The Deformed State

This chapter addresses the features of the plastically deformed state which dominate the process of recrystallization from the perspective of both nucleation and growth. Strain hardening is reviewed as defining the accumulation of dislocation density, which provides the driving force for recrystallization. The heterogeneities that appear during deformation such as orientation gradients, slip bands, transitions bands, and twinning provide a source of new grains (nuclei). At the scale of the dislocation substructure, fluctuations in dislocation density strengthen with strain into a cell structure and, at high temperature, subgrain boundaries. The characteristic accumulation of dislocations of one sign results in misorientation increase across cell walls, which results in a distinction between statistically stored versus geometrically necessary dislocations. The effect of particles on deformation heterogeneity is described. Coarse particles above about one micron promote local heterogeneity, which leads to accelerated nucleation, known as particle stimulated nucleation. Fine particles tend to stabilize dislocation networks and minimize shear banding, for example. Mechanical twinning is important in lower symmetry materials, most notably unalloyed hexagonal metals, and typically provides significant interfacial strengthening.