GTBL042-08 GTBL042-Callister-v3 October 4, 2007 11:51
2nd Revised Pages254 • Chapter 8 / Deformation and Strengthening MechanismsFigure 8.10 Slip lines on the surface
of a polycrystalline specimen of copper
that was polished and subsequently
deformed. 173×. [Photomicrograph
courtesy of C. Brady, National Bureau
of Standards (now the National
Institute of Standards and Technology,
Gaithersburg, MD).]orientation (i.e., the highest shear stress). This is exemplified by a photomicrograph
of a polycrystalline copper specimen that has been plastically deformed (Figure
8.10); before deformation the surface was polished. Slip lines^1 are visible, and it
appears that two slip systems operated for most of the grains, as evidenced by two
sets of parallel yet intersecting sets of lines. Furthermore, variation in grain orien-
tation is indicated by the difference in alignment of the slip lines for the several
grains.
Gross plastic deformation of a polycrystalline specimen corresponds to the com-
parable distortion of the individual grains by means of slip. During deformation,
mechanical integrity and coherency are maintained along the grain boundaries; that
is, the grain boundaries usually do not come apart or open up. As a consequence,(^1) These slip lines are microscopic ledges produced by dislocations (Figure 8.1c) that have
exited from a grain and appear as lines when viewed with a microscope. They are analogous
to the macroscopic steps found on the surfaces of deformed single crystals (Figures 8.8 and
8.9).