Microstructural characterization of atomized U 3 Si 2 powders with different silicon contents (7.4-7.8 wt%)
Abstract
Atomization processes are used to produce uranium-based powders for the manufacturing of dispersion fuel for nuclear research reactors. Whereas this process is considered worldwide as a reference for U-Mo powder production, its use for U 3 Si 2 is still limited. In this paper, the microstructure of as-atomized nearly stoichiometric (7.4 wt% Si) and hyperstoichiometric (7.6 and 7.8 wt% Si) U 3 Si 2 powders is studied in detail. A wide range of analytical techniques were applied: X-ray diffraction (XRD), scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM), electron backscattered diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDS) at both micrometer (SEM) and nanometer (STEM) scales. These analyses lead to an original description of the microstructure of these as-atomized U 3 Si 2 particles. It is shown that most of atomized particles contain only a few U 3 Si 2 grains, some being even monocrystalline. The main secondary phase present in hyper-stoichiometric batches is an U 20 Si 16 C 3like phase. Other minor phases are also encountered, some of them containing metallic impurity elements. These features are attributed to the uranium raw material composition and to a slight contamination by carbon during the powder synthesis. The nature and morphology of secondary phases present in U 3 Si 2 atomized particles appear thus to be linked not only to the silicon excess but also to the presence of impurities which probably strongly segregate during the very fast solidification of the alloy droplets. A slight superficial oxidation of particles also occurs and induces a local redistribution of silicon.
