The Creep Properties Of Nickel-based Single Crystal Superalloy Under Overheating And Overload Conditions

Turbine blades,as the components in hot end of aerospace engines,determine the performance of aerospace engines to some extent.It shows from a lot of analyses that overheating and overload is one of the important reasons for the early failure of engine turbine blades.The overheating refers to the phenomenon that parts suffer from overheating due to abnormal operating conditions exceeding their normal operating temperature.The overheating process is often accompanied by changes in material structure properties and increased deformation of the components,so that the strength and life of the turbine blades are reduced.In addition,when the engine is in different working conditions,the change of rotating speed directly leads to the change of blade load.In the complete working cycle of the engine,there are actually many overload cycles,which have an impact on the damage and life of the blade.Therefore,studying the influence of overheating and overload on the performance and life of turbine blades has become an indispensable task for safety and economic service.Based on the above reasons,this article mainly conducts the following research work:1.The overheating creep test under different pre-creep time tpre was carried out,andits creep characteristics and deformation mechanism were studied.Based on theanalysis of the test data,the evolution rule of the microstructure of material wasobtained,and the microscopic mechanism of the influence of overheating on thecreep properties was revealed.An improved creep constitutive model was proposedunder the framework of crystal plasticity theory by introducing continuous damageevolution and material hardening.The creep behavior of nickel-based single crystalsuperalloy under different tpre was simulated.Comparison with the testexperimental showed that this model could reflect the creep behavior of nickel-based single crystal superalloy under overheating.2.Overheating creep tests were carried out under different overheating temperatureTover?1050?,1100? and 1150??and overheating duration tover?30 min,45min and 60 min?.The effects of overheating temperature Tover and overheatingduration tover on creep performance and deformation mechanism were analyzed.The macroscopic and microscopic fracture analysis of the sample and themicrocosmic evolution rule in the process of overheating test were completed bySEM.Finally,considering the effects of overheating temperature T_over andverheating duration t_over on the?channel width and dislocation density,theconstitutive model proposed in Chapter 2 is improved.And the creep behaviorunder different Tover and tover was simulated.3.The overload creep test of nickel-based single crystal superalloy under the twoloading modes was carried out.Loading mode A:The effects of different time ofstage I t I?20h,40h,60h and 80h?and different stress of stage ??_??280MPa,300MPa and 320MPa?on the overload creep behavior were studied.Loading modeB:The effects of different time of stage ? t_??20h and 40h?,different time of stage? t_??20h and 40h?and different stress of stage II?_??300MPa and 320MPa?on theoverload creep behavior were studied.The macroscopic fractures of the overloadedspecimens under the two loading modes and the microstructure evolution of the?/?'phases during the overloading process were observed by SEM4.The internal state variables elastic strain,plastic strain,cumulative plastic strainand damage are introduced to describe the thermodynamic irreversible process ofthe system.An anisotropic creep damage evolution equation represented by thedamage strain energy release rate Y was deduced by the energy conversion method.Combined with the crystal plasticity theory,a new creep constitutive model wasestablished.The constitutive model is accessed to the finite element softwareAbaqus via the Umat interface of the user material subroutine.The overloadexperimental results are used to verify the proposed constitutive model,and thecreep behavior and life with different conditions were predicted using theestablished model.5.Based on the previous study on creep behavior under overheating and overload,creep tests were carried in complex environment with overheating-overloadcoupling.The coupling effect of overheating and overload was studied.Meanwhile,the damage and failure mechanism of the material and the is revealed.Theoverheating-overload coupling creep is decomposed into overheating creep,overload creep and coupling effect,and the influence mechanism of the three onthe overheating-overload coupling deformation is given.