The effect of zirconium-microalloying on hydrogen diffusion behavior and hydride phase transformation in yttrium: A first principles study

Yttrium hydride has been perceived as an excellent neutron moderator for high-temperature nuclear reactor environments, owing to the high density of hydrogen (similar to neutron mass) and superior thermal stability. There is a growing demand for large-scale, crack-free bulk yttrium hydride for the micro-nuclear reactor applications. However, fabricating crack-free yttrium hydride with desired geometry and configuration is challenging and the relatively high neutron absorption cross section of yttrium prevents its widespread use. Zirconium microalloying offers a promising solution by mitigating cracking during hydriding and enhancing neutronic performance due to zirconium’s low neutron absorption cross section, thus broadening the application of yttrium-based materials. A fundamental understanding of atomic-scale interactions between hydrogen atoms and alloying elements is essential for optimizing the processing of yttrium-based alloys and mitigate hydrogen-induced cracking. In this study, the first-principles calculations were employed to investigate hydrogen diffusion behavior and hydrogen-induced phase transformation in pure yttrium and yttrium-zirconium alloys. The results show that an optimal concentration of zirconium enhances the diffusion coefficient of interstitial hydrogen in the bulk material. Moreover, the preferred phase transformation pathway for yttrium hydride was identified, as {0001}_HCP/{111}_FCC & <1$\overline{2}$10>_HCP/<$\overline{1}$10>_FCC. These findings highlight the pivotal role of hydrogen in phase transformation process and demonstrate that low zirconium concentrations promote the phase transformation of yttrium hydride. The mechanistic insights gained will aid in the development of yttrium-based alloys for high-temperature moderators in advanced nuclear reactors.