Science and Design of Engineering Materials 2nd Edition-1.pdf

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THE SCIENCE AND

DESIGN OF

ENGINEERING

MATERIALS

SECOND EDITION

... ...

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PHYSICAL DATA FOR THE ELEMENTS

Atomic Melting Density of Crystal Atomic weight point solid, 20ⴗC structure,

Element Symbol number (amu) (ⴗC) (gm/cm3) 20ⴗC

Aluminum Al 13 26.98 660.452 2.7 FCC Antimony Sb 51 121.75 630.755 6.69 Rhomb. Argon Ar 18 39.95 ⫺189.352 — — ArsenicAs 33 74.92 603 5.78 Rhomb. Barium Ba 56 137.33 729 3.59 BCC Beryllium Be 4 9.012 1289 1.85 HCP Boron B 5 10.81 2092 2.47 — Bromine Br 35 79.9 ⫺7.25 — — Cadmium Cd 48 112.4 321.108 8.65 HCP Calcium Ca 20 40.08 842 1.53 FCC Carbon C 6 12.01 3826 2.27 Hex. Cesium Cs 55 132.91 28.39 1.91 BCC Chlorine Cl 17 35.45 ⫺100.97 — — Chromium Cr 24 52 1863 7.19 BCC Cobalt Co 27 58.93 1495 8.8 HCP Copper Cu 29 63.55 1084.87 8.93 FCC Fluorine F 9 19 ⫺219.67 — — Gallium Ga 31 69.72 29.7741 5.91 Ortho.

Germanium Ge 32 72.59 938.3 5.32 Dia. cub.

Gold Au 79 196.97 1064.43 19.28 FCC Helium He 2 4.003 ⫺271.69 — — Hydrogen H 1 1.008 ⫺259.34 — — Iodine I 53 126.9 113.6 4.95 Ortho. Iridium Ir 77 192.22 2447 22.55 FCC Iron Fe 26 55.85 1538 7.87 BCC

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PHYSICAL DATA FOR THE ELEMENTS (Concluded)

Atomic Melting Density of Crystal Atomic weight point solid, 20ⴗC structure,

Element Symbol number (amu) (ⴗC) (gm/cm3) 20ⴗC

Lanthanum La 57 138.91 918 6.17 Hex. Lead Pb 82 207.2 327.502 11.34 FCC Lithium Li 3 6.941 180.6 0.533 BCC Magnesium Mg 12 24.31 650 1.74 HCP Manganese Mn 25 54.94 1246 7.47 Cubic Mercury Hg 80 200.59 ⫺38.836 — — Molybdenum Mo 42 95.94 26.23 10.22 BCC Neon Ne 10 20.18 ⫺248.587 — — Nickel Ni 28 58.71 1455 8.91 FCC Niobium Nb 41 92.91 2469 8.58 BCC Nitrogen N 7 14.01 ⫺210.0042 — — Oxygen O 8 16 ⫺218.789 — — Phosphorus P 15 30.97 44.14 1.82 Ortho. Platinum Pt 78 195.09 1769 21.44 FCC Potassium K 19 39.1 63.71 0.862 BCC

Silicon Si 14 28.09 1414 2.33 Dia. cub.

Silver Ag 47 107.87 961.93 10.5 FCC Sodium Na 11 22.99 97.8 0.966 BCC Sulfur S 16 32.06 115.22 2.09 Ortho. Tin Sn 50 118.69 231.9681 7.29 BCT Titanium Ti 22 47.9 1670 4.51 HCP Tungsten W 74 183.85 3422 19.25 BCC Uranium U 92 238.03 1135 19.05 Ortho. Xenon Xe 54 131.3 ⫺111.7582 — — ZincZn 30 65.38 419.58 7.13 HCP

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James P. Schaffer

James P. Schaffer is an associate professor of Chemical Engineering at Lafayette College in Easton, Pennsylvania. After receiving his B.S. in mechanical engineering (1981)and his M.S. (1982)and Ph.D. (1985)in materials science and engineering from Duke University, he taught at the Georgia Institute of Technology for five years before moving to Lafayette in 1990. He has taught an introductory materials engineering course to more than 1200 undergraduate students using the integrated approach taken in this text.

Dr. Schaffer’s field of research is the characterization of atomic scale defects in materials using positron annihilation spectroscopy along with associated techniques. Professor Schaffer holds two patents and has published more than 30 papers. He has received a number of teaching awards including the Ralph R. Teetor Educational Award (SAE, 1989), Jones Lecture Award (Lafayette College, 1994), Distinguished Teaching Award (Middle Atlantic Section of ASEE, 1996), Superior Teaching Award (Lafayette Student Government, 1996), Marquis Distinguished Teaching Award (Lafayette College, 1996), and the George Westinghouse Award (ASEE, 1998). He is a member of ASEE, ASM International, TMS, Tau Beta Pi, and Sigma Xi.

Ashok Saxena

Ashok Saxena is currently professor and chair of the School of Materials Science and Engineering at the Georgia Institute of Technology. Professor Saxena received his M.S. and Ph.D. degrees from the University of Cincinnati in materials science and metallurgical engineering in 1972 and 1974, respectively. After eleven years in industrial research laboratories, he joined Georgia Tech in 1985 as a professor of materials engineering. He assumed the chairmanship of the school in 1993. From 1991 to 1994, he also served as the director of the Campus-Wide Composites Education and Research Center.

Dr. Saxena’s primary research area is mechanical behavior of materials, in which he has published over 125 scientific papers and has edited several books. His research in the area of creep and creep-fatigue crack growth has won international acclaim; he was awarded the 1992 George Irwin Medal for it by ASTM. He is also the recipient of the 1994 ASTM Award of Merit. Professor Saxena is an ASTM Fellow, a Fellow of ASM International, and a member of ASEE, TMS, Sigma Xi, and Alpha Sigma Mu.

Stephen

D.

Antolovich

Stephen D. Antolovich is currently a professor of Mechanical and Materials Engineering at Wash-ington State University, where he also serves as director of the School of Mechanical and Materials Engineering. He received his B.S. and M.S. in metallurgical engineering from the University of Wisconsin in 1962 and 1963, respectively, and a Ph.D. in materials science from the University of California–Berkeley in 1966. He joined the Georgia Institute of Technology in 1983, where he served as professor of materials engineering, director of the Mechanical Properties Research Laboratory (MPRL), and director of the School of Materials Science and Engineering.

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In 1988 Dr. Antolovich was presented with the Reaumur Medal from the French Metallurgical Society. In 1989 he was named Professeur Invite by CNAM University in Paris. In 1990 he was presented with the Nadai Award by the ASME. Dr. Antolovich regularly makes presentations to learned societies in the United States, Europe, Canada, and Korea and has carried out funded research/consultation for numerous government agencies. Dr. Antolovich has published over 100 archival articles in leading technical journals. His major research interests are in the areas of deformation, fatigue, and fracture, especially at high temperatures. He is a member of ASME, ASTM, and AIME, and a Fellow Member of ASM International.

Thomas H. Sanders, Jr.

Thomas H. Sanders, Jr., is currently Regents’ Professor in the School of Materials Science and Engineering at the Georgia Institute of Technology. Professor Sanders received his B.S. and M.S. in ceramic engineering from Georgia Tech in 1966 and 1969, respectively. In 1974 he completed his research for his Ph.D in metallurgical engineering at Georgia Tech and joined the Physical Metallurgy Division of Alcoa Technical Center, Alcoa Center, Pennsylvania. While at Alcoa Center his major research efforts were directed toward developing and implementing processing microstructure–properties relationships for high-strength aluminum alloys used in aerospace appli-cations. He was on the faculty in Materials Science and Engineering at Purdue University from 1980 to 1986 and joined the faculty at Georgia Tech in 1987. He was awarded the W. Roane Beard Outstanding Teacher Award for 1994.

Dr. Sanders’s primary research area is physical metallurgy of materials with primary emphasis on aluminum alloys. He has published approximately 100 scientific papers and has edited several books. He was awarded a Fulbright grant in 1992 to conduct research at Centre National de la Recherche Scientifique (ONERA), Chaˆtillon, France. Professor Sanders is a member of TMS and a Fellow of ASM.

