Competitive precipitation and recrystallization in U-7.5Nb-2.5Zr alloy

Metallic nuclear fuel plates are nowadays an alternative to the ceramic ones in the sense that the uranium density can be increased at lower enrichment. Higher thermal conductivity is also a key factor favouring such fuels for power reactors. Uranium reacts promptly with oxygen and nitrogen at high temperatures to catastrophic corrosion due to non-protective oxide layers, which imparts hot forming processes. The gamma phase body centred cubic structure can be retained at room temperature by annealing the U-7.5Nb-2.5Zr (wt.%) alloy followed by quenching, where the deformation can be extensive. The resulted highly deformed gamma supersaturated structure is subjected further to competitive recovery/recrystallization and phase precipitation phenomena whose are studied in the work. The U-7.5Nb-2.5Zr alloy was melted into plasma and induction furnaces and afterwards annealed to gamma phase. The normalized alloy was cold rolled and underwent isochronal and isothermal treatments. The microstructure evolution was monitored by optical microscopy, X-ray diffraction analysis and hardness measurements. The results show the precipitation events of α" and α+γ₃ phases are dominant over recovery in the range 200°C < T < 500°C. Above 500°C the recrystallization is the main process leading to softening and initial Vickers hardness recovery. One refined gamma phase grain structure was obtained (∼8.0 μm) after annealing at 700°C for 2.5 hours.