Hydrogen diffusion in the bcc metals

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Abstract
Hydrogen diffuses rapidly in the bcc metals even below room temperature. Thus, hydrides can easily form at low temperatures. We have determined the thermodynamic parameters of equilibrium between the bcc solid solutions and the hydride phases in the temperature range 150°K to 300°K for the V−H and Ta−H systems. The presence of other interstitials, especially oxygen, greatly affects the hydride formation, especially for the Ta−H binary system. The oxygen atoms, which are immobile, appear to trap the diffusing hydrogen atoms in O−H complexes, greatly retarding their diffusion rate. Thus, supercooling occurs readily, and the establishment of thermodynamic equilibrium requires very slow cooling and heating rates. In addition, the slower diffusion rate of deuterium, even in oxygen-free tantalum, demands slow cooling and heating rates for attainment of equilibrium. Data obtained in this experiment together with other information concerning the Nb−H system, shows significant trends in the binary systems V−H, Nb−H, and Ta−H.