Solute distribution and second phase particle characteristics in a novel Zr-Nb-Sn-V-Fe alloy

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Abstract
A novel Zr-Nb-Sn-V-Fe alloy has been studied using electron microscopy with energy-dispersive X-ray spectroscopy, atom probe tomography (APT), and X-ray diffraction to examine the role of V additions to Zr alloys used in the nuclear industry. The focus of this investigation is on solute distribution and second phase particles (SPPs), both of which influence the in-reactor performance of Zr alloys. Zr-V spatial distribution maps obtained via APT suggest that V is an interstitial element in α-Zr with a very low solid solubility on the order of 80 atomic ppm. Therefore, the majority of V precipitates out of solid solution as SPPs. Two distinct SPP types were identified. A body-centred cubic β-Nb phase containing small amounts of V (∼6 at.%) was observed to have a Nb- and V-enriched region surrounding the SPP approximately 4 nm from the interface. It is hypothesised that localised strains in the matrix around the SPP restrict solute transport from the matrix to the growing SPP. The second SPP type identified had an average composition of 30Zr-42V-14Nb-13Fe (at.%) determined via APT, although the chemical composition was heterogeneous throughout individual SPPs. Based on the measured composition, a stoichiometric designation of Zr(V,Nb,Fe)2 is proposed. Crystallographic analysis showed structural heterogeneity within the SPPs, with evidence indicating the hexagonal C14 Laves phase. The observed chemical and structural variations suggest that this phase is metastable or undergoing a sluggish phase transformation during the final heat treatment. These findings help further the understanding of material microstructure and phase transformations in next generation Zr cladding alloys.

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