We have investigated the dissolution, segregation and diffusion of hydrogen (H) in a tungsten (W) grain boundary (GB) using a first-principles method in order to understand the GB trapping mechanism of H. Optimal charge density plays an essential role in such a GB trapping mechanism. Dissolution and segregation of H are directly associated with the optimal charge density, which can be reflected by the H solution and segregation energy sequence for the different interstitial sites. To occupy the optimal-charge-density site, H can be easily trapped by the W GB with the solution and segregation energy of −0.23 eV and −1.11 eV, respectively. Kinetically, such a trapping is easier to realize due to the much lower diffusion barrier of 0.13–0.16 eV from the bulk to the GB in comparison with the segregation energy, suggesting that it is quite difficult for the trapped H to escape out of the GB. However, the GB can hold no more than 2 H atoms because the isosurface of optimal charge density almost disappears with the second H atom in, leading to the conclusion that H2 molecule and thus H bubble cannot form in the W GB. Taking into account the lower vacancy formation energy in the GB as compared with the bulk, we propose that the experimentally observed H bubble formation in the W GB should be via a vacancy trapping mechanism.
Effect of carbon on hydrogen behaviour in tungsten: first-principle calculations