The evolution of the grain structure during superplastic deformation of alloy sheet, especially at its mid-thickness cross-section, was investigated by means of the optical microscopy and transmission electron microscopy of specimens which had been deformed to various strains at a constant strain-rate (0.001/s) at 530C. A specially designed  in situ  quenching apparatus was used to preserve the deforming microstructure, and characterize dislocation activity during superplastic deformation. It was found that the evolving microstructure was characterized by grain growth, by a grain-shape change from initially pancake-like to almost-equiaxed, and by a grain misorientation change from predominantly low-angle (less than 15) to high-angle (greater than 15). There was also a weakening of the texture from strong to random, and dislocation activity throughout the superplastic deformation process. It was concluded that, in addition to grain boundary sliding, dislocation mechanisms could play a more important role than the mere accommodation of grain boundary sliding during superplastic deformation.

W.Fan, M.C.Chaturvedi, N.C.Goel, N.L.Richards: Materials Science Forum, 1997, 243-245, 563-8