Phase equilibria in the niobium-vanadium-hydrogen system

Abstract

The effect of vanadium additions to niobium on the metal-hydrogen phase equilibria has been studied. Measurements of the equilibrium H 2(D 2) pressure-composition-temperature isotherms for Nb 1 − x V x alloys with 0 ⩽ x < 0.2 were used to determine the depression of the α — α' critical temperature with increasing vanadium concentration. A simple lattice-fluid model guided reduction of the data. Changes in the triple point temperature as well as the shift of the ζ → ϵ phase transition were determined by differential scanning calorimetry measurements. A rapid overall depression was found, of the order of 7 K (at.% substituted V) −1, for the metal-hydrogen (deuterium) phase boundary structure when compared with the Nb-H system in the hydrogen concentration range of interest. The results explain the enhanced terminal solubility of hydrogen in this system found by Miller and Westlake. The changes in the phase equilibria are discussed in terms of the effect of hydrogen trapping and compared with the results of a cluster-variation calculation for random-field systems of MacGillivray et al., taking into account a distribution of H-site energies due to alloying.