Creep-fatigue Mechanism Research Of The Directionally Solidified Nickel-base Superalloy4706DS

As the world’s most advanced third generation passively safe pressurized water reactor, large-scale advanced pressurized water reactor AP1000will be widely promoted in China and other countries in the world. Self-sustaining nuclear fission reaction in reactror releases energy, which is carried out by cooling medium in the form of heat, boiling steam drive the turbine blade to rotate to generate electricity. As new-generation directionally solidified nickel-base superalloy developed by our own country,4706DS is one of the best candidate materials for AP1000turbine blade.Alloy4706DS is the research object of the experimental investigation in this thesis, creep-fatigue tests have been conducted on specimens of different preparation procedure. After creep-fatigue tests, detailed analysis and characterization of microstructure and microchemistry of creep-fatigue fracture specimens have also been conducted, including metallographic tests, XRD tests, SEM tests, TEM/STEM tests, in order to study the creep-fatigue fracture mechanism of4706DS.Based on the analysis of aging and non-aging creep-fatigue specimens, we can conclude as follows. During aging process, γ’ phase of irregular shape turns into γ’ phase of spherical shape, and γ’ phase aggregate to grow up, the effect of second phase dispersion strengthening decreases; the content of MC carbides increases by separating out from the matrix, the content of M23C6carbides increases by separating out from the matrix and the degenerating reaction of MC carbides, the carbides aggregate and grow up during the aging process, as a result, the effect of second phase dispersion strengthening decreases; the absolute value of the degree of mismatch decreases, the coherent strengthening degree increases. However, the decreasing of second phase dispersion strengthening plays a main role, as a result, the ability of resisting creep-fatigue fracture decreases. Based on analysis and characterization of microstructure and microchemistry of specimens at different aging temperatures, we can conclude as follows. With the increase of aging temperature, the size of γ’ phase increases, the degree of dispersion decreases, the effect of second phase dispersion strengthening decreases; the content of carbides increases, but the size of particles increases, the degree of dispersion decreases, the effect of second phase dispersion strengthening decreases; the absolute value of the degree of mismatch decreases, the coherent strengthening degree increases. When the holding time is3分é'Ÿ, the decreasing of second phase dispersion strengthening plays a main role, as a result, the ability of resisting creep-fatigue fracture decreases. When the holding time is0.6分é'Ÿor30分é'Ÿ, the coherent strengthening plays a main role, as a result, the ability of resisting creep-fatigue fracture increases.Based on the analysis of stress state on the creep-fatigue behavior, we can conclude that, from the transition arc R to the middle location M to the fracture location B, the stress value acting on the specimen gradually increases, as a result, MC carbides resolve, the content of M23C6carbides increases, M23C6carbides aggregate and grow up; γ’ phase aggregate and grow up, resulting in the decrease of the effect of second phase dispersion strengthening; the absolute value of the degree of mismatch increases, the coherent strengthening degree decreases; the density of dislocations increases. As a result, the fracture breaks first.