Uncovering the true 3D structure of zirconium hydrides using 3D-EBSD

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Abstract
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.

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