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Title: Understanding the mechanisms controlling stress corrosion cracking through high-resolution characterization
Author: Shen, Zhao
ISNI:       0000 0004 7960 0243
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2018
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Austenitic alloys have been extensively used in the nuclear industry as structural components due to their combination of excellent mechanical properties and high corrosion resistance. Although these alloys have a good service record, many can become susceptible to stress corrosion cracking (SCC) under pressurized water reactor (PWR) primary water conditions. After several decades of study, a considerable number of factors have been revealed that affect SCC susceptibility, such as material composition, pre-existent cold-work, temperature or water chemistry. Although the effects of these factors on SCC have been extensively studied, a better mechanistic understanding is still needed. High-resolution characterization techniques can reveal key processes involved in SCC crack propagation. For this reason, analytical transmission electron microscopy (ATEM) and transmission Kikuchi diffraction (TKD) have been used to characterize the crack tip region. Several SCC precursors have been identified: intergranular oxidation zone (IOZ) ahead of the crack tip, Cr-Fe depletion ahead of the IOZ, grain boundary migration along the crack and ahead of the crack tip, and localized deformation around the crack tip. The first objective of this thesis was to understand the underlying connections between these precursors and SCC propagation. Intergranular carbides and grain boundary migration were observed to be able to restrain SCC propagation. In addition, two mechanisms were confirmed to play important roles in the SCC propagation: one being a diffusion-based and the other one mechanical deformation-based. The second objective of this thesis was to obtain a better mechanistic understanding of the temperature dependence of crack growth rate (CGR) in 316SS and Alloy 600 in the temperature range of 320-360ºC. It was found that the two proposed mechanisms seem to have different weights on each alloy at "low" and "high" temperatures. The third objective was to explain the effect of Ni content on SCC CGR, also known as the "Coriou effect". Four alloys with different content of Ni, including 316SS, Alloy 16Ni, Alloy 60Ni, and Alloy 600 were characterized. The results showed that although the elemental diffusion increased with the increase of Ni content in the Fe-based alloys (316SS and Alloy 16Ni), the mechanical deformation-based mechanism appeared to play a dominant role, leading to the CGR decrease with the increase of Ni content in the Fe-based alloys. Both the diffusion-based mechanism and the mechanical deformation-based mechanism increased the CGR in Ni-based alloys (Alloy 60Ni and Alloy 600) with the increase of Ni content. The excellent SCC resistance of the alloys in the range of 20-45%wt. Ni was mainly contributed by the low extent of localized deformation around the crack tip, although the elemental diffusion in these alloys was not very low.
Supervisor: Lozano-Perez, Sergio Sponsor: China Scholarship Council ; Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available