Microstructure Properties And Oxidation Resistance Research Of A Nickel-based Superalloy For Internal Combustion Engine Exhaust Valve

Internal combustion engines are widely used to national economy in various fields,including many kinds of machinery,vehicles and ship machines.Improving the internal combustion engine operating temperature of the combustion chamber can effectively improve fuel efficiency and reduce CO₂ emission.Increased working temperature put forward more stringent requirements on materials,including high temperature oxidation resistance,high temperature strength,wear resistance and microstructure stability during thermal exposure period.A new developed Ni-Cr-Fe wrought nickel-based superalloy was chose for internal combustion engine exhaust valve steel.Nickel-based superalloys have been widely used to the hottest applications,which exhibit excellent corrosion resistance and microstructure stability.Optical microscope（OM）,field-emission scanning electron microscope（FE-SEM）,X-ray diffraction（XRD）,transmission electron microscope（TEM）,high-resolution transmission electron microscope（HRTEM）and JMatPro thermodynamic calculation software were employed to study the solid solution and aging heat treatments,microstructure stability during 750°C long-term thermal exposure and high temperature oxidation behaviour,etc.,to explore the intrinsic relationship between the microstructure characteristics and macro mechanical properties.T₁,T₂,T₃,and T₄ four kinds of heat treatments were designed to study the microstructure and mechanical properties.The precipitateγ′particles show an spherical shape and uniformly distribute on the substrate,the average size of particles are 50.10 nm,15.04 nm,23.59 nm,and 49.10 nm,respectively.High temperature aging+low temperature aging（850°C×4h+730°C×4h）two stage treatment can significantly increase the strengthening particle size.The L1₂ ordered structureγ′phase exhibits a coherent orientation relationship withγmatrix on the{100}and{110}atomic cells after solution-aging treatment,the phase interface grid stripe shows continuous transition.Two types of carbides exist in the matrix,including MC carbide（Nb and Ti elements rich）and M₂₃C₆（Cr and Mo elements rich）.The room temperature fracture of solid solution state specimen shown transgranular crack characteristic and the solution-aging treatment specimen fracture crack extended along the grain boundary location,the dimples size on the fracture surface decreased significantly,while the elongation keeping more than 20%.During 750°C long term thermal exposure period,the coarsening process ofγ′particles conforms to LSW theory,which is controlled by the diffusion of solute atoms inγmatrix.Theγ′phase forming elements concentration gradient,between the small particles’surrounding matrix and the large particles’surrounding matrix,is the driving force ofγ′coarsening process.The precipitation of equilibriumα-Cr phase will change the relative concentration of solute atoms in theγmatrix,which will influent the coarsening process ofγ′particles.The morphology ofγ′particles are controlled by the surface energy and the elastic strain energy competition at the same time.In the center of the grain,theγ′particles maintain spherical shape to minimize the surface energy,while on the surface of MC carbides andα-Cr phase surface,the particles spread out into a disc down two-dimensional direction in order to reduce the elastic strain energy.α-Cr phase and the spreadingγ′particles on its surface form a“sandwich”morphological structure.The precipitation contents ofα-Cr phase increased significantly after 750°C-2000h,which lead to the significant reduction of room temperature plasticity.The fracture surface of the long-term specimen is covered with net-likeα-Cr phases,and nearly none dimples exists.During the long-term oxidation process at 750°C,the weight gain process showed a piecewise characteristic.After 2000 h,the oxidation rate of the weight gain process showed an upward trend.Theα-Cr phase precipitated and changed the solute atomic concentration of the matrix,especially the concentration gradient of Cr element near the oxide layer,which may lead to the increase of oxidation rate.The outer oxide layer consists of Cr₂O₃ and TiO₂particles.The addition of rare earth element Y can modify density of the outer oxide,reduce the inward diffusion rate of O element,decrease the thickness of the internal oxide layer,and reduce the thickening rate of the oxide layer.During the 900°C-300h oxidation process,the oxidation layer did not flake off,and the relative content of TiO₂ in the outmost oxide film was higher than that of Cr₂O₃ as the time was prolonged.During the oxidation process at 1000°C,Cr₂O₃ became unstable and evaporated,the surface oxide layer became loose,and surface oxide peeling off occurred in the specimen cooling process.The addition of Y element can significantly reduce the oxidation rate at 1000°C exposure atmosphere.The internal oxide layer is composed of Al₂O₃ and TiO₂.The former is preferentially formed in the low oxygen partial pressure,and then extends to the interior along the grain boundary.To sum up,the microstructure,phases transformation process and high temperature oxidation behavior of this type of nickel-based superalloy have been studied in detail.The theoretical basis for the further optimization of the properties of this type of nickel-based superalloy can include the retardation of the coarsening of theγ′phase,the reduction of theα-Cr phase precipitation,and the improvement of thermal stability of the oxide layer.