The influence of L12 ordered precipitates on hydrogen embrittlement behavior in CoCrNi-based medium entropy alloys

The recently emerged multicomponent (or medium/high entropy) alloys have generated considerable excitement globally in the last 10 years because of their excellent mechanical and functional properties, particularly in terms of strength-ductility combinations that can surpass most other metallic materials. However, the achieved high strength level (above 1 GPa in many cases) fuels strong concerns about hydrogen embrittlement (HE). Detailed investigation in this field is still scarce, especially pertaining to the face-centered cubic medium entropy alloys (MEA) that are typically strengthened by ordered precipitates. Here, we unravel the effect of γ' (L12) ordered precipitates on H-induced damage behavior and the associated HE resistance in CoCrNi-based MEAs. Compared with the equi-molar CoCrNi MEA, the precipitation-hardened (CoCrNi)94Al3Ti3 MEA shows an enhanced HE resistance even at a higher strength level. Both alloys are fractured due to H-assisted intergranular cracking at the initial failure stage when loaded in the presence of H. The formation of intergranular cracks is primarily attributed to the H-induced decohesion at grain boundaries, where a high stress/strain concentration accompanied by a more intensive dislocation planar slip (or stacking fault formation) caused by H was observed. The presence of γ' precipitates serves to slow down the internal diffusion/migration of H due to the trapping effects. The precipitates with a relatively larger size (∼50 nm) also hinder dislocation planar slip thus decreasing the number of pile-up dislocations at grain boundaries. Both effects collectively reduce the tendency of H-induced intergranular cracking, leading to the improved HE resistance. The work reveals the positive role of ordered precipitates in H tolerance and thus provides some insights in further microstructure design of medium/high entropy alloys for applications in H-abundant environment.