Effect of impurity content on superplastic flow in the Zn-22% Al alloy

Abstract

A detailed investigation was conducted on the superplastic Zn-22% Al alloy to study the effect of impurity content on the creep behavior in region II (intermediate-stress region) and region I (low-stress region) of the sigmoidal plot between stress and strain rate which was previously reported for the alloy. In conducting the investigation, three grades of Zn-22% Al, containing different levels of impurities, were used. The experimental results show that at intermediate stresses, where region II exists, the creep rates of the three grades are similar and that the stress exponent, n, and the activation energy, Q, are insensitive to impurity level; for all three grades, n = 2.5 and Q ∼ Q gb , where Q gb is the activation energy for grain boundary diffusion. By contrast, the experimental results reveal that there are significant differences in the creep behavior of the three grades at low stresses and that the emergence of region I at low stresses, along with its stress exponent and activation energy, is affected by the purity level of Zn-22% Al. Analysis of the experimental data for the three grades of the alloy suggests that the creep behavior in region I is a consequence of the presence of a threshold stress which decreases strongly with increasing temperature and which depends on impurity content. These two characteristics of the threshold stress appear to be compatible with those of a threshold stress for sliding whose origin is related to strong segregation of impurity atoms at boundaries.