Investigation On The Relationship Between Temperature-Stress-interaction And Microstructure Stability Of Typical Ni-based Superalloy

Nickel base superalloys have been widely used in the fields of aerospace,ultra supercritical power plants because of their excellent performance at elevated temperature.With the development of science and technology,their service conditions become increasingly harsh.This requires superalloys to maintain high performance and microstructure stability during long-term service.Therefore,investigations are carried on the microstructure stability of Ni-based superalloys.This study mainly focuses on the evolution of ?' phase,inter-and intra-granular precipitates under the condition of temperature-stress-interaction,which can offer the theoretical and experimental evidences for engineering application of Ni-based superalloy.The evolution of microstructure and mechanical properties of 617B alloy during long-term thermal exposure up 12000 h near service temperature(650-750?)is studied systematically,and the relationship between the properties and microstructure is revealed.The ?' phase is the main factor affecting the strength of 617B alloy.Due to the coarsening of ?' phase,the strength of 617B alloy decreases when aged at temperature higher than service condition.The decrease of plasticity and obvious coarsening of grain boundary carbides cause the reduction of impact properties during aging.The rules of alloying elements mutual-diffusion and redistribution between the main precipitates and surface oxides in the near service condition are established.The evolution of ?' phase and grain boundary carbides under different temperature-stress-interaction conditions is further studied,and the effect of temperature-stress-interaction on the microstructure stability of 617B alloy is revealed.It is considered that the temperature and stress have different contribution degree to the microstructure stability of under service condition.It is proposed that there is a sensitive temperature at which both temperature and stress play a leading role on microstructure stability,and the sensitive temperature of 617B alloy is about 725?.Comparing 617B and 740H alloy indicats that the y' phase is more stable in 740H alloy.Therefore,it can be inferred that the sensitive temperature of 740H alloy should be higher than 750?.Furthermore,the microstructure stability of high-content y' phase with octet-cubic and irregular morphology is analyzed in GH4975 alloy during aging at 725?.Compared with 617B and 740H alloy,GH4975 alloy has higher ?' phase microstructure stability.Hence,it can be inferred that GH4975 alloy has a higher sensitive temperature at which the temperature and stress both play a leading role.The net-basket-like microstructure composed of long needle-like ? phase is obtained,and the stability of this kind of structure is discussed.It is indicated that the net-basket-like structure can keep excellent stability at 800?for 500 h.The stability of primary carbides in the hard-deformed alloy with high C(carbon)and high carbide-forming element content is emphasized.It is revealed that the primary carbide in the nickel-based superalloy with high C and high carbide-forming element changes and significantly improves the thermal deformation behavior during the high temperature diffusion heat treatment.The primary carbide in GH4975 alloy turns to rounded morphology after the heat treatment at 1180?/50 h,and the elastic modulus difference between the primary carbide and the matrix decreases obviously.Which improves the coordinated deformation ability and reduces the thermal deformation cracking tendency caused by the primary carbide.It can be proposed that the thermal processability of the high-alloyed hard-deformed superalloy can be improved by proper high temperature heat treatment to improve the coordinated deformation ability of the primary carbide.In conclusion,the evolution of ?' phase,inter-and intra-granular precipitates under different conditions is investigated.The effects of temperature and temperature/stress interaction on the microstructure stability of nickel-based superalloys are obtained by studying the evolution of y 'phase and grain boundary carbides under different conditions,which provides experimental and theoretical basis for the engineering application of superalloys.