( )) traces; numerator, no stress; denominator, stress of 300 MPa.

An increase in the duration of heating led to the stabilization of the β-phase, its amount decreased. The intermetallic compound Ti–Mn was not observed. Despite a considerable decrease of ductility and an increase of strength after a prolonged heating, the presence of the α2-phase was not observed, either, but its effect on the mechanical properties was rather significant. The mechanical properties and phase composition of the annealed test alloy containing 7.3% Al and 1.2% Mn (plate specimens, 3 mm thick) after a prolonged heating without stress and under stress are given in Table 9. The α2-phase in the Ti–7.3% Al–1.2% Mn test alloy was found by the X-ray diffraction analysis only at an Al content more than 7.8% or stress more than 300 MPa applied for no less than 10 000 h. The character of the stability of solid α-solution can be more vividly illustrated by example of test high-temperature titanium alloys VT18. The alloys have the following chemical composition:

24

VALENTIN N. MOISEYEV

Alloy

Ti

Al

Mo

Zr

Nb

Si

VT18

basis

7.8–8.3

0.4–0.8

7.8–8.5

0.8–1.2

0.16–0.30

Heat treatment of the alloys consists of annealing at 900–980°C for 1–4 h and cooling in air. After this annealing, one more heating at 600°C for 6 h with subsequent cooling in air is admissible. This heat treatment can stabilize the structure to some extent. However, the subsequent prolonged heating at working temperatures leads to a sharp decrease of ductility (Table 10). Table 10 Mechanical properties and phase composition of alloy VT18 after heating at 600°C for various times. Duration of heating, h

Phase composition

Initial state 100 200 500 1000

α +((β)) α +((β)) + ((α2)) α +((α2)) α +((α2)) α +((α2))

Mechanical properties σB, kgf/mm2

δ, %

ψ, %

108 110 110 113 115

15 4 4 3 3

28 10 9 9 8

Note: 1. Test specimens from a rod 30-mm dia. 2. (( )), traces.

As it follows from these data, the amount of aluminum in alloy VT18 is more than its solubility in α-titanium. Stabilization of the structure at 600°C leads to the formation of the α-phase, which results in the decrease of ductility. A prolonged heating under stress can, probably, also cause the formation of a chemical compound, which contributes to the decrease of ductility. The oxidation of the surface at a prolonged heating in air can also affect the ductility of the metal. It should be noted that transformations in solid α-solution at a prolonged stabilizing annealing and under stress, related to the formation of the α2-phase, is also observed in the known alloy Ti–8% Al–1% V–1% Mo. The thermal instability of solid β-solution can be illustrated by example of pseudo-β-titanium alloy VT15 (Ti–3% Al–7% Mo–11% Cr), which is a structural analog of the known alloy V120-VCA, containing 15% V instead of molybdenum. Despite the high content of an eutectoid element (Cr), the β-phase in this group of alloys is rather stable and undergoes a eutectoid transformation only under certain conditions. Table 11 gives the structure and mechanical properties of alloy VT15 after heat treatment in the following sequence: 750°C for 30 min, cooling in water, aging at 480°C for 20 h; then 560°C for 15 min, cooling in air, after heating without stress and under stress of σB = 600 MPa, which causes residual deformation 0.2%. Heating of the alloy at temperatures up to 350°C for up to 1000 h is accompanied by a

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25

decrease of ductility. Application of stress in the first 100 h leads to the embrittlement of the specimen (Table 11). Table 11 Phase composition and mechanical properties of alloy VT15 (plate specimens, 2.5 mm thick) in thermally hardened state after long-term heating without stress and under stress. State

Initial 300°C, 100 h 300°C, 1000 h 300°C, 100 h, 600 MPa 300°C, 1000 h, 600 MPa

Established phase composition β β β+α α+β

Mechanical properties σB, MPa

σ0.2, MPa

δ5, %

astandard, kgf/cm2

1380 1310 7 0.8 1370 1320 6 0.7 1420 1340 4 0.4 Embrittlement without signs of ductility Embrittlement without signs of ductility

The above considerations indicate the need for strict regulation of the amount of alloying elements and impurities in alloys in the cases when their contents are within the limits close to the solubility of solid α- and β-solutions. On the other hand, real conditions in which alloys are to be operated should also be taken into account.

