Diffusion-accommodated flow and superplasticity

Abstract 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, in which case the constitutive equation resembles the Nabarro-Herring-Coble equation but predicts strain-rates which are roughly an order of magnitude faster. Or it may be controlled by an interface reaction—roughly speaking, by the restricted ability of a boundary to act as a sink or source for point defects, or by its restricted ability to slide. 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.