Study On Micro-Nano Mechanical Behavior Of Nickel-Based Single Crystal Superalloys Based On Molecular Dynamics Simulation

Nickel-based single crystal superalloys are the preferred materials for manufacturing aeroengine turbine blades because of their excellent high temperature mechanical properties,which play a vital role in the development of modern aviation industry.Turbine blades are often subjected to extremely complex loads in service,such as the repeated action of highintensity centrifugal force and high-temperature airflow,resulting in fatigue and shock damage.The unique phase interface microstructure deformation mechanism of nickel-based single crystal superalloys fundamentally determines their mechanical properties.To improve the fatigue and shock mechanical properties of turbine blades,it is necessary to conduct in-depth study by combining the evolution of interface microstructure and mechanical properties,thereby revealing the microscopic deformation mechanisms under fatigue and shock loads.This thesis starts from the unique phase interface microstructure of nickel-based single crystal superalloys,the mechanical behavior and microscopic deformation mechanism of nickel-based single crystal superalloys are investigated by molecular dynamics simulations under uniaxial tensile load,cyclic load and shock load,and two aspects of quasi-static load and dynamic load are discussed,respectively.The main research contents and results are as follows:1.Since the turbine blades are subjected to complex and multi-directional loads,the uniaxial tensile mechanical properties and micro-deformation mechanisms of nickel-based single crystal superalloys with different orientations are studied.The results show that the deformation mechanism is dislocation and stacking fault shearing γ’ phase under uniaxial tensile load.When the yield point is reached,the combined structure of dislocation and stacking fault on both sides of the phase interface merges into a stacking fault band running through the whole γ’ phase.The order of the elastic modulus in each orientation is(111)>(110)>(100),and the order of the yield strength is(111)>(100)>(110).The above findings are basically consistent with the experimental results in the literature.The anisotropy of mechanical properties is attributed to the differences in the stability of the interfacial dislocation network,the number of slip systems and the orientation factor at different orientations.2.Considering the fatigue failure of turbine blades caused by the cyclic reciprocating stress during operation,the cyclic deformation behaviors of nickel-based single crystal superalloys at different temperatures,strain rates and loading waveforms are studied,and the effects of the above factors on fatigue mechanical properties and micro-mechanisms are analyzed.The research results show that there are three main deformation mechanisms,namely,the dislocation and stacking fault shear γ’ phase deformation mechanism in the low temperature range,the Orowan by-pass and climbing mechanism in the high temperature range,and the dislocation and stacking fault shearing γ’ phase gradually transforms into the Orowan bypass and climb mechanism in the middle temperature range.All the above conclusions are the same as the experimental results.The saturation stress amplitude of nickel-based single crystal superalloys increases firstly and then decreases with the increase of temperature under cyclic loading.The increase of the strain rate causes the dislocation density and the stacking fault ratio to increase,with the result that the nickel-based single crystal superalloys reach cycle saturation faster and have higher stress amplitudes.Sine and trapezoidal waves have larger stress amplitudes than that of triangle and sawtooth waves.3.In view of turbine blades are subjected to the strong dynamic loads such as highintensity centrifugal force and scouring of high-temperature and high-pressure gas during service,the dynamic responses and micro-deformation mechanisms of nickel-based single crystal superalloys at different shock velocities are studied.The results show that the nickelbased single crystal superalloys will appear a typical elastic-plastic double-wave structure during the shock wave propagation.The deformation mechanism is mainly based on the slip and drag of dislocations when the shock velocity is less than or equal to 0.5 km/s.The phase transition from the face-centered cubic structure to the body-centered cubic structure mainly occurs when the shock velocity is between 0.5 km/s and 2.5 km/s.When the shock velocity is larger than 2.5 km/s,the atomic structure transforms from a face-centered cubic structure to an amorphous structure.4.Given that spalling under shock loading is a common failure mode,the shock spalling process of nickel-based single crystal superalloys is studied,and the effects of shock velocity,porosity and initial temperature on spalling strength are discussed.The research results show that two modes of classical spalling and micro-spalling appear under different shock velocities,and with the increase of shock velocity,the spalling mode gradually changes from classical spalling to micro-spalling.With the increase of shock velocities,the spalling strength of nickel-based single crystal superalloys decreases in three stages,corresponding to the microstructural deformation characteristics at different shock velocities.In addition,the spalling velocity threshold and spalling strength of nickel-based single crystal superalloys are associated with the initial temperature.The high temperature environment will significantly reduce the spalling velocity threshold and spalling strength,which is mainly related to the high temperature softening effect caused by the initial high temperature environment.The conclusions obtained in this paper are helpful to further understand the fatigue,shock mechanical properties and micro deformation mechanisms of nickel-based single crystal superalloys.Our works will provide a theoretical basis for the research,development,design and comprehensive performance evaluation of nickel-based single crystal superalloys turbine blades.