abstract: Hydrogen embrittlement (HE) is a phenomenon that affects both the physical and chemical properties of several intrinsically ductile metals. Consequently, understanding the mechanisms behind HE has been of particular interest in both experimental and modeling research. Discrepancies between experimental observations and modeling results have led to various proposals for HE mechanisms. Therefore, to gain insights into HE mechanisms in iron, this dissertation aims to investigate several key issues involving HE such as: a) the incipient crack tip events; b) the cohesive strength of grain boundaries (GBs); c) the dislocation-GB interactions and d) the dislocation mobility. The crack tip, which presents a preferential trap site for hydrogen segregation, was examined using atomistic methods and the continuum based Rice-Thompson criterion as sufficient concentration of hydrogen can alter the crack tip deformation mechanism. Results suggest that there is a plausible co-existence of the adsorption induced dislocation emission and hydrogen enhanced decohesion mechanisms. In the case of GB-hydrogen interaction, we observed that the segregation of hydrogen along the interface leads to a reduction in cohesive strength resulting in intergranular failure. A methodology was further developed to quantify the role of the GB structure on this behavior. GBs play a fundamental role in determining the strengthening mechanisms acting as an impediment to the dislocation motion; however, the presence of an unsurmountable barrier for a dislocation can generate slip localization that could further lead to intergranular crack initiation. It was found that the presence of hydrogen increases the strain energy stored within the GB which could lead to a transition in failure mode. Finally, in the case of body centered cubic metals, understanding the complex screw dislocation motion is critical to the development of an accurate continuum description of the plastic behavior. Further, the presence of hydrogen has been shown to drastically alter the plastic deformation, but the precise role of hydrogen is still unclear. Thus, the role of hydrogen on the dislocation mobility was examined using density functional theory and atomistic simulations. Overall, this dissertation provides a novel atomic-scale understanding of the HE mechanism and development of multiscale tools for future endeavors. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2015

Nos travaux se focalisent sur l’impact des hétérogénéités structurales sur la diffusion de l’hydrogène dans les métaux. Dans ce cadre, des essais de perméation sont simulés par la méthode des éléments finis, afin de comprendre l’impact des caractéristiques métallurgiques sur les données extraites lors de ce type d’analyse. Afin de pouvoir séparer les différents mécanismes intervenant lors de la diffusion, l’étude est conduite en plusieurs étapes. A l’échelle de la membrane, les effets du piégeage et de la présence d’une couche d’oxyde à la surface du matériau sont considérés. Tandis que le piégeage et la couche d’oxyde diminuent tous deux la diffusivité effective, leurs effets sont opposés sur les concentrations en hydrogène en subsurface mesurées. D’autre part, les effets du piégeage lors de la désorption de l’hydrogène sont plus particulièrement étudiés. Il s’avère nécessaire de prendre en considération les fréquences de saut des atomes d’hydrogène afin de se rapprocher des données expérimentales. Ces premières études ayant portées sur des membranes homogènes, nous nous sommes consacrés, dans une dernière partie, aux effets de la microstructure sur la diffusion. Pour cela, nous considérons l’influence des joints de grains dits « généraux », qui se comportent comme des courts-circuits de diffusion pour l’hydrogène. La microstructure entraîne l’apparition d’effets d’échelle, lorsque l’épaisseur de la membrane se rapproche de la taille de grains. Qui plus est, en considérant un modèle à trois dimensions, les triples joints amplifient ces effets d’échelle, notamment dans le cas de matériaux nanocrystallins. / Our work focuses on the impact of structural heterogeneities on the diffusion of hydrogen in metals. In this context, permeation tests are simulated using the finite element methods, to understand the effects of the metallurgical properties on the data extracted with such analysis. To separate the different mechanisms occurring during diffusion, the study is led by several steps. At the scale of the membrane, the effects of trapping and the presence of an oxide layer at the surface of the material are considered. While the trapping and the surface layer both slow down the effective diffusivity, their effects are opposed on the measured hydrogen subsurface concentrations. On the other hand, the effects of trapping during the desorption are more specifically studied. It appears that taking into account the jump frequencies of the hydrogen atoms is required to get closer to the experimental data. Since the first studies dealt with homogeneous membranes, we focus in a last part on the effects of the microstructure on hydrogen diffusion. To do so, we consider the influence of “random” grain boundaries, acting as hydrogen diffusion short-circuits. Scale effects appear due to the microstructure when the membrane thickness approaches the grain size. Moreover, by using a three-dimensional model, triple junctions emphasize the scale effects, especially for nanocrystalline materials.