[SY-E2] Contribution of defects on the anisotropic diffusion behaviour of hydrogen in nickel single crystals
It has been proven that hydrogen contributes significantly to the embrittlement of metals. In particular, several experimental studies have shown that hydrogen promotes the formation of superabundant vacancies, which can act as an embrittlement mechanism. Recently, anisotropic hydrogen diffusion behaviour has been put in forward experimentally in external stress-free Ni single crystals [1]. Therefore, it has been suggested that defects, including superabundant vacancies and vacancy clusters, and their elastic displacement fields may be responsible to the observed anisotropic diffusion behaviour.
In this study, we conduct DFT-based ab initio calculations to study hydrogen diffusion in an elastic strain field induced by the solute, vacancies and vacancy clusters in Ni single crystal. Temperature effects have been taken into account from the extension of the calculations to the free energy including vibration and electronic excitations contributions. The diffusion tensor in the solid under stress is determined from the hydrogen elastic dipole and the total strain field induced by the defect. The latter is calculated in an anisotropic elasticity media according to the previous work of Larché and Cahn [2]. We found that the elastic displacement fields induced by the solute and the vacancies are not strong enough to reproduce the anisotropic diffusion behaviour observed experimentally. Therefore, we turn our study to the effect of the vacancy clusters, which can act as gas bubbles. These vacancy clusters have been observed using TEM on H-charged Ni single crystals [3]. The presence of such clusters and their elastic displacement fields can reproduce the diffusion coefficient tensor observed experimentally and suggests the large contribution of these defects on the anisotropic diffusion of hydrogen in Ni single crystal. Finally, we discuss some implications of the presence of vacancy clusters on the initiation of embrittlement based on the instability of such defects according to the work of Bockris and Reddy [4].
[1] J. Li, et al., Sci Rep, vol 7, 45041, 2017
[2] F. C Larché et J. W Cahn, Acta Metall, vol 30, p 1835-1845, 1982
[3] G. Hachet, et al., Acta Mater, vol 148, p 280-288, 2018
[4] J. O’M Bockris and S. U. M. Khan, Surface Electrochemistry: A Molecular Level Approach, Plenum Press, 1993
In this study, we conduct DFT-based ab initio calculations to study hydrogen diffusion in an elastic strain field induced by the solute, vacancies and vacancy clusters in Ni single crystal. Temperature effects have been taken into account from the extension of the calculations to the free energy including vibration and electronic excitations contributions. The diffusion tensor in the solid under stress is determined from the hydrogen elastic dipole and the total strain field induced by the defect. The latter is calculated in an anisotropic elasticity media according to the previous work of Larché and Cahn [2]. We found that the elastic displacement fields induced by the solute and the vacancies are not strong enough to reproduce the anisotropic diffusion behaviour observed experimentally. Therefore, we turn our study to the effect of the vacancy clusters, which can act as gas bubbles. These vacancy clusters have been observed using TEM on H-charged Ni single crystals [3]. The presence of such clusters and their elastic displacement fields can reproduce the diffusion coefficient tensor observed experimentally and suggests the large contribution of these defects on the anisotropic diffusion of hydrogen in Ni single crystal. Finally, we discuss some implications of the presence of vacancy clusters on the initiation of embrittlement based on the instability of such defects according to the work of Bockris and Reddy [4].
[1] J. Li, et al., Sci Rep, vol 7, 45041, 2017
[2] F. C Larché et J. W Cahn, Acta Metall, vol 30, p 1835-1845, 1982
[3] G. Hachet, et al., Acta Mater, vol 148, p 280-288, 2018
[4] J. O’M Bockris and S. U. M. Khan, Surface Electrochemistry: A Molecular Level Approach, Plenum Press, 1993