Advanced Structural Materials: Properties, Design Optimiza 1574446347, 9781574446340, 9781420017465

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q 2006 by Taylor & Francis Group, LLC

q 2006 by Taylor & Francis Group, LLC

q 2006 by Taylor & Francis Group, LLC

q 2006 by Taylor & Francis Group, LLC

Preface In recent years, the concept of advanced structural materials has changed from advanced composites and intermetallics to microelectromechanical systems (MEMS), cellular materials, biomaterials, shape memory alloys, amorphous alloys, and nanostructured materials. Many of the intermetallic and composite systems that appeared promising just a decade ago are no longer considered by many to be serious candidates for near-term applications in the next decade. Within this context, a number of existing structural materials, such as titanium and cobalt alloys, have also embraced advanced applications in ways that enhance their status as advanced structural alloys/systems. This book is written for both the non-specialist and the specialist. It is written for those who want to develop an understanding of the breadth and depth of current advanced structural materials. Although each chapter has been written by an expert in the field, no prior knowledge of a given material system is assumed. Each chapter, therefore, presents the fundamental concepts (structure and properties of materials) and the applications of advanced structural materials. Due to the huge nature of the field, we have been forced to define advanced structural materials as cutting-edge systems that are currently in structural use, or future systems that appear to have the promise for near-term structural applications. Hence, we do not include chapters on advanced intermetallics and ceramics, which remain as long-term contenders for future large-scale structural applications. Nevertheless, we hope that the rich array of selected topics will provide readers with useful insights into the structure, properties, and applications of some of the systems that are currently considered advanced structural materials. The book is divided into four sections. In section I, a broad introduction to advanced structural materials is presented. This is followed by section II, in which materials at the frontiers of emerging applications are presented. These include some aspects of biomaterials, MEMS, amorphous materials, and nanotechnology. In section III, existing advanced structural alloys are described before focusing on high temperature structural materials in section IV. We are grateful to authors for taking time out of their busy schedules to prepare their chapters. We are also grateful to Ms. Betty Adam of Princeton University for her tireless efforts in coordinating the correspondence with the authors, and synthesizing their inputs into a coherent document. It is hard to imagine how this book could have been completed without her skilled help. This book was initiated by Dawn Wechsler and Janet Sachs of Marcel Dekker. Since their initial efforts, we have been guided by Shelley Kronzek of Taylor & Francis Books, CRC Press. We would like to thank her for her vision and her patience. We hope that this book will be useful to senior undergraduate and graduate students, practicing materials scientists and engineers, researchers, and those who simply want to learn more about advanced structural materials. Much of my current understanding of advanced structural materials has been nurtured by program managers who have supported my research over the past two decades. I would, therefore, like to thank Charles Whitesett (McDonnell Douglas), Dick Lederich (McDonnell Douglas), Shankar Sastry (McDonnell Douglas/Washington University), Oscar Dillon (NSF), Dan Davis (NSF), Jorn-Larsen Basse (NSF), George Yoder (ONR), Julie Christodolou (ONR), Chuck Ward (AFOSR), Bruce MacDonald (NSF), Majia Kukla (NSF), Ulrich Strom (NSF), Tom Rieker (NSF), Joe Akkara (NSF), Tom Weber, Lance Haworth, Adriaan Graaf, and Carmen Huber (NSF) for their support of my efforts. I would also like to thank NSF (DMR Grant No. 0231418) for providing the financial support used to coordinate the preparation of this book. Finally, I would like to thank my dear wife, Morenike, and my children, Rotimi, Deji, and Wole´ for allowing me the time to work on yet another book project. I hope that the time spent on this project will help to enrich the lives of others, just as it has enriched mine. Wole´ Soboyejo Princeton, NJ q 2006 by Taylor & Francis Group, LLC

