Diffusional and dislocation accommodation mechanisms in superplastic materials

Significant developments in the microstructural characterization of superplasticity have been achieved in the 2010s, which can be attributed to advanced electron microscopy and well-controlled mechanical experiments. The objective of this overview is to describe the scientific advances in the microscopic to nanoscopic mechanisms of superplasticity. The particular emphasis is placed on the accommodation mechanisms (i.e. atomic diffusion and/or dislocation activities) associated with grain boundary sliding (GBS) in quasi-single phase materials during superplastic (region II) and near-superplastic (regions II/III similar to III) flows. Superplasticity in region II is attributed to the cooperative process between GBS and grain boundary diffusion, which play the straining and accommodating roles, respectively. On the other hand, near-superplastic flow in regions II/III similar to III is typically attributed to shear strain propagation between GBS and dislocation slip in mantle regions (i.e. Gifkins' core-mantle model, 1976) and across grain interiors (i.e. Ball and Hutchison's transgranular model, 1969) with increasing strain rate. These accommodation mechanisms lead to dynamic microstructural evolutions as represented by continuous dynamic recrystallization during the near-superplastic flow. (c) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.