Pressure-composition-temperature relationships in niobium alloy-hydrogen systems

Abstract

Pressure-composition-temperature relationships have been determined for the solution of hydrogen in a series of alloys of niobium with Ti, Mo, Cu, Pd, Ni, Sn, and Al, over the temperature range 623–1123 K and at pressures up to 1 atm. All the niobium alloys, except with titanium, have a lower affinity for hydrogen than has pure niobium. This is reflected both as a decrease in the solubility of hydrogen at any reference temperature and pressure, and also as an increase in the relative partial molar enthalpy of solution of hydrogen at infinite dilution (ΔH̄∞H).The extent to which each alloying element affects the solubility of hydrogen in niobium cannot be accounted for on the basis of changes in the electron/atom ratio of the metal. Instead, the results are interpreted as evidence that the band structure and density of states of niobium are modified in different ways, depending on whether the alloying metal is a transition or non-transition element.It is shown that the large changes in ΔH̄∞H between titanium, niobium and molybdenum, are not due to the differences in the density of states of the metals. The difference in the density of states of titanium and molybdenum could account for only about 10 K J mol−1 of the observed 210 K J mol−1 change in ΔH̄∞H.