Influence of scandium on superplastic ductilities in an Al-Mg-Sc alloy

Ultrafine grain sizes, of the order of approximately 0.2 μm, may be introduced into Al-Mg-Sc alloys by subjecting the material to severe plastic deformation through the process of equal-channel angular pressing (ECAP). Experiments were conducted to evaluate the influence of the solution treatment temperature on the ductility of an Al-3% Mg-0.2% Sc alloy after ECAP. The results show the highest ductilities are achieved when the solution treatment temperature is within the narrow range of approximately 878 to about 883 K, immediately below the temperature associated with the onset of partial melting. These high temperatures serve to maximize the amount of scandium in solid solution and this leads, on subsequent heating, to an extensive precipitation of fine secondary Al₃Sc particles which inhibit grain growth at the higher temperatures. Conversely, solution treatments at temperatures below approximately 878 K give less Sc in solid solution within the matrix and the precipitation of the Al₃Sc particles is then insufficient to retain a uniform ultrafine microstructure.