Grain refinement and superplasticity in an aluminum alloy processed by high-pressure torsion

Disks of an Al-3% Mg-0.2% Sc alloy were processed by high-pressure torsion (HPT) to refine the grain size to ∼0.15 μm. Inspection of the disks after processing revealed a central core region having a relatively coarse and ill-defined microstructure. The size of this core region decreased with increasing numbers of turns in HPT. Measurements showed the hardness increased with increasing applied pressure and/or increasing numbers of turns. In addition, the hardness increased with increasing distance from the center of the disk and stabilized at distances greater than ∼2-3 mm. The values of the saturation hardness in the outer regions of the disks were similar at higher applied pressures and after larger numbers of turns. This saturation hardness was ∼3× the hardness in the solution-treated condition. Within the region of hardness saturation, the microstructure was reasonably homogeneous and consisted of ultrafine grains separated by high-angle grain boundaries. Tensile testing demonstrated the occurrence of high strain rate superplasticity after HPT with elongations to failure that were similar to those obtained in samples of the same alloy processed by equal-channel angular pressing (ECAP).