1.6 PHASE TRANSFORMATIONS IN TITANIUM ALLOYS Phase transformations in α+β titanium alloys are characterized by a great diversity and complexity. This concerns, first of all, solid-solution alloys with transition elements used most often for alloying in commercial titanium alloys. The metastable structural state in titanium alloys occurs during operations associated with heating and cooling of the metal – hot deformation, heat treatment, welding, etc. Metastable structures formed have a significant effect on the physical and mechanical properties and this is to be taken into account when processing titanium alloys. Structural transformations occurring during the sharp cooling of titanium alloys with various contents of β-stabilizing elements can be followed using a generalized “titanium–β-stabilizing element” phase diagram (Fig. 13). The diagram is valid for isomorphic β-stabilizing elements, i.e., for elements forming no chemical compounds with titanium. To an approximation, this diagram can be also used for β-stabilizing elements forming eutectoid or peritectoid systems with titanium. The generalized diagram consists of two curves originating from one common point, which corresponds to the titanium allotropic transformation temperature. The lower curve restricts the region of the existence of solid α-solution, and the crosssection of this curve with the abscissa axis in point Cα corresponds to the ultimate concentration of solid solution at room temperature. The upper curve determines the border between the (α+β)- and β-phases, and the point of its cross-section with the

26

VALENTIN N. MOISEYEV

σB, kgf/mm2

100

Fe Mn

Al

Mo

80

V

Sn

60

Cr Co

Ta Zr

40

Nb

40

Zr

Nb

Ta

Mn Cr

δ5, %

30 Sn

20 Al

V

Mo

Co Fe

10 0 0

2

4

6

8

Al, Zr, Sn, %

10

0

2

4

6

8

Ta, Nb, V, Mo, %

10

0

2

4

6

8

10

Fe, Cr, Mo, Co, %

Figure 13 Change of phase composition for titanium–β-stabilizing element alloys as a function of the quenching temperature.

abscissa axis, Cβ, corresponds to the minimal concentration of the β-stabilizing element required to form solid β-solution stable within the entire temperature range. This diagram is valid both for binary titanium alloys with β-stabilizing elements and for alloys with several alloying elements, which stabilize the β-phase in titanium. Under equilibrium conditions, the diagram consists of three regions of solid solutions: α, (α+β) and β. As the concentration of the β-stabilizing element is gradually increased, the first process observed during quenching from the β-region is martensite transformation, i.e., the β-phase is not fixed in quenching. As the concentration is increased further, at some point no martensite transformation will occur and a 100% structure will be observed. The concentration of the β-stabilizing element, corresponding to this point, is called the critical concentration (Ccr). If a vertical line is drawn through point Ccr, it will cross the border of the β-region at the point corresponding to the critical temperature, Tcr. As martensite transformation occurs in a temperature range, the dashed lines show the onset (Mo) and end (Me) of martensite transformation. A metastable ω-phase is formed at a concentration of the alloying element within the range from Ccr to C2 at a sharp cooling from the β-region. As this transformation is never complete, alloys have the β+ω phase composition. At a concentration exceeding the value of C2, only the high-temperature β-phase is registered as the result of quenching. A change of the quenching temperature in the (α+β)-region is accompanied by a change in the phase composition of the alloy of a given concentration. Four groups of alloys are to be distinguished by the character of the change of the phase composition at a sharp cooling from various temperatures. The first group includes alloys with the concentration of β-stabilizing elements of up to C1, i.e., alloys which in quenching from the β-region have an exceptionally

