Distribution of alloying elements in irradiated Zircaloy-2 by atom probe tomography and transmission electron microscopy

In order to investigate the distribution of solute atoms in a Zircaloy-2 matrix after neutron irradiation, atom probe tomography (APT) and transmission electron microscopy/energy dispersive X-ray spectroscopy (TEM/EDS) analyses were performed on irradiated Zircaloy-2 samples, which were irradiated in a commercial boiling water reactor (BWR). The neutron fluences were 0.3×10²⁵ n/m², 2×10²⁵ n/m², and 6×10²⁵ n/m² (E > 1MeV). The APT analysis revealed the formation of nanosized dense regions (clusters) of Fe in the Zr matrix; Cr was also detected in these clusters at grain boundaries of the Zr matrix. The number density and the Guinier radius of the clusters were estimated by the maximum separation method. The number density roughly increased with the neutron fluence, while the average Guinier radius increased with the neutron fluence. According to the TEM/EDS analysis, amorphization of Zr(Fe, Cr)₂ second phase particles (SPP) was observed and the composition ratio Fe/(Fe+Cr) gradually decreased at the fluence of 0.3×10²⁵ n/m². Moreover, Fe/(Fe+Ni) in Zr₂(Fe, Ni) SPP decreased at the fluence of 6×10²⁵ n/m². The Fe atoms dissolved from the Zr(Fe, Cr)₂ SPP at low fluence and from the Zr₂(Fe, Ni) SPP at high fluence. (author)