Diffusion-Accommodated Flow and Superplasticity.

Polycrystalline matter can deform to large strains by grain-boundary sliding with diffusional accommodation. A new mechanism for this sort of deformation is described and modelled. It differs fundamentally from Nabarro-Herring and Coble creep in a topological sense grains switch their neighbors, and do not elongate significantly. A constitutive equation describing the mechanism is derived from the model. The strain-rate may be diffusion controlled, or it may be controlled by an interface reaction. The flow behavior of superplastic alloys can be explained as the superposition of this mechanism and ordinary power-law creep dislocation creep. The combined mechanisms appear to be capable of explaining not only the observed relation between strain-rate and stress, but most of the microstructural and topological features of superplastic flow as well. Author