Molecular Dynamics Simulation Of Crack Propagation Behaviors On Nickel Base Single Crystal

Due to the excellent high-temperature performance,nickel base single crystal alloy has been widely used in the manufacture of aircraft engine blades.Although numerous studies have been conducted on crack propagation of nickel base single crystal at macroscopic scales,there is limited knowledge of crack propagation at the atomic scales.Emergency is caused by crack propagation,which often leads to serious consequences.In this paper,systematic studies on crack propagation behavior and microstructure evolution regularity of the pre-cracking nickel base single crystal alloy have been conducted under the combined effects of loading and temperature.This paper aims at providing theoretical basis for the design and application of material as well as explanation for experimental phenomena.The main results and conclusions obtained are as follows:(1)When tensile and shear load are applied to the pre-cracking model,it can be found that crack propagation is determined by the dislocation slip behavior in front of the crack,and the cracks always extend along the directions which are easy for dislocations to slip.The ability of resisting crack expand is quite excellent when crack located in γ phase,while it is poor in γ′ phase.(2)The effects of temperature and tensile rate on crack propagation of pre-cracking nickel base single crystal alloy are studied by utilizing molecular dynamics simulation.When temperature is low,the dislocations inside material are stable and ordered,voids which nucleate at the intersection of slip system greatly facilitate crack propagation.As temperature increases,the thermal activation effect is more obvious and the critical shear stress of dislocations slip decreases.The dislocations are complex and the plasticity of material is greatly enhanced,which inhibits the crack propagation.When tensile rate is low,there is enough time for dislocations to slip which contributes to sufficient plastic deformation of nickel base single crystal alloy,so crack propagation is quite difficult.When tensile rate gradually increases,crack starts to extend before dislocations slip sufficiently.(3)The effects of ambient temperature and shear rate on crack propagation are researched by applying shear loading to the pre-cracking model.When temperature is 300 K,crack extends quickly,while crack propagation is restrained by dislocation pile-up and twinning at 600 K.At 900 K,crack propagation is hindered by dislocation pile-up at the early stage of shearing,and then the initial crack gradually heals up under the effect of atomic thermal motion,which makes crack propagate quite slowly all the time.The emergence of voids and new crack inside the material accelerates crack propagation when shear rate is low.As shear rate rises,crack propagation rate is much slower under the combined influence of twinning and dislocation pile-up.(4)Studies are conducted on crack propagation behaviors of nickel base single crystal alloy under different modes of fatigue,and the effects of initial strain amplitude on crack extension are also researched.For a system under constant strain amplitude cyclic loading,crack starts to propagate after multiple cycles,while the system under increasing strain amplitude cyclic loading suffers fracture within a few cycles.As the initial strain amplitude gradually increases,crack expansion is more rapid,which is attributes to the decrease of dislocations and increase of voids inside the material.