Microstructural evolution during superplastic deformation of a 7475 Al alloy

Grain refinement of a superplastic 7475 Al alloy is observed at strain rates of 10-2s-1 or higher. Metallographic observation shows that the average grain size is changed from 14 μm to 10 μm after 100% elongation. Two-stage strain-rate tests were performed on the 7475 Al alloy to correlate grain refinement with an improvement of superplasticity. The optimum first strain rate and strain in the first stage were determined through tensile superplastic tests. Superplasticity was improved significantly through two-stage strain-rate testing. This is believed to be related to the refinement of the initial grains at high strain rate. The specimen tested at a strain rate of 2.1×10-4s-1 revealed dispersoid-free zones (DFZs) near grain boundaries normal to the stress axis. When a higher strain rate was applied to the specimens with DFZs, no grain refinement was observed. The absence of grain refinement is due to the concentration of plastic deformation in the weak DFZs. © 1998 Kluwer Academic Publishers