Phase Stability and Microstructure of Neutron-Irradiated Substoichiometric Yttrium Dihydrides

Abstract

The impact of the neutron-displacement damage on phase stability and microstructure of substoichiometric yttrium dihydrides (YHx, x <2) were investigated to assess their use as solid moderator in high-temperature mobile nuclear reactors. YHx specimens were, thus, subjected to neutron irradiations in the range of 0.1–2 displacements per yttrium atom (dpa-Y) in the temperature range of 536–878°C at the Oak Ridge National Laboratory's (ORNL's) High Flux Isotope Reactor (HFIR). YHx specimens were initially prepared at stoichiometry (H/Y) ratios of 1.69 and 1.83. HFIR-irradiated specimens were subjected to dimensional, optical, electron back scatter diffraction (EBSD), transmission electron microscopy, thermal desorption spectroscopy (TDS), and high-energy x-ray diffraction (HE-XRD) characterizations to investigate H retention characteristics. Overall, YHx exhibited notable structural and phase stability under short-term neutron-irradiation, except for the samples with significant silicon carbide interaction at high doses and temperatures. Basic dimensional and mass measurements were misleading for accurate assessment of H retention, as confirmed by EBSD phase maps, XRD line profiles, and TDS signals. Thus, it was noted that a robust H retention metric is needed for irradiated hydrides. Nanoscale cavities were observed as a result of the neutron irradiation in all samples. Although no clear impact of dose and irradiation temperature was determined, the initial H/Y ratio had an impact on the cavity number density where low H/Y specimens had high-resistance to cavity formation. The Y-vacancy cluster formation at the collision stage of the displacement cascade and the stabilization by H were determined to be the underlying mechanisms of the observed cavity microstructure.

Keywords: yttrium hydride, neutron irradiation, post-irradiation examination, cavity formation, irradiation-induced defects, electron microscopy