In Situ Neutron Diffraction Study of Crystallographic Evolution and Thermal Expansion Coefficients in U-22.5 at.%Zr During Annealing

A uranium-22.5 at.%zirconium (U-10 wt.%Zr) ingot was manufactured by traditional arc casting. The sample was characterized by high-pressure preferred-orientation time-of-flight neutron diffractometry with in situ heating reaching 900°C. The phase transitions, lattice parameter ratios, and linear coefficients of thermal expansion were investigated experimentally. Contradictory to commonly referenced phase diagrams, the results of this work showed that the β-U+γ″-UZr dual-phase region is not present in U-22.5 at.%Zr. Rather, only two phase transformations were observed upon cooling. These transformations occurred at 655°C and 600°C and corresponded to γ-UZr → α-U+γ″-UZr → α-U+δ-UZr₂, respectively. The linear coefficients of thermal expansion were measured for α-U, δ-UZr₂, and γ-UZr. The thermal expansion of α-U was shown to be anisotropic in nature with significant lattice contraction in the 010_b direction. Similar anisotropy albeit to a lesser extent was observed in δ-UZr₂ with contraction in the 001_c direction.