Dislocation Theory

When one first studies crystalline solids, great emphasis is placed on the striking regularity and perfection of the atomic arrangement therein, a perfection extending in all directions. This is a good starting point, but in studying physical properties such as strength or transformation rates, we soon learn that the rare, local deviations from perfection are just as important. The simplest type of such defects occurs at a point in the lattice_ For example, if one lattice site is unoccupied we say the site is vacant and call the defect a vacancy. If there is one more atom than lattice sites in a region, the resulting point defect is called an interstitial (see Fig. I-I). [n the next chapter we will study diffusion and find that the easiest ways to move atoms in a metal crystal is by means of these point defects. A second common class of defects extends along a line in the crystal.These are called dislocations. Their existence was hypothesized in the early 1930s to explain the fact that metal single crystals shear at loads thousands of times smaller than would be calculated for an absolutely perfect crystal. In the intervening years, dislocations have proved to be of value in explaining a wide range of phenomena connected not only with plastic deformation, but with transformations, grain boundaries, diffusion, etc. In the last decade, various microscopic techniques have been developed which allow the study of the shape, density, and motion of individual dislocations, so they have long since passed from the realm of hypothesis to that of established fact.