Author Wole´ Soboyejo was educated in England. He received his bachelor’s degree in mechanical engineering from King’s College, London University in 1985. He then went on to Cambridge University, where he received his Ph.D. degree in materials science and metallurgy in 1988. Between 1988 and 1992, he was a research scientist at the McDonnell Douglas Research Laboratories. He then worked briefly as a principal research engineer at the Edison Welding Institute (EWI) before joining the faculty in the Department of Materials Science and Engineering at The Ohio State University. Between 1997 and 1998, he was a Visiting Martin Luther King Professor in the Department of Materials Science and Engineering and the Department of Mechanical Engineering at MIT. In 1999, Professor Soboyejo moved to Princeton University and was appointed full professor in the Department of Mechanical and Aerospace Engineering and the Princeton Institute of Science and Technology of Materials (PRISM). He is the director of the Undergraduate Materials Program at PRISM. He is the director of the US/Africa Materials Institute (USAMI) and the chair of the African Scientific Committee of the Nelson Mandela Institution. Professor Soboyejo is the recipient of two National Young Investigator Awards (NSF and ONR) and the Bradley Stoughton Award for Excellence in the Teaching of Materials Science. He is a Fellow of ASME and the Materials Society of Nigeria. He has published more than 350 papers and one textbook on the mechanical properties of engineered materials.

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Contributors Seyed M. Allameh Department of Physics and Geology Northern Kentucky University Highland Heights, Kentucky Tiffany Biles NASA Glenn Research Center Cleveland, Ohio Yifang Cao Department of Mechanical and Aerospace Engineering Princeton Institute for the Science and Technology of Materials (PRISM) Princeton University Princeton, New Jersey Kwai S. Chan Southwest Research Institute Mechanical and Materials Engineering Division San Antonio, Texas

Peter K. Liaw Department of Material Sciences and Engineering The University of Tennessee Knoxville, Tennessee Jun Lou Department of Mechanical Engineering and Materials Science Rice University Houston, Texas Fred McBagonluri Siemens Hearing Piscataway, New Jersey Ronald Noebe NASA Glenn Research Center Cleveland, Ohio Santo A. Padula II NASA Glenn Research Center Cleveland, Ohio

M. Freels Department of Materials Science and Engineering The University of Tennessee Knoxville, Tennessee

J. H. Perepezko Department of Materials Science and Engineering University of Wisconsin-Madison Madison, Wisconsin

L. Jiang Corporate Research and Development Center General Electric Company Schenectady, New York

R. Sakidja Department of Materials Science and Engineering University of Wisconsin-Madison Madison, Wisconsin

K. S. Kumar Department of Materials Science Brown University Providence, Rhode Island D. L. Klarstrom Haynes International, Inc. Kokomo, Indiana John Lewandowski Department of Materials Science and Engineering Case Western University Cleveland, Ohio q 2006 by Taylor & Francis Group, LLC

Gary J. Shiflet Department of Materials Science and Engineering University of Virginia Charlottesville, Virginia W. O. Soboyejo Department of Mechanical and Aerospace Engineering Princeton Institute for the Science and Technology of Materials (PRISM) Princeton University Princeton, New Jersey

T. S. Srivatsan Department of Mechanical Engineering University of Akron Akron, Ohio

Jikou Zhou Materials Science and Technology Division Lawrence Livermore National Laboratory Livermore, California

Satish Vasudevan Department of Mechanical Engineering University of Akron Akron, Ohio

Aiwu Zhu Department of Materials Science and Engineering University of Virginia Charlottesville, Virginia

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Table of Contents SECTION 1: Chapter 1

Introduction

Introduction to Advanced Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

W. O. Soboyejo SECTION 2: Chapter 2

Novel Materials

Small Scale Contact and Adhesion in Nano- and Bio-Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Yifang Cao and W. O. Soboyejo Chapter 3

Mechanical Characterization of Thin Film Materials for MEMS Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Jun Lou Chapter 4

Silicon-Based Microelectromechanical Systems (Si-MEMS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Seyed M. Allameh Chapter 5

Porous Metallic Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

Jikou Zhou SECTION 3: Chapter 6

Advance of Structural Materials

A Thermodynamic Overview of Glass Formation Abilities: Application to Al-Based Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

