Vacancy trapping mechanism for hydrogen bubble formation in metal

We reveal the microscopic vacancy trapping mechanism for H bubble formation in W based on first-principles calculations of the energetics of H-vacancy interaction and the kinetics of H segregation. Vacancy provides an isosurface of optimal charge density that induces collective H binding on its internal surface, a prerequisite for the formation of ${\text{H}}_2$ molecule and nucleation of H bubble inside the vacancy. The critical H density on the vacancy surface before the ${\text{H}}_2$ formation is found to be ${10}^{19}–{10}^{20}\text{H}$ atoms per ${\text{m}}^2$. We believe that such mechanism is generally applicable for H bubble formation in metals and metal alloys.

Figure 1

(Color online) Isosurface of optimal charge for H binding at a vacancy. The larger blue balls and the smaller red ball represent W and H atoms, respectively. (a) Without H. The crosses mark the 6 minimum-energy H binding sites on the isosurface. (b) With one H.

Figure 2

(Color online) Atomic configuration and the isosurface of optimal charge for H for different number of embedded H atoms at the monovacancy. The atom labels are the same as in Fig. 1.

Figure 3

(Color online) The average H-embedding energy per H as a function of the number of H embedded at the monovacancy in W. The zero-point energy is the energy of H in the TIS far away from the vacancy.

Figure 4

(Color online) H diffusion energy profile and diffusion paths (arrows) in W. (a) H diffusion from one TIS to another in intrinsic W. (b) H diffusion toward an empty vacancy. Sites 1, 2, and 3 are the 3NN, 2NN, and 1NN TIS of the vacancy, respectively. Site 4 is one of the 6 most stable sites at the vacancy. (c) H diffusion toward a vacancy occupied with 6 H. Site 1, 2, and $3^{\prime }$ are the 3NN, 2NN, and 1NN of the vacancy, respectively. Site $4^{\prime }$ is the most stable site for the 7th H. The atom labels are the same as in Fig. 1.