High Temperature Oxidation Behavior Of Al_xCoCrFeNi (x=10,20 At.%) High Entropy Alloy

High entropy alloy has excellent physical properties,high temperature strength,corrosion resistance and high temperature oxidation resistance,which is expected to be applied to the structural components of aerospace engines,the protective coatings for gas turbine engine blades,and the bonding coatings for thermal barrier coatings.High entropy alloy will be oxidized under high temperature service environment,forming loose and porous oxides on the surface of the alloy,and leading to the decrease of the oxidation resistance of the alloy and even the failure of the alloy.At present,some researches have been carried out on the high-temperature oxidation behavior of high entropy alloys,but the scholars mainly focus on the phase and morphology characteristics of the oxides,ignoring the relationship between the phase development and morphology development of oxides.Therefore,in this thesis,Al_xCoCrFeNi（x=10,20 at.%）high entropy alloys are employed to study the microstructure characteristic of alloys after oxidation at 950℃in O₂environment（with Ar-20%）for 1h,4h,12h and 24h.The mechanism of phase evolution and the formation and healing of interfacial pores are discussed.The main research contents and conclusions are as follows:（1）XRD,SEM,TEM and EDS are employed to analyze the phase evolution of two oxidized samples.The oxides of Al₁₀Co Cr Fe Ni sample have a three-layer structure:firstly,α-Al₂O₃in the inner layer is formed by the reaction of thermodynamically preferred Al with O;and then Cr₂O₃in the middle layer is formed by the reaction of Cr with O;finally,Fe,Ni and Co also diffuse outward and form oxides due to the dynamical factors,which further react with Cr₂O₃to form the spinel in the outermost layer.The oxides of Al₂₀Co Cr Fe Ni sample exhibit a double layer structure:a loose metastableγ-Al₂O₃layer is formed on the surface of the alloy by the reaction of thermodynamically preferred Al and O,and then theγ-Al₂O₃near the alloy substrate gradually transforms into a denseα-A₂O₃layer.（2）According to the phase development of oxides,the mechanism models of interface pores formation and healing are proposed.The formation mechanism of pores at the interface betweenα-Al₂O₃and alloy substrate is that the metal cations diffuse outward,resulting in the aggregation of vacancies at the interface between metal and oxide.The formation of pores atγ-Al₂O₃/α-Al₂O₃interface and Cr₂O₃/spinel is attributed to the volume shrinkage caused by the phase transition.The healing of the pores at the interface betweenα-Al₂O₃and the alloy substrate is due to the diffusion of O from the outside to the pores at the interface through the oxide grain boundary,and the metastableθ-Al₂O₃is formed by the reaction of Al in the alloy with O to fill the pores.Some of the metal cations in the oxide also diffuse outward to the surface of the pore and form Cr₂O₃/spinel,leading to the pores healing.