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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 of
Materials, 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 for
Engineering, 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 by
theory.
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Solidification
Figure 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 the
crystallographic orientationof each site is different. (b)and (c) Growth of crystals assolidification continues. (d)Solidified metal, showingindividual grains and grain
boundaries; note the differentangles at which neighboringgrains meet each other.
Source: W. Rosenhain.
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Grain Sizes
TABLE 1.1
ASTM No. Grains/mm2
Grains/mm3
3
2
1
0
1
2
3
4
5
6
78
9
10
11
12
1
2
4
8
16
32
64
128
256
512
1,0242,048
4,096
8,200
16,400
32,800
0.7
2
5.6
16
45
128
360
1,020
2,900
8,200
23,00065,000
185,000
520,000
1,500,000
4,200,000
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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 1.2
Process T/Tm
Cold workingWarm working
Hot working
< 0.30.3 to 0.5
> 0.6