Fundamentals of Superplasticity and Its Application

Abstract

The term superplasticity has been used to describe extraordinary elongations (several hundred percent) obtained during tensile deformation of polycrystalline materials. Mostly a scientific curiosity fifteen years ago, superplastic materials are now being used in a number of industrial applications, including near-net-shape forming and solid state welding. Two types of superplastic flow have been observed: internal stress superplasticity and fine structure superplasticity. Both rely on one common characteristic: a high sensitivity of the flow stress to the strain rate. This is a necessary but not sufficient condition for superplasticity. The various structural prerequisites for fine structure superplasticity are evolving rapidly and the phenomenology of superplastic flow is well documented. This knowledge, coupled with advances made in understanding normal plastic flow in crystalline solids (diffusion controlled dislocation creep), permits predicting methods for enhancing and optimizing superplasticity in materials.