7 - Recrystallization

Abstract

Dislocation hardening is accompanied by dynamic recovery. Dynamic recovery is not the only (dynamic) restoration mechanism that may occur with dislocation hardening. Recrystallization can also occur and this process can also “restore” the metal and reduce the flow stress. Often, recrystallization during deformation is observed at relatively high strain-rates which is outside the common creep realm. However, any complete discussion of elevated temperature creep, and particularly, a discussion of high-temperature plasticity must include this restoration mechanism. An understanding of the hotworking requires an appreciation of both dynamic recovery and recrystallization processes. During deformation, energy is stored in the material mainly in the form of dislocations. This energy is released in three main processes, those of recovery, recrystallization, and grain coarsening. The usual definition of recrystallization is the formation and migration of high-angle boundaries, driven by the stored energy of deformation. The definition of recovery includes all processes releasing stored energy that do not require the movement of high-angle boundaries. Typical recovery processes involve the rearrangement of dislocations to lower their energy, and annihilation of dislocation line length in the subgrain interior. Grain coarsening is the growth of the mean grain size driven by the reduction in grain-boundary area. Recrystallization can occur under two broad conditions; static and dynamic. Basically, static occurs in the absence of external plasticity during the recrystallization. The most common case for static is heating cold-worked metal leading to a recrystallized microstructure.