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CHAPTER 1

The Structure of Metals

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Chapter 1 Outline

Figure 1.1 An outline of the topics described in Chapter 1

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Body-Centered Cubic Crystal Structure

Figure 1.2 The body-centered cubic (bcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) singlecrystal with many unit cells. Source: W. G. Moffatt, et al., The Structure and Properties of Materials, Vol. 1,John Wiley & Sons, 1976.

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Face-Centered Cubic Crystal Structure

Figure 1.3 The face-centered cubic (fcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) singlecrystal with many unit cells. Source: W. G. Moffatt, et al., The Structure and Properties of Materials, Vol. 1,John Wiley & Sons, 1976.

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Hexagonal Close-Packed Crystal Structure

Figure 1.4 The hexagonal close-packed (hcp) crystal structure:(a) unit cell; and (b) singlecrystal with many unit cells.Source: W. G. Moffatt, et al., TheStructure and Properties ofMaterials, Vol. 1, John Wiley &Sons, 1976.

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Slip and Twinning

Figure 1.5 Permanent deformation (alsocalled plastic deformation) of a singlecrystal subjected to a shear stress: (a)structure before deformation; and (b)permanent deformation by slip. The sizeof the b/a ratio influences the magnitudeof the shear stress required to cause slip.

Figure 1.6 (a) Permanent deformation of a singlecrystal under a tensile load. Note that the slip planestend to align themselves in the direction of the pullingforce. This behavior can be simulated using a deck ofcards with a rubber band around them. (b) Twinningin a single crystal in tension.

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Slip Lines and Slip Bands

Figure 1.7 Schematic illustration of slip linesand slip bands in a single crystal (grain)subjected to a shear stress. A slip band consistsof a number of slip planes. The crystal at thecenter of the upper illustration is an individualgrain surrounded by other grains.

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Edge and Screw Dislocations

Figure 1.8 Types of dislocations in a single crystal: (a) edge dislocation; and (b) screw dislocation.Source: (a) After Guy and Hren, Elements of Physical Metallurgy, 1974. (b) L. Van Vlack, Materials forEngineering, 4th ed., 1980.

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Defects in a Single-Crystal Lattice

Figure 1.9 Schematic illustration of types of defects in a single-crystal lattice: self-interstitial, vacancy, interstitial, and substitutional.

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Movement of an Edge Dislocation

Figure 1.10 Movement of an edge dislocation across the crystal lattice under a shear stress.Dislocations help explain why the actual strength of metals in much lower than that predicted bytheory.

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SolidificationFigure 1.11 Schematicillustration of the stagesduring solidification ofmolten metal; each smallsquare represents a unit cell.(a) Nucleation of crystals atrandom sites in the moltenmetal; note that thecrystallographic orientationof each site is different. (b)and (c) Growth of crystals assolidification continues. (d)Solidified metal, showingindividual grains and grainboundaries; note the differentangles at which neighboringgrains meet each other.Source: W. Rosenhain.

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Grain Sizes

TABLE 1.1ASTM No. Grains/mm2 Grains/mm3

–3–2–10123456789101112

1248163264128256512

1,0242,0484,0968,200

16,40032,800

0.72

5.61645128360

1,0202,9008,20023,00065,000

185,000520,000

1,500,0004,200,000

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Preferred Orientation

Figure 1.12 Plastic deformation ofidealized (equiaxed) grains in aspecimen subjected to compression(such as occurs in the rolling or forgingof metals): (a) before deformation; and(b) after deformation. Note htealignment of grain boundaries along ahorizontal direction; this effect isknown as preferred orientation.

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Anisotropy

Figure 1.13 (a) Schematic illustration of a crack in sheet metal that has been subjected to bulging(caused by, for example, pushing a steel ball against the sheet). Note the orientation of the crack withrespect to the rolling direction of the sheet; this sheet is anisotropic. (b) Aluminum sheet with a crack(vertical dark line at the center) developed in a bulge test; the rolling direction of the sheet was vertical.Source: J.S. Kallend, Illinois Institute of Technology.

(b)

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Annealing

Figure 1.14 Schematic illustration of theeffects of recovery, recrystallization, andgrain growth on mechanical propertiesand on the shape and size of grains. Notethe formation of small new grains duringrecrystallization. Source: G. Sachs.

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Homologous Temperature Ranges for VariousProcesses

TABLE 1.2Process T/TmCold workingWarm workingHot working

< 0.30.3 to 0.5> 0.6