Page 56 - Mechanical Behavior of Materials

Section 2.5 Inelastic Deformation 57

Figure 2.20 Slip caused by the motion of an edge dislocation.

Figure 2.21 Slip caused by the motion of a screw dislocation. (From [Felbeck 96] p. 114;
c 1996 by Prentice Hall, Upper Saddle River, NJ; reprinted with permission.)

For a given crystal structure, such as BCC, FCC, or HCP, slip is easier on certain planes,
and within these planes in certain directions. For metals, the most common planes and directions
are shown in Fig. 2.23. The preferred planes are those on which the atoms are relatively close
together, called close-packed planes, such as the basal plane for the HCP crystal. Similarly, the
preferred slip directions within a given plane are the close-packed directions in which the distances
between atoms is smallest. This is the case because a dislocation can more easily move if the
distance to the next atom is smaller. Also, atoms in adjacent planes project less into the spaces
between atoms in the close-packed planes than in other planes, so there is less interference with slip
displacement.