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The contribution of the grain boundary engineering to the problem of intergranular hydrogen embrittlement

Abstract : The mobility of hydrogen in metals is a key parameter for understanding the basic mechanisms of hydrogen embrittlement (HE). This problem is directly related to the mechanisms of diffusion and trapping of hydrogen within a crystal lattice. These mechanisms depend on the various microstructural heterogeneities and in particular the crystalline defects. In our work, we have focused on the diffusion and trapping of hydrogen in two elementary systems: nickel single crystals and bi-crystals. We developed a methodology combining experimental tools (electrochemical permeation / TDS, HRTEM, EBSD) and numerical methods (FEM-COMSOL / EAM-LAMMPS). The results obtained on the single crystals show a dependence of the diffusion coefficient of hydrogen with the crystallographic orientation and the hydrogen content. The thermodynamic analysis of the nickel-hydrogen-vacancy system shows a dependence of the chemical potential of hydrogen with the stress state induced by the formation of clusters of vacancies associated with the presence of hydrogen. The anisotropic character of the diffusion is then explained by the anisotropy of the elastic properties of the crystal lattice and the presence of these clusters. Moreover, we have characterized the processes of diffusion and trapping of hydrogen for nickel bi-crystals with different free volumes. The segregation energy of hydrogen depends on the nature of the site (the local free volume and the mechanical energy associated with the incorporation of solute). The diffusion of hydrogen is directly influenced by the nature of the grain boundary (the free volume and the distribution of the segregation sites). Our results, at the atomic scale, show a correlation between the solubility and the free volume of the grain boundary. The grain boundaries with a higher free volume have more favorable diffusion paths for hydrogen than in the crystal lattice and at the same time more segregation sites.
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Jiaqi Li. The contribution of the grain boundary engineering to the problem of intergranular hydrogen embrittlement. Materials. Université de La Rochelle, 2017. English. ⟨NNT : 2017LAROS037⟩. ⟨tel-01941726⟩

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