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Abstract

Hydrogen embrittlement of iron and steels has been studied extensively in experiment and theory, but due to the complexity of the mechanisms involved it is still not fully understood. Two of the most discussed mechanisms for hydrogen embrittlement are hydrogen enhanced decohesion (HEDE) and hydrogen enhanced local plasticity (HELP). To address these mechanisms from a microscopic point of view it is important to obtain a detailed understanding of the stability and mobility of hydrogen in the vicinity of point and extended defects.

On an atomistic scale the diffusion of hydrogen in iron can be considered as a rare event compared to the time scale of molecular vibrations. This requires to study the dynamical evolution over an extended time scale up to seconds or longer going beyond the limits of classical molecular dynamics. Here we employ kinetic Monte Carlo (kMC) simulations with energetics based on density functional theory (DFT).

To investigate hydrogen diffusion close to vacancies and interfaces in bcc-Fe we use an adaptive kMC approach where the stable states and energy barriers are determined on-the-fly during the simulation and an a priori mapping to a lattice is not required. In our simulations we study the evolution of the local hydrogen concentration around these defects. In the case of vacancies we also consider the formation of H-vacancy clusters and the influence of hydrogen on the mobility of the vacancy itself.