It was noted that polycrystalline materials could deform via a mechanism that involved grain shape changes which occurred by the diffusive flow of material. Since grains maintained contact during flow, grain-boundary sliding was inevitable. This had confused understanding of the overall mechanism to the extent that it was claimed that deformation occurred entirely by sliding. The main evidence for diffusive shape changes was offered by observations of precipitate-containing materials. The precipitates acted as inert markers, accumulated on vacancy sink boundaries and left zones denuded of precipitate on boundaries those that acted as sources. The zone size was related to the diffusive strain of each grain, but comparisons with theoretical predictions were complicated by the intervention of grain rotation. In the theory of diffusive flow, the creation and annihilation rates of vacancies were assumed to be constant over any boundary. Thus, any zone-width should also be constant. However, if grain rotation occurred, the rate of vacancy creation was not constant over the boundary and it could then act simultaneously as a source and sink at different positions along the boundary. This had important implications concerning the prediction of denuded-zone formation. The mechanism of grain rotation via an interfacial diffusion mechanism was analyzed. The fundamental aspects were first explored with regard to a bicrystal under a bending moment. The analysis was then extended to a simple polycrystalline structure.

Diffusional Rotation of Crystals about a Common Interface. B.Burton: Philosophical Magazine A, 2002, 82[1], 51-64