Pre-Damage Assessment And Creep Residual Life Prediction Of Nickel-Based Single Crystal Superalloy

Turbine blade is one of the most important hot end components of advanced aeroengines,which is generally made of Ni-based single crystal superalloy.The harsh working environment and load make the microstructure inevitably produce a change called "rafting" during service,which leads to a decrease in the mechanical properties of the alloy and the weakening of the blade’s ability to resist damage.The cost of turbine blade is very high,and the easy abandonment of rafted blades will make the maintenance cost of engines expensive;if we continue to use the rafted blade at will,it will affect the service safety of the engine.Therefore,the rafting behavior has always been the research focus of the strengthening mechanism of Ni-based single crystal superalloys.In addition,the contents of W,Mo,Ta,Re and other high melting point elements in the single crystal superalloy are significantly increased in order to improve the temperature bearing capacity of the turbine blades.However,the microstructure degradation is easy to occur in the long-term service at high temperature,thus reducing the mechanical properties of the alloy.Therefore,the microstructure stability at high temperature is one of the important indicators to be investigated in the development and application of the alloy.Based on the above reasons,the main research work of this paper is as follows:1)The creep properties of heat-treated specimens with cubic γ’ phase and pre-rafted specimens with N-type raft structure were compared.The creep interruption test and creep rupture test at 980 °C/300 MPa were performed on samples with two kinds of γ/γ’ microstructure.The results show that the specimen with the cubic γ′phase has lower creep rate and longer rupture life than that of the raft-shape γ′phase.The microstructure of the two samples was observed and analyzed by scanning electron microscope(SEM).The microstructure parameters,such as the channel width of the γ matrix,were obtained by the relevant image processing.A creep damage model considering the continuous damage mechanism and the Orowan stress was established.The model based on the crystal slip theory and can predict the creep behavior of the pre-rafting Ni-based single crystal at high temperature.The raft effect was incorporated into the model by the dependence of Orowan stress on the channel width of the γ matrix.2)The creep properties of [001] and [011] oriented Ni-based single crystal superalloys were studied by means of stress-free annealing and stress-assisted annealing pretreatment.The creep properties of the pretreated alloy and the original alloy were compared,and it is found that the two oriented alloys have shorter creep rupture life and higher minimum creep strain rate after different pretreatments.After different pretreatments,the [011] oriented specimens show:(i)significantly decreased in creep rupture life,(ii)higher steady-state creep rate and(iii)lower creep rupture strain.SEM was used to observe and analyze the microstructure evolution law of different orientation pretreated alloy during creep test,and the macroscopic fracture and microstructure after fracture were analyzed.Based on the improved creep damage equation,the von Mises stress distribution in γ/γ’ two-phase interface of different orientation alloys with or without external load was analyzed by finite element method.The effect of crystal orientation on creep strength and the effect of microstructure degradation after pretreatment on creep life are discussed.3)The microstructure evolution,γ/γ’ lattice misfit and their effect on the creep life of Ni-based single crystal superalloy during long-term thermal exposure at 1100 °C were investigated in detail.The microstructure evolution mainly included the coarsening of γ′ precipitates and the precipitation of TCP phase.The size of γ′precipitates and the precipitation amount of TCP phase increased with the increase of thermal exposure time,and coarsening of the γ′ precipitates led to the increase of the matrix channel width simultaneously.The lattice constants of γand γ′ phases were measured by XRD,and the lattice misfit of the two-phase alloy was determined.The absolute value of the lattice misfit decreased with the increase of exposure time,and the corresponding dislocation networks spacing increased.The creep rate increased linearly with the increase of dislocation networks spacing.In addition,the size and morphology of γ′ phase,the precipitation of TCP phase,γ/γ′ lattice misfit and Orowan stress related to the size of γ phase on the creep life.Finally,a physical analysis model related to the relationship between γ′ phase evolution and creep rupture life during thermal exposure in superalloy was established,and the creep remaining life was predicted based on this model.4)The creep rupture life of a Ni-based single-crystal superalloy was evaluated under980 °C/270 MPa after the superalloy was thermally exposed to temperatures of900 °C,950 °C and 1100 °C for 100–500 h.The results showed that increasing of thermal exposure temperature can cause coarsening and coalescence of γ’precipitates along with a decreasing in their volume fraction.The coarsening rate of the γ’ phase increased with the thermal exposure temperature,and the activation energy of coarsening,Q,was approximately 270.9 k J/mol;this means that element diffusion controlled the coarsening of the γ’ phase.In addition,an increase in the thermal exposure temperature accelerated the precipitation process of the topologically close-packed(TCP)phase.Because of the degradation of the microstructure,the creep rupture life decreased from 113 h after thermal exposure at 900 °C to 95 h after exposure at 950 °C,and finally to 29 h after exposure at1100 °C when exposed for 500 h.In view of the experimental results,the influence of the microstructure evolution on the creep rupture life was investigated in detail.Finally,a creep constitutive model based on the dislocation density was combined with a continuous damage model that considers the cavity damage,to obtain a creep damage model.The creep remaining life prediction model of the single-crystal superalloy was established to predict the remaining life by introducing initial damage terms(the damage caused by the coarsening of the γ’ phase and that caused by the precipitation of the TCP phase during long-term thermal exposure,respectively)to the creep damage model.5)Tensile tests were carried out at room temperature,980 ℃ and 1100 ℃,and the uniaxial tensile plastic strain was introduced into the samples.The effect of plastic pre-strain at room temperature on the high temperature tensile properties of Ni-based single crystal superalloy was studied.The results showed that the yield stress and tensile strength both increase with the increase of the plastic pre-strain level,and the elongation after fracture decreases with the increase of the plastic pre-strain value.The plastic pre-strain weakens the significance of strain rate sensitivity.The strain hardening exponent also decreases with plastic pre-strain,and the relationship between strain hardening exponent and plastic pre-strain is nonlinear.The effect of plastic and creep strain on directional coarsening of Ni-based single crystal superalloy was studied by finite element analysis.The results showed that the directional coarsening was dependent on the inelastic strain gap between the γ and γ′ phases,which was attributed to an inhomogeneous relaxation of the local misfit stress field.The difference in the effects of plastic and creep strain in the coarsening was observed to quantitatively correspond to the difference in the localized inelastic deformations.