Thickness inhomogeneity in hardness and microstructure of copper after the compressive stage in high-pressure torsion

Distinction between plastic deformation occuring in compression and compression-torsion stages is important for understanding the properties and microstructures of materials processed by high-pressure torsion (HPT). In the present study, remarkable through-thickness inhomogeneities of hardness and microstructure were found in the samples processed by compression stage of HPT. Three regions on the radial direction plane of compressed disks were defined to display the inhomogeneity: edge zone (high hardness), radial medium zone (uniform hardness) and center zone (low hardness near the surface and high hardness in the thickness central plane). A low hardness region in the center near the surface was detected and found to stretch along the upper and bottom surfaces of the disks compressed by low pressure. This low hardness region was also found to decrease with increasing the pressure. Not only the hardness but also the microstructure through-thickness inhomogeneity is attributed to stress and strain distribution in the disk as well as to a huge friction between the anvil and the disk during processing.