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NonferrousMetalsand Alloys:

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Properties, andApplications Nonferrous metals include a wide variety of materials, ranging from aluminumto zinc, with special properties that are indispensable in many consumer andcommercial products. This chapter introduces each class of nonferrous metal and its alloys, andbriefly describes their methods of production. Their physical and mechanical properties are then summarized, along withgeneral guidelines for their selection and applications, together with severalexamples Shape-memory alloys, amorphous alloys, and metal foams are also described,with examples of their unique applications.

6.| IntroductionNonferrous metals and alloys cover a wide range, from the more common metals(such as aluminum, copper, and magnesium) to high-strength, high-temperaturealloys (such as those of tungsten, tantalum, and molybdenum). Although generallymore expensive than ferrous metals (Table 6.1), nonferrous metals and alloys havenumerous important applications because of properties such as good corrosion resist-ance, high thermal and electrical conductivity, low density, and ease of fabrication(Table 6.2). Typical examples of nonferrous metal and alloy applications are alu-minum for cooking utensils and aircraft bodies, copper wire for electrical powercords, zinc for galvanized sheet metal for car bodies, titanium for jet-engine turbineblades and for orthopedic implants, and tantalum for rocket engines.

As an example, a turbofan jet engine for the Boeing 757 aircraft typically con-tains the following nonferrous metals and alloys: 38 Ti, 37 Ni, 12 Cr, 6 Co,5 Al, 1 Nb, and 0.02 Ta. Without these materials, a jet engine (Fig. 6.1) couldnot be designed, manufactured, and operated at the power and efficiency levelsrequired.This chapter introduces the general properties, the production methods, andthe important engineering applications for nonferrous metals and alloys. The manu-facturing properties of these materials (such as formability, machinability, and weld-ability) are described in various chapters throughout this text.

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Introduction |5|Aluminum and AluminumAlloys |52Magnesium andMagnesium Alloys |57Copper and CopperAlloys |58Nickel and NickelAlloys |60Superalloys |6|Titanium and TitaniumAlloys |62Refractory Metals andAlloys |63Beryllium |64Zirconium |64Low-me|tingA|loys |64Precious Metals |66Shape-memory Alloys(Smart Materials) |66Amorphous Alloys(Metallic Glasses) |676.I5 Metal Foams |67

EXAMPLE:6.| An All-aluminumAutomobile |56

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Chapter 6 Nonferrous Metals and Alloys: Production, General Properties, and ApplicationsTABLE 6.IApproximate Cost-per-unit-volume forWroughtMetals and Plastics Relativeto the Cost of Carbon SteelGold 30,000 Magnesium alloys 4-6Silver 600 Aluminum alloys 2-3Molybdenum alloys 75-100 High-strength low-alloy steels 1.4Nickel 20 Gray cast iron 1.2Titanium alloys 20-40 Carbon steel 1Copper alloys 8-10 Nylons, acetals, and silicon rubber 1.1-2Zinc alloys 1.5-3.5 Other plastics and elastomers* 0.2-1Stainless steels 2-9*As molding compounds.Note: Costs vary significantly with quantity of purchase, supply and demand, size and shape, and otherfactors.

TABLE 6.2General Characteristics of Nonferrous Metals and AlloysMaterial CharacteristicsNonferrous alloys

Aluminum

MagnesiumCopperSuperalloysTitaniumRefractory metalsPrecious metals

More expensive than steels and plastics; Wide range of mechanical,physical, and electrical properties; good corrosion resistance;high-temperature applicationsAlloys have high strength-to-weight ratio; high thermal andelectrical conductivity; good corrosion resistance; goodmanufacturing propertiesLightest metal; good strength-to-Weight ratioHigh electrical and thermal conductivity; good corrosionresistance; good manufacturing propertiesGood strength and resistance to corrosion at elevated temperaturescan be iron-, cobalt-, and nickel-based alloysHighest strength-to-Weight ratio of all metals; good strength andcorrosion resistance at high temperaturesMolybdenum, niobium olumbium), tungsten, and tantalum; highstrength at elevated temperaturesGold, silver, and platinum; generally good corrosion resistance

6.2 Aluminum and Aluminum AlloysThe important advantages of aluminum Al) and its alloys are their high strength-to-weight ratios, resistance to corrosion by many chemicals, high thermal and electricalconductivities, nontoxicity, reflectivity, appearance, and ease of formability andmachinability; they are also nonmagnetic. The principal uses of aluminum and itsalloys, in decreasing order of consumption, are in containers and packaging aluminumcans and foil), architectural and structural applications, transportation ircraft andaerospace applications, buses, automobiles, railroad cars, and marine craft), electricalapplications as economical and nonmagnetic electrical conductors), consumerdurables ppliances, cooking utensils, and furniture), and portable tools Tables 6.3and 6.4). Nearly all high-voltage transmission Wiring is made of aluminum. In its struc-tural load-bearing) components, 82 of a Boeing 747 aircraft and 70 of a Boeing777 aircraft is aluminum. The frame and the body panels of the new Rolls Royce


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Section 6.2 Aluminum and Aluminum Alloys

Low-pressure compressor High-pressureFan T alloy Ti or AI alloy turbine LOW_preSSureN all0Y turbine

N IIHig|q_p| 9SSU|re COlT1lf)US1lOl1 3 Oycompressor Chamber

Inlet caseAI alloy

Ti or Ni alloy Ni all0Y

Turbine Turbineblades exhaust caseNi alloy Ni alloy

Accessory sectionAl alloy or Fe alloy

FIGURE 6.I Cross section of a jet engine (PW/2037), showing various components and thealloys used in manufacturing them. Source: Courtesy of United Aircraft Pratt SC Whitney.

TABLE 6.3Properties of Selected Aluminum Alloys at Room Temperature ElongationUltimate tensile Yield strength in 5 0 mmAlloy (UNS) Temper strength (MPa) (MPa) ( )1100 (A91100) O 90 35 35-451100 H14 125 120 9-202024 (A92024) O 190 75 20-222024 T4 470 325 19-203003 (A93003) O 110 40 30-403003 H14 150 145 8-165052 (A95052) O 190 90 25-305052 H34 260 215 10-146061 (A96061) O 125 55 25-306061 T6 310 275 12-177075 (A97075) O 230 105 16-177075 T6 570 500 11

Phantom coupe are made of aluminum, improving the cars strength-to-Weight andtorsional rigidity-to-Weight ratios.Aluminum alloys are available as mill products-that is, as Wrought productsmade into various shapes by rolling, extrusion, drawing, and forging (Chapters 13through 15). Aluminum ingots are available for casting, as is aluminum in powderform for powder-metallurgy applications (Chapter 17). Most aluminum alloys canbe machined, formed, and welded with relative ease.


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|54 Chapter 6 Nonferrous Metals and Alloys: Production, General Properties, and ApplicationsTABLE 6.4Manufacturing Characteristics and Typical Applications of SelectedWrought Aluminum Alloys

Characteristics*Corrosion

Alloy resistance Machinability Weldability Typical applications1100 A -D A Sheet-metal work, spun hollowware, tin stock2024 C -C B-C Truck wheels, screw machine products, aircraft structures3003 A -D A Cooking utensils, chemical equipment, pressure vessels,sheet-metal work, builders hardware, storage tanks5052 A -D A Sheet-metal work, hydraulic tubes, and appliances; bus,truck, and marine uses6061 B -D A Heavy-duty structures where corrosion resistance is needed;truck and marine structures, railroad cars, furniture, pipelines,

bridge railings, hydraulic tubing7075 C B-D D Aircraft and other structures, keys, hydraulic fittings*A, excellent; D, poor.

There are two types of wrought alloys of aluminum:I. Alloys that can be hardened by cold working and are not heat treatable.2. Alloys that can be hardened by heat treatment.

Designation ofWrought Aluminum Alloys. Wrought aluminum alloys are identi-fied by four digits and by a temper designation that shows the condition of thematerial. See also Unified Numbering System later in this section.) The major alloy-ing element is identified by the first digit:

1xxx-Commercially pure aluminum: excellent corrosion resistance, highelectrical and thermal conductivity, good workability, low strength, notheat treatable2xxx-Copper: high strength-to-weight ratio, low resistance to corrosion,heat treatable3xxx-Manganese: good workability, moderate strength, generally not heattreatable4xxx-Silicon: lower melting point, forms an oxide film of a dark gray tocharcoal color, generally not heat treatable5xxx-Magnesium: good corrosion resistance and weldability, moderate tohigh strength, not heat treatable6xxx-Magnesium and silicon: medium strength; good formability, machin-ability, weldability, and corrosion resistance; heat treatable7xxx-Zinc: moderate to Very high strength, heat treatable8xxx-Other element

The second digit in these designations indicates modifications of the alloy. Forthe 1xxx series, the third and fourth digits stand for the minimum amount of alu-minum in the alloy. For example, 1050 indicates a minimum of 99.50 Al, and 1090indicates a minimum of 99.90 Al. In other series, the third and fourth digits identifythe different alloys in the group and have no numerical significance. For instance, atypical aluminum beverage can may consist of the following aluminum alloys, all inthe H19 condition which is the highest cold-worked state): 3004 or 3104 for the canbody, 5182 for the lid, and 5042 for the tab. These alloys are selected for their man-ufacturing characteristics as well as for economic reasons.


