Precipitation and aging phenomena in an ultrafine grained Al-Zn alloy by severe plastic deformation

The post-deformation aging response and underlying precipitation kinetics in ultrafine-grained, age-hardened alloys processed by severe plastic deformation are critical and heretofore poorly understood phenomena that ultimately influence mechanical behavior. In this study, a supersaturated solid solution 7075 Al alloy was processed by equal-channel-angular pressing (ECAP) route Bc at 250 °C for 6 passes to attain ultrafine grained (UFG) structures. Thermal and mechanical analyses revealed that there are three sequential phase transitions that govern post-deformation aging: first, dissolution reactions of Guinier-Preston (GP) zones, metastable ηp and η′ precipitates which correspond to slight decreases in strength and ductility; second, afresh precipitating of GP zones, metastable ηp and η′/stable η phases corresponding to an increase in strength with significant decrease in ductility; and third, subsequent η transformation of the metastable η′ and ηp as well as coarsening of stable η phases corresponding to a dramatic decreases in strength. Microstructural analysis suggests that the significant decrease in ductility may be attributed to grain boundary precipitation of the metastable ηp and η′/stable η phases.