Aiwu Zhu and Gary J. Shiflet Chapter 7

NiTi-Based High-Temperature Shape-Memory Alloys: Properties, Prospects, and Potential Applications . . . . . . . . . . . . . . . . . 145

Ronald Noebe, Tiffany Biles, and Santo A. Padula II Chapter 8

Cobalt Alloys and Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

M. Freels, Peter K. Liaw, L. Jiang, and D. L. Klarstrom Chapter 9

The Science, Technology, and Applications of Aluminum and Aluminum Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225

T. S. Srivatsan and Satish Vasudevan Chapter 10

Metal Matrix Composites: Types, Reinforcement, Processing, Properties and Applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275

T. S. Srivatsan and John Lewandowski q 2006 by Taylor & Francis Group, LLC

Chapter 11

Titanium Alloys: Structure, Properties, and Applications . . . . . . . . . . . . . . . . 359

Fred McBagonluri and W. O. Soboyejo

SECTION 4: Chapter 12

High Temperature Materials Niobium Alloys and Composites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401

Kwai S. Chan Chapter 13

Mo-Si-B Alloys for Ultrahigh Temperature Applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 437

J. H. Perepezko, R. Sakidja, and K. S. Kumar Chapter 14

Nickel-Base Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475

W. O. Soboyejo

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Introduction to Advanced Materials W. O. Soboyejo Department of Mechanical and Aerospace Engineering, Princeton Institute for the Science and Technology of Materials (PRISM), Princeton University

CONTENTS 1.1 1.2

Introduction ..............................................................................................................................1 Applications of Advanced Materials .......................................................................................2 1.2.1 Materials in Aeroengines..............................................................................................2 1.2.2 Materials for the National Aerospace Plane ................................................................4 1.2.3 Materials in Sporting Goods ........................................................................................8 1.2.4 Materials for Human Prosthetic Devices......................................................................8 1.2.5 Materials for Automotive Applications .......................................................................8 1.3 Engineering of Balanced Properties ......................................................................................11 1.4 Summary.................................................................................................................................12 References .......................................................................................................................................13

1.1 INTRODUCTION Advanced structural metallic materials have had a considerable impact on the development of a wide range of strategic technologies. However, only a few specialists are aware of the basic scientific concepts that have guided the design of new alloys, intermetallics, and metal matrix composites. These concepts are described in this book in an effort to make such knowledge widely accessible to engineers and scientists who have a strong tendency to become rather focused on a few particular systems. We also hope that the book will serve as a useful overview to the public at large who have faith in these advanced materials without knowing much about the properties that make them suitable for structural applications. We hope that this book will provide a simple picture of the basic concepts that guide the development of new alloys and composite materials. For this reason, excessive detail has been avoided in the individual sections in an effort to retain clarity in the presentation of key ideas. However, the more inquiring reader is provided with an extensive list of references at the end of each chapter. Finally, it is important to emphasize that this book is not intended to be a comprehensive overview of advanced metallic materials. No authors could possibly provide a complete review of all the relevant material available in the literature. Instead, we hope that this text will provide a useful picture of the basic ideas used to control the fracture properties of engineered structural

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Advanced Structural Materials: Properties, Design Optimization, and Applications

metallic materials. We also hope that our limited choice of examples will serve to illustrate the critical role that advanced structural metallic materials will play in the aerospace, biomedical, automotive, sporting goods, and other industries in the twenty-first century. An overview of advanced metallic materials is presented in this chapter.

