The rate-controlling deformation mechanisms in superplasticity—a critical assessment

Abstract

The phenomenon of micrograin superplasticity is critically reviewed in the context of the dominant microstructural characteristics,i.e., grain boundary sliding and migration, grain rotation and rearrangement, and dislocation activity. Existing theoretical models consider the accommodation process for grain boundary sliding to be either purely diffusional or due to dislocation motion. The latter can be in the form of individual dislocations or dislocations in the pile-up arrays in the interior of the grains or in grain interfaces. The mechanical properties,i.e., stress, strain-rate, activation energy, threshold stress, and so forth, are compared with the prediction of these models. The extreme sensitivity of the activation energy for superplasticity (which usually equals that for grain boundary diffusion) to alloy or impurity content is emphasized. The very large influence to prior thermal and mechanical history of the specimens on the mechanical data is discussed in the context of alteration of the significant details of grain boundary substructure.