Molecular Dynamics Simulations On The Mechanical Properties Of Gradient Nano-grained Cu And Cu/Ni Alloy

The mechanical properties and deformation mechanism of nano-metallic materials have been a hot research topic in the field of materials, especially about the influence of grain size on strength and plastic of materials. In general, grain refinement will increase the strength of materials, but on the other hand, the ductility of material will be greatly reduced. The ideal engineering structure materials should have high strength and high plasticity, however, it is difficult to achieve. High plastic material often have a low strength, and high plastic material often have a poor ductility. Nanocrystalline material is a typical high-strength material. The strength of nanocrystalline is higher than the coarse-grained metals by an order of magnitude,but the ductility almost close to zero. The ductility of coarse-grained metal is better, but strength is lower. How to improve the tensile plastic deformation ability of nanocrystalline become a major problem in the field of material. The microstructure and composition content of material are the crucial on macroscopic mechanical properties. Composition, properties and structure of gradient material continuous variation, and there is no obvious interface in gradient material. Some researches show that gradient nano-grained metal not only have a high strength but also have a high ductility. Therefore, it is meaningful to research the microscopic deformation mechanism and mechanical properties of gradient material. With the rapid development of computer, computer simulation has became a effective method on material performance prediction and design. Molecular dynamics simulation method has been used to study the mechanical properties of gradient nano-grained copper and Cu-Ni alloy.Firstly, molecular dynamics simulations were employed to study the mechanical properties and deformation mechanism of nanocrystalline copper and gradient nano-grained copper under uniaxial tension. The results showed that the yield strength and ductility of gradient nano-grained copper are better than nanocrystalline copper. During the simulation,the cracks appeared in smaller grain size area firstly, and then in larger grain size area. The ductility of monocrystalline area is best. When material failure, there is still no cracks appeared in this place, only deformation motion and stacking fault existed.Secondly, molecular dynamics(MD) simulations of nanocrystalline Cu-Ni alloys with different gradient distribution of Ni content under uniaxial tensile straining were performed to study their deformation behaviors and mechanical properties. The results indicate that, with the increase of concentration gradient of the Ni, the elasticity young’s modulus of Cu/Ni alloy increase gradually, and the yield strength and ductility of the alloy were impacted by the change of Ni content. When tensile direction perpendicular to the direction of concentration gradient, the cracks appeared in triple junction firstly after yield stage, and then in the area of Ni content close to 50 percent. When concentration gradient of Ni is very high, rich Cu areawill crack easily.