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Section 6.2 Aluminum and Aluminum AlloysDesignation of Cast Aluminum Alloys. Designations for cast aluminum alloysalso consist of four digits. The first digit indicates the major alloy group, as follows:

1xx.x-Aluminum 99.00 minimum)2xx.x-Aluminum-copper3xx.x-Aluminum-silicon with copper and/or magnesium)4xx.x-Aluminum-silicon5xx.x-Aluminum-magnesium6xx.x-Unused series7xx.x-Aluminum-zinc8xx.x-Aluminum-tinIn the 1xx.x series, the second and third digits indicate the minimum aluminumcontent, as do the third and fourth in wrought aluminum. For the other series, thesecond and third digits have no numerical significance. The fourth digit to the right

of the decimal point) indicates the product form.Temper Designations. The temper designations for both wrought and cast alu-minum are as follows:

F-As fabricated by cold or hot working or by casting) O-Annealed from the cold-worked or the cast state) H-Strain hardened by cold working for wrought products only) T-Heat treated W-Solution treated only unstable temper)

Unified Numbering System. As is the casewith steels, aluminum and other nonfer-rous metals and alloys now are identified internationally by the Unified NumberingSystem UNS), consisting of a letter indicating the general class of the alloy, followedby five digits indicating its chemical composition. For example, A is for aluminum, Cfor copper, N for nickel alloys, P for precious metals, and Z for zinc. In the UNS des-ignation, 2024 wrought aluminum alloy is A92024.Production. Aluminum was first produced in 1 825. It is the most abundant metallicelement, making up about 8 of the earths crust, and is produced in a quantity sec-ond only to that of iron. The principal ore for aluminum is bauxite, which is ahydrous water-containing) aluminum oxide and includes various other oxides. Afterthe clay and dirt are washed off, the ore is crushed into powder and treated with hotcaustic soda sodium hydroxide) to remove impurities. Next, Alumina aluminumoxide) is extracted from this solution and then dissolved in a molten sodium-fluorideand aluminum-fluoride bath at 940 to 980C. This mixture is then subjected to di-rect-current electrolysis. Aluminum metal forms at the cathode negative pole), whileoxygen is released at the anode ositive pole). Commercially pure aluminum is up to99.99 Al, also referred to in industry as four nines aluminum. The productionprocess consumes a great deal of electricity, which contributes significantly to the costof aluminum.Porous Aluminum. Blocks of aluminum have been produced that are 37 lighterthan solid aluminum and have uniform permeability microporosity). This charac-teristic allows their use in applications where a vacuum or differential pressure hasto be maintained. Examples are the vacuum holding of fixtures for assembly andautomation, and the vacuum forming or thermoforming of plastics Section 196).These blocks are 70 to 90 aluminum powder; the rest is epoxy resin. They can bemachined with relative ease and can be joined together using adhesives.


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|56 Chapter 6 Nonferrous Metals and Alloys: Production, General Properties, and Applications

EXAMPLE 6.| An All-aluminum AutomobileAluminum use in automobiles and in light trucks hasbeen increasing steadily. As recently as 1990, therewere no aluminum-structured passenger cars in pro-duction anywhere in the world, but in 1997 therewere seven, including the Plymouth Prowler and theAudi A8 (Fig. 6.2). With weight savings of up to 47%over steel vehicles, such cars use less fuel, create lesspollution, and are recyclable.New alloys and new design and manufacturingmethodologies had to be developed. For example,welding and adhesive bonding procedures had to be

refined, the structural frame design had to beoptimized, and new tooling designs (to allow formingof aluminum) had to be developed. Because of thesenew technologies, the desired environmental savingswere able to be realized without an accompanyingdrop in performance or safety. In fact, the Audi A8 isthe first luxury-class car to earn a dual five-star(highest safety) rating for both driver and front-seatpassenger in the National Highway TransportationSafety Administration (NHSTA) New Car AssessmentProgram.

ia)Fiobotically applied, advanced arc-welding processesprovide consistent high-quality assembly of castings,extrusions, and sheet componentsDie-cast nodes are thin walledto maximize weight reduction,yet provide high performance

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..3",;_, .a _ to ` Z ffm .~~ 1: fmt. E. f .sy ` ~ ~&Z;i ,;: rrcc_pgl /, gg . .,,,,

- pggg 6, Q V ,, _,Z7/ fi: ' __ W / ee.. Qatar ccrrre ~ liia `ii lliiii .. t ,:f;fr~, i/z; ri,. ix: "1=~>;r:...,u lliliifiiisg. in "*= ~- reeppp Strong, thin-walled extrusions fi exhibit high ductiiity, energy i.. ,ai absorption, and toughnessAdvanced extrusion bending processessupport complex shapes and tight radii(bi

FIGURE 6.2 ia) The Audi A8 automobile, which has an all-aluminum body structure. (b) The aluminum body structure,showing various components made by extrusion, sheet forming, and casting processes. Source: Courtesy of ALCOA, Inc.


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Section 6.3 Magnesium

6.3 Magnesium and Magnesium AlloysMagnesium Mg) is the lightest engineeringmetal available, and it has good vibration-damping characteristics. Its alloys are used in structural and nonstructural applica-tions wherever weight is of primary importance. Magnesium is also an alloyingelement in various nonferrous metals.

Typical uses of magnesium alloys are in aircraft and missile components,material-handling equipment, portable power tools, ladders, luggage, bicycles, sport-ing goods, and general lightweight components. Like aluminum, magnesium is find-ing increased use in the automotive sector, mainly in order to achieve weight savings.Magnesium alloys are available either as castings such as die-cast camera frames) oras wrought products such as extruded bars and shapes, forgings, and rolled platesand sheets). Magnesium alloys are also used in printing and textile machinery tominimize inertial forces in high-speed components Section 3.2).

Because it is not sufficiently strong in its pure form, magnesium is alloyed withvarious elements Table 6.5) in order to gain certain specific properties, particularlya high strength-to-weight ratio. A variety of magnesium alloys have good casting,forming, and machining characteristics. Because they oxidize rapidly i.e., they arepyrop/ooric), a fire hazard exists, and precautions must be taken when machining,grinding, or sand-casting magnesium alloys. Products made of magnesium and itsalloys are, nonetheless, not a fire hazard during normal use.Designation of Magnesium Alloys. Magnesium alloys are designated with thefollowing:a. One or two prefix letters, indicating the principal alloying elements.b. Two or three numerals, indicating the percentage of the principal alloying ele-ments and rounded off to the nearest decimal.c. A letter of the alphabet except the letters I and O) indicating the standardizedalloy with minor variations in composition.d. A symbol for the temper of the material, following the system used for alu-minum alloys.For example, consider the alloy AZ91C-T6:

The principal alloying elements are aluminum A, at 9 , rounded off) andzinc Z, at 1 ). The letter C, the third letter of the alphabet, indicates that this alloy was thethird one standardized later than A and B, which were the first and second

and Magnesium Alloys l57

TABLE 6.5Properties and Typical Forms of Selected WroughtMagnesium Alloys

Ultimate0 tensile Yield Elongation

COIHPOSIIIOH M strength strength in 50 mmAlloy Zn Mn Zr Th Condition Pa) MPa) ) Typical formsAZ31B 3.0 1.0 0.2 _ - F 200 ExtrusionsH24 220 Sheet and platesAZSOA 8.5 0.5 0.2 - - T5 275 Extrusions and forgingsHK31A - - 0.7 3 H24 200 Sheet and platesZK60A 5.7 - 0.55 T5 300 Extrusions and forgings


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Chapter 6 Nonferrous Metals and Alloys: Production, General Properties, and Applicationsalloys that were standardized respectively). Specific information related to thisstandardized alloy can then be obtained.

T6 indicates that this alloy has been solution treated and artificially aged.Production. Magnesium is the third-most-abundant metallic element 2 ) in theearths crust, after iron and aluminum. Mostmagnesium comes from seawater, whichcontains 0.13 magnesium in the form of magnesium chloride. First produced in1808, magnesium metal can be obtained electrolytically or by thermal reduction. Inthe electrolytic inet/ood, seawater is mixed with lime calcium hydroxide) in settlingtanks. Magnesium hydroxide precipitates to the bottom, is filtered and mixed withhydrochloric acid. The resulting solution is subjected to electrolysis as is done withaluminum), producing magnesium metal, which is then cast into ingots for furtherprocessing into various shapes. In the thermal-reduction met/ood, magnesium oresdolomite, magnesite, and other rocks) are broken down with reducing agents suchas powdered ferrosilicon, an alloy of iron and silicon) by heating the mixture in avacuum chamber. As a result of this reaction, vapors of magnesium form, and theycondense into magnesium crystals, which are then melted, refined, and poured intoingots to be processed further into various shapes.

6.4 Copper and Copper AlloysFirst produced in about 4000 B.C., copper Cu, from the Latin cuprurn) and its alloyshave properties somewhat similar to those of aluminum and its alloys. In addition,they are among the best conductors of electricity and heat Tables 3.1 and 3.2), andthey have good corrosion resistance. Copper and its alloys can be processed easily byvarious forming, machining, casting, and joining techniques.

Copper alloys often are attractive for applications in which a combination ofelectrical, mechanical, nonmagnetic, corrosion-resistant, thermally conductive, andwear-resistant qualities are required. Applications include electrical and electroniccomponents, springs, coins, plumbing components, heat exchangers, marine hard-ware, and consumer goods such as cooking utensils, jewelry, and other decorativeobjects). Although aluminum is the most common material for dies in polymerinjection molding Section 19.3), copper often is used because of its better thermalproperties. Pure copper also can be used as a solid lubricant in hot-metal-formingoperations.Copper alloys can acquire a wide variety of properties by the addition of alloy-ing elements and by heat treatment, to improve their manufacturing characteristics.The most common copper alloys are brasses and bronzes. Brass an alloy of copperand zinc) is one of the earliest alloys developed and has numerous applications, in-cluding decorative objects Table 6.6). Bronze is an alloy of copper and tin Table 6.7).There are also other bronzes, such as aluminum bronze an alloy of copper andaluminum) and tin bronzes. Beryllium copper or beryllium bronze) and phosphorbronze have good strength and hardness for applications such as springs and bearings.Other major copper alloys are copper nickels and nickel sili/ers.Designation of Copper Alloys. In the Unified Numbering System, copper is iden-tified with the letter C, such as C26200 for cartridge brass. In addition to beingidentified by their composition, copper and copper alloys are known by variousnames Tables 6.6 and 6.7). The temper designations such as 1/Z hard, extrahard, extra spring, and so on) are based on degree of cold work such as by rollingor drawing).


