The evolution of homogeneity in processing by high-pressure torsion

Disks of high-purity aluminum were processed by high-pressure torsion (HPT) at room temperature under different conditions of imposed pressure and numbers of turns. Measurements were taken of the microhardness values both along diameters in each disk and following a rectilinear grid pattern to give color-coded maps of the hardness distributions. The results show the hardness increases by a factor of ∼2 in the first turn of HPT but the microhardness distribution is inhomogeneous because higher values of hardness are recorded in the central regions of the disks. This central region of inhomogeneity decreases with increasing numbers of turns so that the hardness distribution becomes essentially homogeneous after five turns. The results are different from earlier reports in HPT where the central regions of the disks have a lower hardness. The results are interpreted using a model in which the degree of hardness depends upon the rate of recovery in the material.