Chapter 1 - Introduction to Materials Science & Engineering PDF

The principal goals of a materials scientist and engineer are to (1) make existing materials better and (2) invent or discover new phenomena, materials, devices, and applications. Breakthroughs in the materials science and engineering field are applied to many other fields of study such...

The principal goals of a materials scientist and engineer are to (1) make existing materials better and (2) invent or discover new phenomena, materials, devices, and applications. Breakthroughs in the materials science and engineering field are applied to many other fields of study such as biomedical engineering, physics, chemistry, environmental engineering, and information technology. The materials science and engineering tetrahedron shown here represents the heart and soul of this field, and its use is illustrated for the production of steel for an automotive chassis. As shown in this diagram, a materials scientist and engineer’s main objective is to develop materials or devices that have the best performance for a particular application. In most cases, the performance-to-cost ratio, as opposed to the performance alone, is of utmost importance. This concept is shown as the apex of the tetrahedron and the three corners are representative of A—the composition, B—the microstructure, and C—the synthesis and processing of materials. These are all interconnected and ultimately affect the performance-to-cost ratio of a material or a device. The accompanying micrograph shows the microstructure of a dual phase steel. The microstructure of dual phase steels is engineered to absorb energy during automotive collisions. Hard particles of a phase called martensite (dark) are dispersed in a matrix of relatively soft, ductile ferite (light). For materials scientists and engineers, materials are like a palette of colors to an artist. Just as an artist can create different paintings using different colors, materials scientists create and improve upon different materials using different elements of the periodic table, and different synthesis and processing routes. (Michael Shake/Shutterstock.com / Digital Vision/Getty Images / Digital Vision/Getty Images / Metals Handbook, Desk Edition (1998), ASM International, Materials Park, OH 44073- 0002. Reprinted with permission of ASM International. All rights reserved. www.asminternational.org.) Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Chapter Introduction to Materials 1 Science and Engineering Have You Ever Wondered? What do materials scientists and engineers study? From a materials stand point, how do you significantly improve the fuel efficiency of a commercial jet airliner? Can we make flexible and lightweight electronic circuits using plastics? Why do jewelers add copper to gold? What is a “smart material?” Chapter Learning Objectives The key objectives of this chapter are to Understand the primary concepts that define materials science and engineering. Understand the role of materials science in the design process. Classify materials by properties. Classify materials by function. I n this chapter, we will first introduce you to the field of materials science and engineering using different real-world examples. We will then provide an intro- duction to the classification of materials. Although most engineering programs require students to take a materials science course, you should approach your study of materials science as more than a mere requirement. A thorough knowledge of materials science and engineering will make you a better engineer and designer. Materials science underlies all technological advances, and an understanding of the basics of materials and their applications will not only make you a better engineer, but will help you during the design process. In order to be a good designer, you must learn what materials will be appropriate to use in different applications. You need to be capable of choosing the right material for your application based on its properties, and you must recognize how and why these properties might change over time and due to processing. Any engineer can look up materials properties in a book or search databases for a material that meets design specifications, but the ability to innovate Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 4 Chapter 1 Introduction to Materials Science and Engineering and to incorporate materials safely in a design is rooted in an understanding of how to manipulate materials properties and functionality through the control of the material’s structure and processing techniques. The most important aspect of materials is that they are enabling; materials make things happen. For example, in the history of civilization, materials such as stone, iron, and bronze played a key role in mankind’s devel­opment. In today’s fast- paced world, the dis­covery of silicon ­ single crystals and an understanding of their properties have enabled the information age. In this book, we provide compelling examples of real-world applications of engineered materials. The diversity of applications and the unique uses of materials illustrate why a good engineer needs to understand and know how to apply the principles of materials science and engineering. 