On the intrinsic resistance to hydrogen embrittlement of equiatomic VCoNi medium entropy alloy

Recent electrochemical hydrogen charging (EHC) experiments and mechanistic theory suggest that VCoNi is much more resistant to hydrogen embrittlement (HE) than other FCC alloys. Here, HE of VCoNi is studied under controlled gaseous hydrogen charging (GHC) and partial discharging conditions. Under the same GHC conditions, VCoNi absorbs 3–4 times more H than SS316L and CrCoNi, suffers catastrophic intergranular cleavage, and exhibits a HE threshold concentration of 3000 appm comparable to CrCoNi but much lower than SS316L. These experiments are supported by H thermodynamic/kinetic properties calculated by density-functional theory (DFT), Monte Carlo (MC), and kinetic MC simulations. Diffusion profiles with predicted H diffusivities show that H-penetration during EHC at room temperature for typical durations of 24–72 h is limited to surface regions leading to apparent HE resistance, while H-penetration is complete under GHC at 573 K for 24 h at the same total H content. DFT calculations show very high H-affinities/low absorption-energies and high H-absorption in good agreement with experiments. The mechanistic theory including short-range order (SRO) in VCoNi predicts HE thresholds in reasonable agreement with experiments, indicating that prior high HE threshold is due to the incorrect assumption of a random solid solution. High H-segregation is also predicted at grain boundaries (GBs), similar to CrCoNi exhibiting intergranular cleavage. Thus, VCoNi is not a superior alloy for resistance to HE and is far inferior to SS316L. These results highlight the importance of appropriate H-charging processes for assessing true HE resistance and of careful consideration of SRO and H-GB-segregation in developing mechanistic theories of HE.