Grain-boundary diffusion and solubility of helium in submicrocrystalline palladium

The diffusion and solubility of helium in palladium with a submicrocrystalline structure are investigated by thermal desorption of helium from He-saturated specimens at temperatures T = 293–508 K and saturation pressures P = 0.1–35 MPa. As the saturation pressure rises, the effective diffusion coefficient increases, exhibits a plateau, and then decreases to its initial value. Along with the four plateaus discovered earlier, the solubility versus saturation pressure dependence in the range 25.5–35.0 MPa demonstrates a fifth plateau, where the solubility is as high as (3.0 ± 0.4) × 1017 cm−3. It is shown that the helium diffuses along grain boundaries, at which clusters (traps) consisting of eight to ten vacancies are localized, and dissolves in these clusters. The high value of Ceff in the fifth plateau is explained by pairwise merging of adjacent vacancy clusters. From the Deff(P) dependences, the vacancy clusters concentration is estimated as C* = 2.32 × 1016 cm−3. Within the experimental error, this value coincides with that obtained from the solubility data. Calculations of the energy of helium-defect interaction in submicrocrystalline Pd that are made using the molecular dynamics method support the experimental results.