Phase Transformations and Microstructural Development in the U-10 Wt Pct Mo Alloy with Varying Zr Contents After Heat Treatments Relevant to the Monolithic Fuel Plate Fabrication Process

Decomposition of metastable, isotropic, body-centered cubic γ-phase in the U-10 wt pct Mo alloys with varying Zr contents was investigated as function of heat treatment parameters (i.e., temperature and time) relevant to the fabrication of monolithic fuel plate in development for research and test reactors. Phase constituents and microstructural characterization was performed using X-ray diffraction via Rietveld refinement, scanning electron microscopy with quantitative image analysis, and analytical transmission electron microscopy. Results were compared with the amount of equilibrium phases calculated using the lever rule from the available ternary phase diagrams. After sequential three-step heat treatment (900 °C for 168 hours, 650 °C for 3 hours, and 560 °C for 1.5 hours) decomposition of γ-phase was observed in alloys containing 2 wt pct or higher Zr. Decomposition of γ-phase occurred by the depletion of Mo in γ-phase due to the formation of Mo₂Zr. Formation of Mo₂Zr at 900 °C and 650 °C produced γ-phase with low-Mo content which in turn promoted a faster onset of eutectoid decomposition at 560 °C. The U-10Mo-4Zr and U-10Mo-10Zr alloys had the highest amount of Mo₂Zr, and exhibited the largest amount of γ-phase decomposition after the heat treatment at 560 °C. Cooling rate from the heat treatment at 560 °C did not influence the phase constituents of the U-10Mo-xZr alloys, but the slower cooling promoted the better-defined lamellae microstructure associated with eutectoid decomposition. Trace amount of δ-phase was also observed in the microstructure after the heat treatment at 560 °C, presumably due to local inhomogeneity in alloy compositions.