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MAX phase ceramics for nuclear applications
[Thesis]. Manchester, UK: The University of Manchester; 2018.
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Abstract
The PhD research presented here is part of the new nuclear manufacturing (NNUMAN) research group, looking into novel materials and manufacturing processes. NNUMAN is an EPSRC funded (EP/J021172/1) program with direct support from Rolls Royce plc. The fabrication of MAX phase coatings by different means and assessing their tribological properties was the original theme of this thesis. However, due to issues with fabrication processes the overriding direction was moved towards assessing the response of bulk MAX phases to irradiating and corrosive environments. It is believed that this research will contribute to the understanding of suitable compositions and applications within nuclear. The MAX phases comprise an early transition metal (M), an A-group element (A) and either carbon or nitrogen (X). They have an inherently nano-layered structure with alternating ceramic (MX) and metallic (A) layers. The unique mix of both ceramic and metallic properties have made MAX phases a proposed material for nuclear applications. Proton irradiation is performed on Ti3SiC2, Ti3AlC2, Ti2AlC and Cr2AlC bulk MAX phases to 0.1 dpa at 350oC. Crystallographic instabilities are observed through x-ray diffraction (XRD) analysis after irradiation. A mechanism for irradiation induced point defect swelling is proposed, by matching XRD and modelling data. Anti-site point defects are proposed to be a major contributor to anisotropic lattice parameter changes in Ti3SiC2. Additional carbon interstitial defects are also proposed in Ti3AlC2, as well as anti-site defects, making them less radiation tolerant. Further proton irradiations on Ti3SiC2 and Ti3AlC2 were performed at elevated temperatures to propose a temperature at which lattice changes are not observed. It is concluded that high proton fluences require temperatures in excess of ~700oC and ~1,000oC for Ti3SiC2 and Ti3AlC2, respectively. Corrosion of bulk MAX phases in simulated primary water also suggests a deleterious response to normal light water reactor (LWR) operations. Advanced scanning transmission electron microscopy (STEM) techniques, such as energy dispersive x-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS), is employed to understand corrosion mechanisms. Selective oxidation of A-layers is observed, with no evidence of passivation for Ti3SiC2, Ti3AlC2 and Ti2AlC. The response of Cr2AlC was most promising as little oxidation occurred, which is confirmed also with XRD analysis. A comprehensive corrosion mechanism is proposed, whereby compositions of MAX phase in LWR coolants is limited.