Steven B. Warner

Steven B. Warner is Professor and Chairperson of the Textile Sciences Department, University of Massachusetts, Dartmouth. Dr. Warner earned his combined S.B. and S.M. degrees in metallurgy and ceramics in 1973 from the Massachusetts Institute of Technology. In 1976 he was awarded an Sc.D. from the Department of Materials Science and Engineering at MIT. He was a research scientist from 1976–1982 at Celanese Research Co. and from 1982–1988 at Kimberly-Clark Corp. In 1987 he joined Georgia Institute of Technology as Adjunct Professor in Chemical Engineering; in 1988 he became Associate Professor in Materials Engineering; and from 1990–1994 he was a faculty member in Textile and Fiber Engineering.

Dr. Warner’s research interests are the structure-property relationships of materials, especially polymers. He has published more than 30 scientific papers, holds six U.S. patents, and is the author of Fiber Science. In addition he has been a technical expert in a number of patent cases.

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F O R E W A R D

If one’s technical library were to contain only a single book on materials, this is the book to have. The authors have succeeded in covering the field of materials science and engineering in even its broadest aspects. They have captured both the science of the discipline as well as the engineering and design of materials. All classes of materials are treated; metals, semiconductors, ceramics, and polymers, as well as composites made of combinations of these. As urged in the National Research Council’s recent study of materials science and engineering, processing and synthesis also are included, as are the subjects of machinability and joining. (No material, however outstanding its properties, is likely to be very useful if it can’t be produced, shaped, or attached to other compo-nents.)

The breadth of The Science and Design of Engineering Materials, which reflects the varied fields of expertise of the authors, makes it an ideal text for a survey course for students from all fields of engineering. Because of the depth as well as the breadth with which the topics are treated, the text also is an excellent choice for introductory courses for materials science and engineering majors. Graduates of these introductory and survey classes will value The Science and Design of Engineering Materials as a resource book for years to come. The clear explanations and frequent examples allow the practicing engineer, on his or her own, to become acquainted with the materials field or update his/her knowledge of it. Care and skill have been exercised in the choice of illustrations. Numerous drawings and graphs augment explanations in the text, and clearly reproduced micrographs provide real-life examples of the phenomena being described. The examples and questions are especially noteworthy. While a portion of the questions are of the “one right answer” kind, and are intended to reinforce and clarify the material in the text, others are of the open-ended, design type that require creative thought and more closely resem-ble real-life situations. They can form the bases for useful and provocative class discus-sions.

This new edition of The Science and Design of Engineering Materials is a valuable addition to the materials literature. It will contribute to the materials education of engi-neers and scientists for years to come.

Julia Weertman

Walter P. Murphy Professor of Materials Science and Engineering Northwestern University

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A society’s ability to develop and use materials is a measure of both its technical sophis-tication and its technological future. This book is devoted to helping all engineers better understand and use materials to ensure the future of technology.

THE INTENDED MARKET

The book is intended for undergraduate students from all engineering disciplines. It assumes a minimal background in calculus, chemistry, and physics at the first-year college level. The text has been used successfully in a variety of situations including:

■ A traditional 40- to 42-lecture single-semester/quarter course

■ A yearlong course sequence

■ A foundation course for materials engineering majors

■ A service course with students from multiple engineering disciplines

■ A service course targeted at a specific audience (for mechanical or electrical

engineers only)

■ A section composed of only first- and second-year students

■ As a refresher course for materials engineering graduate students with a B.S.

degree in another engineering discipline.

Though only some of the chapters might be used in a single-semester/quarter course, experience suggests that students benefit from reading the entire text. The authors have intentionally made no effort to mark optional sections or chapters, since topic selection is a function of many factors, including instructor preferences, the background and needs of the students, and the course sequence at a specific institution.

THE AUTHOR TEAM

The field of materials engineering is so vast that no single individual can master it all. Therefore, a team was assembled with expertise in ceramics, composites, metals, poly-mers, and semiconductors. The author team has the collective expertise to explain clearly all the important aspects of the field in a single coherent package. The authors teach or have taught in chemical, materials, mechanical, and textile engineering departments. We teach at small colleges, where the engineering program is within a liberal arts setting, as well as major technological universities. Just as a composite combines the best features of its constituent materials, this book combines the varied strengths of its authors.

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THE INTEGRATED APPROACH

The book is organized into four parts. Part I, Fundamentals, focuses on the structure of engineering materials. Important topics include atomic bonding, thermodynamics and kinetics, crystalline and amorphous structures, defects in crystals, and strength of crys-tals. The concepts developed in these six chapters provide the foundation for the remain-der of the course. In Part II, Microstructural Development, the important processing variables of temperature, composition, and time are introduced, along with methods for controlling the structure of a material on the microscopic level. Part III focuses on the engineering properties of the various classes of materials. It builds upon the understanding of structure developed in Part I and the methods used to control structure set forth in Part II. It is in the properties section of the text that our approach, termed the integrated approach, differs from that of most of the competing texts.

Traditionally, all the macroscopic properties of one type of material (usually metals) are discussed before moving on to describe the properties of a second class of materials. The process is then repeated for ceramics, polymers, composites, and semiconductors. This traditional progression offers several advantages, including the ability to stress the unique strengths and weaknesses of each material class.

As authors, we believe most engineers will be searching for a material that can fulfill a specific list of properties as well as economic, processing, and environmental require-ments and will want to consider all classes of materials. That is, most engineers are more likely to “think” in terms of a property class rather than a material class. Thus, we describe the mechanical properties of all classes of materials, then the electrical properties of all classes of materials, and so on. We call this the integrated approach because it stresses fundamental concepts applicable to all materials first, and then points out the unique characteristics of each material class. During the development of the book the authors found that there were times when “forcing integration” would have degraded the quality of the presentation. Therefore, there are sections of the text where the integrated approach is temporarily suspended to improve clarity and emphasize the unique characteristics of specific materials.

The fourth and final part of the book deals with processing methods and with the overall materials design and selection process. These two chapters tie together all the topics introduced in the first three parts of the book. The goal is for the student to understand the methods used to select the appropriate material and processing methods required to satisfy a strict set of design specifications.

EMPHASIS ON DESIGN AND APPLICATIONS

Students are better able to understand the theoretical aspects of materials science and engineering when they are continually reinforced with applications and examples from their personal experiences. Thus, we have made a substantial effort to include both familiar and technologically important applications of every concept introduced in the text. In many cases we begin a discussion of a topic by describing a familiar situation and asking why certain results occur. This approach motivates the students to learn the details of the quantitative models so that they can solve problems, or understand phenomena, in which they have a personal interest.

The authors believe that most engineering problems have multiple correct solutions and must include environmental, ethical, and economic considerations. Therefore, our homework problems include both numerical problems with a single correct answer and design problems with multiple valid solution techniques and “correct” answers. The

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sample exercises within the text are divided into two classes. The Examples are straightfor-ward applications of concepts and equations in the text and generally have a single correct numerical solution. In contrast, Design Examples are open-ended and often involve select-ing a material for a specific application.

We have used a Case Study involving the design of a camcorder as a continuous thread throughout the manuscript. Each of the four parts of the text—Fundamentals, Microstruc-tural Development, Properties, and Design—begins with the identification of several mate-rials issues associated with the camcorder that can only be understood using concepts developed in that portion of the text. This technique allows students to get a view of the forest before they begin to focus on individual trees. The ongoing case also permits us to form bridges between the important aspects of the course within a context that is familiar to most students.

The authors’ belief in the importance of materials design and selection is underscored by the inclusion of an entire chapter on this subject at the end of the book. We recommend strongly that the instructor have the students read this chapter even if the schedule does not permit its inclusion in lecture. We find that it “closes the loop” for many of our students by helping them to understand the relationships among the many and varied topics introduced in the text. The design chapter contains 10 case studies and addresses issues such as life-cycle cost analysis, material and process selection, nuclear waste disposal, inspection criteria, failure analysis, and risk assessment and product liability.

