Unveiling the formation of nanotwin-mediated metastable ζ-hydrides in fatigued Zr-1Nb-0.01Cu cladding tube

The impact of internal pressure fatigue treatment on the α-Zr matrix and hydride precipitates in Zr-1Nb-0.01Cu cladding tube has been systematically investigated using synchrotron-based high-energy X-ray diffraction, electron backscatter diffraction and transmission electron microscopy techniques. The evolution in d-spacings of the basal plane (0002), the pyramidal plane {10 1 ¯ 1} and the prismatic plane {10 1 ¯ 0} at azimuths from 0° to 360° indicates that the internal pressure fatigue treatment has imparted a degree of plastic deformation upon the α-Zr phase, which is further confirmed by the presence of {10 1 ¯ 2} deformation twins within the α-Zr phase. Concurrently, the prior internal pressure fatigue treatment at 400 °C significantly increased nucleation sites for zirconium hydrides during the subsequent furnace cooling process. High-density dislocations can be seen in both the δ-hydride precipitate ({111}<110> type) and the α-Zr matrix ({10 1 ¯ 0}< a> type) near the hydride-matrix interface. Additionally, δ-hydride {111}<11 2 ¯ > nanotwins were observed within the micro-sized δ-hydrides in the fatigued cladding tube, resulting from the nucleation and growth of intra-granular δ-hydrides at different regions of the same parent α-Zr grain. Furthermore, the HCP-structured ζ-hydride phase was detected at the δ-hydride nanotwin forefront along the growth direction and at the curved δ-hydride nanotwin boundaries, providing direct experimental evidence to the notion that the metastable ζ-hydride serves as the precursor to the stable δ-hydride phase during the precipitation process. The current experimental findings contribute to the fundamental understanding of microscopic mechanisms underlying the delayed hydride cracking and the stress-induced hydride reorientation in Zr alloys.