Processes of microstructural evolution during superplastic deformation

Microstructural changes occurring at the surface and in the bulk of superplastically deformed materials have been considered. Surface studies showed formation of macroscopic surface steps and fibers, which determine surface roughness and can affect corrosion properties, respectively. Bulk microstructural changes include morphological and crystallographic changes, as well as defect accumulation. Phase/grain growth, phase spatial distribution, and phase/grain shape changes control the morphology of the phase constituents. Weakening of preexisting texture and an increase in the ratio of high angle grain boundaries determine crystallographic changes. Defect accumulation is related to cavity formation; density of lattice dislocation is superplastically deformed materials is low. Various explanations proposed for these processes of microstructural evolution in superplastic materials are considered. It is shown that these processes are closely related to cooperative grain boundary sliding—that is, the sliding of grain groups.