Investigation of the mechanical behaviour of nuclear-grade SiC fibre reinforced SiC matrix composite and Cr-coated zircaloy-4 cladding materials under extreme conditions

Abstract

SiC fibre-reinforced SiC matrix composite (SiCf-SiCm) and Cr-coated zircaloy materials are widely recognized as promising candidate cladding materials for the nuclear accident tolerant fuels (ATF). For their future industrial application, it is critical to understand their mechanical behaviour and failure processes under extreme conditions. Therefore, this project mainly focuses on the deformation and fracture processes of two types of SiCf-SiCm claddings (one with single-layer outer/inner SiC coatings; one with multi-layer SiC coating), and four types of Cr-coated zircaloy-4 claddings (three types of Cr coatings manufactured by cold-sprayed (CS) method with different manufacturing parameters, one Cr coating manufactured by physical vapour deposition (PVD) method). Real-time synchrotron micro X-ray computed tomography (μXCT) is employed in current project to monitor the crack formation and propagation via C-ring compression tests of these materials at ambient and high temperatures.
Both types of SiCf-SiCm claddings were tested at room temperature (RT) and 1200°C. Both materials processed temperature-dependent variations in failure processes. For both types of materials tested at both temperatures, cracks initiated in the outer SiC coating, and various cracks’ toughening mechanisms occurred simultaneously rather than in a sequence. These are significantly diverging from the widely assumed toughening theory in such SiCf-SiCm materials tested via uniaxial tension loading. Before the μXCT experiments, local property and residual stress of individual components in the materials were respectively estimated by nanoindentation and Raman spectroscopy. Lastly, the correlation between microstructures, local properties, residual stresses and macro-scale performance of these materials are studied.
Four types of Cr-coated zircaloy-4 claddings were tested at both RT and 345°C. Post-failure coating cracks’ patterns was analyzed via scanning electron microscope (SEM) imaging under higher resolution. Prior to the real-time testing, local properties of Cr coatings and underlying zircaloy-4 substrate were measured by nanoindentation method, and the microstructures of Cr grain in these Cr coatings were analyzed by electron backscatter diffraction (EBSD) method. It was found that, the different manufacturing processes of CS and PVD methods consequently lead to different Cr grains’ microstructures in the coating, which subsequently affect local properties of these materials, as well as their mechanical behaviour. Additionally, the different manufacturing parameters of CS process could also result in different mechanical behaviour and coating crack patterns. Finally, the relationship between microstructure, local properties and high temperature mechanical behaviour of these Cr-coated zircaloy-4 materials are investigated.

Date of Award	18 Jun 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Lilly Liu (Supervisor)