Kieran Lynch

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.

Hydride precipitation in zirconium alloys is an inherently three-dimensional process; however, much of the current mechanistic understanding is derived from two-dimensional observations that cannot capture true hydride morphology, connectivity, or the associated strain fields. In this work, we present the first three-dimensional electron backscatter diffraction (EBSD) dataset of the zirconium–hydride system, enabling full volumetric reconstruction of hydrides, grain boundaries, and the surrounding zirconium matrix. The analysis reveals that all hydride packets studied either form at, or are connected to grain boundaries, even when they appear intragranular in two-dimensional (2D) sections. Hydrides exhibit complex, curved geometries and contain S3 variant boundaries within them whose presence and spatial distribution cannot be reliably identified from individual 2D slices. Localised misorientation within both the zirconium matrix and the hydride phase demonstrates that plastic strain localises preferentially in the zirconium grain that shares an orientation relationship with the hydride. Intense, localised KAM hotspots are consistently observed in the α-Zr matrix, especially where the hydride protrudes into the matrix and near hydride variant boundaries, suggesting that variant formation may be driven by strain within the matrix rather than within the hydride itself. This study further highlights the need for caution when correlating grain boundary character with hydride precipitation behaviour, as the true three-dimensional boundary morphology and connectivity cannot be inferred from a single two-dimensional section. Together, these findings establish 3D-EBSD as a critical tool for accurately interpreting hydride precipitation mechanisms and for underpinning experimentally informed modelling of hydride behaviour in zirconium alloys.