Effects Of Re On Microstructure Evolution And Damage Failure Of Nickel-based Single Crystal Superalloys

As the core hot-end component of aero-engine,the turbine blade determines the performance and efficiency of aero-engine to some extent.Nickel-based single crystal superalloys have been widely used in the hot part of aero-engines because of their excellent mechanical properties.At present,nickel-based single crystal superalloys have been developed to the sixth generation.The fourth generation single crystal superalloys in china are still under development.The third generation nickel-based single crystal superalloys have been in service in some engines,its remarkable feature is the addition of high content of refractory element Re.The addition of Re increases the creep resistance of nickel-based single crystal superalloys,but also promotes the precipitation of TCP phases in the service of nickel-based single crystal superalloys due to the low diffusion rate.TCP phases seriously weaken the mechanical properties of nickel-based single crystal superalloys.Studying the mechanical properties,precipitation mechanism,growth mechanism and its influence on the microstructure evolution and failure mechanism of TCP phase can effectively give full play to advantages and avoid disadvantages,and then improve the high temperature properties of nickel-based single crystal superalloys.Re is mainly segregated in the γ phase of the dentrite,hinders the movement of dislocations and influences the microstructure of single crystal.The study on the combiniation of the diffusion of Re and crystal slip theory,the the quantitative analysis of TCP phase growth mechanism provide a new and useful technical protocols for better studying the microstructure evolution of single crystal superalloys.The research on the Re content,diffusion mechanism and its intrinsic relation with the movement of dislocations and microstructure evolution is indispensable in the development of single crystal superalloys,which can promote the upgrade of nickel-based single crystal superalloys.In this paper,the study on microstructure evolution of single crystal superalloys focuses on the correlation analysis of internal key factors(Re element,TCP phase,γphase,γ′phase,dislocation,etc.),and integrates the dispersed individuals within the framework of microstructure evolution,so that the research results have better integrity and dialectical relationship.Based on the above reasons,this article mainly conducts the following research works:1.Based on the crystal slip theory and the diffusion of elements in solids,the rafting of γ′phase,the diffusion of Re,the formation mechanism of TCP phase and the initiation and propagation of crack in TCP phase of the second-generation nickel-based superalloy were researched by the scanning electron microscopy(SEM),the transmission electron microscope(TEM),the energy dispersive X-ray spectroscopy(EDS)and the electron back-scatter diffraction(EBSD).The comparative study under different experimental conditions(1050oC/200 MPa and 1100oC/140MPa)eliminated the dispersion of the research results.2.The tensile and compressive creeps of the third-generation nickel-based superalloy at1100 o C and 140 MPa were performed.A nanoindenter with SEM was used to measure the mechanical properties of γ phase,γ′ phase and TCP phase under different test conditions.The three-phase finite element model which corresponds to the creep microstructure is modeled for numerical simulation to study the stress and damage distribution caused by TCP phase at the combination of crystal plasticity theory and finite element analysis.3.In the combination of the rafting,the movement of dislocations and crystal slip theory,the tensile and compressive creeps of the third-generation nickel-based superalloyat 1100 oC and 140 MPa were performed to study the rafting process,the change of matrix channel,the dislocation morphology in different crystal planes,the diffusion of Re,the precipitation of TCP phase,the failure mechanism and the effect of crystal orientation deflection on the microstructure evolution.The study on Re diffusion and TCP phase growth in the manner of crystallology provides a new perspective to explore the microstructure evolution of single crystal superalloys.4.The high temperature creep and long-term aging tests of the second-generation and the third-generation single crystal superalloys with different Re contents were carried out.The effects of Re,dislocations,and TCP phases and their interaction on creep properties and microstructure evolution were researched.This study of their interaction provides a new way to clarify the reinforcement mechanism of Re.Based on the diffusion of elements in solids and the nature of plastic deformation(dislocation motion),the segregation of Re and the precipitation of TCP phase were investigated.5.Considering the influence of temperature,stress and creep time,the high-temperature creep tests of the second-generation nickel-based single crystal superalloy DD6 and the third-generation nickel-based single crystal superalloy DD9 under different experimental conditions were carried out.The effects of Re content,stress and temperature on microstructure evolution of γ matrix channel during high-temperature creep were studied by observing the width and morphology of γ matrix channel.The effect of the combination critical point of stress and temperature on the evolution of channel width was researched.