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

The effect of impurity type on boundary sliding behavior in the superplasti c Zn-22 pet Al alloy was investigated using two grades of the alloy: Zn-22 pet Al-0.13 pet Cu (grade Cu) and Zn-22 pet Al-0.14 pet Fe (grade Fe). In t he investigation, boundary sliding offset measurements in both grades were made at strain rates ranging from 5 x 10(-7)/s to 10(-1)/s. This range of s train rate covered region I (the low strain rate region), region II (the in termediate 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, xi in the two grades of Zn-22 pet Al are about 20 and 52 pet at high (region III) and intermediate (region I I) strain rates, respectively. By contrast, the experimental data reveal th at xi in grade Cu at low strain rates (52 pet) is essentially equal to that at intermediate strain rates (region Il), while xi in grade Fe at low stra in rates (24 pet) is considerably lower than that at intermediate strain ra tes (56 pet). It is demonstrated that the difference in sliding behavior be tween grade Fe and grade Cu at low strain rates corresponds well with the d ifference in superplastic behavior between the two grades. In addition, con sideration of the present and earlier data on sliding behavior in Zn-22 pet Al provides a correlation between two roles played by boundaries during su perplastic deformation: the ability of boundaries to contribute to deformat ion through the process of boundary sliding and their ability to serve as f avorable sites for the accumulation of impurities, i.e., boundary segregati on.