Kinetics of the δ to γ zirconium hydride transformation in Zr-2.5Nb Original Research Article

When the concentration of hydrogen exceeds the solubility limit in a metal matrix, metal hydrides may appear as precipitates that degrade the performance of the material. Neutron diffraction was combined with microscopy to study the δ to γ phase transformation of zirconium hydride precipitates in Zr-2.5 wt.% Nb. Specimens were heated to dissolve all hydrides, then cooled to holding temperatures ranging from 17–100 °C, to investigate the kinetics of transformation from the high-temperature δ-hydride to the low-temperature γ-hydride. The δ to γ transformation proceeds over a period of many hours, with a rate that increases as the holding temperature is decreased. Transmission Electron Microscopy images indicate that the boundary regions of hydride precipitates transform to the γ-phase, leaving a shrinking core of the δ-phase. The crystallographic orientations of the hydrides appear to be determined by the texture of the α-Zr matrix, even after complete dissolution and re-precipitation.