Effect of β Phase Decomposition on Recrystallization in α-Zr Region in Zr-2.5%Nb Pressure Tube Material

The effects of beta phase decomposition on recrystallization and texture variation in Zr-2.5% Nb alloy pressure tube material were investigated. Isothermal annealing was conducted at 350 to 550 degrees C for 240 hours, and isothermal annealing was performed at 500 degrees C for 240 to 3,000 hours. The recrystallization and texture variation were analyzed by inverse pole figure variation using the XRD and EBSD methods. Annealing in alpha-Zr region at below 610 degrees C induced recrystallization and texture variation in the alpha-Zr. These results differ from those from a previous annealing study of the alpha+beta region at 750-830 degrees C. Annealing above 400 degrees C for 240 hours caused beta-Zr decomposition into beta-Nb. The decomposition of the beta-phase by annealing above 475 degrees C caused a contraction of 7.5% in the d((110)) spacing in the beta-phase, and a reduction in the volume fraction of the beta phase by about 80%. It seems that the stress internally formed by the lattice contraction of the beta-phase provides the driving force for recrystallization. In addition, it suggests that the newly formed alpha-Zr produced by beta phase decomposition provides new nucleation sites for recrystallization and causes texture variation in the alpha-Zr. The reason why the recrystallization and the texture variation occurs only in the alpha-Zr stable region at below 610 degrees C is discussed.