Samuel A. J. Armson

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.

Understanding the in-reactor corrosion behavior of zirconium alloys is essential for optimizing the lifetime of fuel assemblies. Recent advances in available experimental methods have enabled the characterization of oxide morphology, crystallography, and chemical heterogeneity with unprecedented detail for both autoclave and reactor formed oxides. Advanced high-resolution techniques have already improved the understanding of zirconium alloy corrosion performance. However, they are carried out on small volumes of material and require preparation of thin samples, which can lead to changes in the phase distribution in the oxide and often show varied results from different regions of a single bulk specimen. The present study utilizes high-spatial-resolution electron backscatter diffraction (EBSD) performed on bulk samples to produce spatially resolved microtexture data from nanograined zirconium oxide over a large area, which has not previously been possible. This advanced method of plan-view oxide texture analysis, alongside targeted focused ion beam cross-section measurements and substrate EBSD analysis, has revealed well-defined regions of monoclinic oxide grains that exhibit different textures depending on the orientation of the substrate grain on which they have formed. The observed variations in oxide texture have significant implications on any conclusions drawn solely from methods that are limited to the characterization of small areas—especially where sampling areas are smaller than the substrate grain size. Two competing mechanisms of oxide grain growth and nucleation are discussed, and detailed EBSD analysis illustrates a correlation between local oxide texture and corrosion rate. This analysis is performed on specimens of autoclave-tested Zircaloy-2 and ZIRLO and highlights differences in oxide texture development between the two alloys, indicating the significance of material composition and thermomechanical processing on corrosion behavior.