| As a novel candidate for the accident tolerant fuel cladding materials,Cr coated zirconium alloy has shown excellent corrosion resistance and high temperature steam oxidation resistance beyond the design basis accident scenarios.Compared with the coating preparation technologies such as magnetron sputtering,plasma spraying,arc ion plating,3D laser coating,and cold spraying,the electroplating technology is more effective and economical for depositing Cr coating due to high deposition rate,low equipment costs,low energy consumption,making it more suitable for industry-scale production.In this thesis,electroplating method was used to prepare Cr coated zirconium alloy.The effects of the concentration of each composition of plating solution and the parameters of the plating conditions on the electrodeposition process were discussed.The influence of the pretreatment process before the plating on the microstructure and mechanical properties of the interface were analyzed.Besides,the micro structure evolution behavior of Cr coatings and the interface between coating and Zrmatrix was in-situ investigated during He+irradiation by the Multiple Ion Beam In-situ TEM Analysis Facility.The successful preparation of Cr coated Zr-4 alloy by electrodeposition is reported for the first time in the world.The concentration values of each composition of plating solution and the parameters of the plating conditions were optimized by Hull Cell experiment and square tank experiment to obtain an electrodeposition process capable for preparing a Cr coating on the surface of Zr-4 alloy with excellent interfacial microstructure and bonding force.The optimum bath compositions and plating conditions are as follows:Cr2(SO4)3·6H2O(50 g/L),H2C2O4·2H2O(10 g/L),H3BO3(90 g/L),Na2SO4(100 g/L),PEG(2 mL/L),SPS(200 mg/L),temperature 45℃,pH 3.5,the current density of 8～10 A/dm2,and magnetic stirring.Under the above conditions,the deposition rate can reach 0.205μm/min.A Cr coating with the thickness of approximately 6 μm can be deposited within 30 minutes by electroplating.Different surface pretreatment process before plating have significant effects not only on the microstructure of the surface and the interface,but also on the bonding strength between Cr coating and Zr-4 substrate.The activation process can effectively remove the oxide film on the surface of the Zr-4 alloy and etch rich nucleation sites for rapid nucleation growth of the Cr coating.What’s more,the activation process also significantly reduces the surface roughness,forming a dense and uniform surface and combining Cr coating with Zr-4 alloy matrix.The microstructure analysis showes that the intermediate transition nickel layer and heat treatment improve the microstructure of the interface,and no defects can be found at the interface between each layer.Cr coating can be well combined with Ni layer in a lattice matching mode.The relationship of crystallographic orientation between Cr coating and Ni layer are[111]Cr//[011]Ni and(110)Cr//(111)Ni.Meanwhile,the intermediate transition Ni layer and the Zr-4 alloy are combined in a metallurgical bonding(Ni5Zr,Ni7Zr2,Ni3Zr,Ni10Zr7,NiZr and NiZr2 in sequence).Scratch testing is used to test the bonding strength between the coating and the substrate.The results show that the activation process significantly increases the bonding strength from 1.9 N(before activation treatment)to 48.15 N(after activation treatment).Preplated Ni layer further improves the bonding strength to 75.8 N.After heat treatment,because of the brittle phase of the formed NixZry intermetallic compounds,the bonding strength is slightly reduced.The microstructure evolution behavior of Cr coating and the interface are in-situ investigated during He+irradiation by Multiple Ion Beam In-situ TEM Analysis Facility in the College of Energy,Xiamen University.The in-situ irradiation experiments are carried out under 30 keV He ions with a flux of 1×1013 He+/cm2·s at 400℃.FEI Technai G2 F30 TWIN transmission electron microscope is used for in-situ observation.After helium irradiation,the size and distribution characteristic of helium bubbles are different in each layer,indicating different resistance ability of helium irradiation.Among them,the Ni layer has the best helium irradiation resistance.Furthermore,helium bubbles show different aggregation states at different interfaces that mainly aggregate at the interface between Ni layer and Ni5Zr layer,however,a helium bubble denuded zone can be observed adjacent to the interface between Cr coating and Ni layer.In-situ observations of the Cr coating show that both size and density of helium bubbles are increased in Cr coating with increase of helium concentration,and the average size of helium bubbles at grain boundaries is larger than that inside Cr grains.To be noted,helium atoms preferentially precipitate along grain boundaries to form helium bubbles that further gather to form short bubble microcracks.The bubble microcrack will be regarded as a channel for the transport of the oxidizing species towards Zr-4 alloy matrix,but the interlayers including pure Ni layer and NixZry intermetallic compound layers show an excellent irradiation resistance of helium beam,which can act as a protective barrier to delay further diffusion of oxidizing species.Finally,the growth model of helium bubbles is discussed.The migration and coalescence of helium bubbles in Cr coating is mainly controlled by surface diffusion(MC/S).The surface diffusivity coefficient(Ds)is 1.740×10-14 cm2/s when the in-situ irradiation temperature is at 673K.Besides,the diffusion coefficient is related to the average radius of helium bubbles and the irradiation temperature.When the irradiation temperature is constant,the diffusion coefficient is inversely proportional to the fourth power of the average radius of helium bubbles.At the same temperature,the smaller the size of helium bubbles is,the higher rate of bubble movement will be. |
PDF Full Text Request