Molecular Dynamics Simulation and Thermodynamics Calculation on Surface Segregation in Ni-Cu Nano-Films under Stress

The surface segregation of Cu atoms in a Ni-Cu system was investigated using molecular dynamics simulations. Thermodynamic calculations were performed to verify the results of the molecular dynamics simulations. For the thermodynamic calculations, a model for evaluating the influence of stress on surface segregation was developed using the modified Darken model in combination with the broken-bond model. Using molecular dynamics simulations, it was found that the enrichment of Cu atoms occurred for a free-standing Ni-10 at.% Cu film consisting of 20 layers. Simultaneously, the stress distribution across the Ni-Cu thin film is obtained. The thermodynamic calculation results show that the influence of stress on the surface segregation cannot be ignored because of the considerable surface stress. Surface tension stress promotes the surface segregation of copper in Cu-Ni alloys due to the larger lattice parameter of copper than nickel, which leads to the reduction of surface strain energy. When the thickness is greater than 31 nm (or the number of layers exceeds 89), the size effect disappears, i.e., the surface concentration doesn’t increase with the increase of thickness. The calculation results obtained by the Bragg-William equation used for the surface segregation in equilibrium are in good agreement with the thermodynamic calculation and molecular dynamics simulation results.