Microstructural saturation, hardness stability and superplasticity in ultrafine-grained metals processed by a combination of severe plastic deformation techniques

A commercial Al-7075 aluminum alloy was used to investigate the significance of the saturation hardness and saturation microstructure after processing by high-pressure torsion (HPT) through up to 20 turns or a combination of equal-channel angular pressing (ECAP) for 4 or 8 passes and HPT. The results show additional grain refinement by initially processing the material by ECAP to produce an ultrafine grain size before conducting the HPT. Thus, the grain size immediately prior to the HPT processing controls the saturation microstructure and hardness. Microstructural images reveal a significant grain refinement from elongated grains in the samples prior to processing with an average grain diameter of ~8 ?m to equiaxed grains with an average grain size of ~200 nm after processing by a combination of ECAP for 8 passes and HPT through 20 turns. This grain refinement leads to an improvement in the mechanical properties of the Al-7075 alloy. Vickers microhardness tests show a significant increase in the microhardness values of the Al-7075 alloy from Hv?102 in the annealed samples to Hv?270 after processing by a combination of ECAP for 8 passes and HPT through 20 turns. Specimens processed by a combination of ECAP and HPT also exhibit excellent superplastic properties with elongations up to >1000% at 673 K