Role of grain-boundary sliding in diffusional creep of polycrystals

The kinetics of deformation caused by grain-boundary sliding and diffusion is discussed for a two-dimensional model polycrystal. It is shown that boundary sliding is a prerequisite to induce the bulk diffusion. The sliding must also occur concurrently with the diffusion for a polycrystal to continue diffusional creep. Rate equations for deformation of the polycrystal are also formulated when both of the bulk and boundary diffusions are operative in addition to the boundary sliding. It is shown that the two diffusional processes are exclusive in general. Only when boundaries slide extremely fast, the strain rate of the polycrystal in a steady state becomes the simple sum of the strain rate achieved by the bulk diffusion and that by the boundary diffusion.