Materials Science and Engineering: An Introduction PDF

Chapter 1 Introduction © iStockphoto/Mark Oleksiy...

Chapter 1 Introduction © iStockphoto/Mark Oleksiy © blickwinkel/Alamy A familiar item fabricated from three different material types is the © iStockphoto/Jill Chen beverage container. Beverages are marketed in aluminum (metal) cans (top), glass (ceramic) bottles (center), and plastic (polymer) bottles (bottom). © iStockphoto/Mark Oleksiy © blickwinkel/Alamy 1 Learning Objectives After studying this chapter, you should be able to do the following: 1. List six different property classifications of mate- 4. (a) List the three primary classifications rials that determine their applicability. of solid materials, and then cite the 2. Cite the four components that are involved in distinctive chemical feature of each. the design, production, and utilization of materi- (b) Note the four types of advanced materials als, and briefly describe the interrelationships and, for each, its distinctive feature(s). between these components. 5. (a) Briefly define smart material/system. 3. Cite three criteria that are important in the ma- (b) Briefly explain the concept of nanotechnol- terials selection process. ogy as it applies to materials. 1.1 HISTORICAL PERSPECTIVE Materials are probably more deep seated in our culture than most of us realize. Transportation, housing, clothing, communication, recreation, and food production— virtually every segment of our everyday lives is influenced to one degree or another by materials. Historically, the development and advancement of societies have been intimately tied to the members’ ability to produce and manipulate materials to fill their needs. In fact, early civilizations have been designated by the level of their materials development (Stone Age, Bronze Age, Iron Age).1 The earliest humans had access to only a very limited number of materials, those that occur naturally: stone, wood, clay, skins, and so on. With time, they discovered techniques for producing materials that had properties superior to those of the natural ones; these new materials included pottery and various metals. Furthermore, it was discovered that the properties of a material could be altered by heat treatments and by the addition of other substances. At this point, materials utilization was totally a selection process that involved deciding from a given, rather limited set of materials, the one best suited for an application by virtue of its characteristics. It was not until relatively recent times that scientists came to understand the relationships between the structural elements of materials and their proper- ties. This knowledge, acquired over approximately the past 100 years, has empowered them to fashion, to a large degree, the characteristics of materials. Thus, tens of thousands of dif- ferent materials have evolved with rather specialized characteristics that meet the needs of our modern and complex society, including metals, plastics, glasses, and fibers. The development of many technologies that make our existence so comfortable has been intimately associated with the accessibility of suitable materials. An advance- ment in the understanding of a material type is often the forerunner to the stepwise progression of a technology. For example, automobiles would not have been possible without the availability of inexpensive steel or some other comparable substitute. In the contemporary era, sophisticated electronic devices rely on components that are made from what are called semiconducting materials. 1.2 MATERIALS SCIENCE AND ENGINEERING Sometimes it is useful to subdivide the discipline of materials science and engineering into materials science and materials engineering subdisciplines. Strictly speaking, materi- als science involves investigating the relationships that exist between the structures and 1 The approximate dates for the beginnings of the Stone, Bronze, and Iron ages are 2.5 million bc, 3500 bc, and 1000 bc, respectively. 2  1.2 Materials Science and Engineering 3 properties of materials. In contrast, materials engineering involves, on the basis of these structure–property correlations, designing or engineering the structure of a material to produce a predetermined set of properties.2 From a functional perspective, the role of a materials scientist is to develop or synthesize new materials, whereas a materials engi- neer is called upon to create new products or systems using existing materials and/or to develop techniques for processing materials. Most graduates in materials programs are trained to be both materials scientists and materials engineers. Structure is, at this point, a nebulous term that deserves some explanation. In brief, the structure of a material usually relates to the arrangement of its internal components. Subatomic structure involves electrons within the individual atoms and interactions with their nuclei. On an atomic level, structure encompasses the organization of atoms or molecules relative to one another. The next larger structural realm, which contains large groups of atoms that are normally agglomerated together, is termed microscopic, mean- ing that which is subject to direct observation using some type of microscope. Finally, structural elements that can be viewed with the naked eye are termed macroscopic. The notion of property deserves elaboration. While in service use, all materials are exposed to external stimuli that evoke some type of response. For example, a specimen subjected to forces experiences deformation, or a polished metal surface reflects light. A property is a material trait in terms of the kind and magnitude of response to a specific imposed stimulus. Generally, definitions of properties are made independent of mate- rial shape and size. Virtually all important properties of solid materials may be grouped into six differ- ent categories: mechanical, electrical, thermal, magnetic, optical, and deteriorative. For each, there is a characteristic type of stimulus capable of provoking different responses. Mechanical