CHANGES TO THE SECOND EDITION

Five new features have been added to the second edition of the text:

1. Each chapter begins with a motivational insert called Materials in Action. This feature is designed to introduce the reader to the important ideas in the chapter through an interesting real-world situation. Examples include a description of how adding 0.4 weight percent carbon to iron increases the strength of the material by two orders of magnitude, a discussion of why directionally solidified nickel-based turbine blades are worth their weight in gold in some aerospace applications, and an illustration of the false economy of using less expensive machining operations if they have a negative influence on fatigue crack initiation. This new feature extends our emphasis on design and applica-tions, which was one of the most popular attractions of the first edition.

2. We have developed a new Materials in Focus CD-ROM to enhance the textbook presentation. The CD-ROM contains a phase diagram tool and over 30 animations designed to help the reader gain an understanding of some of the visual concepts in the book. Examples include “three-dimensional” views of unit cells and polymer molecules, the movement of dislocations through crystals, changes in the population of electron energy levels in semiconductors with temperature, illustrations of polarization mecha-nisms, and examples of processing operations. In addition, the CD-ROM contains all of the photomicrographs in the text, and a series of interactive example problems. For example, in the portions of Chapter 7 on phase diagrams students can select a state point on a phase diagram and have the software help them determine the phases present, the compositions of the phases, and their relative amounts. Every illustration on the CD-ROM is directly linked to an illustration, concept, or problem in the text. In fact, every location in the text that has a link to a CD-ROM animation or example is clearly indicated

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3. Over 225 new homework problems have been added throughout the text. The majority of the chapters contain several design problems (i.e., problems with multiple correct solutions). These homework problems are marked with a “Design Problem” icon

in the margin of the text.

4. We have added an eight-page full color insert near the center of the book. This feature allows us to illustrate several important applications of materials science and engineering that simply are not easily described with either words or two-color illustra-tions.

5. The entire book has been redesigned for enhanced readability. In particular, the use of the icons illustrated below permits the reader to quickly identify several important features of the second edition:

A Design Problems A Design Examples

A Animated CD-ROM Concept

We have made a determined effort to improve the quality of the photomicrographs and to eliminate errors that were present in the first edition. We would like to express our sincere thanks to those of you who spotted problems and pointed them out to us. The book is better for your efforts, and if you have additional suggestions for how to improve the text we would be happy to hear them.

6. A Web site for the book can be found at http://www.mhhe.com. It contains infor-mation about the book and its supplements, Web links, and teaching resources.

ACKNOWLEDGMENTS

This book has undergone extensive revision under the direction of a distinguished panel of colleagues who have served as reviewers. The book has been greatly improved by this process and we owe each reviewer a sincere debt of gratitude. The reviewers for the first edition were:

John R. Ambrose, University of Florida

Robert Baron, Temple University

Ronald R. Bierderman, Worcester Polytechnic Institute

Samuel A. Bradford, University of Alberta

George L. Cahen, Jr., University of Virginia

Stephen J. Clarson, University of Cincinnati

Diana Farkas, Virginia Polytechnic Institute

David R. Gaskell, Purdue University

A. Jeffrey Giacomin, Texas A&MUniversity

Charles M. Gilmore, The George Washington University

David S. Grummon, Michigan State University

Ian W. Hall, University of Delaware

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Phillip L. Jones, Duke University

Dae Kim, The Ohio State University

David B. Knorr, Rensselaer Polytechnic Institute

D. Bruce Masson, Washington State University

John C. Matthews, Kansas State University

Masahiro Meshii, Northwestern University

Robert W. Messler, Jr., Rensselaer Polytechnic Institute

Derek O. Northwood, University of Windsor

Mark R. Plichta, Michigan Technological University

Richard L. Porter, North Carolina State University

John E. Ritter, University of Massachusetts

David A. Thomas, Lehigh University

Peter A. Thrower, Pennsylvania State University

Jack L. Tomlinson, California State Polytechnic University

Alan Wolfenden, Texas A&M University

Ernest G. Wolff, Oregon State University

The reviewers for the second edition are:

Bezad Bavarian, California State University–Northridge

David Cahill, University of Illionois

Stephen Krause, Arizona State University

Hillary Lackritz, Purdue University

Thomas J. Mackin, University of Illinois–Urbana

Arumugam Manthiram, The University of Texas at Austin

Walter W. Milligan, Michigan Technological University

Monte J. Pool, University of Cincinnati

Suzanne Rohde, University of Nebraska–Lincoln

Jay Samuel, University of Wisconsin–Madison

Shome N. Sinha, University of Illinois–Chicago

The authors would also like to thank the members of the editorial team: Tom Casson, publisher; Scott Isenberg; Kelley Butcher, developmental editor; and Gladys True, project manager. We would also like to thank James Mohler of the Department of Technical Graphics, Purdue University, the developer of the Materials in Focus CD-ROM.

SUPPLEMENTS

We have devoted considerable effort to the preparation of a high-quality solutions manual. Our approach is to employ a common solution technique for every homework problem. The procedure includes the following steps:

1. Find: (What are you looking for?)

2. Given: (What information is supplied in the problem statement?)

3. Data: (What additional information is available, from tables, figures, or

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4. Assumptions: (What are the limits on this analysis?)

5. Sketch: (What geometrical information is required?)

6. Solution: (A detailed step-by-step procedure.)

7. Comments: (How can this solution be applied to other similar situations and

what alternative solution techniques might be appropriate?)

The solutions manual is available to adopters of the text. Also, the authors have gained considerable experience using the “integrated” approach in the classroom and are avail-able to discuss implementation strategies with interested colleagues at other institutions.

James P. Schaffer Thomas H. Sanders, Jr.

Ashok Saxena Steven B. Warner

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1 MATERIALS SCIENCE AND ENGINEERING 2

1.1 Introduction 4

1.2 The Role of Materials in Technologically Advanced Societies 4

1.3 The Engineering Profession and Materials 6

1.4 Major Classes of Materials 7 1.4.1 Metals 8

1.4.2 Ceramics 9 1.4.3 Polymers 10 1.4.4 Composites 11 1.4.5 Semiconductors 13 1.5 Materials Properties and Materials

Engineering 14

1.6 The Integrated Approach to Materials Engineering 16

1.7 Engineering Professionalism and Ethics 18

Summary 19

PART

I

FUNDAMENTALS 20

2 ATOMIC SCALE STRUCTURES 22

2.1 Introduction 24 2.2 Atomic Structure 24

2.3 Thermodynamics and Kinetics 28 2.4 Primary Bonds 30

2.4.1 Ionic Bonding 31 2.4.2 Covalent Bonding 34 2.4.3 Metallic Bonding 35

2.4.4 Influence of Bond Type on Engineering Properties 37

2.5 The Bond-Energy Curve 39 2.6 Atomic Packing and Coordination

Numbers 43 2.7 Secondary Bonds 49 2.8 Mixed Bonding 51

2.9 The Structure of Polymer Molecules 52 Summary 54 Key Terms 55 Homework Problems 56

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3 CRYSTAL STRUCTURES 60 3.1 Introduction 62

3.2 Bravais Lattices and Unit Cells 62

3.3 Crystals with One Atom per Lattice Site and Hexagonal Crystals 65

3.3.1 Body-Centered Cubic Crystals 65 3.3.2 Face-Centered Cubic Crystals 68 3.3.3 Hexagonal Close-Packed Structures 69 3.4 Miller Indices 71 3.4.1 Coordinates of Points 72 3.4.2 Indices of Directions 73 3.4.3 Indices of Planes 76

3.4.4 Indices in the Hexagonal System 77 3.5 Densities and Packing Factors of Crystalline

Structures 78 3.5.1 Linear Density 78 3.5.2 Planar Density 80 3.5.3 Volumetric Density 82

3.5.4 Atomic Packing Factors and Coordination Numbers 82

3.5.5 Close-Packed Structures 83 3.6 Interstitial Positions and Sizes 85

3.6.1 Interstices in the FCC Structure 85 3.6.2 Interstices in the BCC Structure 86 3.6.3 Interstices in the HCP Structure 87 3.7 Crystals with Multiple Atoms per Lattice

