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Title: Heavy ion irradiation as a proxy to neutron irradiation in impure Fe and Fe-Cr alloys
Author: Hewitt, Luke
ISNI:       0000 0004 8502 7455
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2018
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The realisation of nuclear fusion energy will require materials that can withstand high doses of neutron irradiation, and one class of candidate materials for structural applications is reduced-activation ferritic-martensitic (RAFM) steels containing around 9% Cr. The present lack of a fusion neutron source has meant that much of the research into radiation damage has been conducted using specimens irradiated in fission test reactors, or by using heavy ion irradiation as a simulation method. This work explores details of the use of this emulation technique, alongside the effect of composition and impurities on radiation damage in these materials. Beam rastering is often employed during heavy ion irradiation in order to deliver a more uniform dose across the target area in comparison to continuous irradiation with a defocused beam, but this practice has been reported to alter the radiation-induced microstructural changes. This work investigated this in Fe and Fe-Cr alloys, using nanoindentation. No difference in post-irradiation hardness was observed between samples irradiated with a rastered or defocussed beam. A direct comparison of the microstructure and mechanical properties of heavy ion and neutron irradiated Fe-9Cr samples was made, and it was found that the segregation to dislocation loops and clustering of impurity elements Si and P reported in the neutron irradiated specimen was also present in the ion irradiated specimen. The α' precipitation reported after neutron irradiation did not occur after ion irradiation. The post-irradiation increase in hardness and yield stress of both samples were found to be the same, measured by nanoindentation and microcantilever testing. An increase in hardness of 0:88 ± 0:21 and 0:81 ± 0:26 GPa was found in the ion and neutron irradiated specimens respectively. The increase in hardness for the neutron irradiated specimen was found to be less then the 1:5 ± 0:28 GPa reported for an identically irradiated Fe-6Cr sample. The impact of carbon concentration on irradiation hardening in Fe was investigated by ion irradiating a high purity Fe sample containing regions of varying carbon concentration. Two samples were irradiated at two different dose rates, but no dose rate effect was observed. The increase in hardness however was found to increase with increasing carbon concentration, with a maximum increase of around 3 times that observed in pure Fe. This work has identified some limitations of and future challenges for the use of heavy ion irradiation as a surrogate to neutron irradiation in the absence of a dedicated fusion materials testing facility.
Supervisor: Roberts, Steve ; Hardie, Chris Sponsor: Culham Centre for Fusion Energy (CCFE)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available