PROCESSES OF MICROSTRUCTURAL EVOLUTION DURING SUPERPLASTIC DEFORMATION

Microstructural changes occurring at the surface and in the bulk of su perplastically deformed materials have been considered. Surface studie s showed formation of macroscopic surface steps and fibers, which dete rmine surface roughness and can affect corrosion properties, respectiv ely. Bulk microstructural changes include morphological and crystallog raphic changes, as well as defect accumulation. Phase/grain growth, ph ase spatial distribution, and phase/grain shape changes control the mo rphology of the phase constituents. Weakening of preexisting texture a nd an increase in the ratio of high angle grain boundaries determine c rystallographic changes. Defect accumulation is related to cavity form ation; density of lattice dislocation is superplastically deformed mat erials is low. Various explanations proposed for these processes of mi crostructural evolution in superplastic materials are considered. It i s shown that these processes are closely related to cooperative grain boundary sliding-that is, the sliding of grain groups. (C) Elsevier Sc ience Inc., 1996.