Study On Mechanical Properties Of Nanocomposite Cu/Ni By Molecular Dynamics Simulation

Nanocrystalline materials show vast application prospects in optics,mechanics,thermodynamics,and electromagnetism due to their small size effect,surface/interface effect and quantum tunnel effect which macroscopic materials do not possess.Cu/Ni nanocomposite systems with stable structure and excellent performance have been focused by scientists recently.In view of the current research status,this thesis explores the mechanical properties of two types of nanocomposites composed of Cu substrate board with impurity Ni and Cu/Ni bilayer structure.The two types of nanocomposites are modeled using molecular dynamics.The mechanical properties of Cu substrate with impurity Ni are analyzed by applying a tensile loading.The surface friction properties and interactions between dislocations and interface of Cu/Ni bilayer are studied by means of nanocutting and nanoindentation.The main research contents are as follows.(1)The impurity Ni is embedded into the substrate Cu and the Cu/Ni composite plate structure is constituted.Then tensile loading is applied to the composite to study the effects of Ni impurity on the materials mechanical properties,and analyze the interactions between dislocations and interface.The strengthening effect of Ni on the tensile deformation stage is revealed from micro perspective.The results indicate that the impurity Ni has lattice mismatch with its surrounding substrate Cu,and it is conducive to the generation of dislocations and makes the decline of yield strength.The impurity can also hinder the movement of dislocations,and furthermore result in the increment of tensile stress in the stage of plastic deformation.For the Cu/Ni composite plate,the size and shape of impurity Ni are changed further to study their effects on yield strength.It is found that the yield strength can be influenced by two factors:one is the quantity of dislocations produced by the lattice mismatch areas between Ni and Cu,and the other is the hindering effects of impurity and interface on the movement of dislocations.The Cu/Ni composite boards with boundary cracks are also modeled,and the relationship of direction of crack propagating with the shape of impurity is analyzed.(2)For the Cu/Ni bilayer,it is divided into Cu-Ni and Ni-Cu according to difference of the processed surface and base.Nano-cutting and nanoindentation processes are conducted to machine the composite bilayers.Single crystals Cu and Ni are also processed to be served as comparisons.In the nanocutting process,Cu/Ni bilayers are subjected to larger friction forces and normal forces than their single crystals.Both friction and normal forces of bilayers and single crystals increase with cutting velocity ranging from 100 m/s to 300 m/s.The friction coefficients of nano films are smaller than that of corresponding macro-materials,and this can be explained based on the formation processes of nanometer chips.The nanoindentation processes of Cu/Ni bilayers can be divided into three stages:loading stage,staying stage and unloading stage.In the loading stage,elastic deformation and plastic deformation occur orderly.The indentation forces of Cu/Ni bilayers are nearly the same as that of single crystals Cu and Ni in the elastic deformation,while the bilayers' indentation forces are larger than that of the single crystals in the plastic deformation and show higher hardness.In the unloading stage,film atoms are attached to the leaving tool and exert attractive forces on it.The relaxed deformation and dislocation movements are also found in this stage.The interface of Cu/Ni bilayer appears as a barrier of the dislocation diffusion.