1-1 What is Materials Science and Engineering? Materials science and engineering (MSE) is an interdisciplinary field that studies and manipulates the composition and structure of materials across length scales to control materials properties through synthesis and processing. The term composition means the chemical make-up of a material. The term structure means the arrangement of atoms, as seen at different levels of detail. Materials scientists and engineers not only deal with the development of materials, but also with the synthesis and processing of materials and manufacturing processes related to the production of components. The term “synthesis” refers to how materials are made from naturally occurring or man-made chemicals. The term “processing” means how materials are shaped into useful components to cause changes in the properties of different materials. One of the most important f­unctions of materials ­scientists and engineers is to establish the relationships between a material or a device’s properties and performance and the microstructure of that material, its composition, and the way the material or the device was synthesized and processed. In materials science, the emphasis is on the underlying relationships between the synthesis and processing, structure, and properties of materials. In materials engineering, the focus is on how to translate or transform materials into useful devices or structures. One of the most fascinating aspects of materials science involves the investigation of a material’s structure. The structure of materials has a profound influence on many properties of materials, even if the overall composition does not change! For example, if you take a pure copper wire and bend it repeatedly, the wire not only becomes harder but also becomes increasingly brittle! Eventually, the pure copper wire becomes so hard and brittle that it will break! The electrical resistivity of the wire will also increase as we bend it repeatedly. In this simple example, take note that we did not change the material’s com- position (i.e., its chemical make-up). The changes in the material’s properties are due to a change in its internal structure. If you look at the wire after bending, it will look the same as before; however, its structure has been changed at the microscopic scale. The structure at the microscopic scale is known as the microstructure. If we can understand what has changed microscopically, we can begin to discover ways to control the material’s properties. Let’s consider one example using the materials science and engineering tetrahedron shown in Figure 1-1. (Another example is shown on the chapter opening page.) For most of the history of commercial air travel, the fuselages of airplanes have been made using aluminum alloys. The fuselage material must possess sufficiently high strength, but must also be lightweight and formable into aerodynamic contours. Aluminum is one material that meets these requirements. In 2011, passengers began traveling on Boeing’s 787 Dreamliner aircraft. One of the primary innovations of the Boeing 787 is the extensive use of composites; composite materials are formed by incorporating multiple components in a material in such a way that the properties of the resultant material are unique and not otherwise attainable. Composite materials comprise half of the Dreamliner’s total weight, and in fact, the fuselage of the Boeing 787 is made from carbon fiber-reinforced plastic. Carbon fiber-reinforced plastic is a composite of carbon fiber in a polymer epoxy resin matrix. Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  1-1 What is Materials Science and Engineering? 5 How much can the weight of the fuselage be reduced by using Performance a composite? Cost What is the passenger capacity of a composite fuselage airplane? What is the cost of fabrication? Is the cost offset by fuel savings? A: Composition What material should be used for the matrix? What material should be used as the reinforcing phase? What should the volume percentage of the reinforcing phase be? C: Synthesis and processing What process should be used to fabricate the fuselage? How can composite components be joined? Can the processing be implemented with sufficient reliability? B: Microstructure How should the reinforcing phase be arranged in the matrix? What features of the structure influence reliability? What controls the strength? Figure 1-1 Application of the materials science and engineering tetrahedron to carbon fiber-reinforced plastic for the fabrication of aircraft fuselages. The composition, microstructure, and synthesis/ processing are all interconnected and affect the performance-to-cost ratio. Clockwise from upper right: the Boeing 787; the interior of an empty Boeing 787 fuselage; a giant autoclave used to bake carbon fiber-reinforced plastic sections; carbon fiber in an epoxy matrix. (Bloomberg via Getty Images / Srinivasa, Vinod, Shivakumar, Vinay, Nayaka, Vinay, Jagadeeshaiaih, Sunil, Seethram, Murali, Shenoy, Raghavendra, & Nafidi, Abdelhakim. (2010). Fracture morphology of carbon fiber reinforced plastic composite laminates. Materials Research, 13(3), 417-424. Retrieved January 06, 2014, from http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1516-14392010000300022&lng=en&tlng=en. 10.1590/S1516 -14392010000300022./ AFP/Getty Images / Aviation Images) After decades of success with their various models of aircraft, Boeing invested billions of dollars to develop a commercial airplane based on a new class of materials. Why would Boeing do this? The driving force behind the move to carbon fiber-reinforced plastic was to reduce the weight of the fuselage, thereby increasing fuel efficiency. This significantly increases the performance to cost ratio of the aircraft. The switch to a composite material involved numerous technical challenges. What would the composite material be? How would the composite fuselage be formed? Decades of data are available for the growth of cracks in aluminum under the cyclic loading of take-offs and landings. Would the composite fuselage be reliable? Would a carbon fiber-reinforced plastic also have the corrosion-resistance that aluminum has or would delamination between the fibers and plastic occur? Aluminum jets have structural panels Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 6 Chapter 1 Introduction to Materials Science and Engineering that are riveted together. How can various structural components made from composites