Similarities and differences in the approaches to structural superplasticity and high temperature creep

Abstract

Structural superplasticity is a high temperature deformation process that leads to large, essentially neck-free elongation (in metals and metallic glasses). It is characterised by the retention of a fine near-equiaxed grain size, predominance of grain boundary sliding, grain rotation and virtual absence of grain elongation. Many models describing superplastic behaviour have been put forth. These models can be classified as those based directly on creep mechanisms, those based on creep mechanisms with suitable modifications, those based on accommodation of grain boundary sliding by dislocation creep mechanisms, with the accommodating step controlling the rate of flow, and those based on entirely different approaches from the ones used for creep mechanisms, those based on a Raschinger grain switching event for two-phase materials and those based on "unaccommodated" 'grain boundary sliding. Examined critically are some of the more important models of each type, bringing out their merits and limitations. The model based on "unaccommodated" grain boundary sliding seems, at present, capable of explaining satisfactorily most of the experimental observations concerning superplastic flow. However, the lack of conclusive experimental evidence regarding the presence or absence of dislocation activity during optimal superplastic flow and the rather poor understanding of the structure of high-angle grain boundaries make universal agreement at this stage on any one approach improbable.