Phase transitions during artificial ageing of segregated as-cast U-Mo alloys

U-Mo alloys are a promising alternative in high-density fuel materials for use in research and test reactors, due to its resistance to swelling associated with the presence of the metastable γ-phase in the microstructure. It has been reported that increasing additions of Mo cause a beneficial delay in the decomposition of the γ-phase in U-Mo alloys during isothermal heat treatments. Analyses emphasized high temperature aging (>400 °C), where the microstructural evolution features were followed by low resolution light microcopy, but little information is available for lower temperature treatments. High resolution microstructural characterization techniques allow a more in depth analysis of the decomposition, as already shown for U-Nb alloys. In this work, phase transitions resulting from the decomposition of the γ-phase, or its variants, were investigated in as-cast U-Mo alloys with 5, 7 and 10 wt.% Mo, aged at 300 and 500 °C. Characterization techniques employed light microscopy, hardness, high resolution FE-SEM and EFM, which establishes contrasts for domains with different physical properties. The ageing of Mo-segregated regions, typical of the as-cast state, allowed the consideration of a broad range of Mo contents. For the low Mo regions, results were similar to those reported for U-Nb alloys, involving transformation twins and disorder-ordering mechanisms in the low temperature ageing and the formation of a non-lamellar constituent (NL3) in the high temperature ageing. The precipitation of oriented acicular or Widmanstätten γ′ platelets over a γ-phase matrix was observed for the hypereutectoid compositions of the U-Mo system, while the interdendritic regions with Mo contents close to the eutectoid composition were initially untransformed. This indicates a maximum in the γ stabilizing effect for the eutectoid composition, in opposition to the commonly accepted increased stabilizing effect for raised Mo additions.