Site 87

3.7.1 Crystals with Two Atoms per Lattice Site 88

3.7.2 Crystals with Three Atoms per Lattice Site 92

3.7.3 Other Crystal Structures 93 3.8 Liquid Crystals 95

3.9 Single Crystals and Polycrystalline Materials 95

3.10 Allotropy and Polymorphism 96 3.11 Anisotropy 98

3.12 X-ray Diffraction 98 Summary 103

Key Terms 104 Homework Problems 104

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4 POINT DEFECTS AND DIFFUSION 110

4.1 Introduction 112 4.2 Point Defects 112

4.2.1 Vacancies and Interstitials in Crystals 112

4.2.2 Vacancies and Interstitials in lonic Crystals 115

4.3 Impurities 116

4.3.1 Impurities in Crystals 117 4.3.2 Impurities in lonic Crystals 121 4.4 Solid-State Diffusion 122

4.4.1 Practical Examples of Diffusion 123 4.4.2 A Physical Description of Diffusion

(Fick’s First Law) 124

4.4.3 Mechanisms of Diffusion in Covalent and Metallic Crystals 128 4.4.4 Diffusion for Different Levels of

Concentration 130

4.4.5 Mechanisms of Diffusion in Ionic Crystals 132

4.4.6 Mechanisms of Diffusion in Polymers 133

4.4.7 Fick’s Second Law 135 Summary 140

Key Terms 141 Homework Problems 141

5 LINEAR, PLANAR, AND VOLUME DEFECTS 146

5.1 Introduction 148

5.2 Linear Defects, Slip, and Plastic Deformation 148

5.2.1 The Shear Strength of Deformable Single Crystals 148

5.2.2 Slip in Crystalline Materials and Edge Dislocations 152

5.2.3 Other Types of Dislocations 156 5.2.4 Slip Planes and Slip Directions in

Metal Crystals 159

5.2.5 Dislocations in Ionic, Covalent, and Polymer Crystals 162

5.2.6 Other Effects of Dislocations on Properties 166

5.3 Planar Defects 167

5.3.1 Free Surfaces in Crystals 167 5.3.2 Grain Boundaries in Crystals 168 5.3.3 Grain Size Measurement 169 5.3.4 Grain Boundary Diffusion 170 5.3.5 Other Planar Defects 171 5.4 Volume Defects 173

5.5 Strengthening Mechanisms in Metals 174 5.5.1 Alloying for Strength 175 5.5.2 Strain Hardening 176 5.5.3 Grain Refinement 178 5.5.4 Precipitation Hardening 179 Summary 179 Key Terms 180 Homework Problems 180

6 NONCRYSTALLINE AND SEMICRYSTALLINE MATERIALS 184

6.1 Introduction 186

6.2 The Glass Transition Temperature 186 6.3 Viscous Deformation 190

6.4 Structure and Properties of Amorphous and Semicrystalline Polymers 192

6.4.1 Polymer Classification 192 6.4.2 Molecular Weight 198 6.4.3 Polymer Conformations and

Configurations 200

6.4.4 Factors Determining Crystallinity of Polymers 202

6.4.5 Semicrystalline Polymers 205 6.4.6 The Relationship between Structure

and Tg 206

6.5 Structure and Properties of Glasses 206 6.5.1 Ionic Glasses 208

6.5.2 Covalent Glasses 211 6.5.3 Metallic Glasses 212

6.6 Structure and Properties of Rubbers and Elastomers 212

6.6.1 Thermoset Elastomers 213 6.6.2 Thermoplastic Elastomers 214 6.6.3 Crystallization in Rubbers 215 6.6.4 Temperature Dependence of Elastic

Modulus 216 6.6.5 Rubber Elasticity 217 Summary 219

Key Terms 220 Homework Problems 220

PART

II

MICROSTRUCTURAL DEVELOPMENT 224

7 PHASE EQUILIBRIA AND PHASE DIAGRAMS 226

7.1 Introduction 228

7.2 The One-Component Phase Diagram 229 7.3 Phase Equilibria in a Two-Component

System 232

7.3.1 Specification of Composition 232 7.3.2 The Isomorphous Diagram for Ideal

Systems 234

7.3.3 Phases in Equilibrium and the Lever Rule 235

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7.3.4 Solidification and Microstructure of Isomorphous Alloys 238

7.3.5 Determination of Liquidus and Solidus Boundaries 241

7.3.6 Specific Isomorphous Systems 242 7.3.7 Deviations from Ideal Behavior 242 7.4 The Eutectic Phase Diagram 247

7.4.1 Definitions of Terms in the Eutectic System 248

7.4.2 Melting and Solidification of Eutectic Alloys 249

7.4.3 Solidification of Off-Eutectic Alloys 250 7.4.4 Methods Used to Determine a Phase

Diagram 255

7.4.5 Phase Diagrams Containing Two Eutectics 257

7.5 The Peritectic Phase Diagram 260 7.6 The Monotectic Phase Diagram 263 7.7 Complex Diagrams 265

7.8 Phase Equilibria Involving Solid-to-Solid Reactions 267

7.8.1 Eutectoid Systems 268 7.9 Phase Equilibria in Three-Component

Systems 271

7.9.1 Plotting Compositions on a Ternary Diagram 272

7.9.2 The Lever Rule in Ternary Systems 274 Summary 275

Key Terms 276 Homework Problems 277

8 KINETICS AND MICROSTRUCTURE OF STRUCTURAL TRANSFORMATIONS 286

8.1 Introduction 288

8.2 Fundamental Aspects of Structural Transformations 289

8.2.1 The Nature of a Phase Transformation 289 8.2.2 The Driving Force for a Phase

Change 290 8.2.3 Homogeneous Nucleation of a Phase 292 8.2.4 Heterogeneous Nucleation of a Phase 296 8.2.5 Matrix/Precipitate Interfaces 298 8.2.6 Growth of a Phase 302

8.3 Applications to Engineering Materials 304 8.3.1 Phase Transformations in Steels 305 8.3.2 Precipitation from a Supersaturated

Solid Solution 320

8.3.3 Solidification and Homogenization of an Alloy 324

8.3.4 Recovery and Recrystallization Processes 330

8.3.5 Sintering 334 8.3.6 Martensitic (Displacive)

Transformations in Zirconia 337 8.3.7 Devitrification of an Oxide Glass 339 8.3.8 Crystallization of Polymers 340 Summary 343

Key Terms 344 Homework Problems 344

PART

III

PROPERTIES 356

9 MECHANICAL PROPERTIES 358

9.1 Introduction 360

9.2 Deformation and Fracture of Engineering Materials 360 9.2.1 Elastic Deformation 361 9.2.2 Deformation of Polymers 364 9.2.3 Plastic Deformation 367 9.2.4 Tensile Testing 368 9.2.5 Strengthening Mechanisms 376 9.2.6 Ductile and Brittle Fracture 377 9.2.7 Hardness Testing 378 9.2.8 Charpy Impact Testing 382 9.3 Brittle Fracture 386

9.3.1 Examples and Sequence of Events Leading to Brittle Fracture 386 9.3.2 Griffith-Orowan Theory for Predicting

Brittle Fracture 388

9.4 Fracture Mechanics: A Modern Approach 390 9.4.1 The Stress Intensity Parameter 391 9.4.2 The Influence of Sample Thickness 393 9.4.3 Relationship between Fracture

Toughness and Tensile Properties 394 9.4.4 Application of Fracture Mechanics to

Various Classes of Materials 395 9.4.5 Experimental Determination of Fracture

Toughness 398 9.5 Fatigue Fracture 399

9.5.1 Definitions Relating to Fatigue Fracture 399

9.5.2 Fatigue Testing 401

9.5.3 Correlations between Fatigue Strength and Other Mechanical Properties 402 9.5.4 Microscopic Aspects of Fatigue 404 9.5.5 Prevention of Fatigue Fractures 406 9.5.6 A Fracture Mechanics Approach to

Fatigue 406

9.6 Time-Dependent Behavior 409

9.6.1 Environmentally Induced Fracture 409 9.6.2 Creep in Metals and Ceramics 410

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9.6.3 Mechanisms of Creep Deformation 412 Summary 416 Key Terms 417 Homework Problems 418 10 ELECTRICAL PROPERTIES 426 10.1 Introduction 428 10.2 Electrical Conduction 428

