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(ZrxSc1-x) precipitates