High deformation ability induced by phase transformation through adjusting Cr content in Co-Fe-Ni-Cr high entropy alloys

To understand the phase transformation and its effect on mechanical properties of high entropy alloys (HEAs) during deformation, Co₄₀Fe₂₀Ni_40-xCr_x (x = 10, 20, 30 at%, hereinafter simplified as HCr10, HCr20 and HCr30, respectively) HEAs were prepared, and the microstructure and tensile properties were investigated. The results show that HEAs are mainly composed of face-centered cubic (FCC) phase, and the σ phase forms in HCr30 due to the valence electron concentration (VEC) decreasing from 8.8 to 8.0 with increasing Cr content. For HCr10, the phase transforms from FCC to hexagonal closed-packed (HCP), but for HCr20 and HCr30, the phase transforms from FCC to HCP and σ phases during tensile deformation. The volume fraction of HCP phase firstly increases from 4.29 to 15.40 vol%, then decreases to 7.67 vol%, and σ phase increases from 0 to 28.89 vol% with the increase of Cr content. The stacking faults (SFs) probability increases with the increase of Cr content, contributing to the occurrence of the phase transformations. Tensile testing shows that the yield strength increases by 50% in HCr30 compared with HCr10, and the ultimate strength and elongation reach the maximum values (718 MPa and 59.05%) in HCr20. HCr20 shows a high strain hardening exponent (n = 0.95) after tensile deformation. The phase transformation induced by tensile deformation mainly accounts for the strength-ductility improvement, whereas an increase in σ phases will reduce the ductility.