Effect of impurity content on boundary sliding behavior in the superplastic Zn-22% Al alloy

A detailed investigation was conducted on the superplastic Zn-22% Al alloy to study the effect of impurity content on grain boundary sliding behavior in region I (low strain rate region), and region II (intermediate strain rate region, or the superplastic region) of the sigmoidal plot between stress and strain rate that was previously reported for the alloy. In conducting the investigation, three grades of Zn-22% Al were used; grades 1 and 2 contain 180 and 100 p.p.m. of impurities, respectively, whereas grade 3 is a high purity grade containing 6 p.p.m. of impurities. The experimental results show that at intermediate strain rates (region II), the sliding behavior of the three grades of Zn-22% Al is similar and that the contribution of boundary sliding to the total strain, xi, is about 60%. By contrast, the experimental data reveal that at low strain rates, the three grades exhibit significant differences regarding the sliding contribution. These differences are manifested in the following observations: (i) xi in grade 3 at low strain rates is essentially equal to that at intermediate strain rates (region II), (ii) xi in grade i or grade 2 is considerably lower than that at intermediate strain rates. and (iii) for the same low strain rate, xi in grade 2 is higher than that in grade i. The above observations regarding the effect of impurity level on boundary sliding behavior in Zn-22% Al are consistent with the concept of the interpretation of superplastic flow at low strain rates (low stresses) in terms of boundary segregation. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.