In many technological applications of advanced materials, hydrogen embrittlement (HE) is a major concern as it can penetrate in most metals and degrade their properties, performances and its lifetime. Despite numerous efforts in the past decades during which many microscopic mechanisms were proposed, a clear understanding of H embrittlement mechanisms has not been achieved yet. Since HE processes occur on an atomic-scale, the exact mechanisms leading to HE are not easily identified experimentally. One possible improvement would be to use atomic-scale simulations to try to capture details of deformation and fracture processes at the atomic level, enabling the investigation of relevant microscopic mechanism. In such context, the goal of this PhD work is to understand and quantify H interactions with defects like vacancies, dislocations and cracks in fcc metals through multi-scale modeling. The study is organized in four main parts.
In the first part, we employed first principle calculations (density functional theory) to describe H interaction with a vacancy in Nickel. More specifically, the segregation energies of multiple H atoms in a single and di-vacancies was computed. Two characteristic energies were found which clarify the experimental peaks observed in Thermal Desorption Spectra in the literature. The equilibrium concentrations of H-vacancy clusters was then evaluated, under conditions relevant to HE and stress corrosion cracking (SCC) of Ni based alloys (nuclear industry), by Monte Carlo simulations and a thermodynamic model developed from our DFT data.
In the second part, we quantified the trapping effect of vacancies on H diffusion in Nickel. With DFT computed jump barriers related to H trapping and de-trapping in vacancies, we employed accelerated Kinetic Monte Carlo (KMC) simulations to evaluate H diffusion coefficient as a function of vacancy concentration and temperature. The effect of equilibrium and out of equilibrium
vacancies on H trapping and H induced damage was also discussed.
In the third part, we studied the diffusion of coupled H-vacancy clusters in Ni based on combined DFT and statistical method. DFT calculations of vacancy jump barriers were performed relevant to clusters containing from one to six H inside the vacancy. With these computed barriers and previous calculated concentrations of H-vacancy clusters, a simple stochastical model similar to the KMC processure was developped to estimate the diffusion coefficient of H-vacancy clusters as a function of H concentration and temperature.
In the last part, we studied the interaction of hydrogen with a blunted crack tip in Aluminum by combined EAM (semi-empirical interatomic potential) and DFT calculations. Embedded atom method (EAM) potential simulations were performed to evaluate the H effect on dislocation emission from a blunted crack tip under mixed mode loading. This phenomenon can be understood by the H induced change of the unstable stacking fault energy (gus) in Rice's model. Therefore, DFT and EAM calculations of gus were performed with effects of H and mixed mode loads taken into account. In addition, H diffusion in the core of an incipient partial dislocation was studied. |