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Section 6.4 Copper and Copper Alloys |59TABLE 6.6Properties and Typical Applications of SelectedWrought Copper and Brasses

UltimateNominal tensile Yield Elongationcomposition strength strength in 50 mm

Type and UNS number ) Pa) MPa) ) Typical applicationsElectrolytic tough-pitch 99.90 Cu, 220-450 70-365 55-4 Downspouts, gutters, roofing, gaskets,copper C11000) 0.04 O auto radiators, bus bars, nails, printing

rolls, rivetsRed brass, 85 85.0 Cu, 270-725 70-435 55-3 Weather stripping, conduits, sockets,C23000) 15.0 Zn fasteners, fire extinguishers, condenserand heat-exchanger tubingCartridge brass, 70 70.0 Cu, 300-900 75-450 66-3 Radiator cores and tanks, flashlightC26000) 30.0 Zn shells, lamp fixtures, fasteners, locks,hinges, ammunition components,plumbing accessoriesFree-cutting brass 61.5 Cu, 340-470 125-310 53-18 Gears, pinions, automatic high-speedC36000) 3.0 Pb, 35.5 Zn screw machine partsNaval brass C46400 60.0 Cu, 39.25 Zn, 380-610 170-455 50-17 Aircraft: turnbuckle barrels, balls,to C46700) 0.75 Sn bolts; marine hardware: propellershafts, rivets, valve stems, condenserplates

TABLE 6.7Properties and Typical Applications of SelectedWrought Bronzes

UltimateNominal tensile Yield ElongationType and UNS composition strength strength in 5 0 mmnumber ) Pa) MPa) ) Typical applicationsArchitectural bronze 57.0 Cu, 3.0 Pb, 415 140 30 Architectural extrusions, storefronts,C38500) 40.0 Zn as extruded) thresholds, trim, butts, hingesPhosphor bronze, 95.0 Cu, 5.0 Sn, 325-960 130-550 64-2 Bellows, clutch disks, cotter pins,5 A C51000) trace P diaphragms, fasteners, wire brushes,chemical hardware, textile machineryFree-cutting phosphor 88.0 Cu, 4.0 Pb, 300-520 130-435 50-15 Bearings, bushings, gears, pinions,bronze C54400) 4.0 Zn, 4.0 Sn shafts, thrust washers, valve partsLow-silicon bronze, 98.5 Cu, 1.5 Si 275-655 100-475 55-11 Hydraulic pressure lines, bolts, marineB C65100) hardware, electrical conduits, heat-exchanger tubing

Nickel silver, 65-10 65.0 Cu, 25.0 Zn, 340-900 125-525 50-1 Rivets, screws, slide fasteners,C74500) 10.0 Ni hollowware, nameplates

Production. Copper is found in several types of ores, the most common being sulfideores. The ores are generally of low grade although some contain up to 15 Cu) andusually are obtained from open-pit mines. The slurry is ground into fine particles in ballmills rotating cylinderswith metal balls inside to crush the ore, as shown in Fig. 17.6b,the resulting particles are then suspended in water to form a slurry. Chemicals and oilare then added, and the mixture is agitated. Themineral particles forma froth, which isscraped and dried. The dry copper concentrate as much as one-third of which is cop-per) is traditionally smelted melted and fused) and refined; this process is known aspyrometallurgy, because heat is used to refine the metal. For applications such as


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|60 Chapter 6 Nonferrous Metals and Alloys: Production, General Properties, and Applicationselectrical conductors, the copper is refined further electrolytically to a purity of at least99.95 oxygen-free electrolytic copper). A more recent technique for processing cop-per is hydrometallurgy, a process involving both chemical and electrolytic reactions.

6.5 Nickel and Nickel AlloysNickel Ni) is a silver-white metal discovered in 1751 and a major alloying elementthat imparts strength, toughness, and corrosion resistance. It is used extensively instainless steels and in nickel-based alloys also called superalloys). Nickel alloys areused in high-temperature applications such as jet engine components, rockets, andnuclear power plants), in food-handling and chemical-processing equipment, incoins, and in marine applications. Because nickel is magnetic, nickel alloys also areused in electromagnetic applications, such as solenoids. The principal use of nickel asa metal is in the electroplating of parts for their appearance and for the improvementof their corrosion and Wear resistance. Nickel alloys have high strength and corro-sion resistance at elevated temperatures. Alloying elements in nickel are chromium,cobalt, and molybdenum. The behavior of nickel alloys in machining, forming, cast-ing, and Welding can be modified by various other alloying elements.

A variety of nickel alloys, with a wide range of strengths at different tempera-tures have been developed Table 6.8). Although trade names are still in wide use,nickel alloys are now identified in the UNS system with the letter N. Thus, HastelloyG is novv N06007. Other common trade names are:

Monel is a nickel-copper alloy. Inconel is a nickel-chromium alloy with a tensile strength of up to 1400 MPa.Hastelloy also a nickel-chromium alloy) has good corrosion resistance and highstrength at elevated temperatures. Nichrome an alloy of nickel, chromium, and iron) has high electrical resistanceand a high resistance to oxidation and is used for electrical heating elements. Invar and Kovar lloys of iron and nickel) have relatively low sensitivity to tem-perature Section 3.6).

TABLE 6.8Properties and Typical Applications of Selected Nickel Ailoys ll Are Trade Names)

UltimateNominal tensile Yield ElongationType and UNS composition strength strength in 50 mmnumber ) Pa) MPa) ) Typical applicationsNickel 200 annealed) None 380-550 100-275 60-40 Chemical and food processing

industry, aerospace equipment,electronic partsDuranickel 301 4.4 Al, 0.6 Ti 1300 900 28 Springs, plastics extrusion equipment,age hardened) molds for glass, diaphragmsMonel R-405 30 Cu 525 230 35 Screw-machine products, water meterhot rolled) partsMonel K-500 29 Cu, 3 Al 1050 750 30 Pump shafts, valve stems, springsage hardened)

Inconel 600 15 Cr, 8 Fe 640 210 48 Gas turbine parts, heat-treatingannealed) equipment, electronic parts, nuclearreactorsHastelloy C-4 16 Cr, 15 Mo 785 400 54 Parts requiring high-temperaturesolution treated stability and resistance to stress-and quenched) corrosion cracking


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Section 6.6 SuperalloysProduction. The main sources of nickel are sulfide and oxide ores, all of whichhave low concentrations of nickel. Nickel metal is produced by preliminary sedimen-tary and thermal processes, followed by electrolysis; this sequence yields 99.95pure nickel. Although nickel also is present in the ocean bed in significant amounts,undersea mining is not yet economical.

6.6 SuperalloysSuperalloys are important in high-temperature applications; hence, they are alsoknown as heat-resistant or high-temperature alloys. Superalloys generally have goodresistance to corrosion, mechanical and thermal fatigue, mechanical and thermalshock, creep, and erosion, at elevated temperatures. Major applications of superal-loys are in jet engines and gas turbines. Other applications are in reciprocatingengines, rocket engines, tools and dies for hot working of metals, and the nuclear,chemical, and petrochemical industries. Generally, superalloys are identified bytrade names or by special numbering systems, and they are available in a variety ofshapes. Most superalloys have a maximum service temperature of about 1000C instructural applications. The temperatures can be as high as 1200C for non-load-bearing components.Superalloys are referred to as iron-based, cobalt-based, or nic/eel-based.

Iron-based superalloys generally contain from 32 to 67 Fe, from 15 to 22Cr, and from 9 to 38 Ni. Common alloys in this group are the Incoloy series.

Cobalt-based superalloys generally contain from 35 to 65 Co, from 19 to30 Cr, and up to 35 Ni. These superalloys are not as strong as nickel-basedsuperalloys, but they retain their strength at higher temperatures. Nickel-based superalloys are the most common of the superalloys and are avail-able in a wide variety of compositions Table 6.9). The proportion of nickel isfrom 38 to 76 . These superalloys also contain up to 27 Cr and 20 Co.Common alloys in this group are the Hastelloy Inconel, Nimonic, Ren, Udimet,Astroloy and Waspaloy series.

TABLE 6.9Properties and Typical Applications of Selected Nickel-based Superalloys at 8T0C ll Are Trade Names)

ElongationUltimate tensile Yield strength in 50 mmAlloy Condition strength Pa) MPa) Typical applications

Astroloy Wrought 770 690 Forgings for high-temperature useHastelloy X Wrought 255 1 80 ]et engine sheet partsIN-100 Cast 885 695 _let engine blades and WheelsIN-102 Wrought 215 200 Superheater and jet engine partsInconel 625 Wrought 285 275 Aircraft engines and structures, chemicalprocessing equipmentInconel 718 Wrought 340 330 jet engine and rocket partsMAR-M 200 Cast 840 760 ]et engine bladesMAR-M 432 Cast 730 605 Integrally cast turbine wheelsRen 41 Wrought 620 550 Jet engine partsUdimet 700 Wrought 690 635 _jet engine partsWaspaloy Wrought 525 515 jet engine parts


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|62 Chapter 6 Nonferrous Metals and Alloys: Production, General Properties, and Applications

6.7 Titanium and Titanium AlloysTitanium Ti, named after the Greek god Titan) is a silvery white metal discovered in1791, but not produced commercially until the 1950s. Although titanium isexpensive, its high strength-to-weight ratio and corrosion resistance at room andelevated temperatures make it attractive for many applications, including aircraft; jetengines see Fig. 6.1); racing cars; golf clubs; chemical, petrochemical, and marinecomponents; submarine hulls; armor plate; and medical applications, such as orthope-dic implants Table 6.10). Titanium alloys have been developed for service at 550Cfor long periods of time and at up to 75 0C for shorter periods.Unalloyed titanium, known as commercially pure titanium, has excellentcorrosion resistance for applications where strength considerations are secondary.Aluminum, vanadium, molybdenum, manganese, and other alloying elements impartproperties such as improved workability, strength, and hardenability.The properties and manufacturing characteristics of titanium alloys are ex-tremely sensitive to small variations in both alloying and residual elements. Therefore,control of composition and processing are important, especially the prevention ofsurface contamination by hydrogen, oxygen, or nitrogen during processing; theseelements cause embrittlement of titanium and, consequently, reduce toughness andductility.The body-centered cubic structure of titanium beta-titanium) is above 880Cand is ductile, whereas its hexagonal close-packed structure alpha-titanium) is some-what brittle and is very sensitive to stress corrosion. A variety of other structuresalpha, near-alpha, alpha-beta, and beta) can be obtained by alloying and heat treat-ing, so that the properties can be optimized for specific applications. Titanium alu-minide intermetallics TiAl and Ti3Al; see Section 4.2.2) have higher stiffness andlower density than conventional titanium alloys and can withstand higher tempera-tures.Production. Ores containing titanium first are reduced to titanium tetrachloride inan arc furnace, then converted to titanium chloride in a chlorine atmosphere. Thiscompound is reduced further to titanium metal by distillation and leaching dissolv-ing). This sequence forms sponge titanium, which is then pressed into billets, melted,and poured into ingots to be processed later into various shapes. The complexity ofthese multistep thermochemical operations the Kroll process developed in the1940-1950s) adds considerably to the cost of titanium. New developments in elec-trochemical extraction processes are taking place to reduce the number of stepsinvolved and the energy consumption, thereby reducing the cost of producingtitanium.

