Abstract
Hydrogen embrittlement finds its origin at the atomic scale, and more precisely comes from the interactions between H and the wide variety of the metal defects (dislocations, grains boundaries, interfaces, vacancy/H clusters, point defects…) [1]. In this work, we specifically focus on the threefold interaction between H, surfaces and vacancies, from both a theoretical and an experimental approach. A variety of experiments are performed on the 3 surface orientations {100}, {110} and {111} to question this synergy through different properties. The single crystals are H charged electrochemically. The crystallographic orientation impact on the subsurface H diffusion gradient is highlighted by a potentiostatic double step technique before and after cyclic deformation. The evolution of the diffusion process on strained surfaces due to slip band emergence and a probably higher vacancy concentration (favoured by both H absorption and fatigue solicitation) is then discussed. We confront such results with atomistic simulations in order to have a deeper understanding of the key mechanisms involved. Density Functional Theory (DFT) calculations are performed for small systems [2] and a force field derived from an empirical potential otherwise [3]. This permits to probe the effect of different H and vacancy concentrations on the properties of interest while allowing a comparison between the two approaches. Then, the energetic landscape of numerous H and H/vacancy systems in the vicinity of multiple surfaces is explored. The H-defects interaction and formation free energy at stable points are calculated to characterize the local H solubility. The same analysis is carried out at saddle points to catch the diffusion process. We show that H and the H/vacancy couple interact with the elastic field of the surface with globally attractive interactions. Afterwards, we confront our simulation results to the standard elasticity theory.