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27

α′(α″) structure. After quenching of these alloys from temperatures of the (α+β)-region, in the range from the temperature of total polymorphic transformation up to T1, their structure is a combination of the α′+α+β-phases. And after quenching from temperatures below Tcr, they have the (α+β) structure. The second group are alloys with the concentration of alloying elements from C1 to Ccr. They have the α′(α″)+β structure and in quenching from the β-region, martensite transformation in them does not complete. Alloys of this group after quenching from temperatures of polymorphic transformation up to Tcr have the (α′+α″+β) structure, and from temperatures below Tcr the (α+β) structure. For alloys of the third group with the concentration of the β-stabilizing elements of Ccr to C2, quenching from temperatures of the β-region or from the polymorphic transformation temperature up to T2 is accompanied by the transformation of part of the β-phase to the ω-phase. Alloys of this type have the (β+ω) structure after quenching. Titanium alloys of a precritical composition, which have the β-phase at the quenching temperature, with the concentration of β-alloying elements from Ccr to C2, may form a small amount of ω-phase during quenching. The effect of this phase on the properties is insignificant. Alloys of this group have an (α+β) structure after quenching from temperatures below T2. Alloys of the fourth group have an exceptionally β structure after quenching from temperatures above total polymorphic transformation; and from temperatures below polymorphic transformation, a (β+α) structure. The above scheme gives a general idea of the transformations occurring in solid-solution titanium alloys at their cooling from various temperatures during heat treatment, thermomechanical treatment, hot deformation, welding and other treatments. The presence of metastable structures in titanium alloys has a significant effect on their physical, mechanical and operating properties. In a manufactured item, titanium alloys are, with rare exceptions, in a stable state, i.e., after annealing or hardening heat treatment. They contain no metastable structural components. However, metastable structural states often occur in various technological operations and in repair work. Some phase components have little effect on the properties, but the effect of others, for instance, the ω-phase, is rather considerable. In alloys with α- and β-stabilizing elements, when their concentration is close to the solubility in α-titanium, a martensite transformation of the β-phase into the α′-phase occurs during accelerated cooling. The α′-phase can be considerably oversaturated by alloying elements. It has a hexagonal structure, the same as α-titanium. The X-ray of the α′-phase features blurred interference lines characteristic of hexagonal titanium, which is due to the generation of internal stress in the lattice. Under an optical or electron microscope, the α′-phase has a typical needle-like structure. The martensite α′-phase has no high hardness and strength, as martensite of steel has. However, formation of the α′-phase in alloys with oversaturation of solid α-solution by alloying elements leads to a noticeable increase of hardness and strength. Another metastable structural component formed in titanium alloys is the α″-phase. It is an even more oversaturated solid solution based on α-titanium and

28

VALENTIN N. MOISEYEV

occurs in alloys containing elements which are isomorphic to α-titanium – Mo, V, Nb and Ta. An X-ray of alloys with the α″-phase structure is characterized by cleavage of some interference lines peculiar to a hexagonal structure. The cleavage grows as the concentration of the β-alloying element increases. There is a similarity between the α″- and α′-phase, but the α″-phase is distinguished by a lower symmetry – rhombic instead of hexagonal. The rhombic α″-phase can be considered to be an intermediate stage between the body-centered and hexagonal structures. Under an optical or electron microscope, the α″-phase has a needle-like martensite structure. The mechanical properties of the α″-phase are close to the properties of a stable mixture of solid α- and β-solutions of a similar chemical composition. An exceptionally metastable β-solution is observed under an optical microscope in titanium alloys with a high concentration of β-stabilizing elements during sharp cooling. The concentration of β-stabilizing elements at which a solid solution is observed is called a critical concentration and corresponds to an electron concentration equal to 4.18–4.21 el/atom. Earlier, mainly based on metallographic studies, it was believed that quenched alloys, in which α′- or α″-phase are not formed, are homogeneous, i.e., have a completely high-temperature solid β-solution. However, a homogeneous solid β-solution is usually characterized by a good ductility and low hardness. Detailed studies of a solid β-solution, close by its composition to a critical solution, showed for some systems of alloys that microstructurally a homogeneous quenched β-solution has an anomally high hardness and, therefore, is very brittle. A higher hardness and a brittleness of such alloys is explained by the presence of an ω-phase which can be formed under certain conditions during the decomposition of solid β-solution in quenching or aging only in titanium alloys with transition elements. Some investigators believe that the ω-phase should not be singled out into an independent phase, because it is coherent with respect to the matrix (β-phase). It should be considered to be a special state of the solid β-solution. Nevertheless, a serious effect of the ω-phase on the physical and mechanical properties of titanium alloys should be taken into account. Metastable solid solutions – α′, α″, ω and β – formed in accelerated cooling, under the action of temperature and stress can undergo transformations, which have a significant effect on the physical, mechanical and technological properties of titanium alloys. Decomposition of metastable α′-, α″-, ω- and β-phases during heating in the temperature range of 450–650°C makes it possible to obtain a considerable increase of strength at satisfactory ductility by disperse hardening. In conclusion, it should be noted that there is information on the use of the metastable state of titanium alloys. Thus, for instance, heat treatment in metastable solid α″- or β-solution makes it possible to significantly increase the deforming properties of (α+β)-titanium alloys and to slightly increase the crack resistance. Heat treatment of martensite-type (α+β)-titanium alloys in α″-phase enables a minor increase of strength with sufficient ductility preserved.

