Effect Of Co?Cr?Mo?Ru Additions On TCP Phase Evolution And Creep Behavior At 950? In 4^thGeneration Ni-base Single Crystal Superalloys

In recent years,the fourth generation Ni-base single crystal superalloy has become one of the important research field in the world because of its excellent comprehensive properties.However,the cost is one of the restriction on the industrial application in fourth generation superalloys as the price of Re and Ru are extremely expensive.Therefore,it is necessary to optimize the composition of the alloy and investigate the exploration and development of low cost(low Re)alloys with high creep performance.At the same time,the study on the influence of alloying elements on the precipitation of TCP phase and the creep properties at 950? are very limited in the fourth generation single crystal superalloys.Based on the previous research in our group,the effect of Co,Cr,Mo and Ru additions on the ? and ?' phase microstructural stability,TCP phase(?,P,? and R)precipitation and evolution,creep properties and microstructural evolution at 950?and 400MPa were investigated in this thesis.The ?-?' lattice misfit of experimental alloys were conducted in Shanghai synchrotron radiation facility.Cr was found to decrease the ?' volume fraction significantly.Increasing the Co,Cr,Mo and Ru additions resulted in more negative lattice misfit.The additions of Co and Ru improved the microstructural stability by inhibiting ?' phase connection and coarsening during long-term thermal exposure(50-1000h)at 1100?,and the effect of Co on microstructural stability is similar to Ru.Thermal exposure were conducted at 950? and 1100? from 50h to 1000h in alloys with lower Co content.Cr and Mo were found to promote ? phase and P phase,respectively.In the high Cr content alloy,the ? phase was stable after thermal exposure at 950? and 1100?,while the P phase was stable after thermal exposure at 1100? of high Mo alloy,The P phase is transformed from ? phase at 950? in high Mo alloy.In addition,the interfacial relationship of a phase and P phase with the matrix showed that the number of ledge steps of ? and the matrix was higher than that of P phase.It demonstrated that the a phase was more easily to nucleate from matrix than that of P phase.In alloys with high level of Co addition,Co-rich needle-like ? phase and lath-like R phase were precipitated after thermal exposure at 950 ? and 1100?.After thermal exposure at 950? the needle-like ? phase precipitated from the matrix and R phase were formed after 1000h.The R phase precipitated rapidly and remained stable after the thermal exposure at 1100?.At the same time,the thermal exposure results and thermodynamic equilibrium phase diagrams showed that the R phase was more stable than that of ? phase at high temperature.In addition,Cr and Mo promoted the precipitation of R phase and ? phase in alloys with high level of Co addition,respectively.The creep behavior and microstructure evolution were conducted at 950 ?and 400MPa.Co was found to increase the creep performance by slightly increased the ?' volume fraction and decreased the y channel widths.The addition of Ru had positive effect on creep resistance by increasing the lattice misfit and interfacial dislocation networks.However,the harmful effect on creep property induced by inferior microstructural stability in alloys with higher Cr and Mo additions overweighed their strengthening effects(improving the ?-?' lattice misfit).The dislocation substructure evolution were investigated in alloys with high content of Co during the creep process at 950? and 400MPa.The alloy with high level of Mo and Ru additions exhibited more negative y-y' lattice misfit and high density of stacking faults in the y channels during creep.Large amounts of stacking faults in the y matrix were served as the barriers to the dislocation movement.The interfacial dislocation networks prevented dislocation from cutting the ?' phases,which were considered as the other reason to improve creep resistance.Based on the investigation of this thesis.This work is helpful to understand the effects of alloying additions on microstructures and creep properties.It provides a scientific basis for the substitution of Ru in Co to reduce the cost of advanced single crystal superalloys.In addition,the generated information about compositional and microstructural effects on creep properties is helpful to provide the reference to the design and optimization of Ru-containing single crystal superalloys.