Effect of Mo doping on the gaseous hydrogen embrittlement of a CoCrNi medium-entropy alloy

By using slow-strain-rate tensile test and post-mortem characterization of microstructure and fractography, we found that Mo-doping can simultaneously improve the strength and hydrogen resistance of an equi-molar CoCrNi medium-entropy alloy. At a high hydrogen concentration of 55.51 mass ppm, CoCrNi exhibited brittle intergranular fracture. By contrast, (CoCrNi)Mo possessed a mixed fracture morphology with lots of dimples and a small portion of intergranular feature. We attributed the enhanced hydrogen resistance to the Mo-promoted twinning-dominated deformation process, which can suppress the redistribution of hydrogen concentration by dislocation motion, and thus inhibit the hydrogen-induced grain boundary decohesion.