Kinetics of hydrogen and vacancy diffusion in iron: A Kinetic Activation Relaxation technique (k-ART) study

Hydrogen embrittlement (HE), the process by which a material is caused to fail prematurely when exposed to hydrogen, has plagued the metal design and application industry since its discovery. Yet, the exact mechanisms underlying the occurrence of HE remain incompletely understood despite extensive research efforts. The challenge stems from the fact that the mechanistic details on H diffusion and segregation within microstructures remain unclear. Consequently, new strategies are required to predict the various H trap states accurately and isolate key mechanisms of H kinetics.

Here, we use the kinetic Activation-Relaxation-Technique (k-ART), an off-lattice kinetic Monte Carlo approach with on-the-fly event catalogue building, to help us address the challenging problem in this subject area. Simulations using k-ART are able to provide migration barriers and collective diffusivities of H on microstructures without bias, including elastic effects both at short and long ranges. An ab initio ARTn calculation is used as well to confirm the validity of the results.

More specifically, we investigate hydrogen (H) and mono and divacancy-hydrogen complexes (VH_x and V₂H_x ) diffusion in BCC iron using k-ART to explore diffusion barriers and associated mechanisms for these defects. We uncover complex diffusion pathways for the bound complexes, with important barrier variations that depend on the geometrical relations between the position of the inserting Fe atom and that of the bound H. Since H is small and brings little lattice deformation around itself, these bound complexes are compact and H is fully unbound at the second neighbor site already. As more H are added, however, vacancies deform and affect the lattice over longer distances, contributing to increasing the VH_x complex diffusion barrier and its impact on its local environment. We find that the importance of this trapping decreases when going from mono to divacancy complexes, although diffusion barriers for these complexes increase with the number of trapped H.