Multiscale modeling of hydrogen diffusion in iron considering the effect of dislocations

Abstract: Modeling hydrogen diffusion and its absorption in traps is a fundamental first step towards the understanding and prediction of hydrogen embrittlement. In this study, a multiscale approach which includes DFT simulations, OkMC, and phase-field dislocations, is developed to study the movement of hydrogen atoms in alpha-iron crystals containing dislocations. At the nanoscale the interaction energies of hydrogen on different sites of the iron lattice are studied using DFT. At the microscale, this information is used to feed a lattice object kinetic Monte Carlo code (OKMC) which aims to evolve the arrangement of a large set of hydrogen atoms into the iron lattice considering point defects and the presence of dislocations. At the continuum level, an array of dislocations is introduced using a phase-field approach to accurately consider their elastic fields and core regions. The OKMC model includes both the chemical energies of H and vacancies and the elastic interactions between these point defects and the dislocations. The elastic interaction is obtained by an FFT-based approach which allows a very efficient computation of the elastic microfields created by the defects in an anisotropic medium. The framework has been used to obtain the diffusivity tensor of hydrogen as a function of the external stress state, temperature, and the presence of dislocations. It has been found that dislocations strongly affect the diffusivity tensor by breaking its isotropy and reducing its value by the effect of the microstresses around the dislocations.