One of the predominant factors causing hydrogen embrittlement is the interaction of H with dislocations, which leads to enhanced fracture. Extensive evidences have shown that H affects dislocation mobility in bcc and fcc materials. Atomistic simulations on the H-affected kinking process of a screw dislocation in Fe and the H-affected cross-slip process of a dissociated screw dislocation in Ni are conducted by the nudged elastic band method. In Fe, we find that when a kink pair nucleates at H, the activation energy is decreased by the transition of H to a stronger binding site, while it is increased by the transition to a weaker binding she. When a kink pair meets H during expanding, the sideward motion of the kink pair is impeded by H. We thus conclude that H-induced softening occurs as the results of H jumping out of the strongest binding site and kink-pair nucleation at the jumped H where the transition of H back to the strongest binding site occurs during kinking. H-induced hardening occurs as the results of kink-pair nucleation at the site where H is in the strongest binding and the transition of H to a weaker binding site during kinking and/or H-impeded sideward motion of kinks. In Ni, we find that the maximum binding energy of H is strongly dependent on the edge components of the partial dislocations. H binding in the stacking fault exerts no effect on the activation energy of cross-slip. H that is bound to the cores of the partial dislocations and moves with the dislocations during cross-slip leads to an increase of the activation energy and thus induces slip planarity. The increase of the activation energy for cross-slip is due to a net decrease of the H binding energy at the curved dislocations in the cross-slip process.
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