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1.7 ENHANCEMENT OF MECHANICAL STRENGTH OF TITANIUM ALLOYS High mechanical strength and high-temperature strength of titanium alloys, which are solid α- and β-solutions, can be achieved by alloying, heat treatment, mechanical hardening or a combination of these treatments. Using a sufficiently high strain hardening (50%), the strength of technical-grade titanium can be increased from 350 to 800 MPa. In strain hardening of titanium alloys based on the β-structure, the effect is even more significant: sheets from β-alloy (VT16) can be strengthened up to 1500 MPa by cold deformation by 50–70%. The strength characteristics of titanium alloys can be noticeably increased by thermomechanical treatment, i.e., by choosing an optimal combination of plastic deformation and heat treatment. Alloying of titanium with various elements makes it possible to obtain titanium alloys whose ultimate strength in annealed state reaches 1200–1300 MPa, and the application of a hardening heat treatment (quenching and aging) increases the strength even more. Properties and structure of binary and more complex titanium alloys with stabilized solid α- and β-solutions have been studied sufficiently well. If one considers alloying additives from the group of β-stabilizers, iron and manganese prove to be the strongest strengtheners, then come chromium, tungsten, molybdenum, vanadium, etc. One can be guided by the following average data of the hardening action of alloying elements, used in practical work with titanium alloys (Table 12). In the case of multicomponent alloys, the hardening effect would, probably, be the sum of hardening effects of all elements. Alloying elements, being dissolved in α- and β-phases, act as strengtheners, but also change the ratio of the phases in the structure of the alloys. Thus, the strength characteristics of solid-solution (α+β)-titanium alloys are determined by the strength of α and β components. Another factor determining the strength of these alloys is the heterogeneity of the structure, i.e., the dispersity of the mixture of α- and β-phases. An increase of the extent of interphase boundaries is accompanied by a significant increase of strength and reaches a maximum in alloys, which are a mixture of an approximately the same amounts of α- and β-phases. Table 12 Increase of ultimate strength of titanium in MPa per percent of alloying additive. Element Aluminum Tin Zirconium Molybdenum Vanadium Tungsten

∆σB, MPa

Element

∆σB, MPa

50.0 25.0 20.0 50.0 35.0 35.0

Manganese Chromium Iron Cobalt Silicon Niobium

75.0 65.0 75.0 55.0 12.0 15.0

30

β

α+β

α 50% α

50% β

t, °C

σB, Post annealing

VALENTIN N. MOISEYEV

α+β α

50% α

β 50% β

Figure 14 Change of the ultimate strength of annealed binary titanium alloys within a broad range of β-stabilizing elements.

This tendency can be illustrated by example of annealed Ti–Mo and Ti–V alloys (Fig. 14) and also by a generalized diagram (Fig. 15). Strength of titanium alloys in annealed and aged state can be significantly increased. The metastable α′-, α″-, ω- and β-phases formed during a sharp cooling of (α+β)-titanium alloys, depending on the content of β-stabilizing elements and → α transformation. Isothermal quenching temperature, are intermediate phases of β ← heating at temperatures over 350–400°C leads to the decomposition of the metastable phases to form an equilibrium (α+β) structure. At the first stage of decomposition at temperatures of 450–600°C, disperse particles of α- and β-phases are formed to lead to a significant hardening of alloys. A high strength at a satisfactory ductility can be achieved in various alloys with (α+β) structures by selecting quenching and aging modes. Depending on the type of β-stabilizing element, its content in the alloy, quenching temperature and aging mode the extent of hardening can differ. Figure 16 demonstrates a change of strength of heat treated alloys of titanium with various β-stabilizing elements within a wide range of compositions (covering α-, (α+β) and β-regions) by example of Ti–Mo and Ti–V alloys, which is typical of other alloys. The alloys were quenched and aged for maximum strength on condition of preserving a certain ductility ( δ 5 ≥ 3% , ( ψ ≥ 6%). As the quenching temperature was increased up to the α+β → β transition boundary and the aging temperature was decreased down to 450°C, the strength increased and ductility decreased in all (α+β) alloys. The data of the figure show that the strength, which can be obtained by

