Abstract
Cr element is considered to have strong solid solution strengthening effect in high entropy alloys (HEAs). However, the formation of brittle σ phase is enhanced with the increase of Cr content, which is harmful to the ductility of alloys. The valence electron concentration (VEC) is highly correlated with phase stability of HEAs, which is used to predict the formation of σ phase. The microstructure and mechanical behavior of Cr-rich non-equal molar Co-Cr-Fe-Ni HEAs were investigated. Results indicate the formation of σ phase could be suppressed with the high VEC, while lattice parameter tends to increase as Cr content increases, which enhances the metastability of Co-Cr-Fe-Ni system to accelerate grain-boundary precipitates of σ phase. Minor precipitates of σ phase strengthen the HEAs and do not cause serious embrittlement. Deformation-induced martensitic transformation (FCC →HCP) occurs during the tensile deformation of alloys with the VEC of 8.0 and 7.9, and the transformation induced plasticity effect is the main reason for good combination of strength and ductility.
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