MICROSTRUCTURAL EVOLUTION AND THRESHOLD STRESS FOR SUPERPLASTIC FLOW IN THE ZN-AL EUTECTOID

The microstructural evolution and the stress-strain rate behaviour of superplastic Zn-Al eutectoid alloy were investigated by prestraining s pecimens at two strain rates corresponding to Regions I and II. Even t hough the scale of microstructure was similar, the stress-strain rate curves of differently prestrained specimens were distinctly different in the lower strain-rate regime. While Region I of low rate sensitivit y was more prominent when prestrained at a lower strain rate of Region I, it was less distinct because of prestrain in Region II. The thresh old stress for superplastic flow, as assessed by an extrapolation proc edure, varied with the nature of prestrain. The interphase boundaries were more rounded (higher mean curvature) on prestraining on Region II , compared to Region I. The correlation between the changes in the mea n curvature of phase boundaries and the threshold stress arising from the nature of prestrain was consistent with the boundary-migration con trolled sliding mechanism to interpret the threshold stress for superp lastic flow.