Correlation between microstructure evolution and cavity stringer formation in the superplastic Zn-22 wt% Al alloy

Abstract

In the present investigation, microstructure evolution during the superplastic deformation of Zn–22 wt% Al was examined to provide insight into the origin of cavity stringers that form parallel to the tensile axis in the alloy. Substructural data show that groups of fine α (Al-rich) and β (Zn-rich) phases are encompassed by former α boundaries (FαcBs) which consist of fine elongated α grains and which divide the microstructure into equiaxed domains (FαB domains). By examining the correspondence between the behaviour of these boundaries and the development of cavity stringers, it is suggested that the alignment of cavities along the tensile axis can be explained in terms of two processes: firstly the preferential nucleation of cavities at FαBs during the initial stages of deformation, and secondly the tendency of FαBs and their associated cavities to change their orientation and align along the tensile axis during superplastic deformation. Evidence in support of this explanation is provided by present experimental results on firstly the correlation between the overall number of aligned FαBs and the total length of cavity stringers, secondly the effect of impurities on both the average size of FαBs domains and the total length of cavity stringers, and thirdly the direct correspondence between the morphology of FαBs and that of cavity stringers in notched specimens which, upon deformation, exhibit a distinctive local strain gradient.