Effect of long-term natural aging on microstructure and room temperature superplastic behavior of UFG/FG Zn-Al alloys processed by ECAP

Two potential superplastic compositions of Zn-Al alloy systems, Zn-22Al and Zn-0.3Al alloys, were chosen and processed by equal-channel angular pressing /extrusion (ECAP /E) in order to achieve high strain rate (HSR) superplasticity at room temperature (RT). ECAP-processed samples of both alloys were then subjected to long-term natural aging up to 1100 days to evaluate the effect of long-term natural aging on their microstructures and superplastic behaviors. Before natural aging, the maximum elongations to failure at RT were 400 % for ultrafine-grained (UFG) Zn-22Al at the strain rate of 5 x 10(-2) s(-1) and 1000 % for fine-grained (FG) Zn-0.3Al at the strain rate of 1 x 10(-4) s(-1). Long-term natural aging did not cause a significant change in the elongation of UFG Zn-22Al alloy with 355 % maximum elongation. However, optimum strain rate giving the maximum elongation decreased to 3 x 10(-3) s(-1). On the other hand, Zn-0.3Al alloy lost more than half of its superplastic elongation and showed an elongation to failure of 435 % at the end of the natural aging period of 1100 days. Microstructural analyses show that grain boundary corrosion occurred in dilute Zn-0.3Al alloy during the natural aging process. Corroded grain boundaries resulted in cavity nucleation during the tensile tests and some of these cavities attained large sizes and caused premature failure.