TABLE 6. I 0Properties and Typical Applications of Selected Wrought Titanium Alloys at Various Temperatures

Ultimate UltimateNominal tensile Yield Reduction tensile Yieldcomposition strength strength Elongation of area Temp. strength strength ) UNS Condition Pa) MPa) C) Pa) MPa)99.5 Ti R50250 Annealed 330 240 300 150 955 Al, 2.5 Sn R54520 Annealed 860 810 300 565 4506 Al, 4 V R56400 Annealed 1000 925 300 725 650Solution + age 1175 1100 300 980 90013 V, 11 Cr, 3 Al R58010 Solution + age 1275 1210 425 1100 830


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Section 6.8 Refractory Metals and Alloys

6.8 Refractory Metals and AlloysThere are four refractory metals: molybdenum, niobium, tungsten, and tantalum.These metals are called refractory because of their high melting points. Although theywere discovered about 200 years ago and have been used as important alloying ele-ments in steels and superalloys, their use as engineering metals and alloys did notbegin until about the 1940s. More than most other metals and alloys, the refractorymetals maintain their strength at elevated temperatures. Therefore, they are of greatimportance in rocket engines, gas turbines, and various other aerospace applications;in the electronic, nuclear-power, and chemical industries; and as tool and die materi-als. The temperature range for some of these applications is on the order of 1100 to2200C, where strength and oxidation are of major concern.6.8.l MolybdenumMolybdenum Mo) is a silvery white metal discovered in the 18th century and has ahigh melting point, high modulus of elasticity, good resistance to thermal shock, andgood electrical and thermal conductivity. Molybdenum is used in greater amountsthan any other refractory metal, in applications such as solid-propellant rockets, jetengines, honeycomb structures, electronic components, heating elements, and diesfor die casting. The principal alloying elements for molybdenum are titaniumand zirconium. Molybdenum is itself also an important alloying element in cast andwrought alloy steels and in heat-resistant alloys, imparting strength, toughness, andcorrosion resistance. A major limitation of molybdenum alloys is their low resist-ance to oxidation at temperatures above 500C, which necessitates the use of pro-tective coatings.Production. The main source of molybdenum is the mineral molybdenite molyb-denum disulfide). The ore first is processed and the molybdenum is concentrated;later, it is chemically reduced-first with oxygen and then with hydrogen. Powder-metallurgy techniques also are used to produce ingots for further processing intovarious shapes.6.8.2 Niobium Columbium)Niobium Nb, for niobium, after Niobe, the daughter of the mythical Greek kingTantalus) was first identified in 1801; it is also referred to as columbium after itssource mineral, columbite). Niobium possesses good ductility and formability andhas greater oxidation resistance than other refractory metals. With various alloyingelements, niobium alloys can be produced with moderate strength and good fabrica-tion characteristics. These alloys are used in rockets and missiles and in nuclear,chemical, and superconductor applications. Niobium is also an alloying element invarious alloys and superalloys. The metal is processed from ores by reduction andrefinement and from powder by melting and shaping into ingots.6.8.3 TungstenTungsten W for u/olfrarn, its European name, and from its source mineral,wolframite; in Swedish, tung means heavy and sten means stone) was firstidentified in 1781; it is the most abundant of all the refractory metals. Tungsten hasthe highest melting point of any metal 3410C). As a result, it is notable for its highstrength at elevated temperatures. However, it has high density hence it is used forbalancing weights and counterbalances in mechanical systems, including self-wind-


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Chapter 6 Nonferrous Metals and Alloys: Production, General Properties, and Applicationsing watches , is brittle at low temperatures, and offers poor resistance to oxidation.As an alloying element, tungsten imparts strength and hardness to steels at elevatedtemperatures.Tungsten alloys are used for applications involving temperatures above165OC, such as nozzle throat liners in missiles and in the hottest parts of jet androcket engines, circuit breakers, welding electrodes, tooling for electrical-dischargemachining, and spark-plug electrodes. The filament Wire in incandescent light bulbsSection 1.1 is made of pure tungsten and is produced by the use of powder-metallurgy and wire-drawing techniques. Tungsten carbide, with cobalt as a binderfor the carbide particles, is one of the most important tool and die materials.Tungsten is processed from ore concentrates by chemical decomposition and is thenreduced. It is further processed by povvder-metallurgy techniques in a hydrogenatmosphere.6.8.4 TantalumIdentified in 1802, tantalum Ta, after the mythical Greek king, Tantalus is charac-terized by its high melting point 3000C , high density, good ductility, and resist-ance to corrosion. However, it has poor chemical resistance at temperatures above15 OC. Tantalum is used extensively in electrolytic capacitors and in various compo-nents in the electrical, electronic, and chemical industries; it also is used for thermalapplications, such as in furnaces and in acid-resistant heat exchangers. A variety oftantalum-based alloys are available in many forms for use in missiles and aircraft.Tantalum also is used as an alloying element. It is processed by techniques similar tothose used for processing niobium.

6.9 BerylliumSteel gray in color, beryllium Be, from the ore beryl has a high strength-to-Weightratio. Unalloyed beryllium is used in rocket nozzles, space and missile structures,aircraft disc brakes, and precision instruments and mirrors. It is used in nuclear andX-ray applications because of its low neutron absorption. Beryllium is also an al-loying element, and its alloys of copper and nickel are employed in various applica-tions, including springs beryllium copper , electrical contacts, and nonsparkingtools for use in such explosive environments as mines and metal-powder productionSection 17.2.3 . Beryllium and its oxide are toxic; their associated dust and fumesshould not be inhaled.

6.l0 ZirconiumZirconium Zr is silvery in appearance; it has good strength and ductility at elevatedtemperatures and has good corrosion resistance because of an adherent oxide film.Zirconium is used in electronic components and in nuclear-power reactor applica-tions because of its low neutron absorption.

6.1 I Low-melting AlloysLow-melting alloys are so named because of their relatively low melting points. Themajor metals in this category are lead, zinc, tin, and their alloys.


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Section 6.1 16.ll.l LeadLead Pb, after plumbum, the root of the word plumber) has characteristic prop-erties of high density, resistance to corrosion by virtue of the stable lead-oxide layerthat forms to protect the surface), softness, low strength, ductility, and good work-ability. Alloying it with various elements such as antimony and tin) enhances itsdesirable properties, making it suitable for piping, collapsible tubing, bearing alloysBabbitt), cable sheathing, foil as thin as 0.01 mm), roofing, and lead-acid storagebatteries. Lead also is used for damping sound and vibrations, radiation shieldingagainst X-rays, ammunition, as weights, and in the chemical industry. Because itcreeps even at room temperature, the use of lead for load-bearing applications isvery limited.The oldest known lead artifacts were made in about 3000 B.C. Lead pipesmade by the Romans and installed in the Roman baths in Bath, England, two mil-lennia ago are still in use. Lead is also an alloying element in solders, steels, and cop-per alloys; it promotes corrosion resistance and machinability. An additional use oflead is as a solid lubricant for hot-metal-forming operations. Because of its toxicity,however, environmental contamination by lead causing lead poisoning) is a majorconcern; major efforts are currently being made to replace lead with other elementssuch as lead-free solders, Section 32.3.1). The most important mineral source oflead is galena PbS); it is mined, smelted, and refined by chemical treatments.

6.l l.2 ZincZinc Zn), is bluish white in color and is the metal that is fourth most utilized indus-trially, after iron, aluminum, and copper. Although its existence was known formany centuries, zinc was not developed until the 18th century. It has three majoruses: 1) for galvanizing iron, steel sheet, and wire, 2) as an alloy in other metals,and 3) as a material in castings.In galvanizing, zinc serves as an anode and protects the steel cathode) fromcorrosive attack should the coating be scratched or punctured. Zinc also is usedas an alloying element; brass, for example, is an alloy of copper and zinc. Majoralloying elements in zinc-based alloys are aluminum, copper, and magnesium; theyimpart strength and provide dimensional control during casting of the metal. Zinc-based alloys are used extensively in die casting for making such products as fuelpumps and grills for automobiles, components for household appliances such asvacuum cleaners and washing machines, kitchen equipment, various machineryparts, and photoengraving equipment. Another use for zinc is in superplastic alloysA very fine grained 78 Zn-22 Al sheet is a common example of a superplasticzinc alloy that can be formed by methods used for forming plastics or metals.Production. A number of minerals containing zinc are found in nature. The prin-cipal mineral source is zinc sulfide, also called zincblende. The ore first is roasted inair and converted to zinc oxide. It then is reduced to zinc either electrolytically withthe use of sulfuric acid) or by heating it in a furnace with coal which causes themolten zinc to separate).

6.l l.3 TinAlthough used in small amounts compared with iron, aluminum, or copper, tinSn, from the Latin stannum is an im ortant metal. The most extensive use of tinpa silver-white lustrous metal is as a rotective coatin on steel sheets tin latesP g

Low-melting Alloys


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Chapter 6 Nonferrous Metals and Alloys: Production, General Properties, and Applicationsused in making containers tin cans) for food and for various other products. Thelow shear strength of the tin coatings on steel sheets improves deep drawabilitySection 16.7.1) and performance in general pressworking. Unlike galvanizedsteels, if this coating is punctured or destroyed, the steel corrodes because the tin iscathodic.Unalloyed tin is used in such applications as a lining material for water distil-lation plants and as a molten layer of metal in the production of float glass plateSection 18.3.1). Tin-based alloys also called white metals) generally contain cop-per, antimony, and lead. The alloying elements impart hardness, strength, and corro-sion resistance. Tin is an alloying element for dental alloys and for bronzecopper-tin alloy), titanium, and zirconium alloys. Tin-lead alloys are commonsoldering materials, with a wide range of compositions and melting points.Because of their low friction coefficients which result from low shear strengthand low adhesion), some tin alloys are used as journal-bearing materials. Thesealloys are known as babbitts after I. Babbitt, 1799-1862) and contain tin, copper,and antimony. Pewter, an alloy of tin, copper, and antimony, was developed in the15th century and has been used for tableware, hollowware, and decorative artifacts.Organ pipes are made of tin alloys. The most important tin mineral is cassiterite tinoxide), which is of low grade. The ore is mined, concentrated by various techniques,smelted, refined, and cast into ingots for further processing.

