Superplasticity at different microstructural scales

Investigations of superplastic (SP) flow were performed on different microstructural scales. Observations on the scale of the entire deformed volume showed macroscopic shear surfaces along which grain-groups slide as blocks and also rotate towards the stress axis during deformation. Observations on the level of grain-groups indicate a sequential nature of the grain sliding at the shear surfaces. Observations on the level of individual grains revealed a variety of accommodational mechanisms providing compatibility of the sliding of grain-groups and coupling of the processes of sliding and grain boundary migration. The observed pattern of the macroscopic shear surfaces is consistent with predictions of slip-lines field theory. The progress of the grain blocks sliding at the scale of grain-groups can be modeled in terms of cellular dislocations. The micromechanism for such sliding and the migration of sliding grain boundaries at the scale of the individual interface might be interpreted from the viewpoint of grain boundary dislocations. Supplementary processes, observed during SP, such as dynamic grain growth, void formation, phase stringers formation, etc., can be linked with occurrence of such cooperative grain boundary sliding.