1.2 APPLICATIONS OF ADVANCED MATERIALS Since the Stone Age, and perhaps before, the ability to process materials has had significant economic and political implications. The shaping of stones into weapons of war often determined success on the battlefield, just as advanced structural materials and superior computer technology can influence the outcome of modern day warfare. Man’s ability to process materials with enhanced properties has therefore provided economic and political advantages to the groups with materials that are relatively advanced compared to those of their counterparts. This was particularly apparent in ancient Japanese society where the Samurai’s ability to process steels with complex laminated microstructures guaranteed victory in the battlefield [1]. Similar examples of superior swordmaking abilities abound in the bible, where the Hittites and other groups used their superior materials to their advantages in wide variety of military adventures. In more recent times, advanced materials have played a key role in the arms race between the countries of the old Eastern and Western blocs. In fact, it is easy to argue that the arms race was the primary driving force behind the development of new materials between 1945 and 1991 when the Berlin Wall came down. Other factors that contributed to the development of new materials include various international space programs, transportation and materials processing industries, emerging biotechnology industries, and the rapid growth of the microelectronics and computer industries during the last quarter of the twentieth century. The historical development of some important materials systems is summarized in Figure 1.1. Advanced materials have therefore played a pivotal role in providing us with many of the products that have become part of our daily lives. However, most people are unaware of the central role advanced materials play in their lives in spite of their ubiquitous nature [2]. For this reason, a few examples of the existing and emerging applications of advanced materials are presented in the remainder of this chapter as follows: 1. 2. 3. 4. 5.

Materials Materials Materials Materials Materials

in aeorengines for the National Aerospace Plane in sporting goods for human prosthetic devices for automotive applications

A wide range of other examples will also be presented in the individual chapters throughout this book. Nevertheless, we hope that the few examples presented in this section will provide a useful introduction to the advanced materials systems that will be described in detail in subsequent chapters.

1.2.1 MATERIALS

IN

AEROENGINES

Our ability to power modern airplanes depends largely on the thrusts generated by aeroengines. Since speed and safety are critical in both commercial and military aircrafts, the selection of materials for aeroengine applications requires a careful balance of performance and risk [3]. Material cost is also an important factor, especially in commercial aeroengines where plane crashes can attract global attention, particularly in cases where human lives are lost. q 2006 by Taylor & Francis Group, LLC

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Evolution of engineering materials 10 000BC 5000BC 0 1000 1500 1800 Gold Copper Metals Bronze Iron

Relative importance

Steels

Wood Skins Fibers

Gules Relative importance Rubber Composites Straw-brick paper Stone Flint Pottery Glass Cement Refractories Ceramics Portland cement 5000BC

1940

1960

0

1000 1500 1800

1980

1990

2000

2010

2020

Metals

Cast iron

Polymers

10 000BC

1900

Glassy metals Al-lithium alloys Dual phase steels Microalloyed steels New super alloys

Alloy steels

Development slow: mostly quality control and processing

Light alloys Super alloys

Polymers

Conducting polymers High temperature Tilanium polymers Zirconium Alloys High Modulus Composites etc. polymers Ceramics composites Bakelite Polyesters Metal-matrix Nylon Epoxies Coposites Kelvar-FRP PE PMMA Acrylics Ceramics CFRP PC PS PP GERP Fused Tough engineering Pyrosilica Cermets ceramics Ceramics (Al2O3, Si3N4, PSZ, etc.) 1900

1940

1960

1980

1990

2000

2010

2020

Year

FIGURE 1.1 Evolution of the historical evolution of the relative importance of the different classes of materials.

