Introduction to Dislocations

Abstract

This chapter deals with dislocation arrays and boundaries in crystals. Plastic deformation of crystalline materials leads to the formation of three-dimensional arrays or distributions of dislocations that are characteristic of crystal structure of the material being deformed, temperature of deformation, strain, and strain rate. Additionally, such features as grain boundaries, precipitates, and stacking fault energy affect the distribution of the dislocations. The hardening of crystals during plastic deformation is due to the increase in dislocation density and the mutual interaction between dislocations. The simplest boundary is the symmetrical tilt boundary. Line defects in interfaces between crystals can play an important role in transformations involving the growth of one crystal at the expense of another, e.g., twinning, martensitic transformations, and precipitation. In the simplest picture, the defect is akin to a step and its motion over the interface is the mechanism that transforms atoms of one crystal to sites of the other. For the movement of a boundary containing interfacial dislocations, several conditions must be satisfied. First, if slip is involved the dislocation must be free to glide on its slip plane, i.e., the plane that contains the line and its Burgers vector. Second, matter must be conserved if a diffusional flux of atoms is required for motion of interfacial defects. Third, the thermodynamic condition that movement results in either a reduction in the energy of the boundary or, in the case of movement induced by an externally applied stress, that the stress does work when the boundary moves. Fourth, the driving force due, for example, to the externally applied stress or the excess vacancy concentration must be sufficient to produce dislocation movement.