The dual role of boron on hydrogen embrittlement: example of interface-related hydrogen effects in an austenite-ferrite two-phase lightweight steel

: Fe-Mn-Al-C steels are of interest because of their low weight and the wide tunability of their strength-ductility combinations. Despite their high mechanical strength, these advanced steels, particularly when equipped with a multiphase microstructure, are often prone to hydrogen embrittlement (HE), a phenomenon characterized by the abrupt loss of the materials’ load-bearing capacity resulting from H ingress. This motivates us to develop strategies to make these alloys HE resistant. Here, we investigate and quantify the influence of interfacial B segregation on the HE resistance in a typical high-Mn, high-Al lightweight steel with a two-phase austenite-ferrite microstructure. Unlike the commonly reported beneficial role of B segregation in suppressing H-induced damage in single-phase alloys, we reveal that B segregation in the studied steel can lead to both detrimental and beneficial effects. On the one hand, it suppresses H-induced cracking along austenite-ferrite phase boundaries, primarily due to segregation-mediated interface strengthening. On the other hand, B segregation appears to promote H diffusion, increasing H contamination across wider microstructure regions and facilitating HE by H-induced cleavage fracture within ferrite—an alternative damage mode to interface cracking. The ingress and penetration of microstructures by H are closely connected with the kinetics and trapping features of H transport along grain boundaries (GBs, γ/γ and α/α) and phase boundaries (α/γ). We observe that interfacial H diffusion can be enhanced in B-doped steels, due to the repulsion and competition between segregated B and H atoms, as exemplified by ab initio calculations. The interplay between these two effects, resulting from interfacial B segregation, leads to an overall diminished HE resistance in the studied material compared to its B-free counterpart. These findings underscore the mechanistic complexity of B segregation in influencing HE resistance in multiphase steels, highlighting its dual role that must be accounted for in the development of future H-tolerant alloys.