Application Of Multi-principal Element Alloy Coating In Protection Of Zirconium Alloy Cladding

Zirconium alloy is used as the nuclear fuel cladding and structural components in the reactor due to its excellent comprehensive properties.However,under the service environment of high temperature and high pressure water coolant,zirconium alloy will be rapidly oxidized and severely destroyed.As for the zirconium alloy as a cladding material,the key is to reduce its oxidation kinetics.One of the effective ways is to prepare a protective coating on the surface of the existing zirconium alloy.In view of the high mixing entropy of the multi-principal element alloy showing excellent performance and the characteristics of dense and borderless amorphous structure,we designed the amorphous multi-principal element alloy coating.In this work,the magnetron sputtering technology is used to prepare binary to six-component amorphous coatings through the"combined target co-sputtering"method.We choose six elements with low thermal neutron absorption cross section（Zr,Nb,Fe,Cr,Si,Al）.The thesis studies the microstructure of the coating and the hydrothermal corrosion behavior at high temperature and high pressure,electrochemical corrosion behavior and high temperature oxidation corrosion behavior.According to the composition and bonding state,the structure is adjusted and the structure-activity relationship related to the protective effect is established.By understanding the corrosion mechanism and performance characteristics of amorphous multi-principal alloy materials,a coating material that is compatible with the zirconium alloy interface and has a good oxygen barrier effect can be obtained.When the Zr Nb coating is prepared in a ratio of 1:1,the coating structure is a nanocrystalline structure.The high temperature and high pressure hydrothermal corrosion experiment（320℃,16 MPa）proved that a stable Zr-Nb-O amorphous film was formed after corrosion and no cracks appeared after corrosion like zirconium alloy.When a Zr Nb Fe coating with an atomic ratio of 1:1:1 is prepared,an amorphous structure is obtained.After hydrothermal corrosion,a stable Zr-Nb-Fe-O amorphous film is formed without cracks.When the iron content in the Zr Nb Fe coating is increased,a significant iron diffusion phenomenon is found.As the corrosion time increases,the Kirkendall effect causes the connection of voids in the oxide film and more obvious defects appear.The Zr Nb Fe Cr amorphous coating with an atomic ratio of 1:1:1:1 has a stable structure and no element diffusion occurs after hydrothermal corrosion.Because of the addition of Cr,an obvious oxide layer structure appears on the surface of the coating,which also brings better oxygen barrier performance.After adding Si,the oxidation and corrosion resistance of the prepared Zr Nb Fe Cr Si coating has been significantly improved.The formed Si passivation oxide film（Si O₂）improves the oxidation and corrosion resistance.For the Zr Nb Fe Cr Si Al coating,The introduction of Al makes the high-entropy amorphous system undergo obvious silicon out-diffusion after hydrothermal corrosion,and only an oxide film with a thickness of about 28 nm is formed on the surface.Due to the increase of Si in the amorphous oxide layer,the number of oxygen vacancies decreases.The decrease in the number of oxygen vacancies reduces the mobility of the vacancies,and therefore also reduces the mobility of oxygen ions,thereby increasing the oxygen barrier capability.The electrochemical corrosion experiment in Li OH solution proved that the Zr Nb Fe Cr Si Al coating is more corrosion resistant than other coatings.The main reason is that the dense and heterogeneous microstructure of the amorphous coating effectively inhibits the corrosion of the corrosive electrolyte.In addition,the three elements of Cr,Al and Si can generate dense and insoluble passivation films to further improve the corrosion resistance.Therefore,the Zr Nb Fe Cr Si Al high-entropy alloy coating can have the best protective effect on the Zr-4 alloy substrate.The oxidation results in high-temperature air show that the Zr Nb Fe Cr coating undergoes violent element diffusion and shedding after being oxidized at 900℃for 4h.The Zr Fe Nb Cr Al Si high-entropy coating with low silicon content is oxidized at900℃for 4 h to generate corresponding nanocrystals and are wrapped by amorphous Si O₂.Amorphous Si O₂provides a barrier to prevent the mutual diffusion and oxidation of other metal atoms.process.And the nanocrystal becomes an annihilation trap of stress and strain to absorb stress and relieve the occurrence of cracks at high temperature.The Zr Fe Nb Cr Al Si_0.5high-entropy coating with high silicon content showed the same element diffusion trend after being oxidized at 800℃-1000℃for 4h,and the inner layer has an Al diffusion-enriched layer.This dense aluminum oxide layer plays an important role in oxygen barrier.Even if it is oxidized at a high temperature of 1000℃for 2 h,the thickness of the oxide film does not exceed 800nm,and it has an extremely slow oxidation kinetics phenomenon.The above explorations are of great significance for improving the oxidation and corrosion resistance of zirconium alloys.