Effect of S on H-induced grain-boundary embrittlement in γ-Fe by first-principles calculations

The effect of interstitial impurities (H and S) on the atomic, electronic structure, and mechanical properties of the y-Fe M5 (021) grain boundary (GB) was investigated via first principles calculations. H atoms act as an intergranular embrittler in the M5 GB due to a reduction in the charge density between the Fe atoms connected to the grains, whereas H and S co-segregation produces more pronounced embrittlement behavior, resulting in intergranular fracture. The S-induced embrittlement plays a crucial role in the H and S segregation, due to a combination of the structural and chemical effects. The fracturing of M5 GB due to S and H segregation is a two-step process. The first step is the breaking of Fe-Fe bonds in the GB, followed by the breaking of the remaining Fe-S bonds in the second step, resulting in the complete separation of the two grains. Moreover, the S atom can slightly compensate for the embrittlement induced by H, because some of the Fe atoms that obtain electrons from the S atoms can provide more electrons to the H atoms, and thus, they can compensate for the electrons that must be acquired from other Fe atoms. We call this "the electrons compensating effect" and this effect is helpful in the design and alloying of steels that are resistant to H embrittlement. (c) 2021 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.