6.l2 Precious MetalsThe most important precious costly) metals, also called noble metals, are thefollowing:

Gold Au, from the Latin aurum) is soft and ductile and has good corrosionresistance at any temperature. Typical applications include jewelry, coinage,reflectors, gold leaf for decorative purposes, dental work, electroplating, andelectrical contacts and terminals. Silver Ag, from the Latin argentum) is ductile and has the highest electrical andthermal conductivity of any metal Table 3.2). However, it develops an oxidefilm that affects its surface characteristics and appearance. Typical applicationsfor silver include tableware, jewelry, coinage, electroplating, electrical contacts,solders, bearing linings, and food and chemical equipment. Sterling silz/er is analloy of silver and 7.5 copper.

Platinum Pt) is a soft, ductile, grayish-white metal that has good corrosion resist-ance even at elevated temperatures. Platinum alloys are used as electrical contacts;for spark-plug electrodes; as catalysts for automobile pollution-control devices;in filaments and nozzles; in dies for extruding glass fibers Section 18.3.4), inthermocouples; and in jewelry and dental work.

6.13 Shape-memory Alloys Smart Materials)Shape-memory alloys are unique in that, after being plastically deformed into vari-ous shapes at room temperature, they return to their original shape upon heating.For example, a piece of straight wire made of such material can be wound into theshape of a helical spring; when heated with a match, the spring uncoils and returns


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Section 6 15 Metal Foams

to the original straight shape. Shape-memory alloys can be used to generate motionand/or force in temperature-sensitive actuators. The behavior of these alloys, alsocalled smart materials, can be reversible; that is, the shape can switch back and forthrepeatedly upon application and removal of heat. A typical shape-memory alloyis 55 Ni-45 Ti Nitmol). Other such alloys are copper-aluminum-nickel,copper-zinc-aluminum, iron-manganese-silicon, and titanium-nickel-hafnium.Shape-memory alloys generally have such properties as good ductility, corrosionresistance, and high electrical conductivity.Applications of shape-memory alloys include various sensors, eyeglass frames,stents for blocked arteries, relays, pumps, switches, connectors, clamps, fasteners,and seals. As an example, a nickel-titanium valve has been made to protect peoplefrom being scalded in sinks, tubs, and showers. It is installed directly into the pipingsystem and brings the water flow down to a trickle within 3 seconds after the watertemperature reaches 47C. New developments include thin-film shape-memory al-loys deposited on polished silicon substrates for use in microelectromechanicalMEMS) devices see Chapter 29).

6.|4 Amorphous Alloys Metallic Glasses)A class of metal alloys that, unlike metals, do not have a long-range crystallinestructure is called amorphous alloys; these metals have no grain boundaries, andtheir atoms are packed randomly and tightly. The amorphous structure first wasobtained in the late 1960s by the extremely rapid solidification of a molten alloySection 11.6). Because their structure resembles that of glasses, these alloys are alsocalled metallic glasses. Amorphous alloys typically contain iron, nickel, and chromium,which are alloyed with carbon, phosphorus, boron, aluminum, and silicon. They areavailable as wire, ribbon, strip, and powder: One application is for faceplate insertson golf-club heads; this alloy has a composition of zirconium, beryllium, copper,titanium, and nickel and is made by die casting. Another application is in hollowaluminum baseball bats coated with a composite of amorphous metal by thermalspraying and is said to improve the performance of the bat.Amorphous alloys exhibit excellent corrosion resistance, good ductility, highstrength, and very low magnetic hysteresis. The latter property is utilized in theproduction of magnetic steel cores for transformers, generators, motors, lamp bal-lasts, magnetic amplifiers, and linear accelerators. The low magnetic hysteresisloss provides greatly improved efficiency; however, fabrication costs are signifi-cant. Amorphous steels are being developed with strengths twice those of high-strength steels, with potential applications in large structures; however, they arepresently cost prohibitive. A major application for the superalloys of rapidly solid-ified powders is the consolidation into near-net shapes for parts used in aerospaceengines.

6.l 5 Metal FoamsMetal foams are material structures where the metal consists of only 5 to 20 ofthe structures volume, as shown in Fig. 6.3. Usually made of aluminum alloys butalso of titanium, tantalum, and others), metal foams can be produced by blowing air


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|68 Chapter 6 Nonferrous Metals and Alloys: Production, General Properties, and ApplicationsW yy*

FIGURE 6.3 Microstructureof a metal foam used in ortho-pedic implants to encouragebone ingrowth. Source: Cour-tesy of Zimmer, Inc.

KEY TERMS

into molten metal and tapping the froth that forms at the surface; this froth thensolidifies into a foam. Other approaches to producing metal foam include a chem-ical vapor deposition Section 34.6.2 onto a carbon foam lattice, b depositingmetal powders from a slurry onto a polymer foam lattice, followed by sinteringSection 17.4 to fuse the metals and burn off the polymer, c doping molten orpowder metals Chapter 17 with titanium hydride TiH2 , which then releaseshydrogen gas at the elevated casting or sintering temperatures, and d pouringmolten metal into a porous salt and, upon cooling, leaching out the salt with acid.Metal foams have unique combinations of strength-to-density and stiffness-to-density ratios, although these ratios are not as high as the base metals themselves.However, metal foams are very lightweight and thus are attractive materials for aero-space applications. Because of their porosity, other applications of metal foams arefilters and orthopedic implants. Recent developments include nicl


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BIBLIOGRAPHYQualitative Problems I69

ASM Handbook, Vol. 2: Properties and Selection:Nonferrous Alloys and Special-Purpose Materials,ASM International, 1990.

ASM Specialty Handbook: Aluminum and Aluminum Alloys,ASM International, 1993.

ASM Specialty Handbook: Copper and Copper Alloys, ASMInternational, 2001.ASM Specialty Handbook: Heat-Resistant Materials, ASMInternational, 1997.ASM Specialty Handbook: Magnesium and Magnesium

Alloys, ASM International, 1999.ASM Specialty Handbook: Nickel, Cobalt, and Their Alloys,ASM International 2000.Cardelli, F., Materials Handbook: A Concise Desk Reference,2nd ed., Springer, 2008.

Donachie, MJ. ed. , Titanium: A Technical Guide, 2nd ed.,ASM International, 2000.

REVIEW QUESTIONS

Donachie, M.]., and Donachie, S.]., Superalloys:A TechnicalGuide, 2nd ed., ASM International, 2002.Farag, M.M., Materials Selection for Engineering Design,Prentice Hall, 1997.Fremond, M., and Miyazaki, S., Shape-Memory Alloys,Springer Verlag, 1996.Kaufman, ].G., Introduction to Aluminum Alloys andTempers, ASM International, 2000.Lagoudas, D.C. ed. , Shape Memory Alloys: Modeling andEngineering Applications, Springer, 2008.

Leo, D.]., Engineering Analysis of Smart Material Systems,Wiley, 2007.Lutjering, G., and Williams, ].C., Titanium, 2nd ed.,Springer, 2007.

6.l. Given the abundance of aluminum in the earths crust,explain why it is more expensive than steel.6.2. Why is magnesium often used as a structural materialin power hand tools? Why are its alloys used instead of puremagnesium?6.3. What are the major uses of copper? What are the alloy-ing elements in brass and bronze, respectively?6.4. What are superalloys? Why are they so named?6.5. What properties of titanium make it attractive for usein race-car and jet-engine components? Why is titanium notused widely for engine components in passenger cars?6.6. Which properties of each of the major refractory met-als define their most useful applications?6.7. What are metallic glasses? Why is the word glassused for these materials?

QLIALITATIVE PROBLEMS

6.8. What is the composition of a babbitts, b pewter, andC sterling silver?6.9. Name the materials described in this chapter that havethe highest a density, b electrical conductivity, c thermalconductivity, d strength, and e cost.6.|0.6.l I. Describe the advantages to using zinc as a coating forsteel.

What are the major uses of gold, other than in jewelry?

6.l2. What are nanomaterials? Why are they being devel-oped?6.|3. Why are aircraft fuselages made of aluminum alloys,even though magnesium is the lightest metal?

6.|4. Explain why cooking utensils generally are made ofstainless steels, aluminum, or copper.6.I5. Would it be advantageous to plot the data in Table 6.1in terms of cost per unit weight rather than cost per unitvolume? Explain and give some examples.6.I6. Compare the contents of Table 6.3 with those in vari-ous other tables and data on materials in this book, thencomment on which of the two hardening processes heattreating and work hardening is more effective in improvingthe strength of aluminum alloys.

6.l 7. What factors other than mechanical strength should beconsidered in selecting metals and alloys for high-temperatureapplications? Explain.6.|8. Assume that, for geopolitical reasons, the price ofcopper increases rapidly. Name two metals with similarmechanical and physical properties that can be substitutedfor copper. Comment on your selection and any observationsyou make.


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|70 Chapter 6 Nonferrous Metals and Alloys: Production, General Properties, and Applications6.l9. If aircraft, such as a Boeing 757, are made of 79aluminum, why are automobiles made predominantly ofsteel?6.20. Portable notebook) computers and digital camerascan have their housing made of magnesium. Why?6.2l. Most household wiring is made of copper wire. Bycontrast, grounding wire leading to satellite dishes and thelike is made of aluminum. Explain the reason.

QUANTITATIVE PROBLEMS|]6.23. A simply supported rectangular beam is 30 mmwide and 1 m long, and it is subjected to a vertical load of40 kg at its center. Assume that this beam could be made ofany of the materials listed in Table 6.1. Select three differentmaterials, and for each, calculate the beam height that wouldcause each beam to have the same maximum deflection.Calculate the ratio of the cost for each of the three beams.6.24. Obtain a few aluminum beverage cans, cut them, andmeasure their wall thicknesses. Using data in this chapter andsimple formulas for thin-walled, closed-end pressure vessels,calculate the maximum internal pressure these cans can with-stand before yielding. Assume that the can is a thin-walled,closed-end, internally pressurized vessel.)