Materials in modern day aeroengines are, therefore, introduced only after extensive materials development and testing have been performed over a period of w15 years. This has led to unprecedented levels of safety and performance in most of the aeroengines that are currently in service. A schematic of a modern day aeroengine is presented in Figure 1.2. This shows the three main sections of the engine, i.e., the fan, compressor, and turbine. Air is sucked in by the fan, which is normally fabricated from a titanium alloy (such as Ti–6Al–4V) or a polymer matrix composite. The air is then passed through multiple stages of the compressor, which consists primarily of disks and blades that are also fabricated primarily from titanium alloys such as Ti–6Al–4V. The temperatures in the fan and aeroengine sections of the aeroengine are relatively low (close to room temperature). However, higher temperatures (w4008C–5008C) may occur in the high-pressure stages. Beyond the compressor, the air is ignited in a combustor, before expanding through multiple stages of the turbine and the nozzles at the back end of the engine. Nickel and cobalt-based superalloys are often used in the combustor and turbine sections of the aeroengine due to high temperatures experienced in these systems (from w5008C to 6508C in the disks to w12008C at the tips of the blades in the turbines). The compositions and microstructures of the superalloys are also tailored to provide the required combinations of strength, creep, fatigue, and oxidation resistance required for service in aeroengines that revolve at angular speeds as high as 20,000 revs/minute. Beyond the turbine section of the aeroengine, air is expanded through nozzles that are typically fabricated from coated niobium-based alloys (Figure 1.3). The above materials systems have been largely optimized by empirical processing and alloy design schemes. However, many of the systems have inherent limits in properties that may not be overcome by alloying and heat treatment. For example, the densities and moduli of metallic materials may not be altered significantly by alloying. Hence, new materials are often needed to exceed the intrinsic limits of the existing structural metallic systems in modern aeroengines [3–6]. The emerging systems that have been identified as potential candidates to replace existing aeroengine materials are listed in Table 1.1, along with existing aeroengine materials. Note that these q 2006 by Taylor & Francis Group, LLC

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Advanced Structural Materials: Properties, Design Optimization, and Applications

Air intake

Compression

Continuous

Combustion

Exhaust

3200 Carbon/carbon composites

3000 2800

Ceramic matrix composites & ceramics

2600 Material surface temperature, °F

Thermal-barrier coatings

2400 2200 2000

Conventionally cast

Fiber-reinforced superalloys ODS superalloys Eutectics

1800

Single crystals

1600

Directionally solidified superalloys

1400 1950

1960

1970 1980 1990 2000 2010 Approximate year of use in engine

2020

FIGURE 1.2 The role of high temperature materials in the historical evolution of the turbine blade surface temperature in aeroengines.

generally have lower densities than the existing aeroengine materials. However, their fatigue and fracture properties are generally not as attractive as those of the existing materials at room temperature. The current text will therefore devote a significant number of sections to the toughening and strengthening strategies that are being used to guide the design of damage-tolerant aeroengine materials. As in most applications, the trick is to design new materials with the required balance of properties. This theme is one that will recur throughout this book.

1.2.2 MATERIALS

FOR THE

NATIONAL AEROSPACE PLANE

The National Aerospace Plane (NASP) vehicle was conceived in the 1980s in the U.S. It was proposed as a Mach-10 hypersonic vehicle that could travel from Tokyo to New York in two hours [9]. As with q 2006 by Taylor & Francis Group, LLC

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Carbon/carbon composites

4000

3000

Ceramics Metal matrix composites

Operating temperature, °F 2000

1000

Ceramics composites Intermeatallics and intermeatallics composites

Conventional materials (titanium & superalloys)

0 1.0 x 106 Strength/weight ratio

FIGURE 1.3 The ranges of operating temperature and strength/weight ratio for different classes of materials.

most aerospace concepts, the main limitation to this hydrogen-powered and hydrogen-cooled vehicle was the availability of materials with the required combinations of mechanical properties and oxidation resistance. This should be immediately apparent to the reader after reviewing Figure 1.4, in which a schematic of the NASP vehicle is presented. Note that the temperature at the nose of the X-33 derivative of the plane is as high as 18008C. Also, the temperatures in the trailing edges of the wing are between

TABLE 1.1 Densities of Existing and Emerging Aeroengine Materials Section(s) of Aeroengine

Existing Alloy System(s)

Density of Existing Emerging Alloy Alloy System(s) Systems(s) (g/cm3)

Fan hubs and disks

Ti–6Al–4V

4.5

Compressors

a and/or b titanium alloys, e.g. Ti–6Al–4V Ni-base superalloys

4.5

Combustors Turbines

Nozzles

Ni–Co-base superalloys

Nb alloys

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8–10 8–10

10–12

Polymer matrix composites Orthorhombic Ti alloys/composites

Density of Emerging Alloy System(s) (g/cm3) 2 4.5–5.0

Ceramic matrix composite Titanium aluminide

2 4.0–4.5

Niobium aluminide Niobium silicide Niobium aluminide Niobium silicide

5.1–5.5 6 5.1–5.5 6

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Advanced Structural Materials: Properties, Design Optimization, and Applications

