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Title: EBSD characterisation of heterogeneous microstructures in zirconium alloys
Author: Tong, Vivian Suetto
ISNI:       0000 0004 6496 0270
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2016
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This thesis explores heterogeneous deformation and grain growth behaviour in zirconium alloys using electron backscatter diffraction techniques. Zirconium alloys are widely used for nuclear power applications and controlling microstructure during manufacture of reactor parts is critical for safe operation of the nuclear reactor. Mechanisms for the formation of abnormally large ‘blocky alpha’ grains in Zircaloy-4, which forms when annealing lightly deformed Zircaloy-4 within the alpha phase, have been explored using uniaxial compression and three point bending. The presence of blocky alpha is a known industrial problem, and adversely affects reactor safety, though there are limited studies addressing this problem. It was found that blocky alpha nucleates via strain induced grain boundary migration and grows via abnormal grain growth. In the presence of a strain gradient, blocky alpha grains nucleate in highly strained regions and grow towards lower strained regions. The final orientations of the blocky alpha grains depend on the pre-annealing strain state. High resolution electron backscatter diffraction (HR-EBSD) has been used to characterise microstructures before and after annealing to produce blocky alpha. To validate observations of increased dislocation density near grain boundaries, the effect of pattern overlap near grain boundaries on accuracy of HR-EBSD measurements is quantified. It was found that HR-EBSD measurements do not produce significant artefacts in this case. A simple calibration model and experiment is described to enable the extent of pattern overlap to be measured in other set-ups. Twinning behaviour in commercially pure zirconium was studied as a function of texture and strain rate. Twin types preferentially activated under high strain rate (10^3 s-1) and quasi-static strain rate (10^-3 s-1) were characterised using electron backscatter diffraction (EBSD) post-processing software. This software was written in-house and explanations of the algorithms used to identify twin boundaries and map intragranular misorientations have been included in this thesis.
Supervisor: Britton, T. Benjamin Sponsor: Rolls-Royce plc
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral