Investigation of the temperature-dependent failure processes in PVD Cr-coated ZIRLO nuclear fuel cladding using in situ X-ray micro-tomography imaging

In this work, an accident tolerant fuel cladding system with Cr coating deposited using physical vapour deposition (PVD) method onto commercial Optimized ZIRLO™ was studied. The cladding tubes were machined into C-rings and loaded to failure under compression with real-time synchrotron micro-X-ray computed tomography (XCT) imaging at room temperature (RT), 345 °C, 650 °C and 950 °C in argon (Ar) atmosphere. The mechanical behaviour and failure processes were found strongly temperature-dependent where coating showed brittle fracture at RT and 345 °C, ductile fracture at 650 °C and a reversion to brittle fracture at 950 °C. Nanoindentation measurements and scanning electron microscopy (SEM) imaging were conducted on the materials after high temperature testing. It was found that recrystallisation of Cr coating occurred at elevated temperatures, which significantly affected the local properties hence the failure behaviour at different temperatures. This work represents the first in situ 3D observation of progressive failure processes in PVD Cr-coated ZIRLO claddings up to 950 °C, providing critical insights into the brittle-to-ductile transition (BDTT) and subsequent ductile-to-brittle reversion behaviour at extreme temperatures. Moreover, results are compared with other PVD Cr-coated Zircaloy materials from open literature, the influences of microstructures and local properties on the coating failure stress/strain and failure processes are discussed.