Low temperature superplasticity in a friction-stir-processed ultrafine grained Al-Zn-Mg-Sc alloy

Friction stir processing (FSP) was used to create a microstructure with ultrafine grains (0.68 (mu)m grain size) in an as-cast Al-8.9Zn-2.6Mg-0.09Sc (wt.%) alloy. The ultrafine grained alloy exhibited superplasticity at relatively low temperatures and higher strain rates. Optimum ductility of 1165% at a strain rate of 3 × 10^-2 s^-1 and 310 °C was obtained. Enhanced superplasticity was also achieved at a temperature as low as 220 °C. Experimentally observed parametric dependencies and microstructural examinations indicated that the operating deformation mechanism might be the Rachinger grain boundary sliding accommodated by intragranular slip. The FSP microstructure became highly unstable at 390 °C onwards, thus, affecting ductility adversely. In situ transmission electron microscopy heating was used to understand the instability phenomenon, which has been attributed to the drop in particle pinning forces due to the dissolution of metastable precipitates and microstructural heterogeneity.