The hydriding kinetics of β-quenched uranium-0.1 wt.% chromium

Different heat pretreatments were applied to pure and alloyed (0.1 wt.%Cr) uranium in order to evaluate the effects of the resulting microstructural changes on the hydriding kinetics. Samples were studied in the temperature range 30-300°C under hydrogen pressures between 10² and 10⁵ Pa. In the high pressure range, well above the equilibrium pressures of the hydride, a significant increase in hydride front velocity was observed for the alloyed samples quenched from the β-phase, as compared with those gradually cooled down. This difference was not observed for pure (unalloyed) uranium samples. No β-phase residue could be identified in the quenched alloyed samples using X-ray diffraction. It is concluded that the rate change effects resulted from the presence of a metastable, highly strained α-phase supersaturated with dissolved chromium. Higher formation equilibrium pressures were obtained for the latter strained phase as compared with the well annealed alloyed samples. Arrhenius plots of the hydride front velocities yielded for the β-quenched alloyed samples, an apparent activation energy of 7.2 kcal g^-1 at. H (0.3 eV), similar to that of the well annealed uranium. The effect of aging of the alloyed β-quenched samples on the hydriding kinetics was also studied and the results were interpreted in view of the metal strain relaxation.