Mechanical Properties of Zirconium Hydrides under Thermo-Mechanical Loading

Abstract

Zirconium and zirconium alloys continue to be the primary structural material for use in nuclear reactor cores forming the primary barrier between the fuel and it's waste products to the environment. Prolonged exposure to the reactor environment leads to the absorption of hydrogen which has low solubility in the zirconium and as a result precipitates to form zirconium hydrides. The presence of hydrides degrades the overall properties of the cladding and has previously lead to through wall cracking by a mechanism called delayed hydride cracking. Despite this, many of the properties of the hydrides, including fundamental parameters, such as a definitive list of phases and their lattice parameters, remain a subject of debate because of the difficulties in studying them. Previous in-situ loading experiments using synchrotron x-ray diffraction showed the evolution of very large lattice strains within hydrides. Two mechanisms that could lead to the extreme lattice strains were previously suggested and were; a load shedding from the zirconium matrix to the hydrides or a stress induced phase transformation occurring through reordering of the hydrogen atoms [1, 2]. To investigate these mechanisms, in-situ synchrotron x-ray diffraction experiments were designed to probe the properties of the phases. Samples were hydrogen charged using a gas phase equilibrium method and quenched with different cooling rates to form either fine, evenly distributed hydrides or long chains of hydrides known as stringers. Subsequent analysis of diffraction patterns showed this quenching process also lead to the formation of the low temperature $\gamma$ phase in Zircaloy-4. A new routine was added to an open source hydride analysis package (HAPPy) to characterise the hydride morphology from electron microscopy images by fitting the hydrides to an ellipse. To test the load shedding hypothesis, the impact of the hydride morphology on the behaviour of the composite was investigated. A shear lag model showed that the different hydride microstructures would have little impact on the response of the material during elastic deformation, consistent with SXRD results, but experimental results show that smaller hydrides are able to support greater transfer of load from the matrix to the hydride on plastic flow of the matrix, demonstrating the importance of taking into account hydride morphology when assessing their impact on alloy performance. Line profile analysis of the diffraction data in conjunction with lattice strains calculated through changes in d-spacing indicate two failure mechanisms are present within the hydrides and are dependent on hydride size and orientation to load. The results support previous predictions that large hydrides fail in a brittle manner when above a critical failure length with a fracture stress of around 650MPa but the response from hydrides below this length indicates significant plastic deformation can occur for preferably oriented hydrides. Increases in the overall strength of the material when fine hydrides are present show the increased ability of these smaller hydrides to pickup stress. This is hypothesised to be due to the reduced level of defects present in these structures compared to the larger hydrides. An elastic-viscoplastic self consistent (EVPSC) model was employed to simulate the strain evolution for multiple hydride planar reflections and investigate the impact of different unit cells and mechanical properties had on the performance of hydrogen-charged Zircaloy-4. A routine to apply internal stresses to the composite from the precipitation of the hydrides matched results from literature and the impact on the overall properties of the composite were investigated. The results of the diffraction data suggest that load shedding from the matrix to the hydride was present and that non-elastic deformation occurred in the hydrides after a period of plastic deformation in the matrix. This results of this work suggest that applie

Date of Award	6 Jan 2025
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philipp Frankel (Supervisor) & Michael Preuss (Supervisor)