Substitutional solute grain boundary segregation enhances resistance to hydrogen embrittlement in compositionally complex alloys

Abstract

The presence or ingression of hydrogen (H) can dramatically embrittle a broad range of intrinsically ductile metals and alloys. Despite extensive research, fundamental understanding of hydrogen embrittlement (HE) and mitigation methods remain far from complete. Here, we present a thermodynamic approach to robustly enhance resistance to HE in CrCoNi with minor degradations of other properties. Specifically, 6 at.$\text{\%}$ W/Mo are doped and induced to segregate into grain boundary (GB) regions, which restores ductile transgranular fracture with dimpled fracture surfaces and tensile ductility losses of $\sim 10-30\text{\%}$ under gas-H-charged conditions. Density functional theory (DFT) calculations and Monte Carlo (MC) simulations reveal W-GB-segregation energies and favourable W-GB-segregations over a wide temperature window, as well as H-dissolution energies in grain interiors and GB regions. MC simulations with these DFT-based energetics show that most H-atoms reside in grain interiors in all alloys, but the GB-H-occupation ratios are $\sim 1-2$ orders-of-magnitude higher in the undoped alloy. In the doped alloys, W-GB-segregations moderately enhance GB cohesion and more importantly, make GB regions less attractive for H-dissolution, which in turn drastically reduces GB-H-occupation at the most critical low/room-temperatures. The stark differences in GB-H-occupation ratios in the doped and undoped alloys corroborate their respective void-coalescence and GB-cleavage dominant fracture mechanisms. The enhanced HE resistance is derived from GB-solute-segregation and reduced GB-H-occupation, both of which are thermodynamic equilibrium properties of the underlying alloy system. The combined experiments and simulations demonstrate a general strategy to design structural alloys for enhanced resistance to HE, which may be applicable to other alloy systems.