SYNTHESIS, DESIGN, AND PROIECTS

6.22. The example in this chapter showed the benefits ofmaking cars from aluminum alloys. However, the averageamount of steel in cars has increased in the past decade. Listreasons to explain these two observations.

6.26. Beverage cans usually are stacked on top of each otherin stores. Use the information from Problem 6.24, and, refer-ring to textbooks on the mechanics of solids, estimate thecrushing load each of these cans can withstand.|]6.27. Using strength and density data, determine theminimum weight of a 900-mm long tension member thatmust support 340 kg if it is manufactured from a) 3003-Oaluminum, b) 5052-H34 aluminum, c) AZ31B-F magne-sium, d) any brass alloy, and e) any bronze alloy.|]6.28. Plot the following for the materials described inthis chapter: a) yield strength vs. density, b) modulus of elas-ticity vs. strength, c) modulus of elasticity vs. relative cost,and d) electrical conductivity vs. density.

6.29. Because of the number of processes involved in mak-ing metals, the cost of raw materials depends on the condi-tion hot or cold rolled), shape late, sheet, bar, tubing), andsize of the metals. Make a survey of the technical literature,obtain price lists or get in touch with suppliers, and prepare alist indicating the cost per 100 kg of the nonferrous materialsdescribed in this chapter, available in different conditions,shapes, and sizes.6.30. The materials described in this chapter have numer-ous applications. Make a survey of the available literature inthe bibliography, and prepare a list of several specific parts orcomponents and applications, indicating the types of materi-als used.6.3 I. Name products that would not have been developed totheir advanced stages s we find them today) if alloys having

high strength, high corrosion resistance, and high creep resist-ance ll at elevated temperatures) had not been developed.6.32. Assume that you are the technical sales manager of acompany that produces nonferrous metals. Choose any oneof the metals and alloys described in this chapter, and preparea brochure, including some illustrations, for use as sales liter-ature by your staff in their contact with potential customers.6.33. Give some applications for a) amorphous metals,b) precious metals, c) low-melting alloys, and d) nano-materials.6.34. Describe the advantages of making products withmultilayer materials. For example, aluminum bonded to thebottom of stainless-steel pots.)


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Polymers: Structure,General Properties,and Applications Polymers display a wide range of properties and have several advantages overmetallic materials, including low cost, good performance, and ease of manufac-turing; for these reasons, polymers continue to be among the most commonlyused materials. This chapter first describes the structure of polymers, the polymerizationprocess, crystallinity, and the glass-transition temperature. Mechanical properties and how they are affected by temperature and deforma-tion rate are then discussed. The chapter describes the two basic types of polymers: thermoplastics andthermosets. Thermoplastics allow a basic manufacturing process of heating them until theysoften or melt, and then shaping them into the desired product. The process for thermosets is to form the precursors to a desired shape andthen set it through polymerization or cross-linking between polymer chains. The chapter also describes the properties and uses of elastomers, or rubbers. The general properties, typical applications, advantages, and limitations ofpolymers are discussed throughout the chapter, with several specific examplesgiven.

1.l IntroductionThe word plastics first was used as a noun around 1909 and commonly is employedas a synonym for polymers, a term first used in 1866. Plastics are unique in that theyhave extremely large molecules macromolecules or giant molecules . Consumer andindustrial products made of plastics include food and beverage containers, packag-ing, signs, housewares, housings for computers and monitors, textiles clothing ,medical devices, foams, paints, safety shields, toys, appliances, lenses, gears, elec-tronic and electrical products, and automobile bodies and components.Because of their many unique and diverse properties, polymers increasingly havereplaced metallic components in applications such as automobiles, civilian and

7.I Introduction |7I7.2 The Structure of

Polymers |737.3 Thermoplastics |807.4 Thermosetting

Plastics |847.5 Additives in Plastics |847.6 General Properties

and Applications ofThermoplastics |857.7 General Propertiesand Applications ofThermosetting

Plastics |887.8 BiodegradablePlastics |907.9 ElastomersRubbers I9lEXAMPLES:7.I Dental and Medical

Bone Cement |777.2 Use of ElectricallyConducting Polymers

in RechargeableBatteries |83

7.3 Materials for aRefrigerator DoorLiner |89

l7l


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Chapter 7 Polymers: Structure. General Properties, and Applicationsmilitary aircraft, sporting goods, toys, appliances, and office equipment. These substi-tutions reflect the advantages of polymers in terms of the following characteristics:

Relatively lovv cost see Table 6.1) and ease of manufacture Corrosion resistance and resistance to chemicals Low electrical and thermal conductivity Lovv density High strength-to-Weight ratio particularly when reinforced) Noise reduction Wide choice of colors and transparencies Ease of manufacturing and complexity of design possibilities Other characteristics that may or may not be desirable depending on the applica-tion), such as low strength and stiffness Table 7.1), high coefficient of thermalexpansion, low useful-temperature range-up to about 350C-and lower di-mensional stability in service over a period of time.The Word plastic is from the Greek vvord plastilzos, meaning capable of beingmolded and shaped. Plastics can be formed, machined, cast, and joined into vari-ous shapes With relative ease. Minimal additional surface-finishing operations, if

any at all, are required; this characteristic provides an important advantage overmetals. Plastics are available commercially as film, sheet, plate, rods, and tubing ofvarious cross-sections.The earliest polymers were made of natural organic materials from animal andvegetable products; cellulose is the most common example. By means of variouschemical reactions, cellulose is modified into cellulose acetate, used in making sheetsTABLE 1.|Range of Mechanical Properties forVarious EngineeringPlastics at RoomTemperature

Youngsmodulus E) Elongation PoissonsMaterial UTS MPa) GPa) l l ratio, 1/Acrylonitrile-butadiene- 28-55 1.4-2.8 75-5 -styrene ABS)

ABS, reinforced 100 7.5 - 0.35Acetal 5 5-70 1.4-3.5 75-25 -Acetal, reinforced 135 10 - 0.35-0.40Acrylic 40-75 1.4-3.5 5 0-5 -Cellulosic 10-48 0.4-1.4 100-5 -Epoxy 35-140 3.5-17 10-1 -Epoxy, reinforced 70-1400 21-52 4-2 -Fluorocarbon 7-48 0.7-2 300-100 0.46-0.48Nylon 55-83 1.4-2.8 200-60 0.32-0.40Nylon, reinforced 70-210 2-10 10-1 -Phenolic 28-70 2.8-21 2-0 -Polycarbonate 55-70 2.5-3 125-10 0.38Polycarbonate, reinforced 110 6 6-4 -Polyester 55 2 300-5 0.38Polyester, reinforced 1 10-160 8.3-12 3-1 -Polyethylene 7-40 0.1-1.4 1000-15 0.46Polypropylene 20-35 0.7-1.2 500-10 _Polypropylene, reinforced 40-100 3.5-6 4-2 -Polystyrene 14-83 1.4-4 60-1 0.35Polyvinyl chloride 7-55 0.014-4 450-40 -


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Section 7.2 The Structure of Polymers

H ill s * ` i f if CPlasticizersStabilizers 3Colorants }Flame retardants ILubricants :

` i t'i i ` ` I Thermop|astics:Acrylics, ABS, nylons_ _ gg ' _ polycarbonates, polyethylenesKjfgr `ii Heat, PVSSUfe polyvinyl chloride etcmars Polymerren Catalyst


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|74 Chapter 7 Polymers: Structure, General Properties, and Applications' `l i'i ` ii ` i ` ` W `ii i ``f i i T P P ` 'i` feature that distinguishes them from mostMonomer Polymer repeating unit other organic chemical compositions.

H H H H Polymers are long-chain molecules that areCZC _C_C_ Polyethylene formed by polynierization_(that is, by the link-ing and cross-linking of different monomers).H H H H n A monomer is the basic building block of apolymer. The word mer (from the GreekH H H H 6, ,,rneros, meaning part ) indicates the smallestC=C -C-C- Polypropylene repetitive unit; the use of the term is similar toH CH H CH n that of unit cell in crystal structures of metals

3 3 (Section 1.3).H H H H The term polymer means many mers(or units), enerall repeated hundreds orC=C _C_C-_ POM/my' Cmonde (PVC) thousands of; times iln a chainlike structure.H (;| H C| n Most monomers are organic materials inwhich carbon atoms are joined in covalentH H H H (electron-sharing) bonds with other atomsC=C -C-C- Polystyrene (such as hydrogen, oxygen, nitrogen, fluorine,chlorine, silicon, and sulfur). An ethylene mol-H CGHS H CGHS n ecule (Fig. 7.2) is an example of a simple|:| |:| |:| |:| monomer consisting of carbon and hydrogenC=C _ C__C_ Plrgygexgafluoroethylene (PTFE) atoms-Fl Fl Fl Fl H 7.2.l Polymerization

Monomers can be linked into polymers inrepeating units to make longer and largerIGURE 7.2 Molecular structure of various polymers. These areexamples of the basic building blocks for plastics. molecules by a chemical process called apolymerization reaction. Polymerizationprocesses are complex; they will be described only briefly here. Although there are

several variations, two polymerization processes are important: condensation andaddition polymerization.In condensation polymerization (Fig. 7.3a), polymers are produced by theformation of bonds between two types of reacting mers. A characteristic of thisreaction is that reaction by-products (such as water) are condensed out (hence thename). This process is also known as step-growth or step-reaction polymerization, be-

cause the polymer molecule grows step-by-step until all of one reactant is consumed.In addition polymerization (also called chain-growth or chain-reaction poly-merization), bonding takes place without reaction by-products, as shown in Fig. 7.3b.