be joined? From this discussion, you can see that many issues need to be considered during the design and materials selection for any product and that the ratio of performance to cost, composition, microstructure, and synthesis and process are all critical factors. Let’s look at one more example of the application of the materials science and engineering tetrahedron by considering the use in microelectronic devices of a class of mate- rials known as semiconducting polymers (Figure 1-2). Many types of displays such as those found in alarm clocks and watches utilize light emitting diodes (LEDs) made from inorganic compounds based on gallium arsenide (GaAs) and other materials; however, semiconduct- ing polymers also have been used more recently. The advantages of using plastics for micro- electronics include their flexibility and ease of processing. The questions materials scientists and engineers must answer with applications of semiconducting polymers are What are the relationships between the structure of polymers and their electrical properties? How can devices be made using these plastics? Will these devices be compatible with existing silicon chip technology? How robust are these devices? How will the performance and cost of these devices compare with traditional devices? These are just a few of the factors that engineers and scientists must consider during the development, design, and manufacturing of semiconducting polymer devices. Figure 1-2 Application of the tetrahedron of materials science and engineering to semiconducting polymers for microelectronics. Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  1-2 Classification of Materials 7 1-2 Classification of Materials There are different ways of classifying materials. One way is to describe five groups (Table 1-1): 1. metals and alloys; 2. ceramics, glasses, and glass-ceramics; 3. polymers (plastics); 4. semiconductors; and 5. composite materials. Table 1-1 Representative examples, applications, and properties for each category of materials Examples of Applications Properties Metals and Alloys Copper Electrical conductor wire High electrical conductivity, good formability Gray cast iron Automobile engine blocks Castable, machinable, vibration-damping Alloy steels Wrenches, automobile chassis Significantly strengthened by heat treatment Ceramics and Glasses SiO2-Na2O-CaO Window glass Optically transparent, thermally insulating Al2O3, MgO, SiO2 Refractories (i.e., heat-resistant Thermally insulating, lining of furnaces) for withstand high containing molten metal temperatures, relatively inert to molten metal Barium titanate Capacitors for High ability to store charge microelectronics Silica Optical fibers for information Low optical losses technology Polymers Polyethylene Food packaging Easily formed into thin, flexible, airtight film Epoxy Encapsulation of integrated Electrically insulating and circuits moisture resistant Phenolics Adhesives for joining plies in Strong, moisture resistant plywood Semiconductors Silicon Transistors and integrated Unique electrical behavior circuits GaAs Optoelectronic systems Converts electrical signals to light, used in lasers, laser diodes, etc. Composites Graphite-epoxy Aircraft components High strength-to-weight ratio Tungsten carbide-cobalt Carbide cutting tools for High hardness, yet good shock (WC-Co) machining resistance Titanium-clad steel Reactor vessels Low cost and high strength of steel with the corrosion resistance of titanium Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 8 Chapter 1 Introduction to Materials Science and Engineering Materials in each of these groups possess different structures and properties. The ­differences in strength, which are compared in Figure 1-3, illustrate the wide range of properties from which engineers can select. Since metallic materials are extensively used for load-bearing applications, their mechanical properties are of great practical interest. We briefly introduce these properties here. The term “stress” refers to load or force per unit area. “Strain” refers to elongation or change in dimension divided by the original dimension. Application of “stress” causes “strain.” If the strain goes away after the load or applied stress is removed, the strain is said to be “elastic.” If the strain remains after the stress is removed, the strain is said to be “plastic.” When the deformation is elastic, stress and strain are linearly related; the slope of the stress strain diagram is known as the elastic or Young’s modulus. The level of stress needed to initiate plastic deformation is known as the “yield strength.” The maximum percent deformation that can be achieved is a measure of the ductility of a metallic material. These concepts are discussed further in Chapters 6 and 7. Metals and Alloys Metals include aluminum, magnesium, zinc, iron, titanium, copper, and nickel. An alloy is a metal that contains additions of one or more metals or non- metals, e.g., steel is an alloy of iron with carbon additions. In general, metals have good electrical and thermal conductivities. Metals and alloys have relatively high strength, high stiffness, ductility or formability, and shock resistance. They are particularly useful for structural or load-bearing applications. Although pure metals are occasionally used, alloys provide improvement in a particular desirable property or permit better combinations of properties. For example, pure gold is a soft metal; thus, jewelers add copper to gold to improve strength so that gold jewelry is not easily damaged. Ceramics Ceramics can be defined as inorganic nonmetallic materials. Beach sand and rocks are examples of naturally occurring ceramics. Advanced ceramics are materials made by refining naturally occurring ceramics and other special processes. Advanced ceramics are used in substrates that house computer chips, sensors and Figure 1-3 Representative strengths of various categories of materials. Compressive strengths are shown for ceramics. Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  1-2 Classification of Materials 9 actuators, capacitors, wireless communications, spark plugs, inductors, and electrical insulation. Some ceramics are used as barrier coatings to protect metallic substrates in turbine engines. Ceramics are also used in such consumer products as paints, plastics, and tires, and for industrial applications such as the oxygen sensors used in cars. Traditional ceramics are used to make bricks, tableware, bathroom fixtures, refractories (heat-resistant materials), and abrasives. In general, ceramics do not conduct heat well; they must be heated to very high temperatures before melting. Ceramics are strong and hard, but also very brittle. We normally prepare fine powders of ceramics and mold into different shapes. New processing techniques make ceramics sufficiently resistant to fracture that they can be used in load-bearing applications, such as impellers in turbine engines. Ceramics have exceptional strength under compression. Can you believe that an entire fire truck can be supported using four ceramic coffee cups? Glasses and Glass-Ceramics Glass is an amorphous material, often, but not always, derived from a molten liquid. The term “amorphous” refers to materials that do not have a regular, periodic arrangement of atoms. Amorphous materials will be discussed in Chapter 3. The fiber optics industry is founded on optical fibers based on high-purity silica glass. Glasses are also used in houses, cars, screens for computers, televisions, and smart phones, and h ­ undreds of other applications. Glasses can be thermally treated (tempered) to make them stronger. Forming glasses and then nucleating (forming) small crystals within them by a special thermal process creates materials that are known as glass-ceramics. Zerodur™ is an example of a glass-ceramic material that is used to make the mirror substrates for large telescopes (e.g., the Chandra and Hubble telescopes). Glasses and glass-ceramics are usually processed by melting and casting. Polymers Polymers are typically organic materials. They are produced using a process known as polymerization. Polymeric materials include rubber (elastomers) and many types of adhesives. Polymers typically are good electrical and thermal insulators although there are exceptions. Although they have lower strengths than metals or ceramics, polymers have very good strength-to-weight ratios. They are typically not suitable for use at high temperatures. Many polymers have very good resistance to corrosive chemicals. Polymers have thousands of applications ranging from bulletproof vests, compact discs (CDs), ropes, and ­liquid crystal displays (LCDs) to clothes and coffee cups. Thermoplastic polymers, in which the long molecular chains are not rigidly connected, have good ductility and formability; thermosetting polymers are stronger but more brittle because the molecular chains are tightly linked (Figure 1-4). Polymers are Figure 1-4 Polymerization occurs when small molecules, represented by the circles, combine to produce larger molecules, or polymers. The polymer molecules can have a structure that consists of many chains that are entangled but not connected (thermoplastics) or can form three-dimensional networks in which chains are cross- linked (thermosets). Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 10 Chapter 1 Introduction to Materials Science and Engineering used in many applications, including electronic devices. Thermoplastics are made by shaping their molten form. Thermosets are typically cast into molds. Plastics contain additives that enhance the properties of polymers. Semiconductors Silicon, germanium, and gallium arsenide-based semicon­ ductors such as those used in computers and electronics are part of a broader class of materials known as electronic materials. The electrical conductivity of semiconducting materials is between that of ceramic insulators and metallic conductors. Semiconductors have enabled the information age. In some semiconductors, the level of conductivity can be controlled to produce electronic devices such as transistors and diodes that are used to build integrated circuits. In many applications, we need large single crystals of semi- conductors. These are grown from molten materials. Often, thin films of semiconducting materials are also made using specialized processes. Composite Materials The main idea in developing composites is to blend the properties of different materials. These are formed from two or more materials, producing properties not found in any single material. Concrete, plywood, and fiberglass are examples of composite materials. Fiberglass is made by dispersing glass fibers in a polymer matrix. The glass fibers make the polymer stiffer, without significantly increasing its density. With composites, we can produce lightweight, strong, ductile, temperature- resistant materials or we can produce hard, yet shock-resistant, cutting tools that would otherwise shatter. Advanced aircraft and aerospace vehicles rely heavily on composites. As discussed earlier in this chapter, the Boeing 787 uses carbon fiber-reinforced plastic in many structural components instead of aluminum, leading to high fuel efficiency. Sports equipment such as bicycles, golf clubs, tennis rackets, and the like also make use of different kinds of composite materials that are light and stiff. 