Study On Mechanism Of Deformation And Damage Of Hole Defect In Nickel-Based Single Crystal Superalloy

Currently common used materials for aero-engine turbine blades are nickel-based single crystal superalloys.Due to its excellent high temperature mechanical properties,nickel-based single crystal alloy materials are widely utilized in hot end structural components of aero-engines and gas turbines,such as turbine disks,compressor disks and turbine blades.The strength and life of nickel-based single crystal superalloys directly determine the service life of aircraft engines and even the entire aircraft.The excellent high temperature mechanical properties of nickel-based single crystal superalloy is closely related to its typical microstructure.However,micro-defects,third-phase inclusions,micro-cracks,etc.are inevitably present in the nickel-based single crystal superalloy during the process of casting and subsequent heat treatment due to the influence of process.It has an inestimable impact on the properties of nickelbased single crystal superalloys.Meanwhile,these micro-defects pose great challenge for the mechanical properties evaluation of nickel-based single crystal superalloys and the life prediction of nickel-based single crystal turbine blades.In addition to common defects such as heterocrystals,freckles,orientation deviations and small-angle grain boundaries in nickel-based single crystal alloys,microscopic voids have become a factor that can not be ignored influencing the mechanical properties of alloys.Based on the above reasons,the main research work of this paper is as follows:(1)The tensile behavior of monocrystalline nickel with nano-void was studied via molecular dynamics simulation considering different lattice orientations.A series of simulations were performed to analyze the effect of system size,void volume fraction and lattice orientation on mechanical properties and microstructure evolution.The influence of size of sample on the incipient yield stress is discussed.The results show that void volume fractions have significant effects on Young’s modulus,incipient yield stress and incipient yield strain.The critical stress of[100],[110]and[111]orientation is 6.97GPa,6.77GPa and 7.31 GPa,respectively.The elongation of the sample along[111]orientation is greatest,which illustrate that the sample has good ductility performance along[111]orientation in the same initial damage.The single-void and double-void crystalline Ni atomic systems are employed to investigate inter-void interactions.The simulation results of the dislocation evolution of the three orientations reveal that a relationship exists between the evolution of the dislocation density and the stress-strain curve.At the initial stage of dislocation,the dislocation grows slowly,and consists of Shockley partial dislocation.The dislocation growth rate then increases significantly in the sharply declining stage of the stress-strain curve,where most of dislocations are Shockley partial dislocation.Analysis of the dislocation length during the overall simulation indicates that the dislocation length of the[110]orientation is the longest,followed by that of the[111]orientation and the[100]orientation,which has the shortest dislocation length.(2)Considering the complex microstructure of nickel-based single crystal,the Ni,Ni3Al and Ni/Ni3Al interface model were established to analyze the expansion dynamics of void using molecular dynamics method.The expansion behavior of void shows that a/6<112>Shockley partial dislocations initially nucleates on the free surface of void during the stretching of Ni and Ni3Al model.Whereas,dislocations nucleate from the interface of Ni/Ni3Al interface model.Evolution of void suggested that the plasticity deformation is dominated by movable Shockley partial dislocations and immovable Stair-rod dislocations.Void size effect analysis revealed that the larger void radius is,the smaller yield stress and Young’s modulus are.The mechanical properties of Ni/Ni3Al interface model were controlled by the interaction between void and interface.(3)Molecular dynamics simulation was employed to study the void configurationinduced change in mechanical properties and deformation mechanisms during tensile loading.The results show that void configuration has a significant influence on the yield stress and yield strain,while its effect on the elastic modulus is about 3.14 ± 0.23%.The deformation mechanisms of porous materials with various void configurations at micro-nano scale is proposed:(ⅰ)local plastic deformation;(ⅱ)homogeneous plastic deformation.Further analysis indicates that the difference between the above two deformation mechanisms is mainly caused by the competition and synergy between the stacking faults and dislocation.Local plastic deformation is mainly controlled by stacking faults.Homogeneous plastic deformation is dominated by dislocation motion with only a small amount of stacking shear motion,which gives material a superior plastic elongation.(4)Typical characteristics for creep fracture cleavage plane of nickel-based single crystal were investigated through the experimental observations.The results suggest that the topological shape of cleavage planes is related to the crystal graphical orientation.The cleavage surface exhibit four-fold symmetric in[001]orientation,twofold symmetric in[011]orientation,and triple-symmetric in[111]orientation.It is proved that the topological morphology is associated with the stress field near void through crystal plasticity finite element method.Void size dependence and external stress magnitude dependence of cleavage plane morphology was studied as well.The void size and external stress magnitude have a significant effect on the magnitude of the axial strain,but do not change visually with respect to the distribution of axial strain.The formation of typical characteristics of cleavage plane was discussed by the slip systems activation.The characteristics of dislocation motion around the void were described by molecular dynamics simulation.(5)The inter-hole interference of nickel-based single crystal film-cooling holes specimen during creep deformation were investigated by both experiment and crystal plastic finite element method.The results demonstrated that inter-hole interference promoted the evolution of microstructure,accelerated formation of inter-hole slip zone and accumulation of plastic strain.Based on the above findings,considered the interhole interference effect,the existing McClintock model was modified by stress triaxiality,and improved McClintock model provided a relatively good prediction of deformation of film-cooling holes at complex stress state.The influence of inter-hole interference on the fracture failure mode is further studied,of which is mainly achieved by the plastic slip zone between the holes.During fracture progress,it is seen that micro voids and cracks coalesced in this region.Comparing the stripe plastic slip zone observed in experiments and the fracture failure mode of film cooling hole specimen predicted by the finite element method,it was found that the predicted potential fracture paths showed excellent agreement with the experimental results.