Geometrical study on the inhomogeneous deformation of polycrystals due to grain-boundary sliding

Inhomogeneous deformation of a polycrystal due to grain-boundary sliding was studied geometrically. It was shown that deformation modes such as grain-boundary sliding and separation (diffusional migration) tend to localize owing to inherent inhomogeneity, so that grain boundaries can be classified into three groups according to their deformation modes, where the first group consists of those boundaries that slide, the second of those boundaries that separate (or diffusionally migrate), and the rest of those boundaries that do not deform. It was discussed that there is no definite ratio but some limitation between relative amounts of sliding boundaries and diffusionally migrating boundaries, and that there is fundamentally no limits in the amount of non-deforming boundaries. Superplastic deformation was characterized by the high fraction of sliding boundaries, and ordinary diffusional creep deformation was characterized by the high fractions of diffusionally migrating and non-deforming boundaries. Implications as to the occurrence of region 1 at lower stresses in superplastic deformation was also discussed.