Zirconium hydride phase mapping in Zircaloy-2 cladding after delayed hydride cracking

Hydrogen uptake in zirconium alloys is a major limiting factor of nuclear fuel cladding as subsequent hydride precipitation can lead to potential cladding failure via delayed hydride cracking (DHC). The specific mechanisms of DHC have been a focus in earlier research, however there is still no clear definition of the resulting crystallography and limiting hydride sizes at various temperatures of crack propagation. In this work, a novel thermo-mechanical testing procedure was used to induce radially propagating DHC cracks in thin-walled cladding in a temperature range from 100°C to 320°C. Focused ion beam (FIB)- prepared lamellae were extracted from samples containing the arrested DHC crack for synchrotron micro-beam X-ray diffraction (XRD) mapping of crystallographic phases. The µXRD phase maps provided information about the required hydride cluster size that would lead to fracture (less than 5 µm), i.e. the critical hydride size. Phase mapping shows that DHC-responsible hydrides primarily consist of the δ phase, with slightly increased ɣ/δ ratios at lower cracking temperatures. In multilayer cladding, the liner material shows significantly increased ratios of ɣ-hydride, while on average, the δ-phase remains predominant. This work also highlights that the ɣ-hydride is indeed a stable zirconium hydride phase, which can precipitate during DHC propagation and within the inner liner of multilayer cladding.