Callister - An introduction - 8th edition

206 • Chapter 7 / Dislocations and Strengthening Mechanisms
Direction
of force
Figure 7.8
Macroscopic slip in a single crystal.
Slip plane
parallel to one another and loop around the circumference of the specimen as indicated
in Figure 7.8. Each step results from the movement of a large number of
dislocations along the same slip plane. On the surface of a polished single-crystal
specimen, these steps appear as lines, which are called slip lines. A zinc single crystal
that has been plastically deformed to the degree that these slip markings are
discernible is shown in Figure 7.9.
With continued extension of a single crystal, both the number of slip lines and
the slip step width will increase. For FCC and BCC metals, slip may eventually begin
along a second slip system, the system that is next most favorably oriented with
the tensile axis. Furthermore, for HCP crystals having few slip systems, if the stress
axis for the most favorable slip system is either perpendicular to the slip direction
( 90) or parallel to the slip plane ( 90), the critical resolved shear stress
will be zero. For these extreme orientations the crystal ordinarily fractures rather
than deforming plastically.
Figure 7.9 Slip in a zinc single crystal. (From
C. F. Elam, The Distortion of Metal Crystals,
Oxford University Press, London, 1935.)