Microstructural developments in a spray-cast Al-7034 alloy processed by equal-channel angular pressing

A spray-cast Al-7034 alloy was processed by equal-channel angular pressing (ECAP) to produce an ultrafine submicrometer grain size. The microstructures of the as-received and as-pressed alloy were examined using several different experimental techniques including transmission electron microscopy (TEM), electron back-scatter diffraction (EBSD) and differential scanning calorimetry (DSC). The alloy contains η phase (MgZn₂) and Al₃Zr precipitates and these precipitates serve to inhibit grain growth so that an ultrafine submicrometer grain size is retained at temperatures up to and above 600K. The results demonstrate an increase in the fraction of high-angle boundaries with increasing numbers of passes in ECAP and analysis of the DSC curves leads to the identification of several thermal effects involving the formation, coarsening and dissolution of the η-phase. The rod-like η-phase particles present in the as-received alloy are broken during ECAP and a detailed analysis suggests this breaking occurs primarily in the first pass of ECAP. Tensile tests at elevated temperature demonstrate a potential for achieving superplastic elongations in the as-pressed material.