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.

We investigate the physical origin of H–He interaction in W in terms of optimal charge density by calculating the energetics and diffusion properties using a first-principles method. On the one hand, we show a strong attraction between H and He in W originated from the charge density redistribution due to the presence of He, driving H segregation towards He. This can block the permeation of H into deeper bulk and thus suppress H blistering. On the other hand, we demonstrate that He, rather than H, energetically prefers to occupy the vacancy centre due to its closed-shell structure, which can block H₂ formation at the vacancy centre. This is because He causes a redistribution of charge density inside the vacancy to make it ‘not optimal’ for the formation of H₂ molecules, which can be treated as a preliminary nucleation of the H bubbles. We thus propose that H retention and blistering in W can be suppressed by doping the noble gas elements.

When an impurity is doped in a solid, it inevitably induces a local stress, tending to expand or contract the lattice. Consequently, strain can be applied to change the solubility of impurity in a solid. Generally, the solubility responds to strain “monotonically,” increasing (decreasing) with the tensile (compressive) strain if the impurity induces a compressive stress or vice versa. Using first-principles calculations, however, we discovered that the H solubility can be enhanced by anisotropic strain in some bcc metals, almost independent of the sign of strain. This anomalous behavior is found to be caused by a continuous change of H location induced by anisotropic strain. Our finding suggests a cascading effect of H bubble formation in bcc metals: the H solution leads to H bubble formation that induces anisotropic strain that in turn enhances H solubility to further facilitate bubble growth.