1800° 1090° 980° 870°

Temperature in°C

1450° 980°

760°

1450° 980° 870°

870°

980° 1090° 1780°

FIGURE 1.4 Temperature variations on the skin of the X-33 NASP vehicle.

w10008C and 14508C. Such temperatures clearly exceed the “useful” temperature limits of existing superalloys and refractory Nb alloys. A significant materials development effort [7] was therefore initiated to produce the required high temperature materials for the NASP vehicle in a record time of about 10 years (note that the materials development is generally w15 years). The initial effort was not successful in spite of several billions of dollars of research expenditures. Nevertheless, the X-33 vehicle did fly successfully in 2004, after adopting a more realistic approach to materials and structures. In any case, the NASP program did stimulate extensive materials efforts to develop new high temperature materials with the required balance of mechanical properties. Some of the more promising systems considered for use in the NASP vehicle are listed in Table 1.2, along with their inherent temperature limits. It is important to note that the NASP program did have some very important beneficial effects on other materials development efforts. For example, the gamma-based titanium aluminide systems have been selected for use as shroud seals and nozzle liner files in the engineering of the engine of the YF-22 airplane (Figure 1.5). The frame of this futuristic military jet is produced by the Lockheed–Martin Corporation, while the aeroengine is supplied by Pratt & Whitney. Other potential aerospace applications for g-TiAl alloys include low-pressure turbine blades and small structural

TABLE 1.2 Potential Materials Systems for Hypersonic Vehicles Temperature Range (8C) 400–600 600–650 650–760

650–1200

Materials System

Main Limitation(s)

a/b Ti alloys Orthorhombic Ti alloys Gamma-based titanium Aluminides Niobium aluminide Nickel aluminides Molybdenum disilicides

Oxidation and creep resistance Oxidation resistance and fabricability Damage tolerance and creep Resistance Oxidation resistance Damage tolerance and creep Resistance Damage tolerance Damage tolerance and creep Resistance Damage tolerance Damage tolerance Oxidation resistance

1200–1400

Molybdenum disilicides Niobium silicides

1400–1800 1800–2000

Ceramic matrix composites Carbon–carbon composites

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FIGURE 1.5 Cross-section of aeroengine the YF-22 airplane - applications of gamma titanium aluminides indicated by arrows.

elements (Figure 1.5). It is perhaps the first test-bed for the use of titanium aluminide in an aeroengine application. The Toyota Motor Company of Japan, General Motors, and Ford Motor Company in the U.S. are also exploring possible applications of gamma titanium aluminides in valve and turbo-charged

FIGURE 1.6 Applications of gamma titanium aluminides: (a) turbo-charger, (b), (c) turbine blade, and (d) automotive valve (Courtesy of The Howmet Corporation). q 2006 by Taylor & Francis Group, LLC

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Advanced Structural Materials: Properties, Design Optimization, and Applications

applications. These companies are interest in gamma-based titanium aluminides because improvements in performance can be engineered by the use of lightweight (lighter than the highly alloyed steels that are currently used) titanium aluminide alloys. The use of TiAl valves (Figure 1.6) could also result in reductions in fuel consumption of w1–2 miles per gallon and higher engineering operating pressure/temperatures that can be used to control engine emissions to meet standards imposed by the Environmental Protection Agency (EPA) in the U.S. For this reason, the manufacturers of diesel engines (for large trucks) are currently developing gamma-based titanium aluminide valves for the next generation of diesel truck engines.

