Characterization of the Mechanical Response to Residual Stress in Corroded Zirconium Alloys

Storer, Susan (2011). Characterization of the Mechanical Response to Residual Stress in Corroded Zirconium Alloys. PhD thesis The Open University.

Abstract

Zirconium (Zr) alloys are used in the manufacture of safety critical components such as nuclear fuel tubes. These alloys are suited to structural applications within the aqueous, high pressure environment of a nuclear reactor core, because they are transparent to neutrons, have excellent corrosion resistance and good mechanical properties. However, the zirconium fuel tubes have become the life limiting factor in achieving high burn-ups.

High compressive stresses build up in the oxide due to the large volumetric expansion (>50%) that occurs while the oxide forms. These residual stresses play a key role in stabilising the protective oxide. When stability is lost, cracks often form in the oxide, allowing water ingress, resulting in corrosion of the metal substrate. The tensile stresses existing in the metal substrate, close to the oxide interface are high enough for material creep to occur. This thesis focuses on investigating the mechanical response to residual stress in corroded zirconium alloys.

Novel methodologies used in this study include nanoindentation (NI) and focused ion beam (FIB) milling to determine residual stress. Digital image correlation (DIC), electron back scattering diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDX) were used in the analysis of data.

Mapping NI hardness gradients across the oxide led to a comprehensive, systematic examination into the causes of scatter found in the NI data, with an interesting and surprising outcome. A feasibility study is included using a relatively new method of measuring strain by FIB milling and DIC analysis. It is thought to be the first time this method has been used on zirconium alloys. NI creep testing was used to evaluate time dependent deformation, resulting in high values of the stress exponent (n). An investigation found good correlation between n values from indentation testing and those from load relaxation testing, suggesting a relaxation of deformation modes, possibly activated during the loading process. A comparative analysis was performed on the plastic deformation zone underneath a large Vickers indent and the highly deformed region of the load relaxation tested sample, to ascertain whether twinning mechanisms were dominant in the deformation process.

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