Interface instabilities in hafnium hydride entrained iron metal matrix composites

The chemical interactions in Fe-HfH₂ metal matrix composites (MMCs) are studied across multiple length scales to elucidate the decomposition of the parent phases and corresponding reaction zone physics during direct current sintering. Fe-HfH₂ composites were synthesized with increasing as-mixed hydride contents of Fe-25% HfH₂, Fe-40% HfH₂, Fe-55% HfH₂, and Fe-70% HfH₂ (all in vol. %) to demonstrate the ability to achieve sintered MMCs with target hydride contents. Samples were probed across multiple length scales through a multi-modal workflow employing x-ray diffraction, scanning electron microscopy and segmentation analysis, and synchrotron techniques including hard x-ray fluorescence mapping and nanoprobe x-ray absorption near-edge structure measurements. Under the selected sintering temperature and pressure conditions, hydrogen evolution is seen to evolve through parallel paths: thermal decomposition from during the transformation of HfH₂ to HfH_x<2 and through subsequent reaction with the Fe matrix leading to intermetallic phase formation. Specifically, HfFe and HfFe₂ intermetallic formation accelerates the release of hydrogen with a subsequent HfO₂ phase forming at grain boundaries. For this MMC, the consumption or loss of hydrogen can be considerable in compacts with initial hydride loading of 25%-40% HfH₂ approaching 83% hydrogen loss for the lower volume fraction composites. Increasing the volume fraction of HfH₂ to 70% enhanced the retained hydrogen content to 53% and attributed to the reduced interfacial area intrinsic to the increased HfH₂ loading in this MMC.