Influence of magnesium on grain refinement and ductility in a dilute Al-Sc alloy

Experiments were conducted to determine the influence of magnesium additions on grain refinement and tensile ductility in an Al-0.2% Sc alloy processed by equal-channel angular pressing (ECAP). The experiments show ECAP reduces the average grain size to within the range of ∼0.70 to ∼0.20 μm for alloys containing from 0 to 3% Mg but the as-pressed grain size increases to ∼0.3 μm in an alloy with 5% Mg because it is then necessary to use additional annealing treatments during the pressing process. The ultrafine grains introduced by ECAP are stable to high temperatures in the alloys containing from 0 to 3% Mg: in all alloys, the average grain size is <5 μm after annealing for 1 h at temperatures up to ∼750 K. High superplastic ductilities were achieved in the alloy containing 3% Mg but alloys containing 0.5% and 1% Mg exhibited the enhanced ductilities generally associated with conventional Al-Mg alloys. The results suggest the addition of ∼3% Mg is optimum for achieving superplastic elongations at rapid strain rates in the Al-0.2% Sc alloy.