Multi-Scale Study On The Mechanics Behavior Of Nickel-Based Single Crystal Superalloys

Due to their excellent mechanical properties at elevated temperature,nickel-based single crystal superalloys(NBSCSs)have been widely used in gas turbines and aero engines as the preferred materials of turbine blades.Improved mechanical properties of NBSCSs and rational design of key components are helpful to increase the work efficiency and reliability of gas turbines and aero engines.As we know,excellent high-temperature mechanical properties of NBSCSs originate from their unique two-phase microstructure.Therefore,multi-scale studies on the high-temperature mechanical behavior of NBSCSs and their deformation mechanism play key background role in developing a crystal plasticity(CP)constitutive theory with micro-structural dynamical evolution information.It is obvious that the topics of this doctoral dissertations are of great theoretical significance and potential application prospect,also are frontier of solid mechanics and material science.In order to study the deformation and damage behavior of NBSCSs and their intrinsic physical mechanisms,we pay special attentions to the two-phase microstructure(? matrix phase and ?’ precipitate phase)in the NBSCSs in this work.Firstly,dynamical evolution of misfit dislocation networks on two-phase interface,interaction between gliding dislocations in the matrix phase and misfit dislocations on two-phase interface and their underlying mechanisms are investigated by molecular dynamics(MD)method.Then,the influences of hydrogen atoms on the dynamical evolution of misfit dislocation networks and the hydrogen induced interface damage mechanism are studied by MD method.On these results obtained by MD and existing knowledge by discrete dislocation dynamics(DDD),a microstructure evolution mechanism based crystal-plasticity constitutive model for nickel-based superalloys is developed.The results predicted by this CP model are in good agreement with the experimental data,indicating that the present CP constitutive model can uniformly describe various mechanical behaviors of nickel-based superalloys under different temperatures and loadings.The main works and innovative points of this thesis include:(1)The “bowing mechanism” and “bowing stress” of gliding dislocations in the narrow matrix channel(i.e.? phase)are studied by MD method,with a special focus on how the misfit dislocation networks on the two-phase interface influences the bowing mechanism of matrix dislocation,and a theoretical formula is suggested to predict the critical bowing stress of matrix dislocations.The results show that if the matrix dislocation is close to the horizontal segment of misfit dislocation network,the reaction or strong interaction between the matrix dislocation and the misfit dislocation networks strongly affects the total energies of dislocation system,rendering the critical bowing stress of dislocation in the narrow matrix channel to raise or decrease.When the matrix dislocation glides in hairpin-like shape in the narrow matrix channel,new dislocation segments generate and deposit on the ?/ ?’ interface.If the deposited dislocation on the ?/ ?’ interface is of pure screw character,the reaction between the matrix dislocation and the misfit dislocation networks can significantly lower the critical bowing stress of matrix dislocation;otherwise,if the deposited dislocation on the ?/ ?’ interface is of 60°mixed character,the interaction between the matrix dislocation and the misfit dislocation networks renders the critical bowing stress of matrix dislocation in the matrix channel to increase dramatically.According to these MD results and the classical dislocation theory,the classical Orowan stress formula for dislocation bowing is amended and a theoretical formula is suggested to predict the critical bowing stress of matrix dislocations in narrow channels,which gives good agreement with the MD results.(2)With the application of MD method,the effect of hydrogen on the dynamical evolution of misfit dislocation networks,the nucleation of vacancies,and the damage on the two-phase interface in Ni-based single crystal superalloy are studied,with a full consideration to the temperature effect on them.The results show that the H atoms trapped by two-phase interfaces can facilitate the nucleation of interfacial vacancies,especially in lower temperature and higher H concentration.Furthermore,those captured H atoms can facilitate the reaction and dissociation of interfacial dislocation segments,cause square-like interfacial dislocation networks transfer into hexagonal-like ones,and result in large amount of L-C locks.All of these give heavy damage to the misfit dislocation networks on the interface and the interface itself.(3)Based on the homogenization method and representative volume cell(RVC)model,a CP constitutive model with information of two-phase microstructure and the mechanisms of dislocation dynamical evolution is developed for NBSCSs,and a user material subroutine(UMAT)for this CP constitutive model is coded and integrated into the commercial FEM software ABAQUS.By using this UMAT module,some typical deformation behaviors of NBSCSs are modeled under different temperatures and various loading orientations and good agreement with experimental data is seen.Different from existing CP models,the present CP model fully consider a series of important deformation mechanisms at the two-phase scale,including the octahedral glide and bowing-out of dislocations in the narrow ? channels,the "zig-zag" cross slip,the coupled glide-climb of matrix dislocations on the two-phase interface,the shearing of ?’ precipitate phase by the matrix dislocations and the formation of Kear-Wilsdolf(K-W)lock in the ?’ phase.Due to these full considerations to deformation mechanisms at the two-phase micro-structural scale,the present CP constitutive model can uniformly capture various mechanical behaviors of nickel-based superalloys under different temperatures and loadings.Good agreements between its predicted results and the experimental data indicate that the present CP model have strong ability to predict various high-temperature deformation behaviors of NBSCSs,including monotonic plasticity,cyclic plasticity,creep,flow strength anomaly and non-Schmidt effect.