properties relate deformation to an applied load or force; examples include elastic modulus (stiffness), strength, and toughness. For electrical properties, such as electrical conductivity and dielectric constant, the stimulus is an electric field. The thermal behavior of solids can be represented in terms of heat capacity and thermal conductivity. Magnetic properties demonstrate the response of a material to the ap- plication of a magnetic field. For optical properties, the stimulus is electromagnetic or light radiation; index of refraction and reflectivity are representative optical properties. Finally, deteriorative characteristics relate to the chemical reactivity of materials. The chapters that follow discuss properties that fall within each of these six classifications. In addition to structure and properties, two other important components are in- volved in the science and engineering of materials—namely, processing and perform- ance. With regard to the relationships of these four components, the structure of a material depends on how it is processed. Furthermore, a material’s performance is a function of its properties. Thus, the interrelationship among processing, structure, prop- erties, and performance is as depicted in the schematic illustration shown in Figure 1.1. Throughout this text, we draw attention to the relationships among these four compo- nents in terms of the design, production, and utilization of materials. We present an example of these processing-structure-properties-performance prin- ciples in Figure 1.2, a photograph showing three thin disk specimens placed over some printed matter. It is obvious that the optical properties (i.e., the light transmittance) of each of the three materials are different; the one on the left is transparent (i.e., virtually all of the Processing Structure Properties Performance Figure 1.1 The four components of the discipline of materials science and engineering and their interrelationship. 2 Throughout this text, we draw attention to the relationships between material properties and structural elements. 4 Chapter 1 / Introduction Figure 1.2 Three thin disk specimens of aluminum oxide that have been placed over a printed page in order to demonstrate their differences in light-transmittance characteristics. The disk on the left is transparent (i.e., virtually all light that is reflected from the page passes Specimen preparation, P. A. Lessing through it), whereas the one in the center is translucent (meaning that some of this reflected light is transmitted through the disk). The disk on the right is opaque—that is, none of the light passes through it. These differences in optical properties are a consequence of differences in structure of these materials, which have resulted from the way the materials were processed. reflected light passes through it), whereas the disks in the center and on the right are, respec- tively, translucent and opaque. All of these specimens are of the same material, aluminum oxide, but the leftmost one is what we call a single crystal—that is, has a high degree of perfection—which gives rise to its transparency. The center one is composed of numerous and very small single crystals that are all connected; the boundaries between these small crystals scatter a portion of the light reflected from the printed page, which makes this ma- terial optically translucent. Finally, the specimen on the right is composed not only of many small, interconnected crystals, but also of a large number of very small pores or void spaces. These pores also effectively scatter the reflected light and render this material opaque. Thus, the structures of these three specimens are different in terms of crystal boundaries and pores, which affect the optical transmittance properties. Furthermore, each material was produced using a different processing technique. If optical transmit- tance is an important parameter relative to the ultimate in-service application, the per- formance of each material will be different. 1.3 WHY STUDY MATERIALS SCIENCE AND ENGINEERING? Why do we study materials? Many an applied scientist or engineer, whether mechani- cal, civil, chemical, or electrical, is at one time or another exposed to a design problem involving materials, such as a transmission gear, the superstructure for a building, an oil refinery component, or an integrated circuit chip. Of course, materials scientists and engineers are specialists who are totally involved in the investigation and design of materials. Many times, a materials problem is one of selecting the right material from the thousands available. The final decision is normally based on several criteria. First, the in-service conditions must be characterized, for these dictate the properties required of the material. On only rare occasions does a material possess the maximum or ideal com- bination of properties. Thus, it may be necessary to trade one characteristic for another. The classic example involves strength and ductility; normally, a material having a high strength has only a limited ductility. In such cases, a reasonable compromise between two or more properties may be necessary. A second selection consideration is any deterioration of material properties that may occur during service operation. For example, significant reductions in mechanical strength may result from exposure to elevated temperatures or corrosive environments.  1.3 Why Study Materials Science and Engineering? 