Rheological and metallurgical discussion of superplastic behaviour

Abstract

The analysis of the deformation of superplastic materials shows that the structure tends to evolve towards an equiaxed state and to undergo grain growth. This behaviour influences the constitutive equations : apparent strain-hardening, sigmoidal variation of the Log stress- Log strain rate curve, low apparent activation energy values at intermediate strain rates. This deformation behaviour can be explained when the structural change is taken into account. The analysis described leads to a constitutive equation which presents a metallurgical basis since it takes into account the structural changes. By contrast, the generally proposed relationship is rheological and can be used only within a strictly mechanical approach. An hydrodynamic analogy of superplastic flow is then presented. It shows that superplasticity occurs, on the one hand when the hardest phase is not continuous, and on the other hand when the most ductile phase has a mean free path no greater than a few microns. The analogy also demonstrates that the textural evolution is different for the two phases and accounts for the observed interface sliding. Furthermore, this study suggests that superplastic behaviour with elevated values of the coefficient m can be observed, although large elongations cannot be obtained in cases where the material undergoes cavitation resulting from difficulties in interface sliding