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Introduction
Dislocations are important defects because they influence rate at which crystals grow, as well as the mechanical, electrical, and optical properties of crystals. The two limiting cases for dislocations (edge and screw) are shown in Fig. 1. While a small population of dislocations normally occurs in all crystals, the population can be greatly enhanced by plastic deformation. Figure 1. The atomic structure of the (a) edge dislocation and the (b) screw dislocation. In (b) the smaller filled circles are in the same plane (above the paper) and the larger open circles are in a different plane (below the paper). Because these defects have dimensions on the atomic scale, direct observation is difficult. However, it is possible to locate dislocations by forming visible etch pits at the point where dislocations intersect the crystal surface. Using this method, we will examine the arrangement of dislocations in as received crystals, in crystals after plastic deformation, and in the deformed crystals after annealing. This exercise is a recreation of the classic experiment originally described by C.L. Bauer. A complete description can be found in: C.L. Bauer, "Polygonization of Rock Salt", Trans. Metall. Soc. of AIME 223 [4] (1965) 846-847.
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