5 Finally, probably the overriding consideration is that of economics: What will the finished product cost? A material may be found that has the ideal set of properties but is prohibitively expensive. Here again, some compromise is inevitable. The cost of a finished piece also includes any expense incurred during fabrication to produce the desired shape. The more familiar an engineer or scientist is with the various characteristics and structure–property relationships, as well as the processing techniques of materials, the more proficient and confident he or she will be in making judicious materials choices based on these criteria. C A S E S T U D Y Liberty Ship Failures T he following case study illustrates one role that materials scientists and engineers are called upon to assume in the area of materials performance: experienced a ductile-to-brittle transition. Some of them were deployed to the frigid North Atlan- tic, where the once ductile metal experienced brit- analyze mechanical failures, determine their causes, tle fracture when temperatures dropped to below and then propose appropriate measures to guard the transition temperature.6 against future incidents. The corner of each hatch (i.e., door) was square; The failure of many of the World War II Liberty these corners acted as points of stress concentra- ships3 is a well-known and dramatic example of the tion where cracks can form. brittle fracture of steel that was thought to be duc- German U-boats were sinking cargo ships faster tile.4 Some of the early ships experienced structural than they could be replaced using existing con- damage when cracks developed in their decks and struction techniques. Consequently, it became hulls. Three of them catastrophically split in half when necessary to revolutionize construction methods cracks formed, grew to critical lengths, and then rap- to build cargo ships faster and in greater numbers. idly propagated completely around the ships’ girths. This was accomplished using prefabricated steel Figure 1.3 shows one of the ships that fractured the sheets that were assembled by welding rather day after it was launched. than by the traditional time-consuming riveting. Subsequent investigations concluded one or more Unfortunately, cracks in welded structures may of the following factors contributed to each failure5: propagate unimpeded for large distances, which When some normally ductile metal alloys are can lead to catastrophic failure. However, when cooled to relatively low temperatures, they be- structures are riveted, a crack ceases to propagate come susceptible to brittle fracture—that is, they once it reaches the edge of a steel sheet. experience a ductile-to-brittle transition upon Weld defects and discontinuities (i.e., sites where cooling through a critical range of temperatures. cracks can form) were introduced by inexperi- These Liberty ships were constructed of steel that enced operators. 3 During World War II, 2,710 Liberty cargo ships were mass-produced by the United States to supply food and materials to the combatants in Europe. 4 Ductile metals fail after relatively large degrees of permanent deformation; however, very little if any permanent deformation accompanies the fracture of brittle materials. Brittle fractures can occur very suddenly as cracks spread rapidly; crack propagation is normally much slower in ductile materials, and the eventual fracture takes longer. For these reasons, the ductile mode of fracture is usually preferred. Ductile and brittle fractures are discussed in Sections 8.3 and 8.4. 5 Sections 8.2 through 8.6 discuss various aspects of failure. 6 This ductile-to-brittle transition phenomenon, as well as techniques that are used to measure and raise the critical temperature range, are discussed in Section 8.6. (continued) 6 Chapter 1 / Introduction Figure 1.3 The Liberty ship S.S. Schenectady, which, in 1943, failed before leaving the shipyard. (Reprinted with permission of Earl R. Parker, Brittle Behavior of Engineering Structures, National Academy of Sciences, National Research Council, John Wiley & Sons, New York, 1957.) Remedial measures taken to correct these prob- Improving welding practices and establishing weld- lems included the following: ing codes. Lowering the ductile-to-brittle temperature of In spite of these failures, the Liberty ship program the steel to an acceptable level by improving steel was considered a success for several reasons, the pri- quality (e.g., reducing sulfur and phosphorus im- mary reason being that ships that survived failure were purity contents). able to supply Allied Forces in the theater of operations Rounding off hatch corners by welding a curved and in all likelihood shortened the war. In addition, reinforcement strip on each corner.7 structural steels were developed with vastly improved resistances to catastrophic brittle fractures. Detailed Installing crack-arresting devices such as riveted analyses of these failures advanced the understand- straps and strong weld seams to stop propagating ing of crack formation and growth, which ultimately cracks. evolved into the discipline of fracture mechanics. 7 The reader may note that corners of windows and doors for all of today’s marine and aircraft structures are rounded. 1.4 CLASSIFICATION OF MATERIALS Solid materials have been conveniently grouped into three basic categories: metals, ce- ramics, and polymers, a scheme based primarily on chemical makeup and atomic struc- ture. Most materials fall into one distinct grouping or another. In addition, there are the Tutorial Video: What are the composites that are engineered combinations of two or more different materials. A brief Different Classes explanation of these material classifications and representative characteristics is offered of Materials? next. Another category is advanced materials—those used in high-technology applica- tions, such as semiconductors, biomaterials, smart materials, and nanoengineered mate- rials; these are discussed in Section 1.5.  