TITANIUM ALLOYS: RUSSIAN AIRCRAFT AND AEROSPACE APPLICATIONS

Mn, %

1 1 1.5 3

7 11 13 16 Mn, % 2 2 1

2

2 4

1.5 3 7 11 13 16 9 15 Mn, %20 V, % 1

2 4

9

σB, kgf/mm2

40 140

4

9

50

20

2

4

9

1

2 4 20

9

152

V, % Ta, %

20

50

50

20

Nb, %

120

20

15

V, % 2 1

B

2 , kgf/mm σB, kgf/mm2 σ σ,Bkgf/mm 2

2 σB, kgf/mm σB, kgf/mm2 σ , kgf/mm 2 B

Fe, % Cr, % 160 60 160 1 140 40 1 140 0.5 2 4 9 12 0.5 2 4 1 9 12 15 120 Fe, % Cr, % 2 120 160 100 1 2 100 2 140 80 2 80 120 60 60 100 40 0.5 2 4 9 12 0.5 2 4 9 12 15 40 80 1.5 3Fe, 7 9 11 13 16 20 5 % 30 Cr, % Mo, % 160 60 1 160 140 40 1401.5 3 5 7 9 11 13 16 20 30 2 120 Mo, % 120 160 1 100 100 140 2 80 80 120 60 1 60 100 40 401.5 3 5 7 9 11 132 16 20 30 80 50 4 9 4 9 20 Mo, % Nb, % 160 60

31

Ta, % 1

1 100 2

2

80 60 40 4

9

50

20 Nb, %

4

9

50

20 Ta, %

Figure 15 Ultimate strength of annealed and thermally hardened (quenched and aged) titanium alloys vs the content of β-stabilizing elements.

quenching and aging, increases with the increase of the β-stabilizing element up to the critical concentration. Critical-composition alloys, as a rule, can be heat treated up to maximum strength (Fig. 15). An increase of the amount of the β-alloying element in super-

32

σB, Post quenching and aging

VALENTIN N. MOISEYEV

βinstab → α + β

Ms

t, °

βinstab → α + β α'(α'') → α + β

α

α+β βinstab,α'(α'')

β

βinstab

Figure 16 Change of the ultimate strength of thermally hardened (quenched and aged) binary titanium alloys within the broad range of β-stabilizing elements.

critical-composition alloys contributes to the stabilization of the β-phase and the hardening effect during aging decreases. This change of the mechanical properties in heat-strengthened state is due to the rise of the heterogeneity of the structure and the increase of disperse formations of the α- and β-phases in aging with the concentration of the β-stabilizing element increased up to the critical concentration. It can be said that the maximal amount of β-phase undergoes a dispersive decomposition in alloys of critical composition. If we are to consider the volume of β-phase decomposed in aging as the difference between the volume fixed by quenching and the volume obtained after aging, then it is evident that critical-composition alloys are optimal for a given system of titanium with β-stabilizing element. Figure 16 shows the change of the phase composition of Ti–Mo alloys depending on the composition after quenching from the temperatures of the (α+β)region near polymorphic transformation. Quenching from these temperatures makes it possible to obtain the highest ultimate strength at a satisfactory ductility. The first dashed line shows the quenching temperature per maximal amount of metastable α′(α″)-phase or α′(α″)+β-phase. The second dashed line indicates the quenching temperature per maximal amount of metastable β-phase. Table 13 shows the change of the effect of hardening after quenching and aging per maximal strength, depending on the content of molybdenum in a Ti–Mo alloy. In the table, the amount of decomposed β-phase is a value expressed by the ratio V βquench – V βquench+age -------------------------------------------------------- × 100% , where Vβquench is the volume of β-phase after V βquench+age quenching; Vβquench+age is the volume of β-phase after quenching and aging.

