Abstract
Elemental Cr/C/Al multilayers (stoichiometric ratio: 2:1:1) with and without a Cr overlayer have been synthesized on Zircaloy-4 substrates by magnetron sputtering. The effects of annealing temperatures (400 and 550 °C) on phase/microstructure formation, mechanical properties, and oxidation/corrosion performance have been comparatively studied. Annealing of the multilayers at 400 °C led to the formation of nanocrystalline composite consisting of intermetallic and binary carbide phases. Single-phase Cr2AlC was obtained after 550 °C annealing, but with microcracking of the coatings. Both annealed coatings displayed similar mechanical properties, high-temperature oxidation, and hydrothermal corrosion mechanisms. The composite coatings annealed at 400 °C significantly enhance the high-temperature oxidation resistance (α-Al2O3 scale growth) and hydrothermal corrosion (Cr2O3 passivation layer formation) of a Zircaloy-4 substrate without coating microcracking and delamination. Nanocomposite CrCAl-based coatings are promising candidates for coated ATF applications with acceptable processing temperatures and excellent oxidation/corrosion resistances for a zirconium alloy substrate.
Highlights
The loss of reactor core cooling at the Fukushima Daiichi Nuclear Power Plant in 2011 eventually led to hydrogen denotation, core meltdown, and release of radioactive contamination, revealing the weaknesses of current zirconium alloy fuel cladding under design extension conditions with severe exothermic oxidation and mechanical degradation [1,2]
Materials in the ternary Cr–C–Al system offer promising potential to overcome the aforementioned drawbacks of pure metallic Cr coatings for accident-tolerant fuels (ATF) applications
The results show that annealing of the multilayered films below the crystallization onset temperature of Cr2AlC MAX phase and at typical stress-relief annealing (SRA) temperature (~400 ◦C [31,32]) of Zircaloy cladding tubes can avoid coating microcracking and microstructure/mechanical property modification of the substrate
Summary
The loss of reactor core cooling at the Fukushima Daiichi Nuclear Power Plant in 2011 eventually led to hydrogen denotation, core meltdown, and release of radioactive contamination, revealing the weaknesses of current zirconium alloy fuel cladding under design extension conditions with severe exothermic oxidation and mechanical degradation [1,2]. Deposition of robust, anti-oxidation coatings on the outer surface of zirconium-based alloy fuel claddings represents one short-term ATF strategy [5,6]. This strategy offers the advantages of maintaining the favorable neutronic and irradiation properties of the zirconium alloy cladding underneath, and the fact that the technological and licensing processes can be adopted. A variety of materials have been evaluated and qualified as coatings on Zr-based alloys in terms of high-temperature oxidation and hydrothermal corrosion for coated ATF applications. Materials in the ternary Cr–C–Al system offer promising potential to overcome the aforementioned drawbacks of pure metallic Cr coatings for ATF applications
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