Influence of high-pressure torsion straining conditions on microstructure evolution in commercial purity aluminum

The influence of heat generation during severe plastic deformation on microstructure evolution was investigated in commercial purity aluminum (Al 1050, CP-Al) by using high-pressure torsion (HPT) process. The microstructure was characterized by the observations of the torsion and the longitudinal planes. CP-Al disks were deformed by MPT-straining up to 20 turns (equivalent strain, E-eq, of similar to 600) at 0.2 or 5 rpm at room temperature. To prevent the increase in specimen temperature, HPT-straining was also carried out in liquid nitrogen. In the all conditions, the value of Vicker's microhardness, Hv, was saturated around 0.65 GPa and the microstructure consisted of the equiaxed grains of about 500 nm with quite low dislocation density. The microstructure in the early stage of HPT-straining showed the deformed (sub)structure, and then the equiaxed grain structure with high-angle boundaries formed by grain subdivision, recovery, continuous recrystallization and grain growth with increase in strain amounts and specimen temperature.