Investigation of the thermal stability of U – 9 wt. % Nb – 3 wt. % Zr alloys using drop calorimetry

Uranium metallic alloys are prominent candidates for the next generation of nuclear fuels in some reactor projects. Alloying elements, such as Nb and Zr, are added to improve the stability of the body-centered cubic phase of U (γ-U), but many aspects of the physical metallurgy of these alloys are still to be understood. Present work investigates the decomposition of the γ-U matrix obtained in an U — 9 wt% Nb — 3 wt% Zr alloy, quenched after hot rolling, by means of the Drop Calorimetry (DC) in the temperature range between 483 to 1023 K. The analyzed samples are characterized using Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD). Thermal analysis shows that matrix decomposition can be divided into five stages: (I) for samples treated up to approximately 525 K, no change occurs or eventually the γ-U phase starts transforming into the tetragonally distorted γ0 structure; (II) samples treated at around of 525 K and 645 K show the precipitation of γ0, followed by its partial transformation into the base centered monoclinic α′′ phase; (III) samples treated in the range of 645 and 710 K show the direct transformation γ→α′′, with α′′ now found with cellular morphology and occupying the entire matrix; (IV) samples treated in the range from 710 K to 760 K, show the partial diffusional decomposition of γ into α, with the remaining γ becoming supersaturated (denoted as γ2); (V) finally, samples treated between 760 and 810 K show the discontinuous transformation γ in to the base centered orthorhombic α and the saturated BCC γ3. The results sheds light to complexes diffusional mechanisms governing the nucleation, growth and stability of the phases in the studied ternary system.