A pressure-composition-temperature study of the zirconium/2.5 wt % niobium+ hydrogen system

Abstract

The solubility of hydrogen has been determined in the ( Zr + 2.5 wt % Nb)- H 2 system as a function of composition, temperature (300–950 °C) and hydrogen equilibrium pressure (0.5–760 mm Hg). From the isotherms, the position of the boundaries β−( β + δ) and ( β + δ)− δ were determined. The presence of niobium seems to shift these boundaries to lower hydrogen values and raise the hydrogen equilibrium pressure in the two phase region (β + δ). The equilibrium pressure temperature relation in the two-phase region (β + δ) was derived to be, log 10 p = −12 020/ T + 13.39, where p is measured in mm Hg and T in degrees Kelvin. The heat of formation of δ-hydride from saturated β-Zr is −54.8 kcal/mole H 2, as compared to −49.1 kcal/mole H 2 in unalloyed zirconiumhydrogen.