The High-temperature Oxidation And Thermal Fatigue Behaviors Of Nickel-based Single Crystal Superalloy

Due to the superior mechanics properties,Ni-based single crystal superalloys have been widely used for the hot end components of aircraft engine structures.During the service period,the hot end components require prolonged exposure to high temperatures.Long-term high temperature oxidation would reduce the corrosion and creep resistance of the alloy,induce the initiation of surface cracks and defects,reduce the surface strength and accelerate the failure of the alloy.Transient acceleration and braking process during service could cause transient changes of the temperature in the hot end components,producing transient thermal stresses that could cause local thermal fatigue failure of the components.The combination of long-term high temperature oxidation and thermal fatigue seriously affects the strength and life of hot end components.However,related research is still relatively rare.The study of oxidation and thermal fatigue behavior has important theoretical significance and engineering application value.Based on the above situation,this paper mainly carried out the following research work:1)By virtue of XRD,OM,SEM and EDS,detailed oxidation behaviors for 2000h exposure of DD6 Ni-based single crystal superalloy were investigated at 1050? and1100?.The oxidation kinetic curves both follow the parabolic law in the first stage,during which time an outer continuous compact?-Al₂O₃ film formed immediately on the specimen surface and a large amount of granular(Ni,Co)O first burgeoned in dendrite core area at 1050?.With the oxidation time continuing,the morphologies of the oxides in these two kinds of areas gradually tend to coincide:An outer columnar(Ni,Co)O film,an intermediate layer mainly composed of NiTa₂O₆,CoCo₂O₄,CoWO₄,NiCr₂O₄ and other complex compounds and spinel phases and an inner continuous compact?-Al₂O₃ film.The double diffusion of oxygen anions and Niand Cocations developed inwards and outwards respectively is responsible for the morphology of the oxide scale.At 1100?,the outer Al₂O₃ film formed in the initial time is unstable,discontinuous and incompact and the inner Al₂O₃ layer formed after300h oxidation is porous.2)Detailed long-term of 2000h oxidation behavior of a Ni-based single crystal superalloy was investigated at 1000?.In the first oxidation stage,?-Al₂O₃ formed immediately,and then the mass gain curve obeys the parabolic law,which corresponds to the growth of?-Al₂O₃ film.A single?-Al₂O₃ film kept on the surface of the specimen before 750h oxidation.The oxidation is controlled by the inward diffusion of oxygen anions to react with Al cations at the oxide/metal interface.The shape of the exfoliation region of single?-Al₂O₃ film is closed to an annulus,in which the(Ni,Co)O layer formed due to depletion of Al.The three-layer oxide scale formed rapidly after 750h oxidation.With the continue consumption of Al,a discontinuous inner?-Al₂O₃ layer formed,making oxide scale spall seriously and gain mass rapidly.The states of?-Al₂O₃ film,the controlled film,are different in dendrite core and interdendritic region.3)The high-temperature oxidation dynamics and mechanisms of a Ni-based single crystal superalloy with four kinds of surface roughnesses were investigated by virtue of XRD,OM,SEM and EDS at 1000?.In the initial oxidation stage,outer(Ni,Co)O was mainly produced on the surfaces of the samples with Ra=90nm and 19 nm.Correspondingly,outer Cr₂O₃ and transient?-Al₂O₃ were mainly formed on the surfaces with Ra=509nm and 182 nm.After 180 min oxidation,the values of instantaneous parabolic mass gain coefficients(k_p)of the samples with all surface roughnesses were gradually consistent with the data of the growth parabolic coefficient of?-Al₂O₃.The oxidation mechanisms of Ni-based superalloy with different surface roughnesses were discussed by a model.The external diffusion flux of Al(D_Al)increases with the increases of surface roughness.Thus,the required Al concentration decreases with the increases of surface roughness when the selective oxidation occurrs to form a protective single?-Al₂O₃ film.4)Based on three-dimensional transient thermo-mechanical coupling theory and rate-dependent crystal plasticity theory,a transient thermal shock constitutive model was developed to investigate the crack initiation behavior under transient thermal stress of a V-notched Ni-based single-crystal superalloy at 25?(?)760?/900?/1000?.The evolution and distribution of the resolved shear stress and damage were simulated.Three surface heat transfer stages were considered during the water-cooling process in the simulation.The crack initiation lives and locations in the simulation results both showed good agreement with those observed in the experiments.The growth directions of the two main cracks were approximately 45°with the dendrite orientation,and the{111}<110>slip family was mainly activated.5)Thermal fatigue crack propagation behaviors of a V-notched Ni-based single crystal superalloy DD6 were tested at 25?(?)760?/900?/1000?.The propagation directions of the two main cracks were approximately 45°with the dendrite orientation,and the{111}<110>slip family was mainly activated.Based on3D transient thermo-mechanical coupling theory,combining linear elastic mechanics and rate-dependent crystal plasticity theory,the K and J-integral were calculated to predict the thermal fatigue crack propagation behavior.Three surface heat transfer stages were considered.The predicted crack propagation rates at three heating temperatures all showed good agreement with those obtained in the experiments.