Microstructure and mechanical properties of pure Cu processed by high-pressure torsion

Pure Cu was subjected to severe plastic deformation through high-pressure torsion (HPT) using disc and ring samples. Vickers microhardness was measured across the diameter and it was shown that all hardness values fall well on a unique single curve regardless of the types of the HPT samples when they are plotted against the equivalent strain. The hardness increases with an increase in the equivalent strain at an early stage of straining but levels off and enters into a steady-state where the hardness remains unchanged with further straining. It was confirmed that the tensile strength also follows the same single function of the equivalent strain as the hardness. The elongation to failure as well as the uniform elongation also exhibits a single unique function of the equivalent strain. Transmission electron microscopy showed that a subgrain structure develops at an early stage of straining with individual grains containing dislocations. The subgrain size decreases while the misorientation angle increases and more dislocations are formed within the grains with further straining. In the steady-state range, some grains appear which are free from dislocations, suggesting that recrystallization occurs during or after the HPT process. The mechanism for the grain refinement was discussed in terms of dislocation mobility.