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

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 degreesC, 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. Crown copyright (C) 2003 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc. All rights reserved.