1-3 Functional Classification of Materials We can classify materials based on whether the most important function they perform is mechanical (structural), biological, electrical, magnetic, or optical. This classification of materials is shown in Figure 1-5. Some examples of each category are shown. These categories can be broken down further into subcategories. Aerospace Light materials such as wood and an aluminum alloy (that acciden- tally strengthened the engine even more by picking up copper from the mold used for ­casting) were used in the Wright brothers’ historic flight. NASA’s space shuttle made use of aluminum powder for booster rockets and silica for the space shuttle tiles. The fuselage and wings of Boeing’s 787 aircraft are largely composed of carbon-fiber-­reinforced plastic. Biomedical Our bones and teeth are made, in part, from a naturally formed ceramic known as hydroxyapatite. A number of artificial organs, bone replacement parts, cardiovascular stents, orthodontic braces, and other components are made using different ­plastics, titanium alloys, and nonmagnetic stainless steels. Ultrasonic imaging systems make use of ceramics known as lead zirconium titanate (PZT). Electronic Materials As mentioned before, semiconductors, such as those made from silicon, are essential to making integrated circuits for computer chips. Barium titanate (BaTiO3), tantalum oxide (Ta2O5), and other dielectric materials are used to make ceramic capacitors and other devices. Copper, aluminum, and other metals are used as conductors in power transmission and in microelectronics. Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  1-3 Functional Classification of Materials 11 Energy and Environmental Technology YAG, ITO Figure 1-5 Functional classification of materials. Notice that metals, plastics, and ceramics occur in different categories. A limited number of examples in each category are provided. Energy Technology and Environmental Technology The nuclear industry uses materials such as uranium dioxide and plutonium as fuel. Numerous other materials, such as glasses and stainless steels, are used in handling nuclear materials and managing radioactive waste. Technologies related to batteries and fuel cells make use of many ceramic materials such as zirconia (ZrO2) and polymers. Battery technology has gained significant importance owing to the need for many electronic devices that require longer lasting and portable power. Fuel cells are used in some electric cars. The oil and petroleum industry widely uses zeolites, alumina, and other materials as catalyst substrates. They use Pt, Pt/Rh, and many other metals as catalysts. Many membrane technologies for purification of liquids and gases make use of ceramics and plastics. Solar power is generated using materials such as amorphous silicon (a:Si:H). Magnetic Materials Computer hard disks make use of many ceramic, metallic, and polymeric materials. Computer hard disks are made using cobalt-platinum-­ tantalum-chromium (Co-Pt-Ta-Cr) alloys. Many magnetic ferrites are used to make Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 12 Chapter 1 Introduction to Materials Science and Engineering inductors and components for wireless communications. Steels based on iron and silicon are used to make transformer cores. Photonic or Optical Materials Silica is used widely for making optical fibers. More than ten million kilometers of optical fiber have been installed around the world. Optical ­materials are used for making semiconductor detectors and lasers used in fiber optic ­communications systems and other applications. Similarly, alumina (Al2O3) and yttrium aluminum garnets (YAG) are used for making lasers. Amorphous silicon is used to make solar cells and photovoltaic modules. Polymers are used to make liquid crystal displays (LCDs). Smart Materials A smart material can sense and respond to an external stimulus such as a change in temperature, the application of a stress, or a change in humidity or chemical environment. Usually a smart material-based system consists of sensors and actuators that read changes and initiate an action. An example of a passively smart material is lead zirconium titanate (PZT) and shape-memory alloys. When properly processed, PZT can be subjected to a stress, and a voltage is generated. This effect is used to make such devices as spark generators for gas grills and sensors that can detect under- water objects such as fish and submarines. Other examples of smart materials include magnetorheological or MR fluids. These are magnetic paints that respond to magnetic fields. These materials are being used in suspension systems of automobiles, including models by General Motors, Ferrari, and Audi. Still other examples of smart materials and systems are photochromic glasses and automatic dimming mirrors. Structural Materials These materials are designed for carrying some type of stress. Steels, concrete, and composites are used to make buildings and bridges. Steels, glasses, plastics, and composites also are used widely to make automotives. Often in these applications, combinations of strength, stiffness, and toughness are needed under different conditions of temperature and loading. 1-4 Classification of Materials Based on Structure As mentioned before, the term “structure” means the arrangement of a material’s atoms; the structure at a microscopic scale is known as “microstructure.” We can view these arrangements at different scales, ranging from a few angstrom units to a millimeter. We will learn in Chapter 3 that some materials may be crystalline (the material’s atoms are arranged in a periodic fashion) or they may be amorphous (the arrangement of the material’s atoms does not have long-range order). Some crystalline materials may be in the form of one crystal and are known as single crystals. Others consist of many crystals or grains and are known as polycrystalline. The characteristics of crystals or grains (size, shape, etc.) and that of the regions between them, known as the grain boundaries, also affect the properties of materials. We will further discuss these concepts in later c­ hapters. The microstructure of a dual phase steel is shown on the chapter opening page. 1-5 Environmental and Other Effects The structure-property relationships in materials fabricated into components are often influenced by the surroundings to which the material is subjected during use. This can include exposure to high or low temperatures, cyclical stresses, sudden impact, corrosion, or oxidation. These effects must be accounted for in design to ensure that components do not fail unexpectedly. Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  1-5 Environmental and Other Effects 13 Temperature Changes in temperature dramatically alter the properties of materials (Figure 1-6). Metals and alloys that have been strengthened by certain heat treatments or forming techniques may suddenly lose their strength when heated. A tragic reminder of this is the collapse of the World Trade Center towers on September 11, 2001. Although the towers sustained the initial impact of the collisions, their steel structures were weakened by elevated temperatures caused by fire, ultimately leading to the collapse. High temperatures change the structure of ceramics and cause polymers to melt or char. Very low temperatures, at the other extreme, may cause a metal or polymer to fail in a brittle manner, even though the applied loads are low. This low-temperature em- brittlement was a factor that caused the Titanic to fracture and sink. Similarly, the 1986 Challenger accident, in part, was due to embrittlement of rubber O-rings. The reasons why some polymers and metallic materials become brittle are different. We will discuss these concepts in later chapters. Corrosion Most metals and polymers react with oxygen or other gases, particu- larly at elevated temperatures. Metals and ceramics may disintegrate, and polymers and non-oxide ceramics may oxidize. Materials also are attacked by corrosive liquids, leading to premature failure. The engineer faces the challenge of selecting materials or coatings that prevent these reactions and permit operation in extreme environments. In space applications, we may have to consider the effect of radiation. Fatigue For many applications, components must be designed such that the load on the material is not enough to cause permanent deformation. When we load and unload the material thousands of times, even at low loads, small cracks may begin to develop, and materials fail as these cracks grow. This is known as fatigue failure. In designing load-bearing components, the possibility of fatigue must be accounted for. Strain Rate Many of you are aware of the fact that Silly Putty®, a silicone- based (not silicon) plastic, can be stretched significantly if we pull it slowly (small rate of strain). If you pull it fast (higher rate of strain), it snaps. A similar behavior can occur with many metallic materials. Thus, in many applications, the level and rate of strain have to be considered. In many cases, the effects of temperature, fatigue, stress, and corrosion may be interrelated, and other outside effects can affect the material’s performance. Figure 1-6 Increasing temperature normally reduces the strength of a material. Polymers are suitable only at low temperatures. Some composites, such as carbon- carbon composites, special alloys, and ceramics, have excellent properties at high temperatures. Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 14 Chapter 1 Introduction to Materials Science and Engineering 1-6 Materials Design and Selection When a material is designed for a given application, a number of factors must be con- sidered. The material must acquire the desired physical and mechanical properties, must be capable of being processed or manufactured into the desired shape, and must provide an economical solution to the design problem. Satisfying these requirements in a manner that protects the environment—perhaps by encouraging recycling of the materials—is also essential. In meeting these design requirements, the engineer may have to make a number of trade-offs in order to produce a serviceable, yet marketable, product. As an example, material cost is normally calculated on a cost-per-pound basis. We must consider the density of the material, or its weight-per-unit volume, in our design and selection (Table 1-2). Aluminum may cost more than steel on a weight basis, but it has only one-third the density of steel. Although parts made from aluminum may have to be thicker, the aluminum part may be less expensive than the one made from steel because of the weight difference. In some instances, particularly in aerospace applications, weight is critical, since additional vehicle weight increases fuel consumption. By using materials that are light- weight but very strong, aerospace vehicles or automobiles can be designed to improve fuel utilization. Many advanced aerospace vehicles use composite materials instead of aluminum. These composites, such as carbon-epoxy, are more expensive than the traditional ­aluminum alloys; however, the fuel savings yielded by the higher strength- to-weight ratio of the composite (Table 1-2) may offset the higher initial cost of the aircraft as is also true for the Boeing 787. There are l­ iterally thousands of applications in which similar considerations apply. Usually the ­selection of materials involves trade-offs between many properties. By this point of our discussion, we hope that you can appreciate that the properties of materials depend not only on composition, but also how the materials are made (synthesis and processing) and, most importantly, their internal structure. This is why it is not a good idea for an engineer to refer to a handbook and select a material for a given application. The handbooks may be a good starting point. A good engineer will consider: the effects of how the material was made, the exact composition of the candidate material for the applica- tion being considered, any processing that may have to