Effect of impurity type on boundary sliding behavior in the superplastic Zn-22 pct Al alloy

Abstract

The effect of impurity type on boundary sliding behavior in the superplastic Zn-22 pct Al alloy was investigated using two grades of the alloy: Zn-22 pct Al-0.13 pct Cu (grade Cu) and Zn-22 pct Al-0.14 pct Fe (grade Fe). In the investigation, boundary sliding offset measurements in both grades were made at strain rates ranging from 5×10−7/s to 10−1/s. This range of strain rate covered region I (the low strain rate region), region II (the intermediate strain rate region), and region III (the high strain rate region) of the sigmoidal plot between stress and strain rate that was previously reported for grade Fe. The experimental results show that the contributions of boundary sliding to the total strain, ξ, in the two grades of Zn-22 pct Al are about 20 and 52 pct at high (region III) and intermediate (region II) strain rates, respectively. By contrast, the experimental data reveal that ξ in grade Cu at low strain rates (52 pct) is essentially equal to that at intermediate strain rates (region II), while ξ in grade Fe at low strain rates (24 pct) is considerably lower than that at intermediate strain rates (56 pct). It is demonstrated that the difference in sliding behavior between grade Fe and grade Cu at low strain rates corresponds well with the difference in superplastic behavior between the two grades. In addition, consideration of the present and earlier data on sliding behavior in Zn-22 pct Al provides a correlation between two roles played by boundaries during superplastic deformation: the ability of boundaries to contribute to deformation through the process of boundary sliding and their ability to serve as favorable sites for the accumulation of impurities, i.e., boundary segregation.