Effect of High-Pressure Torsion on Hardness and Electrical Resistivity of Commercially Pure Cu

Commercially pure Cu is processed through high-pressure torsion (HPT) at 6 GPa up to 50 turns and stored under the ambient condition for 21 months. Subsequently, microhardness and electrical resistivity of the Cu samples are measured and correlated with the microstructure. Grain size monotonically decreases with the number of HPT turns and becomes saturated at ≈300 nm after an equivalent strain of ≈40. Considerable fractions of low-angle grain and twin boundaries are observed in the samples processed through HPT to low strains; however, their fractions decrease with increasing HPT turns. Consistently, although the hardness of the processed samples is greater than that of the annealed coarse-grained sample, it first decreases with HPT straining and subsequently increases to a saturated value of ≈160 HV. In contrast, the electrical resistivity of Cu first increases with HPT processing, then decreases and finally becomes saturated at a value of ≈22 nΩ m, which is slightly higher than the least value obtained in this study. The obtained results are compared with the literature on HPT processing of Cu, and the usefulness of the ambient ageing for optimizing the hardness-conductivity relationship is discussed.