Evolution of microstructure and nanoscale chemistry of Zircaloy-2-type alloys during nuclear reactor operation

Sammanfattning: Zirconium alloys are used as fuel cladding tubes in nuclear reactors. During reactor operation, these alloys are degraded by corrosion, hydrogen pickup (HPU), and radiation-induced growth, processes influenced by the alloying elements. The alloy Zircaloy-2, which contains Sn, Fe, Cr, Ni, and O as alloying elements, is commonly used in boiling water reactors (BWRs). This thesis deals with atom probe tomography (APT) investigations of Zircaloy-2 and a similar model alloy, Alloy 2, before and after up to nine years of BWR operation. Alloy 2 contains more Fe and Cr and exhibits lower corrosion and HPU. Less than 10 wt ppm each of Fe, Cr, and Ni was observed in the matrix of as-produced Zircaloy-2 and Alloy 2 of commercial heat treatment, a consequence of very low solubility and formation of second phase particles (SPPs). After reactor exposure, these elements were found in nanoscale clusters that were located at radiation-induced 〈a〉-type dislocation loops. The amount of Fe, Cr, and Ni in clusters increased with increasing fluence. There were two main types of clusters, spheroidal Fe–Cr clusters and disc-shaped Fe–Ni clusters. On average there were no large differences in clusters before and after acceleration in degradation, only small increases in cluster number density, cluster size, and cluster Cr content. 〈c〉-component loops decorated with Sn, Fe, and Ni were observed after but not before acceleration in degradation. Sn formed a network-like structure. No differences in cluster and matrix chemistry between Zircaloy-2 and Alloy 2 were observed after reactor exposure, indicating that the improved properties of Alloy 2 are related to additional Fe and Cr being located in SPPs. It was possible to analyse the materials using voltage-pulsed APT. Voltage pulsing was needed to reliably determine Fe–Ni cluster composition and shape. Fe–Cr clusters were observed also using laser-pulsed APT. Focused-ion-beam (FIB) preparation of APT specimens at room temperature resulted in phase transformation from α-Zr to γ-hydride, whereas cryo-FIB preparation did not. The average number of ions detected before specimen fracture was higher for γ-hydride specimens. There were no significant differences in clustering of Fe, Cr, and Ni between α-Zr and γ-hydride specimens.