be done for shaping the material or fabricating a component, the structure of the material after processing into a component or device, the environment in which the material will be used, and the cost-to-performance ratio. Earlier in this chapter, we discussed the need for you to know the principles of materials science and engineering. If you are an engineer and you need to decide which materials you will choose to fabricate a component, the knowledge of principles of mate- rials science and engineering will empower you with the fundamental concepts. These will allow you to make technically sound decisions in designing with engineered materials. Table 1-2 Strength-to-weight ratios of various materials Strength-to-Weight Material Strength (lb/in.2) Density (lb/in.3) Ratio (in.) Polyethylene 1000 0.030 0.03 3 106 Pure aluminum 6500 0.098 0.07 3 106 Al2O3 30,000 0.114 0.26 3 106 Epoxy 15,000 0.050 0.30 3 106 Heat-treated alloy steel 240,000 0.280 0.86 3 106 Heat-treated aluminum alloy 86,000 0.098 0.88 3 106 Carbon-carbon composite 60,000 0.065 0.92 3 106 Heat-treated titanium alloy 170,000 0.160 1.06 3 106 Kevlar-epoxy composite 65,000 0.050 1.30 3 106 Carbon-epoxy composite 80,000 0.050 1.60 3 106 Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Summary 15 Summary  aterials science and engineering (MSE) is an interdisciplinary field concerned with M inventing new materials and devices and improving existing materials by develop- ing a deeper understanding of the microstructure-composition-synthesis-­processing relationships. Engineered materials are materials designed and fabricated considering MSE principles. The properties of engineered materials depend upon their composition, structure, synthesis, and processing. An important performance index for materials or devices is their performance-to-cost ratio. The structure of a material refers to the arrangement of atoms or ions in the material. The structure at a microscopic level is known as the microstructure. Many properties of materials depend strongly on the structure, even if the composition of the material remains the same. This is why the structure-property or microstructure- property relationships in materials are extremely important. Materials are classified as metals and alloys, ceramics, glasses and glass-ceramics, composites, polymers, and semiconductors. Metals and alloys have good strength, good ductility, and good formability. Metals have good electrical and thermal conductivities. Metals and alloys play an indispensable role in many applications such as automotives, buildings, bridges, aerospace, and the like. Ceramics are inorganic crystalline materials. They are strong, serve as good electrical and thermal insulators, are often resistant to damage by high temperatures and corrosive environments, but are mechanically brittle. Modern ceramics form the ­ underpinnings of many microelectronic and photonic technologies. Glasses are amorphous, inorganic solids that are typically derived from a molten ­liquid. Glasses can be tempered to increase strength. Glass-ceramics are formed by annealing glasses to nucleate small crystals that improve resistance to fracture and thermal shock. Polymers have relatively low strength; however, the strength-to-weight ratio is very favorable. Polymers are not suitable for use at high temperatures. They have very good corrosion resistance, and—like ceramics—provide good electrical and thermal insula- tion. Polymers may be either ductile or brittle, depending on structure, temperature, and strain rate. Semiconductors possess unique electrical and optical properties that make them essential for manufacturing components in electronic and communications devices. Composites are made from two or more different types of materials. They provide unique combinations of mechanical and physical properties that cannot be found in any single material. Functional classification of materials includes aerospace, biomedical, electronic, energy and environmental, magnetic, optical (photonic), and structural materials. Materials can also be classified as crystalline or amorphous. Crystalline materials may be single crystal or polycrystalline. Properties of materials can depend upon the temperature, level and type of stress applied, strain rate, oxidation and corrosion, and other environmental factors. Selection of a material having the needed properties and the potential to be manufactured economically and safely into a useful product is a complicated process requiring the knowledge of the structure-property-processing-composition relationships. Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 16 Chapter 1 Introduction to Materials Science and Engineering Glossary Alloy A metallic material that is obtained by chemical combinations of different elements (e.g., steel is made from iron and carbon). Ceramics A group of crystalline inorganic materials characterized by good strength, especially in compression, and high melting temperatures. Composites A group of materials formed from mixtures of metals, ceramics, or polymers in such a manner that unusual combinations of properties are obtained (e.g., fiberglass). Composition The chemical make-up of a material. Crystalline material A material composed of one or many crystals. In each crystal, atoms or ions show a long-range periodic arrangement. Density Mass per unit volume of a material, usually expressed in units of g/cm3 or lb/in.3 Fatigue failure Failure of a material due to repeated loading and unloading. Glass An amorphous material derived from the molten state, typically, but not always, based on silica. Glass-ceramics A special class of materials obtained by forming a glass and then heat treating it to form small crystals. Grain boundaries Regions between grains of a polycrystalline material. Grains Crystals in a polycrystalline material. Materials engineering