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Abstract

Hydrogen enhanced decohesion (HEDE) is one of the many mechanisms of hydrogen embrittlement of structural materials, such as iron and iron alloys. It is expected that grain boundaries (GBs) play a critical role in this mechanism, as they can provide trapping sites or act as hydrogen diffusion pathways. At the same time, H in the microstructure will interact with different crystallographic defects and both, the total solubility of H in the system and the local concentration at the grain boundaries depend on the chemical potential of H and the stress distribution in the microstructure. The understanding of this process at the atomic level, which enables a prediction of the H distribution, is therefore fundamental for developing methods that can mitigate the detrimental effects of H. Here we present the results of density functional theory calculations of the solubility of H and its effect on the cohesive strength at the Σ5(310)[001] and Σ3(112)[11̄0] symmetrical tilt GBs in bcc Fe [1, 2], as an example of a very open and a very close packed grain boundary structure. To this effect a method to identify the segregation sites for more than one H atom per structural unit at the GB plane is proposed and we investigate the dependency of the H solution energy on local concentration and stress [3]. The results indicate that at local concentrations beyond one monolayer, H leads to a significant reduction of the cohesive strength of the GB planes, more pronounced at the Σ5 GB due to a more open local atomic environment. However, at finite tensile stress H solution becomes more favorable at the Σ3 GB, as opposed to the case of zero stress. This suggests that under certain conditions stresses in the microstructure can lead to a re-distribution of H to the stronger grain boundary, which opens a new pathway to designing H-resistant ferritic steel.