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Table 13 Change of the effect of hardening after heat treatment of an alloy depending on the content of molybdenum. Content of molybdenum in the alloy, %

Amount of decomposed β-phase, %

0 2.5 5.0 7.5 10.0 15.0 20.0 25.0 30.0

0 17 35 50 66 50 33 18 0

Effect of hardening after quenching and aging, % after quenching to α′(α″) or α′(α″)+β-phases

after quenching to β-phase

0 65 70 75 – – – – –

0 25 40 60 80 40 32 28 10

The effect of hardening as the result of quenching and aging is presented as a σ B quench+age – σ B anneal ratio ---------------------------------------------------------- × 100% , where σB quench+age is the tensile strength of σ B anneal

the quenched and aged alloy; σB anneal is the ultimate strength of the annealed alloy. Data of Fig. 16 and Table 14 confirm that the greatest effect of hardening is observed in alloys with the maximally decomposed amount of β-phase, i.e., in alloys of critical composition. Table 14 Effect of hardening during quenching and aging of binary titanium alloys of critical composition with various alloying elements. Alloy

Content of alloying element, %

Amount of decomposed β-phase, %

Effect of hardening after quenching and aging, %

Ti–Fe Ti–Mn Ti–Cr Ti–Mo Ti–V Ti–Nb Ti–Ta

4 7 7 10 15 35 50

73 69 59 66 55 37 28

85 66 63 80 38 28 18

Thus, by the effect of hardening the binary alloys of titanium with investigated β-stabilizing elements can be arranged in the following sequence (with respect to the increasing capability of these elements to stabilize the β-phase in titanium): Ti–Ta, Ti–Nb, Ti–V, Ti–Cr, Ti–Mo, Ti–Mn and Ti–Fe. Any further hardening of solid α- and β-solutions in thermally hardened, as in annealed, alloys, can be achieved by using additives of an α-stabilizing element, for

34

σ0.2, GPa

σB, GPa

VALENTIN N. MOISEYEV

1.4 1.2

VT15 VT14

1.0 VT1-0 VT15

VT14

1.3 1.1 0.9 0.7

OT4 VT16

0.5

VT1-0

0.3 δ 5, %

VT16

OT4

0.8 0.6 0.4

VT1-0

30 20 10

VT15 0

OT4

VT16 VT14

10 20 30 40 50 60 70 Cold deformation, %

Figure 17 The mechanical properties of various titanium alloys vs the extent of cold deformation (sheet, 1.5 mm).

instance, aluminum, or neutral strengtheners dissolving in both α- and β-titanium – tin and zirconium. Increase of strength of titanium alloys by cold deformation (cold work hardening) is used restrictedly. Cold deformation is not widely used for increasing the strength of titanium alloys. Nevertheless, one comes across this technique when dealing with titanium alloys. At present, fastening items (bolts, screws, etc.) from titanium alloy VT16 are manufactured using deformation hardening. This method is rather efficient from the point of view of obtaining a required strength of alloy and labor intensity of the process. Sheet semifinished items from titanium alloys after annealing are planished in rolls with a deformation of 2–5% at temperatures considerably lower than the recrystallization temperatures. A certain hardening should be taken into account in further process treatment and further application of such semifinished items. Hardening due to cold work is determined by an increase of the density of dislocations in the metal. In most perfect monocrystals of pure titanium, it varies from 103 to 105 cm–2. In polycrystalline titanium, the density of dislocations is 107– 109 cm–2, and in deformed titanium it reaches 1010 –1011 cm–2. At the same time, a deformation increase is not always accompanied by a respective increase of the density of dislocations. It rises the most intensively at small deformations. The effect of hardening in titanium alloys with stable α, (α+β) and β structures depending on the extent of cold deformation was assessed in studies of commercial titanium alloys with various structures, which allow a significant plastic deformation – VT1-0 (α structure), OT4 (α + 5% β structure), VT14 (α + 12% β structure), VT16 (α + 25% β structure) and VT15 (β structure) in a stable annealed state. The

Maximal increase of σB, %

TITANIUM ALLOYS: RUSSIAN AIRCRAFT AND AEROSPACE APPLICATIONS

35

60 50 40 30 20 10

0

0.4

0.8

1.2 1.6 Kβ →

2.0

Figure 18 The effect of cold hardening of titanium alloys vs phase composition.

