Influence of scandium and zirconium on grain stability and superplastic ductilities in ultrafine-grained Al-Mg alloys

Experiments were conducted to evaluate the grain refinement introduced by equal-channel angular pressing (ECAP) in three different Al-3% Mg alloys containing either 0.2% Sc, 0.2% Zr or a combination of 0.2% Sc and 0.12% Zr. The results show all three alloys exhibit significant grain refinement with as-pressed grain sizes in the range of ∼0.2-0.3 μm. Tensile testing after ECAP revealed superplastic ductilities in the Al-Mg-Sc-Zr and Al-Mg-Sc alloys at strain rates in the vicinity of ∼10^-2 s^-1 at temperatures of 573 and 673 K but superplasticity was not achieved in the Al-Mg-Zr alloy due to the onset of rapid grain growth at 573 K. At the higher temperature of 773 K, the Al-Mg-Sc-Zr alloy exhibited exceptional superplastic ductilities but superplasticity was no longer achieved in the Al-Mg-Sc alloy. This difference was due to extensive grain growth in the Al-Mg-Sc alloy at temperatures in the vicinity of 773 K whereas the ultrafine-grained microstructure in the Al-Mg-Sc-Zr alloy was stable at this high temperature due to the presence of stable Al₃(Zr_xSc_1-x) precipitates.