Surface studies of Region II superplasticity of AA5083 in shear: Confirmation of diffusion creep, grain neighbour switching and absence of dislocation activity

The deformation mechanisms responsible for superplastic deformation have long remained a controversial subject. This research clarifies the issue by using regular surface grids with submicron and coarser pitches to make quantitative measurements of grain boundary sliding, diffusional transfer of material at grain boundaries and intragranular deformation during superplastic deformation in shear at the optimum superplastic strain rate. Important artefacts associated with surface oxidation and grain boundary migration were identified by removing surface material with a focused ion beam (FIB) to examine the microstructure beneath. The results show that Region II superplasticity occurs by diffusion creep with a grain neighbour switching geometry. Direct and incontrovertible experimental evidence of stress-directed diffusional transfer of matter at grain boundaries and of grain neighbour switching is described. There was no significant deformation within the grains by dislocation motion. The diffusional geometry was consistent with existing models for the process.