10.2.1 Charge per Carrier 432 10.2.2 Charge Mobility 433

10.2.3 Energy Band Diagrams and Number of Charge Carriers 436

10.2.4 The Influence of Temperature on Electrical Conductivity and the Fermi-Dirac Distribution Function 438

10.2.5 Conductors, Semiconductors, and Insulators 444

10.2.6 Ionic Conduction Mechanisms 449 10.2.7 Effects of Defects and

Impurities 451

10.2.8 Conducting Polymers 453 10.2.9 Superconductivity 454 10.2.10 Devices and Applications 456 10.3 Semiconductors 457

10.3.1 Intrinsic and Extrinsic Conduction 457 10.3.2 Compound Semiconductors 464 10.3.3 Role of Defects 464 10.3.4 Simple Devices 465 10.3.5 Microelectronics 470 Summary 472 Key Terms 473 Homework Problems 473

11 OPTICAL AND DIELECTRIC PROPERTIES 478

11.1 Introduction 480 11.2 Polarization 481 11.2.1 Electronic Polarization 481 11.2.2 Ionic Polarization 482 11.2.3 Molecular Polarization 483 11.2.4 Interfacial Polarization 484 11.2.5 Net Polarization 484 11.2.6 Applications 485

11.3 Dielectric Constant and Capacitance 487 11.3.1 Capacitance 487

11.3.2 Permittivity and Dielectric Constant 487

11.3.3 Dielectric Strength and Breakdown 490

11.4 Dissipation and Dielectric Loss 492 11.5 Refraction and Reflection 494

11.5.1 Refraction 495 11.5.2 Specular Reflection 496 11.5.3 Dispersion 499 11.5.4 Birefringence 499 11.5.5 Application: Optical Waveguides 500 11.6 Absorption, Transmission, and

Scattering 502 11.6.1 Absorption 502

11.6.2 Absorption Coefficient 504 11.6.3 Absorption by Chromophores 505 11.6.4 Scattering and Opacity 507 11.7 Electronic Processes 508 11.7.1 X-Ray Fluorescence 508 11.7.2 Luminescence 508 11.7.3 Phosphorescence 510 11.7.4 Thermal Emission 510 11.7.5 Photoconductivity 510 11.7.6 Application: Lasers 511 Summary 512 Key Terms 513 Homework Problems 513 12 MAGNETIC PROPERTIES 518 12.1 Introduction 520

12.2 Materials and Magnetism 520 12.3 Physical Basis of Magnetism 521 12.4 Classification of Magnetic Materials 523 12.5 Diamagnetism and Paramagnetism 523 12.6 Ferromagnetism 525

12.6.1 Magnetic Domains 526

12.6.2 Response of Ferromagnetic Materials to External Fields 528

12.6.3 The Shape of the Hysteresis Loop 530

12.6.4 Microstructural Effects 531 12.6.5 Temperature Effects 531

12.6.6 Estimating the Magnitude of M 531 12.7 Antiferromagnetism and

Ferrimagnetism 532 12.8 Devices and Applications 535

12.8.1 Permanent Magnets 535 12.8.2 Transformer Cores 538 12.8.3 Magnetic Storage Devices 539 12.9 Superconducting Magnets 541 Summary 543 Key Terms 543 Homework Problems 544 13 THERMAL PROPERTIES 548 13.1 Introduction 550

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13.3 Heat Capacity 554

13.4 Thermal Conduction Mechanisms 557 13.5 Thermal Stresses 562 13.6 Applications 566 13.6.1 Bimetallic Strip 566 13.6.2 Thermal Insulation 567 13.6.3 Thermal Shock–Resistant Cookware 567 13.6.4 Tempered Glass 567 13.6.5 Support Structure for Orbiting

Telescopes 569 13.6.6 Ceramic-to-Metal Joints 569 13.6.7 Cryogenic Materials 570 Summary 571 Key Terms 571 Homework Problems 571 14 COMPOSITE MATERIALS 576 14.1 Introduction 578

14.2 History and Classification of Composites 578 14.3 General Concepts 582 14.3.1 Strengthening by Fiber Reinforcement 582 14.3.2 Characteristics of Fiber Materials 583 14.3.3 Characteristics of Matrix Materials 588 14.3.4 Role of Interfaces 589 14.3.5 Fiber Architecture 590 14.3.6 Strengthening in Aggregate Composites 592 14.4 Practical Composite Systems 593

14.4.1 Metal-Matrix Composites 593 14.4.2 Polymer-Matrix Composites 593 14.4.3 Ceramic-Matrix Composites 594 14.4.4 Carbon-Carbon Composites 595 14.5 Prediction of Composite Properties 595

14.5.1 Estimation of Fiber Diameter, Volume Fraction, and Density of the Composite 596

14.5.2 Estimation of Elastic Modulus and Strength 596

14.5.3 Estimation of the Coefficient of Thermal Expansion 600

14.5.4 Fracture Behavior of Composites 601 14.5.5 Fatigue Behavior of Composites 602 14.6 Other Applications of Composites 604

14.6.1 Estimation of Nonmechanical Properties of Composites 606 Summary 607 Key Terms 607 Homework Problems 608 15 MATERIALS-ENVIRONMENT INTERACTIONS 612 15.1 Introduction 614 15.2 Liquid-Solid Reactions 614

15.2.1 Direct Dissolution Mechanisms 616 15.2.2 Electrochemical Corrosion—Half-Cell

Potentials 619

15.2.3 Kinetics of Corrosion Reactions 626 15.2.4 Specific Types of Corrosion 629 15.2.5 Corrosion Prevention 640 15.3 Direct Atmospheric Attack (Gas-Solid

Reactions) 643

15.3.1 Alteration of Bond Structures by Atmospheric Gases 644 15.3.2 Formation of Gaseous Reaction

Products 646

15.3.3 Protective and Nonprotective Solid Oxides 646

15.3.4 Kinetics of Oxidation 649 15.3.5 Using Atmospheric “Attack” to

Advantage 652

15.3.6 Methods of Improving Resistance to Atmospheric Attack 653

15.4 Friction and Wear (Solid-Solid Interactions) 655

15.4.1 Wear Mechanisms 655 15.4.2 Designing to Minimize Friction and

Wear 658 15.5 Radiation Damage 658 Summary 660

Key Terms 661 Homework Problems 661

PART

IV

MATERIALS SYNTHESIS AND DESIGN 666

16 MATERIALS PROCESSING 668

16.1 Introduction 670

16.2 Process Selection Criteria and Interrelationship among Structure, Processing, and Properties 670 16.3 Casting 671 16.3.1 Metal Casting 671 16.3.2 Casting of Ceramics 676 16.3.3 Polymer Molding 676 16.4 Forming 679 16.4.1 Metal Forming 679

Case Study: Process Selection for a Steel Plate 680

16.4.2 Forming of Polymers 686 16.4.3 Forming of Ceramics and

Glasses 687 16.5 Powder Processing 689

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16.5.1 Powder Metallurgy 689 Case Study: Specification of Powder Size

Distribution for Producing Steel Sprockets 691

16.5.2 Powder Processing of Ceramics 692 16.6 Machining 692

16.7 Joining Processes 694

16.7.1 Welding, Brazing, and Soldering 694

16.7.2 Adhesive Bonding 697 16.7.3 Diffusion Bonding 698 16.7.4 Mechanical Joining 699 16.8 Surface Coatings and Treatments 699

16.8.1 Application of Coatings and Painting 700

16.8.2 Surface Treatments 701

Case Study: Material and Process Selection for Automobile Engine Crankshafts 702 16.9 Single-Crystal and Semiconductor

