Phase studies of the Zr-H system at high hydrogen concentrations

Abstract

The phase boundaries of the Zr-H system have been defined at high hydrogen concentrations, on the basis of studies performed on zirconium hydrides containing 4 at % uranium. The methods used include hightemperature X-ray diffraction, electrical resistance, dilatometry, and, to a limited extent, hot stage microscopy. The (δ+ε) two-phase region was found to exist between ZrH 1.64 and ZrHi 1.74 at 24 °C. This region diminishes in width with increasing temperature and closes at 455 °C and ZrH 1.70. From this point, a single boundary, sloping toward higher hydrogen concentrations, separates the δ and ε single-phase regions. Precise lattice parameter measurements indicate that first-order transitions occur at the boundaries of the two-phase region. Theoretical considerations suggest that the δ й ε transition is second order.