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Materials Science and Engineering: An Introduction

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Unformatted text preview: 2nd REVISE PAGES EQA 180 - Chapter 7 1' Dislocations and Strengthening Mechanisms Figure 1.6 (a) A {111] (110) slip system shown within an FCC unit cell. (b)1he (111) plane from (a) and three (110) slip directions (as indicated by arrows) within that plane comprise possible slip (a) (1:) systems. Slip occurs along (110)-type directions within the {111} planes, as indicated by arrows in Figure 7.6. Hence, {111K110} represents the slip plane and direction com- bination, or the slip system for FCC. Figure 7.6!) demonstrates that a given slip plane may contain more than a single slip direction. Thus, several slip systems may exist for a particular crystal structure: the number of independent slip systems represents the different possible combinations of slip planes and directions. For example, for face-centered cubic, there are 12 slip systems: four unique {111] planes and, within each plane, three independent (110) directions. The possible slip systems for BCC and HCP crystal structures are listed in Table 7.1. For each of these structures, slip is possible on more than one family of planes (e.g., {110}, [211}, and {321] for BCC). For metals having these two crystal struc- tures. some slip systems are often operable only at elevated temperatures. Metals with FCC or BCC crystal structures have a relatively large number of slip systems (at least 12). These metals are quite ductile because extensive plastic deformation is normally possible along the various systems. Conversely, HCP met- als, having few active slip systems, are normally quite brittle. The Burgers vector concept was introduced in Section 4.5, and denoted by a I) for edge, screw, and mixed dislocations in Figures 4.3, 4.4, and 4.5, respectively. With regard to the process of slip, a Burgers vector’s direction corresponds to a disloca- tion’s slip direction, whereas its magnitude is equal to the unit slip distance (or interatomic separation in this direction). Of course, both the direction and the mag- nitude of b will depend on crystal structure, and it is convenient to specify a Burg- ers vector in terms of unit cell edge length (a) and crystallographic direction indices. Table 7.1 Slip Systems for Face-Centered Cubic, Body-Centered Cubic, and Hexagonal Close-Packed Metals Number of Metals Slip Plane Slip Direction Slip Systems Face-Centered Cubic _ Cu, Al, Ni, Ag, Au {111} (110) 12 Body-Centered Cubic a—Fe, W, Mo {110} 111 12 a-Fe, w {211} 111 12 cr—Fe, K {321} 111 24 Hexagonal Close-Packed 7 Cd, Zn, Mg,Ti, Be {OOQI} llgf) 3 Ti, Mg, Zr {1010} llgfl 3 Ti, Mg {1011} 1120 6 ...
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