This process was studied geometrically. It was shown that deformation modes such as grain-boundary sliding and separation (diffusional migration) tended to be localized by an inherent inhomogeneity. Thus, the grain boundaries could be classified into 3 groups according to their deformation mode. The first group consisted of those boundaries that slid, the second consisted of those boundaries that separated (or diffusionally migrated), and the third group comprised those boundaries that did not deform. It was found that there was no definite ratio, but merely some limitation as to the relative numbers of sliding boundaries and diffusionally migrating boundaries. There was no fundamental limit on the number of non-deforming boundaries. Superplastic deformation was characterized by a high fraction of sliding boundaries, and ordinary diffusional creep deformation was characterized by high fractions of diffusionally migrating and non-deforming boundaries.

K.Shobu, E.Tani, T.Watanabe: Philosophical Magazine A, 1996, 74[4], 957-64