| Abstract | 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 [delta] to [gamma] 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 [delta]-hydride to the low-temperature [gamma]-hydride. The [delta] to [gamma] 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 [gamma]-phase, leaving a shrinking core of the [delta]-phase. The crystallographic orientations of the hydrides appear to be determined by the texture of the [alpha]-Zr matrix, even after complete dissolution and re-precipitation. |
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