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 degrees 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 eta(p) and eta' precipitates which correspond to slight decreases in strength and ductility; second, afresh precipitating of GP zones, metastable eta(p) and eta'/stable eta phases corresponding to an increase in strength with significant decrease in ductility; and third, subsequent eta transformation of the metastable and eta(p) as well as coarsening of stable eta 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 eta(p) and eta'/stable eta phases. (C) 2020 The Authors. Published by Elsevier B.V.

Automated summary

The post-deformation aging of ultrafine-grained, age-hardened alloys processed by severe plastic deformation involves three sequential phase transitions, leading to significant decreases in ductility and varying levels of strength.