Effect of annealing on mechanical properties of a nanocrystalline CoCrFeNiMn high-entropy alloy processed by high-pressure torsion

Abstract

A CoCrFeNiMn high-entropy alloy (HEA) was processed by high-pressure torsion (HPT) under 6.0GPa pressure up to 10 turns at room temperature. It is shown that there is a gradual evolution in hardness with increasing numbers of turns but full homogeneity is not achieved even after 10 turns. Microhardness measurements reveal that the material reaches a saturation hardness value of ~4.41GPa and in this condition the microstructure shows exceptional grain refinement with a grain size of ~10nm. An ultimate strength value of ~1.75GPa and an elongation to fracture of ~4% were obtained in a sample processed for 5 turns. The nanostructured HEA was subjected to post-deformation annealing (PDA) at 473–1173K and it is shown that the hardness increases slightly to 773K due to precipitation and then decreases up to 1173K due to a combination of recrystallization, grain growth and a dissolution of the precipitates. The formation of brittle precipitates, especially σ-phase, at 873 and 973K significantly reduces the ductility. Short-term annealing for 10min at 1073K prevents grain growth and leads to a combination of high strength and good ductility including an ultimate tensile strength of ~830MPa and an elongation to failure of ~65%.