The growth of artificial voids during superplastic deformation of a ZnAlCu alloy

A series of experiments has been performed on samples of Zn22wt.%Al0.5wt.%Cu alloy at 523 K, at low and intermediate strain rates, with either one or three drilled holes. Similar tests were run for polystyrene, to compare the alloy with a Newtonian viscous material. At low strains and the higher strain rate, the alloy was less sensitive to the presence of the holes than the polymer was. The results are analysed following a modified Hart model of tensile stability, including the effect of hardening due to the grain growth. The instability points, i.e. first when the voids start to grow and then when they start to grow more rapidly in the alloy than in the polymer, are predicted accurately. The main effect of grain growth hardening is to delay the onset of accelerated void growth. From the point of view of microcavitation, the latter implies that the transition between diffusion and plasticity-controlled growth may be delayed