Microstructure and properties of a CoCrFeNiMn high-entropy alloy processed by equal-channel angular pressing

A CoCrFeNiMn high-entropy alloy (HEA) was processed by equal-channel angular pressing (ECAP) for up to four passes at 673K and the results show that the strength increases gradually with increasing straining up to ~ 1GPa with an elongation to failure of ~ 35% after four passes of ECAP. In this condition, the microstructure is a single-phase ultrafine-grained (UFG) CoCrFeNiMn HEA with an average grain size of ~ 100nm and a high dislocation density. This UFG HEA was subjected to post-deformation annealing (PDA) at temperatures of 673–1073K for 60min and it is shown that the hardness increases slightly due to precipitation to 773K and then decreases to 1073K due to a combination of recrystallization, grain growth and a dissolution of precipitates. The formation of brittle σ-phase precipitates improves the strength significantly but with a minor decrease in ductility. Annealing at the peak temperature of 773K produces a very high yield strength of ~ 1015MPa and an ultimate strength of ~ 1080MPa together with an excellent elongation to failure of ~ 30%. An analysis of the data shows that grain boundary strengthening is the most important strengthening mechanism in these ECAP samples both before and after PDA.