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Abstract

Hydrogen enhanced decohesion (HEDE) is one of the many mechanisms of hydrogen embrit- tlement 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 is 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, 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. 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.