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

We investigate hydrogen (H) and mono and divacancy-hydrogen complexes (VH$_x$ and V$_2$H$_x$) diffusion in body-centered-cubic (BCC) iron using the kinetic Activation-Relaxation Technique (k-ART), an off-lattice kinetic Monte Carlo approach with on-the-fly event catalog building, to explore diffusion barriers and associated mechanisms for these defects. K-ART uncovers 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, moreover, 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.