Phase diagram and superabundant vacancy formation in Cr-H alloys

X-ray diffraction measurements on the Cr-H system were made using synchrotron radiation at high hydrogen pressures and high temperatures, and the phase diagram was determined up to p(H-2) = 5.5 GPa and T less than or equal to 1400 K. Three solid phases were found to exist; a bcc phase (alpha) of low hydrogen concentrations, x = [H]/[Cr]less than or equal to0.03 existing at low hydrogen pressures (less than or equal to4.4 GPa), and two high-pressure phases, an hcp (epsilon) phase at lower temperatures and an fee (gamma) phase at higher temperatures, both having high hydrogen concentrations xsimilar to1. A drastic reduction of the melting point is caused by dissolution of hydrogen. A gradual lattice contraction observed in the fee phase indicates the formation of superabundant Cr-atom vacancies (vacancy-hydrogen clusters). Thermal desorption measurements after recovery from high p(H-2), T treatments revealed several desorption stages including those due to the release from vacancy-hydrogen clusters and from hydrogen-gas bubbles, and allowed determination of relevant trapping energies.