Effect of initial annealing temperature on microstructural development and microhardness in high-purity copper processed by high-pressure torsion

The effect of the initial annealing temperature on the evolution of microstructure and microhardness in high purity OFHC Cu was investigated after processing by HPT. Disks of Cu were annealed for one hour at two different annealing temperatures, 400 and 800°C, and then processed by HPT at room temperature under a pressure of 6.0 GPa for 1/4, 1/2, 1, 5 and 10 turns. Samples were stored for 6 months after HPT processing to examine the self-annealing effects. Electron backscattered diffraction (EBSD) measurements were recorded for each disk at three positions: center, mid-radius and near edge. Microhardness measurements were also recorded along the diameters of each disk. Both alloys showed rapid hardening and then strain softening in the very early stages of straining due to self-annealing with a clear delay in the onset of softening in the alloy initially annealed at 800°C. This delay was due to the relatively larger initial grain size compared to the alloy initially annealed at 400°C. The final microstructures consisted of homogeneous fine grain sizes having average sizes of ~0.28 and ~0.34 μm for the alloys initially annealed at 400 and 800°C, respectively. A new model is proposed to describe the behavior of the hardness evolution by HPT in high purity OFHC Cu.