Massive Mo alloying for enhancing resistance to hydrogen-induced crack propagation in medium-entropy CoNiMo alloy

There has been a consistent demand for an alloy design strategy that concurrently enhances both strength and resistance to hydrogen embrittlement (HE). The element Mo is recognized for inducing both lattice distortion and grain boundary strengthening effects, which can simultaneously increase strength and resistance to HE. Accordingly, this study investigates face-centered cubic (FCC) single-phase CoNi and CoNiMo alloys as model systems to unravel the effect of the substantial addition of Mo on resistance to HE. Hydrogen-induced crack propagation behaviors were systematically analyzed using an interrupted tensile test. In the Mo-added alloy, crack propagation increases in width rather than depth, indicating considerable resistance to crack advancement. This reduction of crack propagation rate is attributed to the rapid crack advancement into ductile regions and the activation of deformation twinning near the crack. These phenomena result from the substantial Mo alloying effect, which inhibits hydrogen trapping on grain boundaries, lowers stacking fault energy to facilitate twin formation, and ultimately suppresses plastic instability. Consequently, the addition of Mo into an FCC alloy offers a potential approach for enhancing the strength without significant loss of HE resistance. This strategy presents a viable design approach for developing high-strength FCC single-phase alloy while marginally compromising HE resistance.