Processing 702

16.9.1 Growth and Processing of Single Crystals 703

16.9.2 Oxidation 704

16.9.3 Lithography and Etching 704 Case Study: Mask Selection for Doping of Si

Wafers 705

16.9.4 Diffusion and Ion Implantation 705 16.9.5 Interconnection, Assembly, and

Packaging 707 16.10 Fiber Manufacturing 708 16.10.1 Melt Spinning 709 16.10.2 Solution Spinning 709 16.10.3 Controlled Pyrolysis 711 16.10.4 Vapor-Phase Processes 711 16.10.5 Sintering 712 16.10.6 Chemical Reaction 713 16.11 Composite-Manufacturing Processes 714 16.11.1 Polymer-Matrix Composites (PMCs) 714 16.11.2 Metal-Matrix Composites (MMCs) 715 16.11.3 Ceramic-Matrix Composites (CMCs) 717 Summary 717 Key Terms 719 Homework Problems 719

17 MATERIALS AND ENGINEERING DESIGN 724

17.1 Introduction 726

17.2 Unified Life-Cycle Cost Engineering (ULCE) 727

17.2.1 Design and Analysis Costs 727 17.2.2 Manufacturing Costs 728

17.2.3 Operating Costs 728 17.2.4 Cost of Disposal 728

Case Study: Cost Consideration in Materials Selection 729

17.3 Material and Process Selection 730 17.3.1 Databases for Material

Selection 731 17.3.2 Materials and Process

Standards 732

17.3.3 Impact of Material Selection on the Environment 733

Case Study: Material Selection for Electronic Package Casing 736

Case Study: Material Selection for a Nuclear Waste Container 739 Case Study: Development of Lead-Free,

Free-Cutting Copper Alloy 740 17.4 Risk Assessment and Product Liability 743 17.4.1 Failure Probability Estimation 744 17.4.2 Liability Assessment 746 17.4.3 Quality Assurance Criteria 746 Case Study: Inspection Criterion for Large

Industrial Fans 747 17.5 Failure Analysis and Prevention 748

17.5.1 General Practice in Failure Analysis 749

Case Study: Failure Analysis of Seam-Welded Steam Pipes 752 Case Study: Failure in Wire Bonds in

Electronic Circuits 755 Case Study: Failure in a Polyethylene

Pipe 756

17.5.2 Failure Analysis in Composite Materials 757

17.5.3 Failure Prevention 759

Case Study: Inspection Interval Estimation for an Aerospace Pressure Vessel 760

Case Study: Choosing Optimum Locations for Probes during Ultrasonic Testing 764

Summary 765

Homework Problems 765

APPENDICES

A Periodic Table of the Elements 769 B Physical and Chemical Data for the

Elements 770

C Atomic and Ionic Radii of the Elements 773 D Mechanical Properties 775

E Answers to Selected Problems 790 Glossary 793

References 806 Index 808

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C

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P

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1

MATERIALS SCIENCE

AND ENGINEERING

1.1 Introduction

1.2 The Role of Materials in Technologically Advanced Societies

1.3 The Engineering Profession and Materials

1.4 Major Classes of Materials

1.5 Materials Properties and Materials Engineering

1.6 The Integrated Approach to Materials Engineering

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M A T E R I A L S I N A C T I O N

Building Blocks of Technology

Materials are at the core of all technological advances. Mastering the development, synthesis, and processing of materials opens opportunities that were scarcely dreamed of a few short decades ago. The truth of this statement is evident when one considers the spectacular progress that has been made in such diverse fields as energy, telecommunications, multimedia, computers, construction, and transportation. Travel by jet aircraft would be impossible without the materials that were developed specifically for the jet engine, and there would be no computers as we know them without solid-state microelectronic circuits. Indeed, it has been stated that the transistor has had the most far-reaching impact of any scientific or technological discovery to date. The centrality of materials to advanced technical societies was recognized in a recent report to the U.S. Congress authored by some of the most distinguished educators and scientists in the country. In that report it was stated that

advanced materials and advanced processing of materials are critical to the nation’s quality of life, security, and economic strength. Advanced materials are the building blocks of advanced technologies. Everything Americans use is composed of materials, from semiconductor chips to flexible concrete skyscrapers, from plastic bags to a ballerina’s artificial hip, or the composite structures on spacecraft. The impact of materials extends beyond products, in that tens of millions of manufacturing jobs depend on the availability of high-quality specialized materials.

In that same report it was further stated that

advanced materials are the building blocks of technology. When processed in particular ways, they enable the technological advances that constitute progress. Advanced materials and processing methods have become essen-tial to the enhancement of [the] quality of life, security, industrial productivity and economic growth. They are the tools for addressing urgent problems, such as pollution, declining natural resources and escalating costs.

The ability to develop and use materials is fundamental to the advancement of any society. In this text we will explore how that is done by engineers to improve the well-being of mankind.

Source: Reprinted from Materials Science and Engineering for the 1990s: Maintaining Competitiveness in the Age of

Materials, National Research Council, Washington, D.C. (National Academy Press, 1989).

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1.1 INTRODUCTION

Our purpose in this book is to examine the way in which materials impact society and to show how they are produced, processed, and used in all branches of engineering to ad-vance the well-being of society. In doing this, we will emphasize the relationship between the structure of a material and its underlying properties, and we will develop general principles applicable to all materials. Our goal in following this approach is to enable students to develop a fundamental understanding of material behavior that will help prepare them for a rapidly changing, and sometimes bewildering, environment. Since engineering is essentially an applied activity, practical examples that build on and amplify the fundamentals will also be emphasized for all topics and materials that are considered. The final chapter presents case studies in which the principles and practical information developed in the preceding chapters are integrated into the solution of real, materials-based engineering problems.

In the remainder of this chapter, we will review the fundamental relationship between a society’s economic well-being and its ability to understand and convert materials into usable forms. We will introduce the importance of the relationships between structure, properties, and processing for all classes of (solid) materials in all branches of engineer-ing. The chapter concludes with examples of some of the exciting opportunities and challenges that lie ahead in the areas of mechanical, aerospace, electrical, and chemical engineering.

1.2 THE ROLE OF MATERIALS IN TECHNOLOGICALLY ADVANCED SOCIETIES

Throughout history, most major breakthroughs in technology have been associated with the development of new materials and processes. For example, consider the materials-processing innovations that led to the development of the Damascus sword. Two methods were used to fabricate such swords. In one process, alternating layers of soft iron and steel (in this case Fe with about 0.6% C) were hammered together at high temperatures to produce a blade that had an edge of hard steel to retain a sharp cutting surface and a body of iron that provided resistance to fracture. In Japan, similar results were obtained by hammering steel into a thin sheet and then folding it back upon itself many times. A finished Japanese sword is shown in Figure 1.2–1; the variations in structure are quite

FIGURE 1.2–1 Photographs of the front and back sides of a Japanese sword forged by Hiromitsu in the mid-16th century. The smoothly waving outline was produced by polishing and the contrast enhanced by lighting. The structure of the hard and soft areas (mottled regions) can be seen along the edge from the tip to the midpoint. (Source: Cyril Stanley Smith, A Search for Structure: Selected Essays on Science, Art, and History, MIT Press, Cambridge, MA, copy-right 1992.)

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1250°C 1100°C 1000°C 1100°C 1250°C 1400°C 1100°C 1250°C 1450°C

Lower surface area

Side view 430°C 650°C 1400°C 1100°C 1220°C 1150°C 400°C

clear. The result of either processing method was a novel layered metal structure. Weap-ons produced from metals with this new structure gave their possessors a great advantage in battle. Similarly fabricated weapons in the Middle East provided one basis for the spread of the Syrian empire.

This example illustrates one of the key principles of materials science and engineer-ing— the intimate link between structure,properties,and processing. The structure of the metal resulting from innovative processing methods provided new combinations of prop-erties that offered significant advantage to those who developed the technology. Thus, these swords represent one of the first engineered materials.

More recently,development of processes to obtain precise compositional and struc-tural control has made miniaturized transistor technology possible. The result has been an electronics revolution that produced products such as computers,cellular phones,and compact disk players that continue to affect all aspects of modern life.

Another area where materials provide the springboard for advance is the aerospace industry. Light,strong alloys of aluminum and titanium have fostered the development of more efficient airframes,while the discovery and improvement of nickel-base alloys spurred development of powerful,efficient jet engines to propel these planes. Further improvements are being made as composites and ceramics are substituted for conven-tional materials.

