Microstructure and mechanical properties of CoCrFeNiW_x high entropy alloys reinforced by μ phase particles

In this study, the microstructure and mechanical properties of the as-cast and as-annealed CoCrFeNiW_x (x = 0, 0.2, 0.4 and 0.6, x value in molar ratio) high-entropy alloys were investigated. Both W-addition and annealing treatment were conducive to the formation of the μ phase in CoCrFeNiW_x alloys. With the increase of x value, the microstructures of the as-cast HEAs evolved from single-phase solid solution (x = 0, 0.2) to hypoeutectic structure (x = 0.4), then to hypereutectic structure (x = 0.6). The as-cast CoCrFeNiW_0.4 alloy exhibited a tensile strength of 690.7 MPa with a considerable elongation of 33.1%, which resulted from solid solution strengthening and second phase strengthening. After annealing, the as-annealed CoCrFeNiW_0.4 alloy showed an increased tensile strength of 970.9 MPa, owing to the precipitation of μ-phase particles. Deformed microstructures indicated that cracks generated from μ-phase precipitates of different sizes, and suppressed by the ductile FCC solid solution. Moreover, the high-density dislocation tangles around the μ-phase precipitates lead to a high strain hardening rate, which improved the strength significantly.