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

Transformation kinetics of metastable body-centered cubic gamma-UMo phase in U-10 wt.% Mo alloy during annealing at sub-eutectoid temperatures of 500 degrees C and 400 degrees 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 gamma-UMo grain boundaries. At 500 degrees C, cellular transformation is the main mode of transformation. However, at 400 degrees C, cellular transformation and precipitation of ordered gamma ' U2Mo phase within gamma-UMo grain interior creates a competitive situation. The kinetics of the cellular transformation was found to be very sluggish at 400 degrees C compared to the kinetics observed at 500 degrees C as measured by the transformed area fraction. The hardness of gamma-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. Published by Elsevier B.V.