The authors gratefully acknowledge the EPSRC (Engineering and Physical Sciences Research Council, UK) and Westinghouse Electric Company for their financial support. - <a> loops were present from 0.1 dpa but were not aligned along the basal planes. - Sn is known to repel <a> loops. With low Sn alloying additions in w13, Sn bands were low in concentration. Perhaps this allowed <a> loops to diffuse throughout the microstructure. - In alloys with higher Sn content such as Low Tin ZIRLO and Zircaloy-2, loop alignment becomes progressively stronger. - Discrepancies between XRD- LPA and TEM densities may be due to small <a> loops forming that were not resolvable in TEM. With increasing dose these loops grew to resolvable sizes, decreasing XRD-LPA density with no effect on TEM. - <c> loops were first observed at 12.1 dpa. They increased in density to 33.5 dpa alongside breakaway IIG. - It is proposed that <c> loops form from the transformation of larger vacancy-type <a> loops. - Cu segregated to <c> loops. Cu may diffuse to vacancies, creating Cu-vacancy pairs which later exchange with SIAs. This may help to reduce loop growth rates, delaying breakaway IIG. The effect of Cu on irradiation-induced growth mechanisms in Zr alloys following neutron irradiation C. Andrew (1), W. Zhang (2), A. Carruthers (1), R. Thomas (1), X. Liang (1), Ö. Koç (1), J. Partezana (3), M. Limbäck (3), Z. Shah (3), M. Maric (1), R. W. Harrison (1), P. Frankel (1). (1) - University of Manchester (2) - University of Oxford (3) – Westinghouse Electric Company W13 alloy composition Element Atomic Percent (at.%) Zr 98.49 Nb 0.97 Sn 0.23 Cu 0.23 Fe 0.08 Figure 1: IIG profile of neutron irradiated Zircaloy-2, Low Tin ZIRLO™ and W13 alloys SPP formation in W13 SPP type Mean Diameter (nm) Area fraction (%) Zr2Cu 277 (± 119) 0.55 β-Nb 51 (± 36) 3.40 Zr(Nb, Fe)2 117 (± 71) 1.11 2. Baseline Microstructure Characterisation 3. Irradiated Microstructure Characterisation 4. Dislocation Loop Evolution - The matrix was depleted in Cu, Nb and Fe, and enriched with Sn. - Zr2Cu, β-Nb and Zr(Nb, Fe)2 Laves phase SPPs formed in 1 µm thick bands. Zr2Cu SPPs were coarse but sparsely distributed within the bands. This SPP distribution may have affected the IIG rate of W13. Figure 3: STEM-EDS map of SPP dispersion in the W13 alloy - Zr2Cu SPPs depleted in Cu content, forming a shell from the core to the interface. - Full dissolution of Zr2Cu SPPs was observed at 33.5 dpa. - Cu formed bands along the basal planes of grains, alongside Fe and Sn. Irradiation-induced Nb SPPs formed along the basal planes as well. - Cu likely diffused along the c-axis of grains to sinks on the basal planes to create this banding structure. - Cu banding was strongest near Zr2Cu SPPs. Cu was co-segregated with Fe and anti-segregated with Sn, forming an alternating banding structure. - Segregation between Sn and Cu/Fe bands was not as strong in the matrix. This may be because of the low alloying concentrations of Sn and Fe in the W13 alloy. Additionally, as a β- stabilizer, Cu was confined to the vicinity of SPPs and was not dispersed throughout the matrix. Figure 5: STEM-EDS map of a dissolved Zr2Cu SPP at 33.5 dpa, highlighted by a blue circle Figure 10: Cu segregation to <c> loops (dotted line) at 33.5 dpa Figure 8: <c> loop evolution in the W13 alloy observed through TEM Figure 7: <a> loop evolution in the W13 alloy observed through TEM Acronyms APT – Atom probe tomography DPA – Displacements per atom SIA – Self-interstitial atom SPP – Second phase particle References (1) – R. Konings, R.E. Stoller, Comprehensive Nuclear Materials, Elsevier, 2020. (2) – P.M. Kelly, R.G. Blake, The Characterization of Dislocation Loops in Neutron l Irradiated Zirconium, Philos. Mag. 28 (1973) 415–426. (3) – G. Pan, et al. Advantages gained by Optimized ZIRLO and AXIOM PWR cladding l l materials. ANS, 2016 Figure 2: The texture of the W13 alloy via electron backscatter diffraction analysis Irradiation information Irradiation