THERMODYNAMIC STUDIES OF THE ABSORPTION OF HYDROGEN BY PD-TI(ZR) SOLID-SOLUTION ALLOYS

The solubility of hydrogen in Pd - Ti and Pd - Zr solid solution alloys containing up to 6.0 at.% Ti and Zr has been measured at temperatures between 273 K and 433 K and hydrogen pressures up to 1000 Torr. The relative partial molar enthalpy, DELTA-H(H)0, at infinite dilution for Pd - Ti alloys shows a minor minimum around 4 at.% Ti, in spite of the Pd lattice contraction on Ti substitution. For Pd - Zr alloys the DELTA-H(H)0 values increase in exothermicity regularly with Zr content. The relative partial molar entropy, DELTA-S(H)0, at infinite dilution in both alloy series decreases with the solute contents. The apparent H-H attractive interactions, g1, in Pd - Ti(Zr) alloys decrease with alloying metal content, and the trend of the g1 value with Ti content is dissimilar to those found for other lattice contracted alloys. The stability of the beta-phase hydride in Pd - Ti alloys decreases with Ti content and the beta-phase hydride in Pd - Zr alloys also becomes less stable with Zr content, albeit by a small amount, despite being a lattice expanded alloy system. Solvus compositions at the alpha-max phase boundary in both Pd - Ti and Pd - Zr alloys are almost equal at the same alloying compositions of each alloy series, although increasing with alloying metal content in both cases.