Fundamentals of Materials Science and Engineering - An Integrated Approach

Fundamentals of Materials Science and Engineering - An Integrated Approach

(Parte 4 de 7)

2 Throughout this text we draw attention to the relationships between material properties and structural elements.

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Processing Structure Properties Performance

Figure 1.1 The four components of the discipline of materials science and engineering and their interrelationship.

among these four components in terms of the design, production, and utilization of materials.

We now present an example of these processing-structure-propertiesperformance principles with 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 reflected light passes through it), whereas the disks in the center and on the right are, respectively, 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, it is highly perfect—which gives rise to its transparency. The center one is composed of numerous and very small single crystals that areallconnected;theboundariesbetweenthesesmallcrystalsscatteraportionofthe light reflected from the printed page, which makes this material optically translucent. Finally,thespecimenontherightiscomposednotonlyofmanysmall,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. And, of course, if optical transmittance is an important parameter relative to the ultimate in-service application, the performance of each material will be different.

Figure 1.2 Photograph of 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 (that is, virtually all light that is reflected from the page passes through it), whereas the one in the center is translucent (meaning that some of this reflected light is transmitted through the disk), and the disk on the right is opaque—i.e., 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. (Specimen preparation, P. A. Lessing; photography by S. Tanner.)

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1.4 Classification of Materials • 5

1.3 WHY STUDY MATERIALS SCIENCE AND ENGINEERING?

Why do we study materials? Many an applied scientist or engineer, whether mechanical, civil, chemical, or electrical, will at one time or another be exposed to a design problem involving materials. Examples might include 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 many thousands that are available. There are several criteria on which the final decisionisnormallybased.Firstofall,thein-serviceconditionsmustbecharacterized, for these will dictate the properties required of the material. On only rare occasions does a material possess the maximum or ideal combination of properties. Thus, it may be necessary to trade off one characteristic for another. The classic example involves strength and ductility; normally, a material having a high strength will have 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.

Finally,probablytheoverridingconsiderationisthatofeconomics:Whatwillthe 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 processing techniques of materials, the more proficient and confident he or she will be in making judicious materials choices based on these criteria.

1.4 CLASSIFICATION OF MATERIALS

Solid materials have been conveniently grouped into three basic classifications: metals, ceramics, and polymers. This scheme is based primarily on chemical makeup and atomic structure, and most materials fall into one distinct grouping or another, although there are some intermediates. In addition, there are the composites, combinations of two or more of the above three basic material classes. A brief explanation of these material types and representative characteristics is offered next. Another classification is advanced materials—those used in high-technology applications— viz. semiconductors, biomaterials, smart materials, and nanoengineered materials; these are discussed in Section 1.5.

Metals

Materials in this group are composed of one or more metallic elements (such as iron, aluminum, copper, titanium, gold, and nickel), and often also nonmetallic elements (for example, carbon, nitrogen, and oxygen) in relatively small amounts.3 Atoms in metals and their alloys are arranged in a very orderly manner (as discussed in

3 The term metal alloy is used in reference to a metallic substance that is composed of two or more elements.

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Density (g/cm) (logarithmic scale)

Metals

Platinum

Silver

Copper Iron/Steel

Titanium

Aluminum Magnesium

Composites

Woods

Polymers

PE Rubber

ZrO Al O

SiC,Si N Glass

Concrete

Ceramics

Bar-chart of room-temperature density values for various metals, ceramics, polymers, and composite materials.

Chapter 3), and in comparison to the ceramics and polymers, are relatively dense (Figure 1.3). With regard to mechanical characteristics, these materials are relatively stiff(Figure1.4)andstrong(Figure1.5),yetareductile(i.e.,capableoflargeamounts of deformation without fracture), and are resistant to fracture (Figure 1.6), which accounts for their widespread use in structural applications. 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 1.7) and heat, and are not transparent to visible light; a polished metal surface has a lustrousappearance.Inaddition,someofthemetals(viz.,Fe,Co,andNi)havedesirable magnetic properties.

Figure 1.8 is a photograph that 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 13.

oung’ s) Modulus (in units of gigapascals)] (logarithmic scale)

Composites

WoodsPolymers PVC

Rubbers

PS, Nylon

Metals

Tungsten Iron/Steel

Aluminum Magnesium

Titanium

Ceramics

SiC AI O

Si N ZrO Glass

Concrete

Bar-chart of room-temperature stiffness (i.e., elastic modulus) values for various metals, ceramics, polymers, and composite materials.

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1.4 Classification of Materials • 7

Strength (Tensile Strength, in units of megapascals) (logarithmic scale)

Nylon

Polymers

Steel alloys

Gold

Aluminum alloys

Cu,Ti alloys

Metals

Composites

WoodsGlass

Si N SiC

Ceramics

Al O

Bar-chart of room-temperature strength (i.e., tensile strength) values for various metals, ceramics, polymers, and composite materials.