1.4 Classification of Materials 7 Figure 1.4 40 Metals Bar chart of room- temperature density 20 Platinum values for various Silver Density (g/cm3) (logarithmic scale) metals, ceramics, Ceramics 10 8 Copper polymers, and Iron/Steel 6 ZrO2 composite materials. Al2O3 Titanium 4 Polymers Composites SiC,Si3N4 Aluminum Glass GFRC 2 PTFE Concrete Magnesium CFRC PVC PS 1.0 PE 0.8 Rubber 0.6 Woods 0.4 0.2 0.1 Metals Metals are composed of one or more metallic elements (e.g., iron, aluminum, copper, titanium, gold, nickel), and often also nonmetallic elements (e.g., carbon, nitrogen, oxygen) in relatively small amounts.8 Atoms in metals and their alloys are arranged in a very orderly manner (as discussed in Chapter 3) and are relatively dense in comparison to the ceramics and polymers (Figure 1.4). With regard to mechanical characteristics, these materials are relatively stiff (Figure 1.5) and strong (Figure 1.6), yet are ductile (i.e., capable of large amounts of deformation without fracture), and are resistant to fracture (Figure 1.7), which accounts for their widespread use in structural applications. Tutorial Video: Metals Metallic materials have large numbers of nonlocalized electrons—that is, these electrons are not bound to particular atoms. Many properties of metals are directly attributable to these electrons. For example, metals are extremely good conductors of electricity Figure 1.5 Bar chart of room- Metals Ceramics 1000 Composites temperature stiffness Stiffness [elastic (or Young’s) modulus (in units of Tungsten SiC (i.e., elastic modulus) Iron/Steel AI2O3 CFRC values for various 100 Titanium Si3N4 ZrO2 gigapascals)] (logarithmic scale) metals, ceramics, Aluminum Magnesium Glass GFRC polymers, and Concrete composite materials. Polymers Woods 10 PVC PS, Nylon 1.0 PTFE PE 0.1 Rubbers 0.01 0.001 8 The term metal alloy refers to a metallic substance that is composed of two or more elements. 8 Chapter 1 / Introduction Figure 1.6 Bar chart of room- Metals Composites temperature strength Ceramics (i.e., tensile strength) Strength (tensile strength, in units of Steel megapascals) (logarithmic scale) values for various 1000 CFRC alloys Si3N4 metals, ceramics, Cu,Ti GFRC alloys SiC polymers, and Al2O3 Aluminum composite materials. alloys Gold Polymers 100 Glass Nylon Woods PS PVC PTFE PE 10 (Figure 1.8) and heat, and are not transparent to visible light; a polished metal surface has a lustrous appearance. In addition, some of the metals (i.e., Fe, Co, and Ni) have desirable magnetic properties. Figure 1.9 shows several common and familiar objects that are made of metallic materials. Furthermore, the types and applications of metals and their alloys are discussed in Chapter 11. Ceramics Ceramics are compounds between metallic and nonmetallic elements; they are most fre- quently oxides, nitrides, and carbides. For example, common ceramic materials include aluminum oxide (or alumina, Al2O3), silicon dioxide (or silica, SiO2), silicon carbide (SiC), silicon nitride (Si3N4), and, in addition, what some refer to as the traditional ceramics—those composed of clay minerals (e.g., porcelain), as well as cement and glass. With regard to me- chanical behavior, ceramic materials are relatively stiff and strong—stiffnesses and strengths Tutorial Video: Ceramics are comparable to those of the metals (Figures 1.5 and 1.6). In addition, they are typically very hard. Historically, ceramics have exhibited extreme brittleness (lack of ductility) and are highly susceptible to fracture (Figure 1.7). However, newer ceramics are being engineered to have improved resistance to fracture; these materials are used for cookware, cutlery, and Figure 1.7 Metals Bar chart of room-temperature Steel alloys Composites resistance to fracture 100 Resistance to Fracture (fracture toughness, Titanium (i.e., fracture tough- in units of MPa m) (logarithmic scale) alloys ness) for various Aluminum CFRC GFRC metals, ceramics, alloys polymers, and composite materials. 10 Ceramics (Reprinted from Polymers Engineering Materials Si3N4 Nylon 1: An Introduction to Al2O3 Properties, Applications Polystyrene SiC and Design, third Polyethylene 1.0 edition, M. F. Ashby and Wood D. R. H. Jones, pages Polyester 177 and 178, Copyright Glass 2005, with permission Concrete from Elsevier.) 0.1  1.4 Classification of Materials 9 Figure 1.8 Metals Bar chart of room- 108 temperature electrical Semiconductors 104 Electrical Conductivity (in units of reciprocal conductivity ranges for metals, ceramics, ohm-meters) (logarithmic scale) polymers, and 1 semiconducting materials. 10–4 10–8 Ceramics Polymers 10–12 10–16 10–20 even automobile engine parts. Furthermore, ceramic materials are typically insulative to the passage of heat and electricity (i.e., have low electrical conductivities, Figure 1.8) and are more resistant to high temperatures and harsh environments than are metals and polymers. With regard to optical characteristics, ceramics may be transparent, translucent, or opaque (Figure 1.2), and some of the oxide ceramics (e.g., Fe3O4) exhibit magnetic behavior. Several common ceramic objects are shown in Figure 1.10. The characteristics, types, and applications of this class of materials are also discussed in Chapters 12 and 13. Polymers Polymers include the familiar plastic and rubber materials. Many of them are organic compounds that are chemically based on carbon, hydrogen, and other nonmetallic ele- ments (i.e., O, N, and Si). Furthermore, they have very large molecular structures, often chainlike in nature, that often have a backbone of carbon atoms. Some common and familiar polymers are polyethylene (PE), nylon, poly(vinyl chloride) (PVC), polycar- bonate (PC), polystyrene (PS), and silicone rubber. These materials typically have low densities (Figure 1.4), whereas their mechanical characteristics are generally dissimilar to those of the metallic and ceramic materials—they are not as stiff or strong as these Figure 1.9 Familiar objects made of metals and metal alloys (from left to right): silverware (fork and knife), scissors, coins, a gear, a wedding ring, and a nut and bolt. © William D. Callister, Jr. 10 Chapter 1 / Introduction Figure 1.10 Common objects made of ceramic materials: scissors, a china teacup, a building brick, a floor tile, and a glass vase. © William D. Callister, Jr. other material types (Figures 1.5 and 1.6). However, on the basis of their low densities, many times their stiffnesses and strengths on a per-mass basis are comparable to those of the metals and ceramics. In addition, many of the polymers are extremely ductile and pliable (i.e., plastic), which means they are easily formed into complex shapes. In general, they are