Hydrogen behavior in ultrafine-grained palladium processed by high-pressure torsion

Hydrogen permeation tests were carried out on ultrafine-grained palladium processed by high-pressure torsion (HPT) and the results were compared with those on an annealed coarse-grained state. It is shown that hydrogen diffusion is similar above 200 °C but is enhanced at temperatures below 200 °C in the ultrafine-grained state. All samples were subjected to X-ray diffraction analyses after the hydrogen permeation tests. It is shown that hydride formation occurs at lower temperatures in both coarse- and ultrafine-grained states but the hydride formation temperature is reduced to a lower temperature in the ultrafine-grained state. This study suggests that grain boundaries of palladium act as diffusion pass but not as the sites for hydride formation.