The microstructures and mechanical properties of CoNiCrx medium-entropy alloys

The CoNiCrx (x = 1, 1.4, 1.8, and 2) medium-entropy alloys were synthesized by vacuum arc-melting method to systematically investigate the roles of Cr on microstructures and mechanical properties. The results of microstructural characteristics indicated that both CoNiCr and CoNiCr1.4 alloys have a single face-centered cubic (FCC) structure, while CoNiCr1.8 and CoNiCr2 alloys consist of FCC plus body-centered cubic (BCC) hypoeutectic and eutectic. The formation of FCC and BCC phases could be probably evaluated using the VEC criteria. The tested mechanical properties demonstrated that CoNiCr1.8 alloy exhibited the best comprehensive properties at room and liquid nitrogen temperatures (RT and LNT), whereas the yield strength and hardness of CoNiCr2 alloy were higher than those of other alloys, which may be mainly attributed to the second-phase strengthening and the fine and lamellar microstructure. The yield strength of CoNiCr2 alloy at RT and LNT was 3.8 and 1.9 times (514 MPa and 825 MPa) that of CoNiCr alloy. In addition, the precipitation of dense and ordered FCC (L12) phase could primarily contribute to the enhance of yield strength of CoNiCr1.4 alloy.