relatively inert chemically and unreactive in a large number of environ- ments. One major drawback to the polymers is their tendency to soften and/or decom- pose at modest temperatures, which, in some instances, limits their use. Furthermore, they have low electrical conductivities (Figure 1.8) and are nonmagnetic. Figure 1.11 shows several articles made of polymers that are familiar to the reader. Tutorial Video: Chapters 14 and 15 are devoted to discussions of the structures, properties, applications, Polymers and processing of polymeric materials. Figure 1.11 Several common objects made of polymeric materials: plastic tableware (spoon, fork, and knife), billiard balls, a bicycle helmet, two dice, a lawn mower wheel (plastic hub and rubber tire), and a plastic milk carton. © William D. Callister, Jr.  1.4 Classification of Materials 11 C A S E S T U D Y Carbonated Beverage Containers O ne common item that presents some interesting material property requirements is the container for carbonated beverages. The material used for this beverages. In addition, each material has its pros and cons. For example, the aluminum alloy is relatively strong (but easily dented), is a very good barrier to application must satisfy the following constraints: (1) the diffusion of carbon dioxide, is easily recycled, provide a barrier to the passage of carbon dioxide, cools beverages rapidly, and allows labels to be which is under pressure in the container; (2) be non- painted onto its surface. However, the cans are op- toxic, unreactive with the beverage, and, preferably, tically opaque and relatively expensive to produce. recyclable; (3) be relatively strong and capable of Glass is impervious to the passage of carbon dioxide, surviving a drop from a height of several feet when is a relatively inexpensive material, and may be recy- containing the beverage; (4) be inexpensive, includ- cled, but it cracks and fractures easily, and glass bot- ing the cost to fabricate the final shape; (5) if opti- tles are relatively heavy. Whereas plastic is relatively cally transparent, retain its optical clarity; and (6) be strong, may be made optically transparent, is inex- capable of being produced in different colors and/or pensive and lightweight, and is recyclable, it is not adorned with decorative labels. as impervious to the passage of carbon dioxide as All three of the basic material types—metal aluminum and glass. For example, you may have no- (aluminum), ceramic (glass), and polymer (polyes- ticed that beverages in aluminum and glass contain- ter plastic)—are used for carbonated beverage con- ers retain their carbonization (i.e., “fizz”) for several tainers (per the chapter-opening photographs). All years, whereas those in two-liter plastic bottles “go of these materials are nontoxic and unreactive with flat” within a few months. Composites A composite is composed of two (or more) individual materials that come from the categories previously discussed—metals, ceramics, and polymers. The design goal of a composite is to achieve a combination of properties that is not displayed by any single material and also to incorporate the best characteristics of each of the component ma- terials. A large number of composite types are represented by different combinations of metals, ceramics, and polymers. Furthermore, some naturally occurring materials are composites—for example, wood and bone. However, most of those we consider in our discussions are synthetic (or human-made) composites. One of the most common and familiar composites is fiberglass, in which small glass fibers are embedded within a polymeric material (normally an epoxy or polyester).9 The glass fibers are relatively strong and stiff (but also brittle), whereas the polymer is more flexible. Thus, fiberglass is relatively stiff, strong (Figures 1.5 and 1.6), and flexible. In addition, it has a low density (Figure 1.4). Another technologically important material is the carbon fiber–reinforced polymer Tutorial Video: (CFRP) composite—carbon fibers that are embedded within a polymer. These materials Composites are stiffer and stronger than glass fiber–reinforced materials (Figures 1.5 and 1.6) but more expensive. CFRP composites are used in some aircraft and aerospace applications, as well as in high-tech sporting equipment (e.g., bicycles, golf clubs, tennis rackets, skis/ snowboards) and recently in automobile bumpers. The new Boeing 787 fuselage is pri- marily made from such CFRP composites. Chapter 16 is devoted to a discussion of these interesting composite materials. 9 Fiberglass is sometimes also termed a glass fiber–reinforced polymer composite (GFRP). 12 Chapter 1 / Introduction 1.5 ADVANCED MATERIALS Materials utilized in high-technology (or high-tech) applications are sometimes termed advanced materials. By high technology, we mean a device or product that operates or functions using relatively intricate and sophisticated principles, including electronic equipment (camcorders, CD/DVD players), computers, fiber-optic systems, spacecraft, aircraft, and military rocketry. These advanced materials are typically traditional mate- rials whose properties have been enhanced and also newly developed, high-performance materials. Furthermore, they may be of all material types (e.g., metals, ceramics, polymers) and are normally expensive. Advanced materials include semiconductors, biomaterials, and what we may term materials of the future (i.e., smart materials and nanoengineered materials), which we discuss next. The properties and applications of a number of these advanced materials—for example, materials that are used for lasers, integrated circuits, magnetic information storage, liquid crystal displays (LCDs), and fiber optics—are also discussed in subsequent chapters. Semiconductors Semiconductors have electrical properties that are intermediate between those of electrical conductors (i.e., metals and metal alloys) and insulators (i.e., ceramics and polymers)—see Figure 1.8. Furthermore, the electrical characteristics of these ma- terials are extremely