Preparation And Properties Of Self-passivating Tungsten-Silicon Alloys For Plasma Facing Materials

When a nuclear fusion reactor facing with Loss-of-Coolant（LOCA）and air ingress into the vacuum chamber,W,which is a candidate material for the first wall,is easily oxidized to volatile WO₃.Once the radioactive WO₃leakage into the atmosphere,it will cause great harm to human beings.In order to solve the above-mentioned nuclear safety accident,many researchers have proposed the design ideas for intelligent high-temperature self-passivating W alloys.Si is a typical high-temperature passivation element.Si O₂formed under high-temperature oxidation conditions has the characteristics of high melting point and good thermodynamic stability.It is often used as a protective coating for high-temperature oxidation resistant materials（1000℃and above）.In addition,Si is also a low-radioactive element,and the W-Si alloy thin film has been proved to be a candidate smart self-passivating alloy.It can be seen that the W-Si alloy block is also expected to become a self-passivating plasma facing material.However,the research on the oxidation resistance and mechanism of W-Si alloy block is still blank.In this thesis,the self-passivating W-Si alloy is considered as the research object to study the phase and microstructure evolution progress and physical and mechanical properties of the W-Si alloys,and to study the oxidation kinetic behavior and the evolution of the microstructure of the oxide layer with temperatures rising and time extending.To reveal the high temperature oxidation resistance mechanism of the alloy,and develop a new type of intelligent self-passivating W alloy.The main contents and results are as follows:1.Mechanical alloying（MA）and Spark plasma sintering（SPS）technologies are used to prepare W-Si alloys block.The effect of Si content on phase composition,microstructure,physical and mechanical properties of alloy was studied.In W-3 at.%Si,Si exists in the matrix as W（Si）solid solution and Si O_xparticle.As the Si content increases,W₅Si₃begins to appear and its phase space gradually changes from a dispersed distribution to a continuous distribution state.The tensile strength and ultimate strain of W-32 at.%Si alloy at 600℃can reach 353 MPa and 4.58%,almost twice lower than the pure W.While,the thermal conductivity of W-Si alloy increases with temperature rising,When the temperature reachs600℃,the value of W-3 at.%Si is 89.3±0.3 W·m^-1·k^-1,and the value of W-32 at.%Si is23.0±0.1 W·m^-1·k^-1.2.The high-temperature oxidation resistance of the W-Si alloy enhances with the increase of the Si content.The W-32 at.%Si alloy block oxidized at 1000°C for 10 h can maintain a stable structure,indicating no defects such as holes and cracks were found in the oxide layer,oxide layer/alloy interface and inside the alloy and its oxidation rate at 1000℃in the atmosphere is only 1/18 of that in pure W..3.The high temperature oxidation resistance mechanism of W-Si alloy is related to the content of Si addition.The high temperature oxidation resistance mechanism of low silicon alloy（W-3 at.%Si）is attributed to the nano-Si O₂particles generated by the reaction of solid solution Si and O,which can effectively prevent the growth of WO₃grains and cracks expanding in the oxide laye.But when the temperature reaches 1000℃,as the oxidation time prolongs,more nanoparticles will precipitate out from W and connect and grow to form a cross-linked network structure.This change will cause the destruction of the oxide layer.The high-temperature anti-oxidation mechanism of the high-Si alloy（W-32 at.%Si）is attributed to the synergistic effect of the WO₃/Si O_ycomposite oxide film and the nanoporous amorphous Si O_yoxide film formed by the oxidation of W₅Si₃phase,which can serve stably under high temperature oxidation conditions at 1000℃.The as-formed oxide layer can help to prevent the volatilization of WO₃,prolong W outward diffusion and O₂inwaird diffusion.In addition,the composite oxide layer can absorb the compressive stress in the oxide layer through the adaptive deformation of the oxide film,avoiding the formation and propagation of cracks inside the oxide layer and improving high temperature oxidation resistance of the alloy.4.A new W-Si-Y ternary alloy with good self-passivation behavior was developed,and the effect of Y content on the high-temperature oxidation behavior of the alloy was discussed.W-32 at.%Si-6 at.%Y has the lowest oxidation rate at 1000℃in the atmospheric environment,which is only 1/90 of that of pure W,and it can maintain its structure after isothermal oxidation at 1000℃for 80 hours.The oxide scale on the surface of the alloy is composed of four layers.From the outside to the inside,they are W-Y-O oxide films in a molten state,composite oxide film composed of W-Y-O particle sealed nanoporous oxide film and WO₃particles,coarse loose oxide films composed of WO₃、W-Y-O and Y₂Si₂O₇,nano Y₂O₃particle stabilized dense WO_x/Si O_ycomposite oxide film.The synergistic effect of multilayer oxide film promotes the high temperature oxidation resistance of the alloys.