Superplasticity of ultrafine-grained aluminum alloys processed by equal-channel angular pressing

It is well established that the addition of dilute amounts of scandium to Al-Mg alloys gives a considerable enhancement in the strength and thermal stability of the material and provides the potential for achieving high tensile elongations. In the present investigation, 0.2% Sc was added to pure Al and to an Al-3% Mg solid solution alloy and the materials were subjected to equal-channel angular (ECA) pressing to introduce ultrafine grain sizes. This paper describes the microstructural characteristics of these materials after ECA pressing and examines the thermal stability of the microstructures at elevated temperatures. It is shown that the ultrafine-grained structures are stable in both materials up to temperatures of the order of similar to 700 K but high superplastic ductilities are achieved at high strain rates in the Al-3% Mg-0.2% Sc alloy (e.g. > 1500% elongation at 673 K at 3.3 x 10(-2) s(-1)) whereas the elongations to failure are low in the Al-0.2% Sc alloy. The results demonstrate the importance of the presence of a solute, such as Mg, in order to attain superplastic elongations.