Recrystallization of metals during hot deformation

Abstract

Recovery processes tend to counteract the effects of work hardening during plastic deformation at high temperatures and at strain rates ranging from those of slow creep to those of rapid hot working operations. However, in metals in which recovery is relatively slow, sufficient stored energy can be accumulated to cause the occurrence of dynamic recrystallization during deformation once a critical strain is exceeded. This process then occurs repeatedly with continued straining. If any metal that has been deformed at high temperatures by a dislocation mechanism is held at temperature after deformation, static recrystallization tends to occur with time. The effects of dynamic and static recrystallization on microstructure and on the flow stress or creep rate of the metal are considered in this paper and particular attention is given to the range of deformation conditions under which these recrystallization processes are expected to occur. When metals deform plastically by crystal slip at elevated temperatures, the work hardening produced by deformation tends to be counteracted by recovery processes. These recovery processes cause rearrangement and annihilation of dislocations so that, as strain increases, the dislocations tend to form into two dimensional subgrain walls. In some metals and alloys the recovery entirely balances work hardening, and steady state is achieved and can be maintained to large strains before fracture occurs. In other metals in which recovery is less rapid, certain conditions of stress and temperature of deformation can result in the accumulation of sufficiently high local differences in dislocation density to nucleate recrystallization during deformation. This recrystallization is referred to as dynamic recrystallization to distinguish it from the recrystallization that can occur in all metals when deformation is discontinued but the elevated temperature is maintained, or when deformation is carried out at low temperature and the metal is subsequently annealed. In this paper, both types of recrystallization and their effects on deformation behaviour and microstructure will be outlined and the range of deformation conditions under which they are likely to occur will be considered.