This work was performed as part of the MUZIC-2 (Mechanical Understanding of Zirconium Corrosion) collaboration, established with the goal of understanding the mechanism of hydrogen pickup in zirconium alloys. Hydrogen pickup is one of the least understood and most significant degradation mechanisms affecting zirconium alloys in nuclear reactors. These alloys are used as cladding and structural materials in the reactor core, mainly due to their low thermal neutron absorption cross section and excellent corrosion resistance. This project aims to investigate the effect of oxide texture (i.e. the degree of preferred orientation) on the corrosion performance of zirconium alloys. The texture of the oxide is expected to affect the microstructural development of the oxide, the grain boundary distribution and the stress state. It is therefore considered to be one of the most important factors in determining how the corrosion process occurs, and why different alloys exhibit significantly different corrosion performance. It is hoped that this project will add to the current knowledge of the corrosion process, and in particular hydrogen pickup, so that the route of hydrogen through the protective oxide can be identified. This will lead to the development of a new generation of alloys that provide improved oxidation and hydrogen pickup performance, whilst maintaining the required mechanical properties. This work focuses on four zirconium alloys; Zircaloy-4, ZIRLO™, low-Sn ZIRLO™ and Zr-1.0Nb-0.1Fe. The alloys all have different chemical compositions and therefore exhibit different corrosion performance. The macrotexture of the oxide formed on different alloys was measured by glancing angle X-ray diffraction (XRD) and Electron Backscatter Diffraction (EBSD). A fibre texture was formed in all cases, with the (10-3) to (10-5) planes oriented parallel to the metal-oxide interface for the monoclinic phase. The major orientation was found to be independent of alloy chemistry, substrate orientation and oxidation conditions. The monoclinic texture strength was found to be weakened with increasing oxidation temperature, The major orientation of the tetragonal phase was also found to be a fibre texture, with the (001) planes oriented approximately parallel with the interface. Although significant variation from this texture component was observed. It is suggested that the main driving force for oxide texture development is the transformation stress induced by the Zr-ZrO2 transformation. The microtexture of the oxides was measured using two novel techniques, Transmission Kikuchi Diffraction (TKD) and automated crystal orientation mapping with transmission electron microscopy (TEM). The techniques revealed an oxide microstructure consisting of an outer layer of equiaxed grains with a large range of orientations and an inner layer consisting of well-aligned columnar monoclinic grains. This layer of equiaxed grains was observed to form again after the transition in corrosion kinetics. A large fraction of transformation twin boundaries were observed throughout the oxides. Some of these boundaries were observed to surround large monoclinic grains, providing evidence that the tetragonal grains from which they transformed had grown considerably larger than the critical grain size for stabilisation. Without the presence of Sn, larger monoclinic grains were observed to form with a greater degree of preferred orientation, and with a lower fraction of transformation twin boundaries than Sn-containing alloys. In addition, an increased number of well-oriented tetragonal grains was correlated with the presence of Sn. It is therefore concluded that a reduction in Sn will lead to the formation of an oxide microstructure that is more resistant to both oxidation and hydrogen pickup. An analysis of the crystallography and morphology of a ZrO phase present at the metal-oxide interface was also performed. Finally, the observations have been used to construct a model of the corrosion process.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:664566 |
| Date | January 2015 |
| Creators | Garner, Alistair John |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/investigating-the-effect-of-oxide-texture-on-the-corrosion-performance-of-zirconium-alloys(c98f3395-4ee7-42c5-b9f3-2a55e7923c14).html |
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