sensitive to the presence of minute concentrations of impurity atoms, for which the concentrations may be controlled over very small spatial regions. Semiconductors have made possible the advent of integrated circuitry that has totally revolutionized the electronics and computer industries (not to mention our lives) over the past three decades. Biomaterials Biomaterials are employed in components implanted into the human body to replace dis- eased or damaged body parts. These materials must not produce toxic substances and must be compatible with body tissues (i.e., must not cause adverse biological reactions). All of the preceding materials—metals, ceramics, polymers, composites, and semiconductors— may be used as biomaterials. Smart Materials Smart (or intelligent) materials are a group of new and state-of-the-art materials now being developed that will have a significant influence on many of our technologies. The adjective smart implies that these materials are able to sense changes in their environ- ment and then respond to these changes in predetermined manners—traits that are also found in living organisms. In addition, this smart concept is being extended to rather sophisticated systems that consist of both smart and traditional materials. Components of a smart material (or system) include some type of sensor (which detects an input signal) and an actuator (which performs a responsive and adaptive function). Actuators may be called upon to change shape, position, natural frequency, or mechanical characteristics in response to changes in temperature, electric fields, and/or magnetic fields. Four types of materials are commonly used for actuators: shape-memory alloys, pi- ezoelectric ceramics, magnetostrictive materials, and electrorheological/magnetorheo- logical fluids. Shape-memory alloys are metals that, after having been deformed, revert to their original shape when temperature is changed (see the Materials of Importance box following Section 10.9). Piezoelectric ceramics expand and contract in response to an applied electric field (or voltage); conversely, they also generate an electric field when their dimensions are altered (see Section 18.25). The behavior of magnetostrictive materials is analogous to that of the piezoelectrics, except that they are responsive to  1.5 Advanced Materials 13 magnetic fields. Also, electrorheological and magnetorheological fluids are liquids that experience dramatic changes in viscosity upon the application of electric and magnetic fields, respectively. Materials/devices employed as sensors include optical fibers (Section 21.14), piezoelec- tric materials (including some polymers), and microelectromechanical systems (MEMS; Section 13.9). For example, one type of smart system is used in helicopters to reduce aerodynamic cockpit noise created by the rotating rotor blades. Piezoelectric sensors inserted into the blades monitor blade stresses and deformations; feedback signals from these sensors are fed into a computer-controlled adaptive device that generates noise-canceling antinoise. Nanomaterials One new material class that has fascinating properties and tremendous technological promise is the nanomaterials, which may be any one of the four basic types—metals, ceramics, polymers, or composites. However, unlike these other materials, they are not distinguished on the basis of their chemistry but rather their size; the nano prefix denotes that the dimensions of these structural entities are on the order of a nanometer (10-9 m)—as a rule, less than 100 nanometers (nm; equivalent to the diameter of ap- proximately 500 atoms). Prior to the advent of nanomaterials, the general procedure scientists used to understand the chemistry and physics of materials was to begin by studying large and complex structures and then investigate the fundamental building blocks of these struc- tures that are smaller and simpler. This approach is sometimes termed top-down science. However, with the development of scanning probe microscopes (Section 4.10), which permit observation of individual atoms and molecules, it has become possible to design and build new structures from their atomic-level constituents, one atom or molecule at a time (i.e., “materials by design”). This ability to arrange atoms carefully provides op- portunities to develop mechanical, electrical, magnetic, and other properties that are not otherwise possible. We call this the bottom-up approach, and the study of the properties of these materials is termed nanotechnology.10 Some of the physical and chemical characteristics exhibited by matter may experi- ence dramatic changes as particle size approaches atomic dimensions. For example, materials that are opaque in the macroscopic domain may become transparent on the nanoscale; some solids become liquids, chemically stable materials become combustible, and electrical insulators become conductors. Furthermore, properties may depend on size in this nanoscale domain. Some of these effects are quantum mechanical in origin, whereas others are related to surface phenomena—the proportion of atoms located on surface sites of a particle increases dramatically as its size decreases. Because of these unique and unusual properties, nanomaterials are finding niches in electronic, biomedical, sporting, energy production, and other industrial applications. Some are discussed in this text, including the following: Catalytic converters for automobiles (Materials of Importance box, Chapter 4) Nanocarbons—Fullerenes, carbon nanotubes, and graphene (Section 13.9) Particles of carbon black as reinforcement for automobile tires (Section 16.2) Nanocomposites (Section 16.16) Magnetic nanosize grains that are used for hard disk drives (Section 20.11) Magnetic particles that store data on magnetic tapes (Section 20.11) 10 One legendary and prophetic suggestion as to the possibility of nanoengineered materials was offered by Richard Feynman in his 1959 American Physical Society lecture titled “There’s Plenty of Room at the Bottom.” 