Ceramics

Ceramics are compounds between metallic and nonmetallic elements; they are most frequently oxides, nitrides, and carbides. For example, some of the common ceramic materials include aluminum oxide (or alumina,A l2O3), 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 (i.e., porcelain), as well as cement and glass. With regard to mechanical behavior, ceramic materials are relatively stiff and strong—stiffnesses and strengths are comparable to those of the metals (Figures 1.4 and 1.5). In addition, ceramics are typically very hard. On the other hand, they are extremely brittle (lack ductility) and are highly susceptible

Resistance to Fracture (Fracture Toughness, in units of MPa m) (logarithmic scale)

Composites GFRCCFRC

Woods

Nylon Polymers

Polystyrene Polyethylene

Polyester

Al O SiC

Si N

Glass Concrete

Ceramics

Metals

Steel alloys

Titanium alloys

Aluminum alloys

Figure 1.6 Bar-chart of room-temperature resistance to fracture (i.e., fracture toughness) for various metals, ceramics, polymers, and composite materials. (Reprinted from Engineering Materials 1: An Introduction to Properties, Applications and Design, third edition, M. F. Ashby and D. R. H. Jones, pages 177 and 178, 2005, with permission from Elsevier.)

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Electrical Conductivity (in units of reciprocal ohm-meters) (logarithmic scale)

Ceramics Polymers

Semiconductors

MetalsFigure 1.7 Bar-chart of room-temperature electrical conductivity ranges for metals, ceramics, polymers, and semiconducting materials.

to fracture (Figure 1.6). These materials are typically insulative to the passage of heat and electricity (i.e., have low electrical conductivities, Figure 1.7), and are more resistant to high temperatures and harsh environments than 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 the photograph of Figure 1.9. The characteristics, types, and applications of this class of materials are also discussed in Chapter 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 elements (viz. O, N, and Si). Furthermore, they have very large molecular structures, often chain-like in nature with a backbone of carbon atoms. Some of the common

Figure 1.8 Familiar objects that are 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. (Photography by S. Tanner.)

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1.4 Classification of Materials • 9

Figure 1.9 Common objects that are made of ceramic materials: scissors, a china tea cup, a building brick, a floor tile, and a glass vase. (Photography by S. Tanner.) and familiar polymers are polyethylene (PE), nylon, poly(vinyl chloride) (PVC), polycarbonate (PC), polystyrene (PS), and silicone rubber. These materials typically have low densities (Figure 1.3), whereas their mechanical characteristics are generally dissimilar to the metallic and ceramic materials—they are not as stiff nor as strong as these other material types (Figures 1.4 and 1.5). However, on the basis of their low densities, many times their stiffnesses and strengths on a per-mass basis are comparable to 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 environments. One major drawback of the polymers is their tendency to soften and/or decompose at modest temperatures, which, in some instances, limits their use. Furthermore, they have low electrical conductivities (Figure 1.7) and are nonmagnetic.

The photograph in Figure 1.10 shows several articles made of polymers that are familiar to the reader. Chapters 4, 13, and 14 are devoted to discussions of the structures, properties, applications, and processing of polymeric materials.

Composites

A composite is composed of two (or more) individual materials, which come from the categories discussed above—viz., 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 materials. A large number of composite types exist that are represented by different combinations of metals, ceramics, and polymers. Furthermore, some naturally-occurring materials are also considered to be composites—for example, wood and bone. However, most of those we consider in our discussions are synthetic (or man-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).4 The glass fibers are relatively strong and stiff (but also brittle), whereas

4 Fiberglass is sometimes also termed a “glass fiber-reinforced polymer” composite, abbreviated “GFRP.”

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Figure 1.10 Common objects that are made of polymeric materials: plastic tableware (spoon, fork, and knife), billiard balls, a bicycle helmet, two dice, a lawnmower wheel (plastic hub and rubber tire), and a plastic milk carton. (Photography by S. Tanner.) the polymer is ductile (but also weak and flexible). Thus, the resulting fiberglass is relatively stiff, strong, (Figures 1.4 and 1.5) flexible, and ductile. In addition, it has a low density (Figure 1.3).

Another of these technologically important materials is the “carbon fiberreinforced polymer” (or “CFRP”) composite—carbon fibers that are embedded within a polymer. These materials are stiffer and stronger than the glass fiberreinforced materials (Figures 1.4 and 1.5), yet they are more expensive. The CFRP composites are used in some aircraft and aerospace applications, as well as high-tech sporting equipment (e.g., bicycles, golf clubs, tennis rackets, and skis/snowboards). Chapter 15 is devoted to a discussion of these interesting materials.

(Parte 4 de 7)

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