Molecular Dynamics Simulation On The Mechanical Properties Of The Single Crystal/Polycrystal Nickel

The integral impeller structure is favored by aero-engine researchers because of its simple structure,light weight and high reliability.The integral blisk technology has been used to connect single crystal nickel（SNi）turbine blades and polycrystal nickel（PNi）discs with different performances into an integral blisk,but it has brought a high-risk failure problem in the joint area of the blisk,which seriously restricts the development of the technology.In this thesis,the molecular dynamics（MD）is used to predict the mechanical properties of single crystal/polycrystal nickel（SPSNi）in the dual crystal junction region of the integral impeller.Specific research contents are as follows:（1）The EAM multi body potential is used to simulate the tensile property of the MD model SPSNi in the joint zone.Firstly,the tensile atomic diagram of different crystalline nickels are compared.It is found that the degree of amorphization at the interface after tension is aggravated due to the presence of the single crystal/polycrystalline interface,which easily initiates micro-voids and increases the risk of sudden fracture of SPSNi.Finally,the effects of the strain rate and temperature are especially investigated.When the strain rate is in the rang of 10⁸ s^-1to 10¹⁰ s^-1,the tensile mechanical of SPSNi is almost insensitive to the loading strain rate,and the yield strength rises slightly.After exceeding 10¹⁰ s^-1,the yield strength decreases rapidly with the increase of strain rate.This is because a large number of FCC atoms in SPSNi rapidly transform into disordered amorphous structures at high strain rate,resulting in the rapid decline of SPSNi carrying capacity.The strain rate of 10¹⁰ s^-1 can be as the threshold of SPSNi tensile deformation.At different temperatures,the yield strength of SPSNi decreases linearly with increasing temperature,because the initial structure of dislocation network gradually becomes irregular,and the initial mismatch stress decreases with the increase of temperature under the influence of temperature.（2）The shear property of the MD model SPSNi composites is simulated with EAM multibody potential.Firstly,the influence of crystalline state on the shear property is discussed.Compared with SNi,the overall shear stress of SPSNi composites decreases due to the addition of PNi.It is found that the shear strength of the SPSNi composites is significantly smaller than that of SNi.The results also show that after the shear deformation of different crystal nickels enters the plastic stage,a phenomenon similar to the necking of the macroscopic material occurs,and the atoms near the necking zone undergo partial displacement,rearrangement and amorphization,which is easy to cause breakage.Finally,the effects of shear rate and temperature on the shear mechanical behavior of the SPSNi are examined.The results show that the shear modulus on SPSNi increases with the increase of shear loading rate within a certain range,and the shear modulus of the SPSNi shows a downward trend as the temperature rise.（3）The EAM multibody potential is used to investigate the influence of nanovoids on the mechanical properties of the MD model SPSNi.Firstly,compared with the polycrystal nickel,the addition of SNi may improve the tensile strength of the SPSNi.By comparing tensile atomic snapshots of the SPSNi with prefabricated voids at different positions,it is found that the prefabricated voids in the single crystal side accelerates the fracture of the interface,while the propagation of the prefabricated voids in the polycrystalline side is suppressed by the SNi.In addition,in the process of preparing the integral impeller,if there is a small hole defect on the SNi side of the SPSNi complex,it is more likely to lead to a decrease in the tensile performance of the integral impeller and the combination zone of the integral impeller,which increasing the risk of the integral impeller breaking suddenly.Finally,the influence of the void porosity is investigated.It is found that when the void porosity reaches over 0.8%,the SPSNi tensile strength rapidly decreases.However,when the void porosity is kept to be 0.8%,as the number of voids increases,the elastic modulus of the SPSNi composites increases gradually.It can be concluded that the mechanical properties of the SPSNi composites are not sensitive to small spherical nanovoids and they can still exhibit excellent mechanical performance when the SPSNi composties have small nanovoids.