14 Chapter 1 / Introduction Whenever a new material is developed, its potential for harmful and toxicological interactions with humans and animals must be considered. Small nanoparticles have ex- ceedingly large surface area–to–volume ratios, which can lead to high chemical reactivi- ties. Although the safety of nanomaterials is relatively unexplored, there are concerns that they may be absorbed into the body through the skin, lungs, and digestive tract at relatively high rates, and that some, if present in sufficient concentrations, will pose health risks—such as damage to DNA or promotion of lung cancer. 1.6 MODERN MATERIALS’ NEEDS In spite of the tremendous progress that has been made in the discipline of materials science and engineering within the past few years, technological challenges remain, in- cluding the development of even more sophisticated and specialized materials, as well as consideration of the environmental impact of materials production. Some comment is appropriate relative to these issues so as to round out this perspective. Nuclear energy holds some promise, but the solutions to the many problems that remain necessarily involve materials, such as fuels, containment structures, and facilities for the disposal of radioactive waste. Significant quantities of energy are involved in transportation. Reducing the weight of transportation vehicles (automobiles, aircraft, trains, etc.), as well as increasing engine op- erating temperatures, will enhance fuel efficiency. New high-strength, low-density struc- tural materials remain to be developed, as well as materials that have higher-temperature capabilities, for use in engine components. Furthermore, there is a recognized need to find new and economical sources of energy and to use present resources more efficiently. Materials will undoubtedly play a significant role in these developments. For example, the direct conversion of solar power into electrical energy has been demonstrated. Solar cells employ some rather complex and expensive materials. To ensure a viable technology, materials that are highly efficient in this conversion process yet less costly must be developed. The hydrogen fuel cell is another very attractive and feasible energy-conversion technology that has the advantage of being nonpolluting. It is just beginning to be im- plemented in batteries for electronic devices and holds promise as a power plant for automobiles. New materials still need to be developed for more efficient fuel cells and also for better catalysts to be used in the production of hydrogen. Furthermore, environmental quality depends on our ability to control air and water pollution. Pollution control techniques employ various materials. In addition, materials processing and refinement methods need to be improved so that they pro- duce less environmental degradation—that is, less pollution and less despoilage of the landscape from the mining of raw materials. Also, in some materials manufacturing processes, toxic substances are produced, and the ecological impact of their disposal must be considered. Many materials that we use are derived from resources that are nonrenewable—that is, not capable of being regenerated, including most polymers, for which the prime raw material is oil, and some metals. These nonrenewable resources are gradually becoming depleted, which necessitates (1) the discovery of additional reserves, (2) the development of new materials having comparable properties with less adverse environmental impact, and/or (3) increased recycling efforts and the development of new recycling technologies. As a consequence of the economics of not only production but also environmental im- pact and ecological factors, it is becoming increasingly important to consider the “cradle- to-grave” life cycle of materials relative to the overall manufacturing process. The roles that materials scientists and engineers play relative to these, as well as other environmental and societal issues, are discussed in more detail in Chapter 22.  1.7 Processing/Structure/Properties/Performance Correlations 15 1.7 PROCESSING/STRUCTURE/PROPERTIES/ PERFORMANCE CORRELATIONS As mentioned previously (Section 1.2), the science and engineering of materials in- volves four interrelated components: processing, structure, properties, and perform- ance (Figure 1.1). Inasmuch as the remainder of this text discusses these components Tutorial Video: for the different material types, it has been decided to direct the reader’s attention to Processing/ the treatment of individual components for several specific materials. Whereas some Structure/ of these discussions are found within single chapters, others are spread out over mul- Properties/ tiple chapters. For the latter, and for each material we have selected, a topic timeline Performance of has been created that indicates the locations (by sections) where treatments of the Materials four components are to be found. Figure 1.12 presents topic timelines for the follow- ing materials: steels, glass–ceramics, polymer fibers, and silicon semiconductors. In addition, near the end of each chapter with some discussion of processing, structure, STEELS Isothermal transformation diagrams, continuous-cooling transformation diagrams; heat treating for tempered martensite Diffusion Recrystallization ➣ Heat treatment of steels ➣ ➣ ➣ Processing Crystal structure, Development of microstructure, Microstructure of various polymorphism iron–iron carbide alloys microconstituents ➣ Structure ➣ ➣ Dislocations, slip systems, Phase equilibria, Solid solutions, Mechanical strengthening the iron–iron carbide Mechanical properties of dislocations properties mechanisms phase diagram Fe–C alloys ➣ ➣ ➣ ➣ ➣ Properties Applications of steel alloys ➣ Performance ch 3 ch 4 ch 5 ch 6 ch 7 ch 9 ch 10 ch 11 (a) GLASS–CERAMICS Crystallization, Continuous-cooling fabrication, heat transformation diagrams Concept of viscosity treatment ➣ ➣ ➣ Processing Noncrystalline Atomic structure solids of silica glasses Polycrystallinity ➣ Structure ➣ ➣ Mechanical, thermal, Opacity and translucency in optical properties insulators ➣ ➣ Properties Applications ➣ Performance ch 3 ch 10 ch 12 ch 13 ch 21 (b) Figure 1.12 Processing/structure/properties/performance topic timelines for (a) steels, (b) glass–ceramics, (c) polymer fibers, and (d) silicon semiconductors. 