Effects of annealing process parameters on microstructural evolution and strength-ductility combination of CoCrFeMnNi high-entropy alloy

Abstract

Microstructures and mechanical properties of the CoCrFeMnNi high-entropy alloys (HEAs) treated by various annealing temperatures and times are investigated in this paper. Typical heterogeneous structure which composed of the ultrafine grains in nano/submicron scale and coarse grains can be obtained by annealing at 873∼923 K. The heterogeneity of grain size contributes to the hetero-deformation induced (HDI) strengthening and hardening, as well as the activation of hierarchical nanotwins, leading to the superior tensile property of the CoCrFeMnNi HEA that evades the strength-ductility trade-off. Meanwhile, the rapid multiplication of geometrically necessary dislocations and hierarchical nanotwins induced by the heterogeneous structure are the main factors to facilitate the upturn of strain hardening rate after yielding. The excessive annealing time reduces the content of ultrafine grains and narrows the mechanical incompatibility between the soft and hard zones, which reduces the effect of HDI strengthening and hardening. Consequently, the optimal annealing temperature for the cold-rolled CoCrFeMnNi HEAs with the reduction ratio of ∼96.7% is approximately 873∼923 K, and the holding time should be controlled to retain more ultrafine grains in the heterogeneous structure for superior strength-ductility combination.