The influence of impurity content on creep deformation, cavity stringer formation, microstructure evolution, and boundary segregation in the superplastic zinc-22wt% aluminum eutectoid (Zinc-aluminum)

A detailed investigation was conducted using six grades of Zn-22wt%Al, containing different levels and types of impurities, to study the influence of impurity content on the properties of superplastic flow in the alloy. Aspects of creep deformation and cavitation at low stresses were studied by examining the effect of Fe, Cu, and Mg as selected impurities. Characteristics associated with high purity grades of the alloy, doped with either Cu or Mg, were essentially identical to those reported previously for high-purity Zn-22wt%Al but different from those documented for a grade of the alloy containing a comparable atomic concentration of Fe (1290 at. %). These results suggest that Cu and Mg, unlike Fe, have little or no tendency to segregate at boundaries and clearly indicate that superplastic flow and cavitation at low stresses are controlled not only by impurity level but also by its type. Further evidence for this suggestion was obtained by studying the effect of impurities on former alpha boundaries (FαBs), which are residual grain boundaries that originate during the heat treatment required for grain; refinement in Zn-22wt%Al. In this regard, it was found that the characteristics associated with FαBs directly correspond with the level and type of impurities in Zn-22wt%Al. This correspondence, according to the detailed results obtained on FαB growth kinetics in the six grades of Zn-22wt%Al, results from impurity segregation at FαBs and tends strong support to the interpretation of superplastic behavior at low stresses in terms of phenomena arising from boundary segregation. It is suggested that information on FαB growth kinetics can be utilized to predict low-stress creep characteristics, such as the existence of a threshold stress or the occurrence of extensive cavitation. In addition, the microstructural evolution of FαBs during superplastic deformation was examined to provide insight into the origin of cavity stringers that form parallel to the tensile axis in the alloy. A comparison between the behavior of FαBs and the characteristics of cavity stringers reveals a direct correspondence between these two substructural features. On this basis, a mechanism that explains the formation of cavity stringers in Zn-22wt%Al has also been proposed.