Chapter 8 - Origin and Multiplication of Dislocations

Abstract

Listen

Translate article

When annealed crystals are deformed there is a rapid multiplication of dislocations and a progressive increase in dislocation density with increasing strain. After large amounts of plastic deformation the dislocation density is typically in the range 1014 to 1015 m–2. In the early stages of deformation dislocation movement tends to be confined to a single set of parallel slip planes. Later, slip occurs on other slip systems and dislocations moving on different systems interact. The rapid multiplication leads to work hardening. The origin of dislocations and main mechanisms of dislocation multiplication are described in this chapter. There are two main sources of dislocations in freshly grown crystals. First, dislocations or other defects may be present in the “seed” crystals or other surfaces used to initiate the growth of the crystal. Second, “accidental” nucleation may occur during the growth. When a dislocation is created in a region of the crystal that is free from any defects the nucleation is referred to as homogeneous. This occurs only under extreme conditions because a very large stress is required. To account for the large plastic strain that can be produced in crystals, it is necessary to have regenerative multiplication of dislocations. Two mechanisms are important: Frank-Read type sources and multiple cross glide.