An investigation was made of the dissolution, segregation and diffusion of hydrogen in a tungsten grain boundary using a first-principles method in order to understand the grain boundary trapping mechanism of H. Optimal charge density plays an essential role in such a grain boundary trapping mechanism. Dissolution and segregation of H were directly associated with the optimal charge density, which could be reflected by the H solution and segregation energy sequence for the different interstitial sites. To occupy the optimal-charge-density site, H could be easily trapped by the W grain boundary with the solution and segregation energy of −0.23 and −1.11eV, respectively. Kinetically, such a trapping was easier to realize due to the much lower diffusion barrier of 0.13–0.16eV from the bulk to the grain boundary in comparison with the segregation energy, suggesting that it was quite difficult for the trapped H to escape out of the grain boundary. However, the grain boundary could 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 grain boundary. Taking into account the lower vacancy formation energy in the grain boundary as compared with the bulk, it was proposed that the experimentally observed H bubble formation in the W grain boundary should be via a vacancy trapping mechanism.

Investigating Behaviours of Hydrogen in a Tungsten Grain Boundary by First Principle - from Dissolution and Diffusion to a Trapping Mechanism. H.B.Zhou, Y.L.Liu, S.Jin, Y.Zhang, G.N.Luo, G.H.Lu: Nuclear Fusion, 2010, 50[2], 025016