It is called chain reaction because of the high rate at which long molecules formsimultaneously, usually within a few seconds. This rate is much higher than that incondensation polymerization. In addition polymerization, an initiator is added toopen the double bond between two carbon atoms, which begins the linking processby adding many more monomers to a growing chain. For example, ethylenemonomers (Fig. 7.3b) link to produce the polymer polyethylene; other examples ofaddition-formed polymers are shown in Fig. 7.2.Molecular Weight. The sum of the molecular weights of the mers in a representa-tive chain is known as the molecular weight of the polymer. The higher the molecu-lar weight of a given polymer, the greater the average chain length. Most commercialpolymers have a molecular weight between 10,000 and 10,000,000. Because poly-merization is a random event, the polymer chains produced are not all of equallength, but the chain lengths produced fall into a traditional distribution curve. The


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Section 7.2 The Structure of Polymerso o H H

II Il \ /Ci C CHQ CH2 CHQ CHQ C Ci + /N CH2 CH2 CH2 CHQ CH2 CH2 N\Adipoyl chloride H Hexamethylene diamine H

if Ii T IC CHQ CH2 CHQ CHQ C N CH2 CH2 CHQ CH2 CH2 CHQ N + HClNylon 6,6 Condensate

(3)

H H H Hl_H HlH HHeat, pressure, I I I IC C > C C~rC C-1-C C Polyethylenein Catalyst ||;| l:||H H H | H H : H H rrl _____Mer(b)

FIGURE 1.3 Examples of polymerization. (a) Condensation polymerization of nylon 6,6 and(b) addition polymerization of polyethylene molecules from ethylene mers.molecular Weight of a polymer is determined on a statistical basis by averaging. Thespread of the molecular weights in a chain is referred to as the molecular weightdistribution (MWD). A polymers molecular weight and its MWD have a strong in-fluence on its properties. For example, the tensile and the impact strength, the resist-ance to cracking, and the viscosity (in the molten state) of the polymer all increasewith increasing molecular vveight (Fig. 7.4).Degree of Polymerization. It is convenient to express the size of a polymer chain interms of the degree of polymerization (DP), which is defined as the ratio ofthe molecular Weight of the polymer to the molecular Weight of the repeating :C0mmerCia|unit. For example, polyvinyl chloride (PVC) has a mer weight of 62.5 ; thus, I pO|yn-,ersthe DP of PVC with a molecular Weight of 50,000 is 50,000/62.5 = 800. In Iterms of polymer processing (Chapter 19), the higher the DP, the higher is the Tensile andpolymers viscosity, or its resistance to flow (Fig. 7.4). On the one hand, high Z, Impact ftrenglhviscosity adversely affects the ease of shaping and thus raises the overall cost Q Iof processing. On the other hand, high DP can result in stronger polymers. cg :Bonding. During polymerization, the monomers are linked together bycovalent bonds (Section 1.2), forming a polymer chain. Because of theirstrength, covalent bonds also are called primary bonds. The polymerchains are, in turn, held together by secondary bonds, such as van der Vlscoslly:Waals bonds, hydrogen bonds, and ionic bonds (Section 1.2). Secondary 104 107bonds are weaker than primary bonds by one to two orders of magnitude.In a given polymer, the increase in strength and viscosity vvith molecularWeight is due (in part) to the fact that the longer the polymer chain, thegreater is the energy needed to overcome the combined strength of thesecondary bonds. For example, ethylene polymers having DPs of 1, 6, 35,140, and 1350 at room temperature are, respectively, in the form of gas,liquid, grease, wax, and hard plastic.

Molecular weight, degreeof polymerization

FIGURE 7.4 Effect of molecularWeight and degree of polymerizationon the strength and viscosity ofpolymers.


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Chapter 7 Polymers: Structure, General Properties, and Applications

(a) Linear (b)Branched

e ar *if *T i ****. a*sg * si Q* ii fi; it

*I* s*iza*'S' 5' il ir,i* **i;;s*** *Hifi* *i f *a*,1* lif f in g i i GV iear i**1~a;**ii**Di 1* i** *itiiiilllii* z *U it in .BQ f*is:'** '~r;** #Ta

(c)Cross-linked (d) NetworkFIGURE 1.5 Schematic illustration of polymer chains. (a) Linear structure-thermoplasticssuch as acrylics, nylons, polyethylene, and polyvinyl chloride have linear structures.(b) Branched structure, such as in polyethylene. (c) Cross-linked structure-many rubbers,or elastomers, have this structure, and the vulcanization of rubber produces this structure(d) Network structure, which is basically highly cross-linked-examples are thermosettingplastics, such as epoxies and phenolics.Linear Polymers. The chainlike polymers shown in Fig. 7.2 are called linear poly-mers because of their sequential structure (Fig. 7.5a). However, a linear molecule isnot necessarily straight in shape. In addition to those shown in the figure, other lin-ear polymers are polyamides (nylon 6,6) and polyvinyl fluoride. Generally, a poly-mer consists of more than one type of structure; thus, a linear polymer may containsome branched and cross-linked chains. As a result of branching and cross-linking,the polymers properties are changed significantly.Branched Polymers. The properties of a polymer depend not only on the type ofmonomers, but also on their arrangement in the molecular structure. In branchedpolymers (Fig. 7.5b), side-branch chains are attached to the main chain during thesynthesis of the polymer. Branching interferes with the relative movement of themolecular chains. As a result, their resistance to deformation and stress cracking isincreased. The density of branched polymers is lower than that of linear-chainpolymers, because the branches interfere with the packing efficiency of polymerchains.The behavior of branched polymers can be compared to that of linear-chainpolymers by making an analogy with a pile of tree branches (branched polymers)and a bundle of straight logs (linear polymers). Note that it is more difficult to movea branch within the pile of branches than to move a log within its bundle. The three-dimensional entanglements of branches make movements more difficult, a phenom-enon akin to increased strength.Cross-linked Polymers. Generally three-dimensional in structure, cross-linkedpolymers have adjacent chains linked by covalent bonds (Fig. 7.5c). Polymers with across-linked structure are called thermosets or thermosetting plastics; examples areepoxies, phenolics, and silicones. Cross-linking has a major influence on the proper-ties of polymers (generally imparting hardness, strength, stiffness, brittleness, andbetter dimensional stability; see Fig. 7.6), as well as in the vulcanization of rubber(Section 7.9).


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Section 7.2 The Structure of Polymers |77GlassyA 100 crystalline A Increasing2 S2 Glassy cross-linkingS 3fl) (DE Leathery Increasing E j crystallinity jg Leathery3E l 5s 4, s Fiubbery /bo/` E Flubbery1: ,O B33 600 3 /l/O0E 0 m /_OSSViscious Visfous W/>,(7h9

rg Tm rmTemperature Temperature

(8) (blFIGURE 1.6 Behavior of polymers as a function of temperature and (a) degree ofcrystallinity and (b) cross-linking. The combined elastic and viscous behavior of polymers isknown as viscoelasticity.Network Polymers. These polymers consist of spatial (three-dimensional) net-works of three or more active covalent bonds (Fig. 7.5d). A highly cross-linked poly-mer also is considered a network polymer. Thermoplastic polymers that alreadyhave been formed or shaped can be cross-linked to obtain higher strength by sub-jecting them to high-energy radiation, such as ultraviolet light, X-rays, or electronbeams. However, excessive radiation can cause degradation of the polymer.Copolymers and Terpolymers. If the repeating units in a polymer chain are all ofthe same type, the molecule is called a homopolymer. However, as with solid-solutionmetal alloys (Section 42), two or three different types of monomers can be combinedto develop certain special properties and characteristics, such as improved strength,toughness, and formability of the polymer. Copolymers contain two types of poly-mers (for example, styrene-butadiene, which is used widely for automobile tires).Terpolymers contain three types (for example, acrylonitrile-butadiene-styrene (ABS),which is used for helmets, telephones, and refrigerator liners).EXAMPLE 7.l Dental and Medical Bone CementPolymethylrnethacrylate (PMMA) is an acrylic poly-mer commonly used in dental and medical applica-tions as an adhesive and is often referred to as bonecement. There are a number of forms of PMMA, butthis example describes one common form involvingan addition-polymerization reaction. PMMA is deliv-ered in two parts: a powder and a liquid, which aremixed by hand. The liquid wets and partially dissolvesthe powder, resulting in a liquid with viscosity on theorder of 0.1 Ns/mz, similar to that of vegetable oil.The viscosity increases markedly until a doughystate is reached in about five minutes. The doughfully hardens in an additional five minutes.

The powder consists of high-molecular-weightpoly[(methylmethacrylate)-costyrene] particles about50 /.tm in diameter and containing a small volumefraction of benzoyl peroxide. The liquid consists of amethyl methacrylate (MMA) monomer, with a smallamount of dissolved n,n dimethyl-p-toluidine(DMPT). When the liquid and powder are mixed, theMMA wets the particles (dissolving a surface layer ofthe PMMA particles) and the DMPT cleaves thebenzoyl peroxide molecule into two parts to form acatalyst with a free electron (sometimes referred to as afree radical). This catalyst causes rapid growth ofPMMA from the MMA mers, so that the final


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|78 Chapter 7 Polymers: Structure, General Properties, and Applications

material is a composite of high-molecular-weight An illustration of fully set bone cement is shown inPMMA particles interconnected by PMMA chains. Fig. 7.7.

9 |` PMMAparticle, Polymerized MMA

9 matrixQ IQ 0 l @--lMMAdissoIved e in monomero 6 . e- 9 5FIGURE 1.7 Schematic illustration of the microstructure ofpolymethylmethacrylate cement used in dental and medicalapplications.

7.2.2 CrystallinityPolymers such as PMMA, polycarbonate, and polystyrene are generally amorphous;that is, the polymer chains exist without long-range order (see also amorphous

AmorphousL regionf.%'lXY* x.Crystallineregion' T t * Q * Q

ai1 * g

Q Qc Qs 2, s as* s* ann * fp e 'Q , i'io Q* ** ' * 'Q'geggtfjs Q 1 *,ti, Qn - QW* 4 s-;. ug e_5** *a *)'G=* is . at 4,

1 * an 9 ' eQ - s is g if ,7* g as as 4 in in_G 'QQ , Q* 1 Q

* ,,,s~i= as E1 isgia ssia _ QS`n|;al=ns*,ss. 1 xl ,ing-;l**gg* nx=x11=** *

C G Q|*,,.;a *01. a z Q 5Q I

FIGURE 1.8 Amorphous and crystalline regionsin a polymer. The crystalline region (crystallite) hasan orderly arrangement of molecules. The higherthe crystallinity, the harder, stiffer, and less ductilethe polymer.

alloys, Section 6.14). The amorphous arrangement of polymerchains often is described as being like a bowl of spaghetti orlike worms in a bucket, all intertwined with each other. In somepolymers, however, it is possible to impart some crystallinityand thereby modify their characteristics. This arrangement maybe fostered either during the synthesis of the polymer or by de-formation during its subsequent processing.

