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) x 10(17) 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 C-eff in the fifth plateau is explained by pairwise merging of adjacent vacancy clusters. From the D-eff(P) dependences, the vacancy clusters concentration is estimated as C* = 2.32 +/- 10(16) 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. (c) 2005 Pleiades Publishing, Inc.