LATTICE REARRANGEMENTS IN PD-RICH ALLOYS AT MODERATELY HIGH-TEMPERATURES AND PRESSURES

Research on the hydrogen solubilities in three different forms of Pd3M n are reviewed. These are the two ordered forms: L1(2), long period su perstructure (Al3Zr type), and the disordered form. Similar to the hyd rogen solubilities in Pd3Fe, studied earlier in Baranowski's laborator y, the ordered forms of Pd3Mn dissolve significantly more hydrogen tha n its disordered form. Dissolved H induces metal atom diffusion so tha t the disordered form of Pd3Mn orders at significantly lower temperatu res than in vacuo. It orders in the presence of dissolved H to the L1( 2) rather than the long-period super structure (lps) form, which resul ts from annealing in vacuo. In the alloy system Pd-Rh, dissolved H is shown to enhance metal atom diffusion leading to phase separation in a ccord with the miscibility gap phase diagram. In the case of Pd-Pt all oys dissolved H can lead to phase separation via a ternary equilibrium ; dissolved H is shown to enhance metal atom diffusion in this system.