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

Abstract

A CoCrFeNiMn high-entropy alloy (HEA) was processed by equal-channel angular pressing (ECAP) for up to four passes at 673 K and the results show that the strength increases gradually with increasing straining up to ~1 GPa 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 ~100 nm and a high dislocation density. This UFG HEA was subjected to post-deformation annealing (PDA) at temperatures of 673-1073 K for 60 min and it is shown that the hardness increases slightly due to precipitation to 773 K and then decreases to 1073 K 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 773 K produces a very high yield strength of ~1015 MPa and an ultimate strength of ~1080 MPa 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.

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ORCID for Terence G. Langdon: orcid.org/0000-0003-3541-9250

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