β structure in alloy VT15 (Ti–3% Al–7% Mo–11% Cr) was obtained by heating in the β-region (800°C) followed by cooling in air. The other alloys after annealing were cooled at a rate of 3°C/min. Alloys as sheet specimens were deformed by rolling with various degrees of deformations to obtain sheets 1.5-mm thick to the emergence of visible cracks on the margins. The mechanical properties of cold work-hardened sheets were determined in specimens cut along the rolling direction. Figure 17 presents the changes of ultimate strength, yield strength and relative elongation of the sheets depending on the extent of deformation. The greatest effect of hardening was observed in single-phase titanium alloys with α (VT1-0) or β (VT15) structures. As the content of the β-phase in the series OT4, VT14, VT16 increases, the hardening effect of cold-work hardening decays and is observed only at the initial stage of deformation (Fig. 18). Phase transformations in the process of deformation also affect the hardening of the alloys. Alloy VT16, which is distinguished by the lowest effect of hardening as → β phase transformation. Alloys the result of plastic deformation, features an α ← VT1-0, OT4 and AT15, having the greatest effect of cold-work hardening, featured no phase transformations in deformation.

1.8 ENHANCEMENT OF HIGH-TEMPERATURE STRENGTH OF TITANIUM ALLOYS The general ideas of the theory of metal high-temperature strength extend to titanium alloys. It is generally recognized that the main factors determining the high-temperature

36

VALENTIN N. MOISEYEV

strength of metals are melting temperature, strength of interatomic bonds, diffusion processes, structure and some other factors. However, the specific properties of titanium are a reason for some peculiarities of its behaviour at elevated temperatures. Thus, having the melting temperature (1668°C) higher than that of, for instance, nickel (1455°C), titanium has a lower high-temperature strength. Nickel hightemperature alloys operate at temperatures of up to 900–1100°C, whereas for hightemperature titanium alloys the range of working temperatures is limited by 450–550°C. Table 15 presents the data on the creep resistance during compression for some metals with high melting temperature. The tests were carried out in vacuum (to avoid oxidation) up to 1% residual deformation for 24 h at a temperature of 1000°C. As seen in the table, the creep resistance of titanium, whose melting temperature is higher than that of nickel, is more than half as less. Table 15 Creep resistance of metals at compression. Metal

Melting temperature, °C

Tungsten Iridium Molybdenum Tantalum Chromium Niobium Rhodium Cobalt Iron Nickel Vanadium Titanium Zirconium

3410 2494 2625 3000 1800 2000 1966 1500 1539 1450 1750 1665 1750

Creep strength, kgf/mm2 9.5 9.5 4.7–6.0 4.7–6.0 3.15–4.7 4.3 4.7 1.05 0.7 0.35 0.35 0.14 0.14

Density, g/cm3 19.2 22.4 10.2 16.2 7.2 8.5 12.3 8.9 7.8 8.9 6.0 4.5 6.37

Creep strength to density ratio 0.5 0.43 0.45–0.6 0.3–0.45 0.46 0.5 0.4 0.12 0.09 0.04 0.06 0.03 0.02

Thus, the existing views of the close relation of high-temperature strength to only melting temperature of metals is not absolute. At polymorphic transformation temperature, metals which undergo polymorphic transformation, including titanium, are characterized by a significant weakening of interatomic forces and, as a rule, a sharp decrease of high-temperature strength. Taking into account that in titanium alloys the polymorphic transformation occurs in a temperature range, one should expect a sharp decrease of hightemperature strength within this range. Table 16 compares the values of temperatures of (α+β→β) polymorphic transformation and recommended working temperatures for solid-solution commercial titanium alloys. The column “Recommended working temperature” gives the ultimate recommended working temperature for alloys used in long-life aircraft items. It is

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37

determined based on the analysis of the prolonged high-temperature strength characteristics and on the experience of operation of items from titanium alloys. Table 16 Temperature of (α+β→β) polymorphic transformation of commercial titanium alloys and recommended temperatures for their use in long-life items (in aircraft building and engine building). Alloy

VT20 VT5-1 VT6 OT4 VT14 OT4-1 OT4-0 VT22 VT16 VT35 VT32

Τemperature of α+β→β transformation, °C

980–1020 980–1030 980–1000 960–1000 920–960 910–950 860–920 860–890 840–880 770–790 750–780

Kβ