Effect of impurity content on creep and boundary sliding behavior of the superplastic zinc - 22 percent-aluminum eutectoid

A detailed investigation was conducted to study the effect of impurity content on superplastic flow and grain/phase boundary sliding behavior of Zn-22%Al eutectoid alloy whose creep behavior exhibits a sigmoidal relationship between stress and strain rate. Superplastic flow behavior was studied using six grades of the alloy containing different levels and types of impurities; three of these grades of the alloy were used for the investigation on boundary sliding. The results of the creep test show that the creep characteristics, including the stress exponent, n, and the activation energy for deformation, Q, in region II (intermediate stresses) are independent of purity level of the alloy, whereas those in region I are affected by the impurity content in the alloy. Analysis of the creep data of the six grades of the alloy suggests that the creep behavior observed in region I is due to the existence of a strongly temperature dependent threshold stress which increases with increasing amount of impurities in the alloy. Presence of such a threshold stress appears to result from strong segregation of impurity atoms at the boundaries and their interaction with the boundary dislocations. The results of the boundary sliding measurements support the above suggestion. The results show that strain contribution of boundary sliding to total strain is maximum in region II compared to that in regions I and III, and that the effect of impurity content is significant in region I, where extend of boundary sliding is reduced due to the presence of the impurities.