16 Chapter 1 / Introduction POLYMER FIBERS Polymerization, additives, melting, fiber forming ➣ Melting temperature, factors that affect ➣ Processing Electronic structure, Polymer molecules, polymer interatomic bonding crystals ➣ ➣ Structure Melting temperature, factors that affect ➣ Thermoplastic Mechanical properties, polymers factors that affect Degradation ➣ ➣ ➣ Properties Applications ➣ Performance ch 2 ch 14 ch 15 ch 17 (c) SILICON SEMICONDUCTORS Composition specification Diffusion Integrated circuits ➣ ➣ ➣ Processing Electronic structure, interatomic bonding Electronic band structure ➣ Structure ➣ Electrical properties Properties ➣ Integrated circuits ➣ Performance ch 3 ch 4 ch 5 ch 18 (d) Figure 1.12 (continued) properties, and/or performance for at least one of these four materials, a summary is provided in the form of one or more concept maps. A concept map is a diagram that illustrates the relationships among concepts. We represent these relationships by connecting arrows (frequently horizontal); each arrow points (left to right) from one concept to another. The organization of these connections is hierarchical—that is, a concept to the left of an arrow should be mastered before a concept to the right can be understood. For each map, at least one of its concepts is discussed in its chap- ter; other concepts may be treated in previous and/or later chapters. For example, Figure 1.13 presents a portion of a concept map for the processing of steel alloys that appears in Chapter 10. Iron–iron carbide Isothermal Continuous-cooling Heat treatment phase diagram transformation transformation of steels (Chapter 9) diagrams diagrams (Chapter 11) (Chapter 10) (Chapter 10) Figure 1.13 Portion of a concept map for the processing of a steel alloy found in Chapter 10.  References 17 SUMMARY Materials Science There are six different property classifications of materials that determine their ap- and Engineering plicability: mechanical, electrical, thermal, magnetic, optical, and deteriorative. One aspect of materials science is the investigation of relationships that exist be- tween the structures and properties of materials. By structure, we mean how some in- ternal component(s) of the material is (are) arranged. In terms of (and with increas- ing) dimensionality, structural elements include subatomic, atomic, microscopic, and macroscopic. With regard to the design, production, and utilization of materials, there are four elements to consider—processing, structure, properties, and performance. The per- formance of a material depends on its properties, which in turn are a function of its structure(s); furthermore, structure(s) is (are) determined by how the material was processed. Three important criteria in materials selection are in-service conditions to which the material will be subjected, any deterioration of material properties during operation, and economics or cost of the fabricated piece. Classification of On the basis of chemistry and atomic structure, materials are classified into three Materials general categories: metals (metallic elements), ceramics (compounds between me- tallic and nonmetallic elements), and polymers (compounds composed of carbon, hydrogen, and other nonmetallic elements). In addition, composites are composed of at least two different material types. Advanced Materials Another materials category is the advanced materials that are used in high-tech ap- plications, including semiconductors (having electrical conductivities intermediate between those of conductors and insulators), biomaterials (which must be compat- ible with body tissues), smart materials (those that sense and respond to changes in their environments in predetermined manners), and nanomaterials (those that have structural features on the order of a nanometer, some of which may be designed on the atomic/molecular level). REFERENCES Ashby, M. F., and D. R. H. Jones, Engineering Materials 1: An Jacobs, J. A., and T. F. Kilduff, Engineering Materials Technology, Introduction to Their Properties, Applications, and Design, 5th edition, Prentice Hall PTR, Paramus, NJ, 2005. 4th edition, Butterworth-Heinemann, Oxford, England, 2012. McMahon, C. J., Jr., Structural Materials, Merion Books, Ashby, M. F., and D. R. H. Jones, Engineering Materials 2: Philadelphia, PA, 2004. An Introduction to Microstructures and Processing, 4th Murray, G. T., C. V. White, and W. Weise, Introduction to edition, Butterworth-Heinemann, Oxford, England, 2012. Engineering Materials, 2nd edition, CRC Press, Boca Ashby, M., H. Shercliff, and D. Cebon, Materials Engineering, Raton, FL, 2007. Science, Processing and Design, Butterworth-Heinemann, Schaffer, J. P., A. Saxena, S. D. Antolovich, T. H. Sanders, Jr., Oxford, England, 2007. and S. B. Warner, The Science and Design of Engineering Askeland, D. R., P. P. Fulay, and W. J. Wright, The Science and Materials, 2nd edition, McGraw-Hill, New York, NY, 1999. Engineering of Materials, 6th edition, Cengage Learning, Shackelford, J. F., Introduction to Materials Science for Engineers, Stamford, CT, 2011. 7th edition, Prentice Hall PTR, Paramus, NJ, 2009. Baillie, C., and L. Vanasupa, Navigating the Materials World, Smith, W. F., and J. Hashemi, Foundations of Materials Science Academic Press, San Diego, CA, 2003. and Engineering, 5th edition, McGraw-Hill, New York, Douglas, E. P., Introduction to Materials Science and Engineering: NY, 2010. A Guided Inquiry, Pearson Education, Upper Saddle Van Vlack, L. H., Elements of Materials Science and Engineering, River, NJ, 2014. 6th edition, Addison-Wesley Longman, Boston, MA, 1989. Fischer, T., Materials Science for Engineering Students, White, M. A., Physical Properties of Materials, 2nd edition, Academic Press, San Diego, CA, 2009. CRC Press, Boca Raton, FL, 2012.