Observations of Solute Redistribution Obtained using Atom Probe Tomography Examinations of Bulk Zircaloy Neutron Irradiated at Nominally 410°C

Atom Probe Tomography (APT) examinations were performed on bulk regions of alpha-annealed and beta-treated Zircaloy-2 following irradiation in the Advanced Test Reactor (ATR) to neutron fluences of 2.9-3.1 × 1025 n/m2 (E>1 MeV). Irradiation at higher temperatures of nominally 410°C resulted in about 10-20% less hardening than observed in the literature for irradiations at 260-326°C. Clustering of Fe, Sn, and Cr solute into approximately spherical shaped clusters at features consistent with the size and shape of dislocation loops was observed. In some cases, these clusters were observed to form in layers along the basal plane orientation in the location of loops. The formation of clusters at the dislocation loops produces the high barrier strength that is needed for a hardening model to more closely correlate with the measured irradiation hardening. Individual clusters not located at loops are an additional hardening barrier. The average size, Number Density (ND), and composition of Fe and Sn clusters were generally similar for alpha-annealed and beta-treated Zircaloy-2 with the exception of differences in Sn cluster ND. Local variation in the type and distribution of clusters is observed that is correlated with local differences in microstructure, precipitates, and local microchemistry.