Phase transformation studies in U–Nb–Zr alloy

Highlights: • We employ drop calorimetry under up-quenching mode to detect alloy transformation kinetics for the first time. • Two processing windows for rolling U–Zr–Nb alloy into γ-phase are found at 200 and 400 °C. • Volume changes are measured for the main transformations. • We infer fast transformations from the alloy heating enthalpy curve. - Abstract: Phase transformation diagrams provide fundamental informations for designing thermomechanical processes being a must regarding uranium alloys nuclear fuels. The work shows the evaluation of a kinetic transformation diagram for U–7.5Nb–2.5Zr (wt.%) based on both calorimetry experiments and dilatometry allied to X-ray diffraction analysis. Calorimetry measurements in scanning and drop modes can detect enthalpies of heating and transformation onset points from ambient up to select isotherms while the dilatometer is used to scan for sample volume changes related to phase transformations. The resulted kinetic diagram shows the gamma phase is stable for this alloy, guiding the rolling deformation process to temperature ranges where this phase remains for longer periods. Comparing to the literature results, the low temperature transformation (300–400 °C) is shifted to longer times accordingly to the disclosed TTT kinetic diagram. Therefore, two forming process windows can be proposed at 200 °C and 400 °C neighborhood where gamma-phase remains for enough time to accomplish total reduction