Superplasticity and superplastic forming processes

Abstract

Superplasticity, first observed some seventy years ago, remained a scientific curiosity until about twenty years ago. It is now recognized as a property which can be utilized in forming processes. There are two types of superplastic behaviour, known as fine–grained (or fine–structure) and internal–stress superplasticity. Fine–grained superplastic materials have a strain–rate sensitivity exponent of 0·5, and deform principally by a grain–boundary sliding mechanism. In this paper the microstructural features important in the development of fine structure super plasticity are discussed, and phenomenological equations for describing superplastic flow are presented. The superplastic properties of fine–grained materials can be optimized by promoting grain–boundary sliding and inhibiting slip. A number of fine–grained superplastic materials have been developed for commercial use, and their number is increasing. Internal–stress superplastic materials can have a strain–rate sensitivity exponent as high as unity, i.e. they can exhibit Newtonian viscous behaviour. Internal stresses can be generated by thermal cycling in materials that consist of two phases, or are anisotropic in their thermal–expansion coefficients, or are polymorphic. No commercial applications have yet been found for the superplastic forming of materials by generating internal stress.MST/169