On the Potential of MAX phases for Nuclear Applications

Darin Joseph Tallman

doi:10.17918/etd-6662

Materials within nuclear reactors experience some of the harshest environments currently known to man, including long term operation in extreme temperatures, corrosive media, and fast neutron fluences with up to 100 displacements per atom, dpa. In order to improve the efficiency and safety of current and future reactors, new materials are required to meet these harsh demands. The M_[n+1]AX_n phases, a growing family of ternary nano-layered ceramics, possess a desirable combination of metallic and ceramic properties. They are composed of an early transition metal (M), a group 13-16 element (A), and carbon and/or nitrogen (X). The MAX phases are being proposed for use in such extreme environments because of their unique combination of high fracture toughness values and thermal conductivities, machinability, oxidation resistance, and ion irradiation damage tolerance. Previous ion irradiation studies have shown that Ti₃SiC₂ and Ti3AlC2 resist irradiation damage, maintaining crystallinity up to 50 dpa. The aim of this work was to explore the effect of neutron irradiation, up to 9 dpa and at temperatures of 100 to 1000 °C, on select MAX phases for the first time. The MAX phases Ti₃SiC₂, Ti3AlC2, Ti₂AlC, and Ti2AlN were synthesized, and irradiated in test reactors that simulate in-pile conditions of nuclear reactors. X-ray diffraction, XRD, pattern refinements of samples revealed a distortion of the crystal lattice after low temperature irradiation, which was not observed after high temperature irradiations. Additionally, the XRD results indicated that Ti3AlC2 and Ti2AlN dissociated after relatively low neutron doses. This led us to focus on Ti₃SiC₂ and Ti₂AlC. For the first time, dislocation loops were observed in Ti₃SiC₂ and Ti₂AlC as a result of neutron irradiation. At 1 x 1023 loops/m3, the loop density in Ti₂AlC after irradiation to 0.1 dpa at 700°C was 1.5 orders of magnitude greater than that observed in Ti₃SiC₂, at 3 x 1021 loops/m3. The Ti₂AlC composition appeared more prone to microcracking that Ti₃SiC₂. Additionally, exceptionally large denuded zones, up to 1 [mu]m in width after 9 dpa irradiations at 500 °C, were observed in Ti₃SiC₂, indicating that point defects readily diffuse to the grain boundaries. Denuded zones of this width, to our knowledge, have never been observed. In comparison, TiC impurity particles were highly damaged with various dislocation loops and defect clusters after irradiation. It is thus apparent that the A-layer, interleaved between MX blocks in the MAX phase nanolayered structure, readily accommodates and/or annihilates point defects, providing significant irradiation damage tolerance. Comparison of defect densities, post-irradiation microstructure, and electrical resistivity showed Ti₃SiC₂ to have the highest irradiation tolerance. Diffusion bonding of MAX phases to Zircaloy-4 was studied in the 1100 to 1300 °C temperature range. The out diffusion of the A-group element into Zircaloy-4 formed Zr-intermetallic compounds that were roughly an order of magnitude thicker in Ti₂AlC than Ti₃SiC₂. Helium permeability results suggest that the MAX phases behave similarly to other sintered ceramics. Based on the totality of our results, Ti₃SiC₂ remains a promising candidate for high temperature nuclear applications, and warrants future exploration. This work provides the foundation for understanding the response of the MAX phases to neutron irradiation, and can now be used to finely tune ion irradiation studies to accurately simulate reactor conditions.

On the Potential of MAX phases for Nuclear Applications

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On the Potential of MAX phases for Nuclear Applications

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