Hydrogen induced microstructure, mechanical properties and cracking evolution in a novel CoCrNiMo medium-entropy alloy

Medium-entropy alloys (MEAs) have shown exceptional hydrogen tolerance. Here we systematically investigated the hydrogen resistance evolution of a non-equiatomic Co35Ni36Cr23Mo6 MEA from microstructure, trapping site, mechanical properties and cracking. The ultimate tensile strength and ductility of Co35Ni36Cr23Mo6 at room temperature were about 840 MPa and 71%, respectively. In the case of hydrogen pre-charging for 12 h, the above values were almost unchanged, indicating excellent hydrogen resistance. Grain boundaries were the main initiation and propagation paths for hydrogen-induced secondary cracks, while no secondary crack was on the Co35Ni36Cr23Mo6 alloy without hydrogen. We also found that the non-ductile region in the fracture morphology could be generated only when the hydrogen concentration reached a certain threshold. Moreover, hydrogen not only had negative effect on the MEA, but also promoted the formation of nanotwins during deformation process in this alloy.