1.2.3 MATERIALS

IN

SPORTING GOODS

Advanced materials have played an increasing role in sporting applications during the past 30 years. One sport where the influence has been particularly important has been the ancient Scottish game of golf. State-of-the-art golf clubs are now fabricated from titanium alloys and newly developed amorphous alloy (Figure 1.7) that absorb minimal amounts of energy upon contact with golf balls [8]. The current state-of-the-art amorphous glass alloy is Al–62Zr–10Ni–10Cu–3.5Be (weight %). This alloy has a strength of w1900 MPa and a modulus close to that of titanium. It is used in the fabrication of golf clubs that cost up to a few thousand dollars each! However, they are extremely popular among professional or avid golfers due to their high coefficients of restitution. Similar advantages are also beneficial to the design of baseball bats where very thin sections of precipitation strengthened aluminum alloys are used. However, these baseball bats are almost too effective, i.e., the sudden bounce of the ball from the baseball bat is sudden, and an unanticipated bounce can be dangerous. In fact, there have been recorded cases of players developing detached retinas after being struck by fast recoiling baseballs traveling at unprecedented velocities after being hit by these strengthened aluminum baseball bats. Other sports in which advanced materials/ composites play a strong role include tennis, boat racing, and canoeing.

1.2.4 MATERIALS

FOR

HUMAN PROSTHETIC DEVICES

Few metallic materials are biocompatible with the human body, i.e., most metallic materials that can be used as implants are rejected by the human body [9]. However, titanium and cobalt are two metals that are compatible with the human body. These two metals are therefore used widely in a range of human prosthetic devices (human implants). In many cases, pure titanium and pure cobalt are preferred because most of the possible alloying elements such as vanadium that have been shown to have undesirable toxic side effects are not present. However, the pure metals may not have sufficient balance of strength and other mechanical properties for prosthetic applications in the human body. Titanium and cobalt-based alloys (mixtures with other elements) have, therefore, been developed for applications in which higher levels of strength and fatigue resistance are required [9,10]. Titanium alloys are particularly attractive for hip implants because of their exceptional combinations of corrosion resistance, moderate density (w4.5 g/cm3), yield strength (500–800 MPa), and fracture toughness (w40–100 MPa m1/2). However, titanium alloys have limited wear resistance [10]. Cobalt-based alloys are, therefore, preferred in applications where wear resistance is critical. These include knee implants and interfacial layers between polyethylene caps and titanium hip implants that fit into socket joints. Stainless steels are also used in some corrosion-resistant devices, while carbon–carbon materials tend to be favored in some heart-valve applications that undergo several million fatigue cycles (heartbeats) per year!

1.2.5 MATERIALS

FOR

AUTOMOTIVE APPLICATIONS

Advanced materials offer some unique opportunities in the design of lightweight vehicles with improved fuel consumption and performance. However, low cost is also a very strong factor in the selection and application of materials in the automotive industry [11]. Unlike aerospace materials, q 2006 by Taylor & Francis Group, LLC

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FIGURE 1.7 Liqiudmetal golf club heads (a and b).

cost is perhaps the biggest factor in the selection of new materials for most automotive applications. This is particularly true for high volume compact cars and family automobiles. However, luxury vehicles and high performance sports cars/racing cars have provided a valuable test-bed for the introduction of advanced materials into the next generation of automobiles. One example of an advanced materials system that has been introduced into Toyota motor vehicles is a new class of in situ titanium matrix composites reinforced with TiB whiskers [11–14]. The whiskers in these structures are produced in situ reactions that occur during processing via powder [11,12] or ingot [13,14] metallurgy routes. They are formed by the q 2006 by Taylor & Francis Group, LLC

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Advanced Structural Materials: Properties, Design Optimization, and Applications

FIGURE 1.8 Microstructure of In situ titanium matrix composite reinforced with TiB whiskers (Ti-8Al-1V1Mo+TiB whiskers).

