Evidence for the presence of U–Mo–Al ternary compounds in the U–Mo/Al interaction layer grown by thermal annealing: a coupled micro X-ray diffraction and micro X-ray absorption spectroscopy study

The systematic presence of the ternary phases U₆Mo₄Al₄₃ and UMo₂Al₂₀ is reported in a U–Mo/Al interaction layer grown by thermal annealing. This work shows, therefore, the low Mo solubility in UAl₃ and UAl₄ binary phases; it contradicts the hypothesis of the formation of (U,Mo)Al₃ and (U,Mo)Al₄ solid solutions often admitted in the literature. Using µ-XAS (micro X-ray absorption spectroscopy) at the Mo K edge and µ-XRD (micro X-ray diffraction), the heterogeneity of the interaction layer obtained on a γ-U_0.85Mo_0.15/Al diffusion couple has been precisely investigated. The UMo₂Al₂₀ phase has been identified at the closest location from the Al side. Moreover, µ-XRD mapping performed on an annealed fuel plate enabled the characterization of the four phases resulting from the γ-U_0.85Mo_0.15/Al and (U₂Mo + α-U)/Al interactions. A strong correlation between the concentrations of UAl₄ and UMo₂Al₂₀ and those of UAl₃ and U₆Mo₄Al₄₃ has been shown.