Observation of kinetics of γ zirconium hydride formation in Zr–2.5Nb by neutron diffraction

Neutron diffraction was employed to observe an isothermal transformation among zirconium hydrides in a commercial zirconium alloy. A specimen of Zr–2.5Nb, which contained about 200 mg deuterium/kg of alloy, was heated to 450°C for almost 17 h, then cooled directly to 17°C and held at this temperature for an extended time. A series of neutron diffraction patterns was collected during this thermal cycle. The diffraction patterns show that a small amount of the tetragonal γ-phase zirconium hydride appears soon after cooling, along with a predominant quantity of the cubic δ-phase hydride. Over three subsequent days at 17°C, the amount of γ-phase hydride increases while there is a corresponding reduction in the amount of δ-phase hydride.