reaction with the titanium that is present in titanium alloy mixtures. A micrograph of an in situ titanium matrix composite is presented in Figure 1.8. This shows aligned TiB whiskers in a matrix of Ti–8Al–1V–1Mo. Note that the TiB whiskers were aligned by the extrusion process. Otherwise, the whiskers would be randomly oriented. The in situ titanium matrix composites have higher strength, wear resistance, and stiffness than the unreinforced titanium matrices. They also have improved elevated-temperature strength and creep resistance and moderate density (w4.1 g/cm3). Furthermore, they are relatively easy to process via powder metallurgy, casting, and wrought processing techniques [11–14] at costs that are comparable to those of unreinforced titanium alloys. For these and other reasons, the Toyota Motor Company of Japan has explored the possible application of in situ titanium matrix composites in a wide range of automobile applications. These include applications in connecting rods, gears, and valves (Figure 1.9). The TiB-reinforced values were tried successfully in racing cards in 1997 and are currently being introduced into the next generation of Toyota sports cars and luxury vehicles such as the Lexus. As discussed earlier in Section 1.2.2, titanium aluminide intermetallics based on TiAl are also being considered for valve and turbocharger applications (Figure 1.6) in the next generation of automotive vehicles [15]. These materials offer a temperature advantage (up to w7508C–8008C) over titanium alloys which are generally limited to applications below w5008C–6008C [12]. They also have moderate density (w4.1–4.5 g/cm3) and stiffness (w150 GPa). However, titanium aluminides are relatively brittle (fracture toughness w20–35 MPa m1/2 and ductility w1%–2%) compared to the valve steels that they may replace. There are also some concerns about their cost (compared with valve steels) and wear resistance [15]. Nevertheless, most major automobile manufacturers are currently exploring possible applications of titanium aluminides in intake and exhaust valves. The Howmet Company in Whitehall, MI, is also developing low-cost methods for the fabrication of titanium aluminide valves. In particular, there is considerable interest in the application of titanium aluminides in the diesel engines of large trailer trucks. It is envisaged that such applications could ultimately lead to significant reductions in NOx emissions that are required by the EPA. Materials producers estimate that future sales from such valves could amount to hundreds of millions of dollars over the next few years. An example of an investment cast titanium aluminide valve produced by Howmet is shown in Figure 1.6. This valve can be produced at costs that are close to those of highly alloyed steels used in current vehicles. However, further development work is needed to optimize the production of wear resistant, low cost titanium aluminide valves [15]. q 2006 by Taylor & Francis Group, LLC

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FIGURE 1.9 A range of automotive components fabricated from in situ titanium matrix composites-gears, connecting rods, and inlet/exhaust.

Other projected applications of advanced metallic materials in automotive vehicles include the use of aluminum matrix composites in connecting rods, engine blocks, cylinder liners, and brake calipers [5]. However, existing applications of advanced materials in automotive vehicles are still largely restricted to polymer matrix composites used primarily in high-performance vehicles. There are also some limited applications of particle- or whisker-reinforced particle- or whisker-reinforced aluminum matrix composites in some engine applications [5].

1.3 ENGINEERING OF BALANCED PROPERTIES It should be clear from the above discussion that a balance of properties is required for the safe application of advanced materials in structural and nonstructural applications. In the case of high temperature alloys/composites for transportation systems, a combination of strength and lightweight (lower density) is often a basic requirement that must be satisfied before the more advanced properties (such as fracture toughness and fatigue resistance) are fully considered. Some of the most promising metallic materials systems (those with the basic combinations of strength and density) are shown in Figure 1.10. Note that gamma-based titanium aluminide intermetallics have excellent combinations of low density (w4.5 g/cmK3) and strength for intermediatetemperature applications up to w7608C. However, as discussed earlier, gamma-based titanium aluminides are limited by their room-temperature damage tolerance (fracture toughness and fatigue crack growth resistance). Titanium alloys also have comparable levels of strength and density to gamma-based titanium aluminides. However, they are generally limited to applications at temperatures below 6008C. This is due largely to their limited oxidation and strength at temperatures above this limit. In recent years, Nb–Al–Ti-based intermetallics have been developed for potential structural applications [16,17]. These B2 C orthorhombic intermetallics have combinations of strength and fracture toughness that are comparable to those of steel sand and other structural metallic materials. q 2006 by Taylor & Francis Group, LLC

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Advanced Structural Materials: Properties, Design Optimization, and Applications

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