Molecular statics simulation of the effect of hydrogen concentration on {112}<111> edge dislocation mobility in alpha iron

Abstract

To clarify the whole picture of hydrogen embrittlement (HE), an understanding of the elementary processes occurring during the fracture process is important. As one of the most important elementary processes of HE crack growth, the role of dislocation motion has been intensively studied. In this study, we performed molecular statics calculations to simulate the dislocation velocity in the presence of absorbed hydrogen atom from a gaseous hydrogen atmosphere. The dislocation motion was assumed to be a stress-dependent thermal activation process, and the energy barriers were investigated using the nudged elastic band method for the {112}<111> edge dislocation in alpha iron. In addition to the energy barrier for dislocation motion, those for hydrogen diffusion were also computed to evaluate the competitive motion of dislocations and hydrogen atoms. The hydrogen concentration was also evaluated using the hydrogen trap energy concept. The results indicate that an extremely low hydrogen concentration yields contradictory results such as softening or hardening depending on the applied stress. In contrast, with increasing hydrogen concentration, the dislocation velocity always decreases and results in hardening independent of the applied stress. Focusing on the dislocation motion, we propose a simplified classification map of hydrogen embrittlement mechanisms, which suggests the importance of the environment and stress conditions on the dominant mechanism of HE.