Analysis of EBSD image quality related to microstructure evolution in zirconium-niobium cladding to quantify the degree of recrystallization

Zirconium alloy claddings used in nuclear power plants, when subjected to final annealing after cold-pilgering, exhibit a recrystallization degree that renders them suitable for in-pile applications. Because the degree of recrystallization can influence fuel performance, its accurate and simple measurement is important from the perspective of producing nuclear fuel cladding. Recently, methods for calculating the recrystallization degree using information obtained from electron backscatter diffraction (EBSD) measurements have been reported, including a calculation method based on the distribution of the image quality values. However, no studies have been reported on determining the recrystallization degree of zirconium using image quality. To apply this method to zirconium, we first analyzed the correlation between microstructural changes and image quality of zirconium cladding. The appearance of newly formed microstructures was observed as a change in the image quality frequency distribution, and its shape was analyzed using various results obtained with the aid of EBSD software and previously reported details of the recrystallization behavior of zirconium. On the basis of this information, a new calculation formula suitable for recrystallization calculations for a zirconium-niobium cladding tube was derived and subsequently compared with the calculation formula presented in a previous study.