Recrystallization and grain growth of U-5.5Â wt%Nb alloy

We studied the microstructures of the U-5.5 wt%Nb (U-5.5Nb) alloy with different thickness reductions induced by cold rolling and different heat treatments. The recrystallization and grain growth behavior of the U-5.5Nb alloy with 50% deformation were analyzed. Samples showed the typical rolling microstructure after plastic deformation. Microstructure analysis showed that increasing the deformation, annealing temperature, or annealing time can promote recrystallization. The size of recrystallized grains decreases with the increasing deformation. The isothermal kinetics curve of recrystallization shows a typical S-type behavior. The recrystallization index n is 1.58, which is lower than the ideal Johnson Mehl Avrami Kolmogorov (JMAK) value, i.e., 3. This indicates that the isothermal annealing of U-5.5Nb alloy at 750 °C is a one-dimensional recrystallization process. The recrystallization activation energy of U-5.5Nb alloy after annealing at 750 °C is between 314.47 and 555.84 kJ/mol. The grain growth index at 700 °C and 750 °C was calculated to be 2.24 and 2.21, respectively, which are slightly higher than the ideal value of 2. Values of grain growth activation energy for U-5.5Nb alloy after annealing for 30 min and 60 min between 700 and 900 °C are Q30min = 211.72 kJ/mol and Q60min = 186.56 kJ/mol, respectively. Both values are higher than the diffusion activation energy of niobium atoms in γ uranium, which is Qdiff = 138 kJ/mol. It may be the “solute drag effect” that led to the increase in the activation energy of grain growth with the decrease in the annealing time.