Kinetics of cellular transformation and competing precipitation mechanisms during sub-eutectoid annealing of U10Mo alloys

Transformation kinetics of metastable body-centered cubic γ-UMo phase in U-10 wt.% Mo alloy during annealing at sub-eutectoid temperatures of 500 °C and 400 °C has been determined as a function of time using detailed microstructural characterization by scanning electron microscopy, X-ray diffraction analysis, scanning transmission electron microscopy, and atom probe tomography. Based on the results, we found that the phase transformation is initiated by cellular transformation at both the temperatures, which results in formation of a lamellar microstructure along prior γ-UMo grain boundaries. At 500 °C, cellular transformation is the main mode of transformation. However, at 400 °C, cellular transformation and precipitation of ordered γ′ U2Mo phase within γ-UMo grain interior creates a competitive situation. The kinetics of the cellular transformation was found to be very sluggish at 400 °C compared to the kinetics observed at 500 °C as measured by the transformed area fraction. The hardness of γ-UMo grain interior also was measured and correlated with the observed microstructural evolution. The implication of these results on the low-enriched U-10 wt.% Mo alloy fuel fabrication process development is discussed.