The role of materials in the exploration of space is of central importance. One promi-nent example lies with the U.S. space shuttle. During reentry,extremely high tempera-tures develop as a result of friction between the earth’s atmosphere and the shuttle. These temperatures,which can exceed 1600⬚C,would melt any metal currently used in air-frames. Ceramic tiles,which have the ability to withstand extremely high temperatures and have excellent insulating properties,provide a method for protecting the aluminum frame of the spacecraft.

The approximate temperature distribution developed on the surface of the space shuttle during reentry is shown in Figure 1.2–2. Those regions in which the temperature ranges

FIGURE 1.2–2

Surface temperatures of U.S. space shuttle during reentry into the earth’s at-mosphere. (Source: G. Lewis, Selection of

Engi-neering Materials, Prentice

Hall, Inc., Englewood Cliffs, NJ, 1990.)

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between 400 and 1260⬚C have been protected with about 30,000 silica tiles. The tiles are coated with a layer of black borosilicate glass to both insulate the surface and radiate

thermal energy from the shuttle. In those regions that may reach 1600⬚C, coated

rein-forced carbon/carbon composites (materials composed of carbon fibers surrounded by a carbon matrix) are used. Without such materials, it is doubtful that a reusable space vehicle would be possible. This is an example of the way our highest aspirations are real-ized through our practical ability to develop and work with advanced materials.

Another example of materials providing the vehicle to technological breakthrough occurs in telecommunications. Information that was once carried electrically through copper wires is now being carried optically, through high-quality transparent SiO2fibers

as shown in Figure 1.2–3. The optical properties of the fibers are deliberately and pre-cisely varied across the fiber diameter to provide for maximum efficiency. Using this technology has increased the speed and volume of information that can be carried by orders of magnitude over what is possible using copper cable. Moreover, the reliability of the transmitted information has been vastly improved. In addition to these benefits, the negative effects of copper mining on the environment have been reduced, since the mate-rials and processes used to produce glass fibers have more benign environmental effects.

The centrality of materials to the economic well-being of the United States has been pointed out in the National Research Council study entitled Materials Science and Engineering for the 1990s—Maintaining Competitiveness in the Age of Materials. This document states that “materials science and engineering is crucial to the success of industries that are important to the strength of the U.S. economy and U.S. defense.” A similar position has been adopted by Japan, where the ability to develop, process, and fabricate advanced materials has been declared the cornerstone of the nation’s strategy to maintain a leading technological position.

1.3 THE ENGINEERING PROFESSION AND MATERIALS

In one way or another, materials are a major concern in all branches of engineering. In fact, the definition of engineering according to the Accreditation Board for Engineering and Technology makes this point clearly:

Engineering is the profession in which a knowledge of the mathematical and natural sciences gained by study, experience, and practice is applied with judgment to develop ways to utilize, economically, the materials and forces of nature for the benefit of mankind.

FIGURE 1.2–3

Optical fiber preform used to manufacture lightguides. The rings represent areas having different indices of refraction. When the pre-form is drawn, the final fiber diameter is about 125⫻ 10⫺6m.

(Source: Permission of AT&T Archives.)

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If this definition is accepted, we can see that engineering is a profoundly human activity that touches upon the life of all members of society. We can also see that an engineer is not only an applied scientist but much more. The engineer must have a good business sense, including an understanding of economics.

Important differences exist between the functions and approaches of engineers and scientists. Engineering is essentially an integrating activity, while science is a reductionist activity. The engineer often employs an intuitive, global (and, frequently, empirical) approach as opposed to that of the scientist, who breaks a problem down into its most basic elements to elucidate fundamental principles. In other words, an engineer is fre-quently required to solve problems by synthesizing knowledge from various disciplines and to produce items without a complete fundamental understanding of what he or she is dealing with. In such cases an engineer must define the operating conditions and develop a test program, based on his or her intuition, that will allow the project to move ahead in a safe, orderly, and economical manner.

In carrying out a job, the engineer will be faced with an almost infinite number of materials from which to choose. In some cases the materials will be put into service with little or no modification required, while in other cases additional processing will be nec-essary to obtain the desired properties. In choosing the best material for the job, the best approach is to determine the properties that are required and to then see what material will meet those properties at the lowest cost.

It is important to have a clear understanding of what is meant by the word cost. It does not simply refer to the initial cost of an item. Something may have a high initial cost, yet over the lifetime of the part, the total cost, taking all factors into account, may be low. An approach that considers the lifetime of the component or assembly is commonly referred to as life-cycle cost analysis. Factors such as reliability, replacement cost, the cost of downtime, the cost of environmental cleanup or disposal, and many others must all be considered. Materials play a key role in the life-cycle cost of a part. For example, consider tennis rackets or skis fabricated from composites, or macroscopic mixtures, of carbon fibers embedded in an epoxy matrix. While the initial cost of these items is relatively high, they are very durable and over their (significantly longer)lifetime are much less expensive than the metal or wood items they replaced.

It is also important for the engineer to realize that choice of materials cannot be made on the basis of a single property. For example, if an electrical engineer is designing a component in which the ability to conduct electricity is the principal property, he or she must remember that the material must be capable of being economically fabricated into the required form, be able to resist breaking, and have long-term stability so that the properties will not change significantly with time. Thus, in the majority of cases, choice of a material involves a complex set of trade-offs (including economic factors), and there is seldom one single solution that is “right” for the given application. Alternatively stated, there are often multiple “correct” solutions to a materials-selection problem; engineers must investigate several alternate solutions before making a final selection.

In addition, as we have seen in the case of the space shuttle, the materials selected must function together as a system. While each material is selected for specific properties to fulfill a given need, the materials must also be capable of operating together without degrading the properties of one another.

1.4 MAJOR CLASSES OF MATERIALS

The major classes of engineering materials are considered to be: (1)metals, (2)ceramics, (3)polymers, (4)composites, and (5)semiconductors. Metals with which you are probably

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familiar include iron, copper, aluminum, silver, and gold; common ceramics include sand, bricks and mortar, (window) glass, and graphite; examples of familiar polymers are cellulose, nylon, polyethylene, Teflon, Kevlar, and polystyrene; we have already discussed mixtures of materials known as composites such as carbon/carbon composites used in tiles on the space shuttle and carbon fibers in an epoxy matrix used in tennis rackets and skis; and the simplest semiconductors are silicon and germanium. By understanding the similarities and differences among these classes of materials, you will be in a position to make intelligent materials choices that can meet the challenges of modern technology. Why are materials arranged in the groups listed above? Many materials have similar atomic structures or useful engineering properties or both that make it convenient to classify them into these five broad groups. It should be recognized that these classifi-cations are somewhat arbitrary and may change with new discoveries and advances in technology. Composites, also sometimes called “engineered materials,” provide an excel-lent example of a new classification. These materials are made by combining other (often conventional) materials, using advanced technology, to obtain properties that could not be obtained from the existing classes of materials.

In our discussion in this chapter and throughout the book we will emphasize that the properties of a material are related to its structure. We will deal with structure at many

size scales ranging from the atomic scale (⬃0.1 ⫻ 10⫺9 m or 0.1 nm) through the

microscopic scale (⬃50 ⫻ 10⫺6m or 50␮m), and up to the macroscopic scale (⬃10⫺2m or 1 cm). In the next chapter we will see that the material structure on each of these size scales can be used to understand and explain certain materials properties.

While the properties of a material are related to its structure, it is important to understand that the way in which a material is processed affects the structure and hence the properties. As an example of this important concept, consider the dramatic effect that thermal processing can have on the properties of steel. If slowly cooled from a high temperature, steel will be relatively soft and have low strength. If the same steel is quenched (i.e., rapidly cooled) from the same high temperature, it will be extremely hard and brittle. Finally, if it is quenched and then reheated to some intermediate temperature, it will have an excellent combination of strength and toughness. While we will study this example in depth later in the text, the major point to be made here is that each of the three thermal processes has produced a different structure in the same material, which in turn gives rise to different properties.

Each of the five classes of materials, together with some elementary structure-property relationships, is discussed briefly in the following sections.

