Specimens were observed  in situ in a scanning electron microscope during superplastic deformation in single shear and simple tension. This revealed the sliding of grain groups (cooperative grain boundary sliding), the rotation of grain groups (cooperative grain rotation) and cooperative grain boundary migration (correlated migration of sliding grain boundaries). The macroscopic patterning of cooperative grain boundary sliding surfaces was consistent with the predictions of slip-line field theory. The progress of the sliding of grain blocks at the scale of grain groups could be modelled in terms of cellular dislocations. The micro-mechanisms of such sliding, and the migration of sliding grain boundaries at the scale of the individual interface, could be explained in terms of grain boundary dislocations.

M.G.Zelin, A.K.Mukherjee: Acta Metallurgica et Materialia, 1995, 43[6], 2359-72