An engineering-oriented field that focuses on how to transform materials into useful devices or structures. Materials science A field of science that emphasizes studies of relationships between the microstructure, synthesis and processing, and properties of materials. Materials science and engineering An interdisciplinary field concerned with inventing new materials and improving existing materials by developing a deeper understanding of the ­structure-property-processing-composition relationships between different materials. Materials science and engineering tetrahedron A tetrahedron diagram showing how the ­performance-to-cost ratio of materials depends upon their composition, microstructure, synthesis, and processing. Mechanical properties Properties of a material, such as strength, that describe how well a material withstands applied forces, including tensile or compressive forces, impact forces, cyclical or fatigue forces, or forces at high temperatures. Metal An element that has metallic bonding and generally good ductility, strength, and electrical conductivity. Microstructure The structure of a material at the microscopic length scale. Physical properties Characteristics such as color, elasticity, electrical conductivity, thermal conductivity, magnetism, and optical behavior that generally are not significantly influenced by forces acting on a material. Plastics Polymers containing other additives. Polycrystalline material A material composed of many crystals (as opposed to a single-crystal material that has only one crystal). Polymerization The process by which organic molecules are joined into giant molecules, or polymers. Polymers A group of materials normally obtained by joining organic molecules into giant ­molecular chains or networks. Processing Different ways for shaping materials into useful components or changing their ­properties. Semiconductors A group of materials (e.g., Si, GaAs) having electrical conductivity between ­metals and typical ceramics. Single crystal A crystalline material that comprises only one crystal (there are no grain ­boundaries). Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Problems 17 Smart material A material that can sense and respond to an external stimulus such as change in temperature, application of a stress, or change in humidity or chemical environment. Strength-to-weight ratio The strength of a material divided by its density; materials with a high strength-to-weight ratio are strong but lightweight. Structure The arrangements of atoms or ions in a material. The structure of materials has a pro- found influence on many properties of materials, even if the overall composition does not change. Synthesis The process by which materials are made from naturally occurring or other chemicals. Thermoplastics A special group of polymers in which molecular chains are entangled but not interconnected. They can be easily melted and formed into useful shapes. Normally, these polymers have a chainlike structure (e.g., polyethylene). Thermosets A special group of polymers that decompose rather than melt upon heating. They are normally quite brittle due to a relatively rigid, three-dimensional network structure (e.g., polyurethane). Problems Section 1-1 What is Materials Science and (c) polymers; and Engineering? (d) semiconductors. 1-1 Define materials science and engineering Specify the object that each material is (MSE). found in and explain why the material is 1-2 What is the importance of the engineering used in each specific application. Hint: tetrahedron for materials engineers? One example answer for part (a) would 1-3 Define the following terms: be that aluminum is a metal used in the (a) composition; base of some pots and pans for even heat (b) structure; distribution. It is also a lightweight metal (c) synthesis; that makes it useful in kitchen cookware. (d) processing; and Note that in this partial answer to part (a), (e) microstructure. a specific metal is described for a specific 1-4 Explain the difference between the terms application. materials science and materials engineering. 1-11 Describe the enabling materials property of each of the following and why it is so: Section 1-2 Classification of Materials (a) steel for I-beams in skyscrapers; 1-5 The myriad materials in the world primar- (b) a cobalt chrome molybdenum alloy for ily fall into four basic categories; what are hip implants; they? What are materials called that have (c) polycarbonate for eyeglass lenses; and one or more different types of material (d) bronze for artistic castings. fabricated into one component? Give one 1-12 Describe the enabling materials property example. of each of the following and why it is so: 1-6 What are some of the materials and mech­ (a) aluminum for airplane bodies; anical properties of metals and alloys? (b) polyurethane for teeth aligners (invis- 1-7 What is a ceramic, and what are some of the ible braces); properties that you expect from a ceramic? (c) steel for the ball bearings in a bicycle’s 1-8 Make comparisons between thermoplas- wheel hub; tics and thermosetting polymers (a) on the (d) polyethylene terephthalate for water basis of mechanical characteristics upon bottles; and heating, and (b) according to possible (e) glass for wine bottles. molecular structures. 1-13 What properties should an engineer con- 1-9 Give three examples of composites that sider for a total knee replacement of a can be fabricated. deteriorated knee joint with an artificial 1-10 For each of the following classes of mate- prosthesis when selecting the materials for rials, give two specific examples that are a this application? regular part of your life: 1-14 Write one paragraph about why single- (a) metals; crystal silicon is currently the material of (b) ceramics; choice for microelectronics applications. Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.