1.4.1 Metals

Metals form solids in which the atoms are located in regularly defined, repeating positions throughout the structure. These regular repeating structures, known as crystals and dis-cussed in detail in Chapter 3, give rise to specific properties. Metals are excellent conduc-tors of electricity, are relatively strong, are dense, can be deformed into complex shapes, and are resistant to breaking in a brittle manner when subjected to high-impact forces. This set of mechanical and physical properties makes metals one of the most important classes of materials for both electrical and structural applications. Extensive (and in some cases exclusive) use of metals occurs in automobiles, airplanes, buildings, bridges, ma-chine tools, ships, and many other applications where a combination of high strength and resistance to brittle fracture is required. In fact, it is largely the excellent combination of strength and toughness (i.e., resistance to fracture) that makes metals so attractive as structural materials.

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The basic understanding of metals and their properties is advanced, and they are considered to be mature materials with relatively little potential for major breakthroughs. However, significant improvements have been and continue to be made as a result of advances in processing. Two examples are:

■ Higher operating temperatures in jet engines have been attained through the use

of turbine blades that are produced by controlled solidification processes. The blades are made of alloys (atomic-scale mixtures of atoms) of nickel or other metals and are in wide commercial use. Improvements will continue as proces-ses are refined through use of advanced sensors and real-time computer control.

■ Frequently parts are fabricated from metal powders by compacting them into a

desired shape at high temperature and pressure in a process known as powder metallurgy (PM). An important reason for using PM processing is reduced fabri-cation costs. While some improvement in properties can be obtained through PM, a major benefit is the reduced variation in properties, which will allow the operating loads to be safely increased. Reduced production costs through PM will continue to impact the aerospace and automotive fields.

1.4.2 Ceramics

Ceramics are generally composed of both metallic and nonmetallic atomic species. Many (but not all) ceramics are crystalline, and frequently the nonmetal is oxygen, as in Al2O3,

MgO, and CaO, all of which are typical ceramics. One significant difference between ceramics and metals is that in ceramics, bonding is ionic and/or covalent. As a result there are no “free” electrons in ceramics. They are generally poor conductors of electricity, but are frequently used as insulators in electrical applications. One familiar example is spark plugs, in which a ceramic insulator separates the metal components.

Ionic and covalent bonds are extremely strong. As a result, ceramic materials are intrinsically stronger than metals. However, because of their more complex structure, the ions or atoms cannot easily be displaced as a result of applied forces. Rather than bend to accommodate such forces, ceramics tend to fracture in a brittle manner. This brittleness generally limits their use as structural materials, although recent improvements have been made by incorporating ceramic fibers into a ceramic matrix and other innovative tech-niques. Ceramics’ rigid bond structure confers other advantages, including high tempera-ture stability, resistance to chemical attack, and resistance to absorption of foreign substances. They are thus ideal in high-temperature applications such as the space shuttle, as containers for reactive chemicals, and as bowls and plates for foods where surface contamination is undesirable.

Some ceramics are not crystalline. The most common example is window glass, which is composed primarily of SiO2with the addition of various metal oxides. Optical

proper-ties are of major importance in glass and may be controlled through composition and processing. In addition the thermal and mechanical properties of glass can also be controlled. Safety glass is simply glass that has been subjected to a thermal cycle that leaves the surface in a state of compression and thereby resistant to cracking. In fact, glass treated in this way is even difficult to crack when struck with a hammer!

Some current and potential applications for ceramic materials with a large economic impact are listed below:

■ In the automotive industry the thermal and strength properties of ceramics

make them very attractive for engine components. For example, there are over 60,000 autos in Japan with ceramic turbochargers, which increase the efficiency

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H C H H C H H C H H C H H C H H C H

of the automobile. The materials in this application are Si3N4or SiC processed

to have some ability to resist brittle fracture.

■ Ceramics based on compounds such as YBa2Cu3O7and Ba2Sr2CaCu2Ox have

increased critical superconductingtemperatures to⬎ 95 K. This means that

superconductingfilms may be used as liners in microwave devices and as wires for all kinds of applications. Improvingthe current-carryingcapacity and con-nection technology are essential for widespread application of these materials.

■ Next-generation computers will have ceramic electro-optic components that

will give increased speed and efficiency.

1.4.3 Polymers

Polymers consist of long-chain molecules with repeating groups that are largely cova-lently bonded. Common elements within the chain backbone include C, O, N, and Si. An example of a common polymer with a simple structure, polyethylene, is shown in Fig-ure 1.4–1. The bonds within the backbone are all covalent, so the molecular chains are extremely strong. Chains are usually bonded to each other, however, by means of compar-atively weak secondary bonds. This means that it is generally easy for the chains to slide by one another when forces are applied and the strength is thus relatively low. In addition, many polymers tend to soften at moderate temperatures, so they are not generally useful for high-temperature applications.

Polymers, however, have properties that make them attractive in many applications. Since they contain common elements and are relatively easy to synthesize, or exist in nature, they can be inexpensive. They have a low density (in part because of the light elements from which they are constituted) and are easily formed into complex shapes. They have thus replaced metals for molded parts in automobiles and aircraft applications, especially where the load-bearingrequirements are modest. Because of these properties, as well as their chemical inertness, they are used as beverage containers and as pipingin plumbingapplications.

Like metals and ceramics, their properties can be modified by compositional changes and by processing. For example, substitution of a benzene ring for one in four hydrogen atoms converts polyethylene, shown in Figure 1.4–1, to polystyrene, Figure 1.4–2. Polyethylene is pliable and is used for applications such as “squeeze bottles.” In poly-styrene, the comparatively large benzene side group restricts the motion of the long-chain molecules and makes the structure more rigid. If the benzene group in polystyrene is replaced with a Cl atom (intermediate in size between H and the benzene ring), poly-vinylchloride is produced. The Cl atom will restrict the chain mobility more than an H atom but less than a benzene ring. A leathery material is produced with somewhat intermediate properties between polyethylene and polystyrene. These three polymers illustrate the fundamental principle, applicable to all materials, of the relationship be-tween material structure and properties.

FIGURE 1.4–1 Schematic of the structure of polyethylene. The mer or basic repeating unit in the polymer is the @ C2H4@ group.

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C H H C H C H H C H C H H C H Fiber Matrix Fiber

Some current and potential applications for polymers include the following:

■ The development of biodegradable polymers offers the potential for minimizing

the negative impact on our environment that results from the tremendous amount of waste our society generates.

■ Advances in liquid-crystal polymer technology may permit development of

light-weight structural materials.

■ Electrically conducting polymers may be able to replace traditional metal wires

in weight-critical applications such as electrical cables in aerospace vehicles.

1.4.4 Composites

Composites are structures in which two (or more) materials are combined to produce a new material whose properties would not be attainable by conventional means. Examples include plywood, concrete, and steel-belted tires. The most prevalent applications for fiber-reinforced composites are as structural materials where rigidity, strength, and low density are important. Many tennis rackets, racing bicycles, and skis are now fabricated from a carbon fiber –epoxy composite that is strong, light, and only moderately expensive. In this composite, carbon fibers are embedded in a matrix of epoxy, as shown in Fig-ure 1.4–3. The carbon fibers are strong and rigid but have limited ductility. Because of their brittleness, it would not be practical to construct a tennis racket or ski from carbon alone. The epoxy, which in itself is not very strong, plays two important roles. It acts as a medium to transfer load to the fibers, and the fiber-matrix interface deflects and stops small cracks, thus making the composite better able to resist cracks than either of its constituent components.

FIGURE 1.4–2 Schematic of the structure of polystyrene. This polymer has the same basic structure as the polyethylene shown in Figure 1.4–1 except that a benzene ring (C6H5) has been substituted for one of the four

H atoms. As a result of the larger side group, which hinders the sliding motion of adjacent polymer chains, polystyrene is stiffer than polyethylene.

FIGURE 1.4–3

A cross-sectional view of a carbon-epoxy composite showing the strong and stiff graphite fibers embed-ded in the tough epoxy ma-trix. (Source: Bhagwan D. Agarwal and Lawrence J. Broutman, Analysis and

Performance of Fiber Com-posites, 2nd ed., copyright 䊚 1990 by John Wiley &

Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.)

Figure

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References

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