The characteristics of microcavitation in high strain rate superplasticity

Experiments were conducted on an Al-2124 composite reinforced with Si3N4 particulates under conditions appropriate for optimum high strain rate superplasticity (HSR SP) and at a temperature where there is experimental evidence for partial melting. It is shown that small but stable microcavities, having diameters of <1 μm, are formed in the composite during deformation under these conditions. These microcavities are nucleated relatively easily at the interfaces between the reinforcement and the matrix alloy but the growth of the cavities along the interfaces is very slow because the presence of a liquid phase serves to relax the stress concentrations and, in addition, the process of plasticity-controlled growth is partially inhibited because the cavities impinge on the Si3N4 particulates. Quantitative measurements show that the orientations of the cavities are random with respect to the tensile axis.