The crystalline regions in polymers are called crystallites(Fig. 7.8). These crystals are formed when the long moleculesarrange themselves in an orderly manner, similar to the folding ofa fire hose in a cabinet or of facial tissues in a box. A partiallycrystalline (semicrystalline) polymer can be regarded as a two-phase material, one phase being crystalline and the otheramorphous.

By controlling the fatC of solidification during cooling andthe chain structure, it is possible to impart different degrees ofcrystallinity to polymers, although never 100%. Crystallinityranges from an almost complete crystal (up to about 95% byvolume in the case of polyethylene) to slightly crystallized


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Section 7.2 The Structure of Polymers I 79(mostly amorphous) polymers. The degree of crystallinity also is affected by branch-ing. A linear polymer can become highly crystalline, but a highly branched polymercannot, although it may develop some low level of crystallinity. It will never achievea high crystallite content, because the branches interfere with the alignment of thechains into a regular crystal array.Effects of Crystallinity. The mechanical and physical properties of polymers aregreatly influenced by the degree of crystallinity: as crystallinity increases, polymersbecome stiffer, harder, less ductile, more dense, less rubbery, and more resistant tosolvents and heat (Fig. 7.6). The increase in density with increasing crystallinity iscalled crystallization shrinkage and is caused by a more efficient packing of the mol-ecules in the crystal lattice. For example, the highly crystalline form of polyethylene,known as high-density polyethylene (HDPE), has a specific gravity in the range of0.941 to 0.970 (80 to 95% crystalline). It is stronger, stiffer, tougher, and less ductilethan low-density polyethylene (LDPE), which is about 60 to 70% crystalline andhas a specific gravity of about 0.910 to 0.925.

Optical properties of polymers also are affected by the degree of crystallinity.The reflection of light from the boundaries between the crystalline and the amor-phous regions in the polymer causes opaqueness. Furthermore, because the index ofrefraction is proportional to density, the greater the density difference between theamorphous and crystalline phases, the greater is the opaqueness of the polymer.Polymers that are completely amorphous can be transparent, such as polycarbonateand acrylics.7.2.3 Glass-transition TemperatureAlthough amorphous polymers do not have a specific meltingpoint, they undergo a distinct change in their mechanical behav-ior across a narrow range of temperatures. At low tem-peratures, they are hard, rigid, brittle, and glassy; at hightemperatures, they are rubbery or leathery. The temperature at (D Amorphouswhich a transition occurs is called the glass-transition tempera- 5 polymersture (Tg), also called the glass point or glass temperature. The term glass is used in this description because glasses, which Goomg-_ ;_ 5f';?;||ineare amorphous solids, behave in the same manner. (See metallic E; ,aw-_\ ____ posiymersglasses, Section 6.14.) Although most amorphous polymers cg- _,_-- 'exhibit this behavior, an exception is polycarbonate, which is w /neither rigid nor brittle below its glass-transition temperature.Polycarbonate is tough at ambient temperatures and is used forsafety helmets and shields.

To determine Tg, a plot of the specific volume of the poly-mer as a function of temperature is produced; Tg occurs where '-lg Q-mthere is a sharp change in the slope of the curve (Fig. 7.9). In thecase of highly cross-linked polymers, the slope of the curvechanges gradually near Tg; hence, it can be difficult to deter-mine Tg for these polymers. The glass-transition temperaturevaries with different polymers (Table 7.2) and can be above orbelow room temperature. Unlike amorphous polymers, partlycrystalline polymers have a distinct melting point, Tm (Fig. 7.9;see also Table 7.2). Because of the structural changes (first-order changes) that occur, the specific volume of the polymerdrops suddenly as its temperature is reduced.

mperatureFIGURE 7.9 Specific volume of polymers as afunction of temperature. Amorphous polymers,such as acrylic and polycarbonate, have a glass-transition temperature, Tg, but do not have aspecific melting point, Tm. Partly crystallinepolymers, such as polyethylene and nylons, contractsharply while passing through their meltingtemperatures during cooling.


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Chapter 7 Polymers: Structure, General Properties, and ApplicationsTABLE 7.2Glass-transition and Melting Temperaturesnf Some PolymersMaterial Tg C) Tm C)Nylon 6,6 57 265Polycarbonate 150 265Polyester 73 265PolyethyleneHigh density -90 137Low density -110 115

Polymethylmethacrylate 105 -Polypropylene - 14 1 76Polystyrene 100 239Polytetrafluoroethylene -90 327Polyvinyl chloride 87 212Rubber f 73 -7.2.4 Polymer BlendsThe brittle behavior of amorphous polymers below their glass-transition tempera-ture can be reduced by blending them, usually with small quantities of an elastomerection 7.9). The tiny particles that make up the elastomer are dispersed through-out the amorphous polymer, enhancing its toughness and impact strength byimproving its resistance to crack propagation. These polymer blends are known asrubber-modified polymers.Advances in blending involve several components, creating polyblends thatutilize the favorable properties of different polymers. Miscible blends mixing with-out separation of two phases) are created by a process similar to the alloying ofmetals that enables polymer blends to become more ductile. Polymer blends accountfor about 20 of all polymer production.

7.3 ThermoplasticsIt was noted earlier that within each molecule, the bonds between adjacent long-chain molecules secondary bonds) are much weaker than the covalent bondsbetween mers primary bonds). It is the strength of the secondary bonds that deter-mines the overall strength of the polymer; linear and branched polymers have weaksecondary bonds.

As the temperature is raised above the glass-transition temperature, Tg, ormelting point, Tm, certain polymers become easier to form or mold into desiredshapes. The increased temperature weakens the secondary bonds hrough thermalvibration of the long molecules), and the adjacent chains can then move more easilywhen subjected to external shaping forces.When the polymer is cooled, it returns toits original hardness and strength; in other words, the process is reversible. Polymersthat exhibit this behavior are known as thermoplastics, common examples of whichare acrylics, cellulosics, nylons, polyethylenes, and polyvinyl chloride.The behavior of thermoplastics depends on other variables as well as theirstructure and composition. Among the most important are temperature and rateof deformation. Below the glass-transition temperature, most polymers are glassybrittle) and behave like an elastic solid. That is, the relationship between stress and


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strain is linear, as shown in Fig. 2.2.) The particular behaviordepends on the polymer. For example, PMMA is glassy below

Section Thermoplastics I 8 I

Tough and ductileABS, nylon)

Soft and flexiblepolyethylene, PTFE )

its Tg, whereas polycarbonate is not glassy below its T8: Rigid andWhen the applied stress is increased further, polycarbonate brittleeventually fractures, just as a piece of glass does at ambient melamine.temperature. U, phenohc)Typical stress-strain curves for some thermoplastics and thermosets at room temperature are shown in Fig. 7.10. Note 5that these plastics exhibit various behaviors, which may be de-scribed as rigid, soft, brittle, flexible, and so on. The mechani-cal properties of several polymers listed in Table 7.1 indicatethat thermoplastics are about two orders of magnitude less stiffthan metals. Their ultimate tensile strength is about one order l/of magnitude lower than that of metals see Table 2.2).Effects of Temperature. If the temperature of a thermo-plastic polymer is raised above its Tg, it first becomes lent/veryand then, with increasing temperature, rubbery Fig. 7.6).Finally, at higher temperatures e.g., above Tm for crystallinethermoplastics), it becomes a viscous fluid, and its viscositydecreases with increasing temperature. As a viscous fluid, itcan be softened, molded into shapes, resolidified, remelted,and remolded a number of times. In practice, however, re-

Strain

FIGURE 1.I0 General terminology describing thebehavior of three types of plastics. PTFE poly-tetrafluoroethylene) has Teflon as its trade name.Source: After R.L.E. Brown.

-25C60 I70 f 00peated heating and cooling causes degradation, or thermal Q; 50aging, of thermoplastics. 250The typical effect of temperature on the strength and 5 40elastic modulus of thermoplastics is similar to that of metals: Q 30With increasing temperature, the strength and the modulus of U3 A 5020 65.elasticity decrease and the ductility increases Fig. 7.11). The -\ 7800effect of temperature on impact strength is shown in 10Fig. 7.12; note the large difference in the impact behaviors of 0

O 5 1 O 1 5 20 25 30various polymers.Effect of Rate of Deformation. When deformed rapidly, thebehavior of thermoplastics is similar to metals, aswas indicatedby the strain-rate sensitivity exponent nfz in Eq. 2.9).Thermoplastics in general have high 771 values, indicating thatthey can undergo large uniform deformation in tension beforefracture. Note in Fig. 7.13 how unlike in ordinary metals) thenecked region elongates considerably. This phenomenon can be

Strain )FIGURE 1.l| Effect of temperature on the stress-strain curve for cellulose acetate, a thermoplastic.Note the large drop in strength and the large increasein ductility with a relatively small increase intemperature. Source: After T.S. Carswell and H.K.Nason.

easily demonstrated by stretching a piece of the plastic holder for a 6-pack of beveragecans. Observe the sequence of necking and stretching behavior shown in Fig. 7.13a.This characteristic which is the same in the superplastic metals, Section 2.2.7) enablesthe thermoforming of thermoplastics ection 19.6) into such complex shapes as meattrays, lighted signs, and bottles for soft drinks.Orientation. When thermoplastics are deformed say, by stretching), the long-chainmolecules tend to align in the general direction of the elongation; this process is calledorientation. As in metals, the polymer becomes anisotropic see also Section 1.6), sothe specimen becomes stronger and stiffer in the elongated stretched) direction thanin its transverse direction. Stretching is an important technique for enhancing thestrength and toughness of polymers and is especially exploited in producing high-strength fibers for composite materials, as discussed in Chapter 9.


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I 82 Chapter 7 Polymers: Structure, General Properties, and ApplicationsCreep and Stress Relaxation. Because of their viscoelastic behav-

|_0W-denSity High-impact ior, thermoplastics are particularly susceptible to creep and stresspolyethylene polypropylene relaxation, and to a larger extent than metals. The extent of thesephenomena depends on the polymer, stress level, temperature, and-.gy time. Thermoplastics exhibit creep and stress relaxatio

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