Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

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2nd Revised Pages

8.3 Basic Concepts of Dislocations • 245

Direction

of motion

Direction

of motion









(a)

(b)

Figure 8.2 The formation of a step on the surface of a crystal by the motion of (a) an edge

dislocation and (b) a screw dislocation. Note that for an edge, the dislocation line moves

in the direction of the applied shear stressτ; for a screw, the dislocation line motion is

perpendicular to the stress direction. (Adapted from H. W. Hayden, W. G. Moffatt, and

J. Wulff,The Structure and Properties of Materials,Vol. III,Mechanical Behavior,p. 70.

Copyright©c1965 by John Wiley & Sons, New York. Reprinted by permission of John Wiley

& Sons, Inc.)

dislocation through some particular region of the crystal, the atomic arrangement

is ordered and perfect; it is only during the passage of the extra half-plane that the

lattice structure is disrupted. Ultimately this extra half-plane may emerge from the

right surface of the crystal, forming an edge that is one atomic distance wide; this is

shown in Figure 8.1c.

The process by which plastic deformation is produced by dislocation motion is

slip termedslip;the crystallographic plane along which the dislocation line traverses is

theslip plane,as indicated in Figure 8.1. Macroscopic plastic deformation simply cor-

responds to permanent deformation that results from the movement of dislocations,

or slip, in response to an applied shear stress, as represented in Figure 8.2a.

Dislocation motion is analogous to the mode of locomotion employed by a cater-

pillar (Figure 8.3). The caterpillar forms a hump near its posterior end by pulling in

Figure 8.3 Representation of the analogy between caterpillar and dislocation motion.