Effect of surface stress on Ni segregation in (110)NiAl thin films

Molecular statics and Monte Carlo simulations in conjunction with an embedded-atom potential are applied to study surface stresses and surface segregation in (110) NiAl free-standing thin films with different thicknesses. Ni always segregates to the surface for both stoichiometric and Ni-rich bulk compositions. The amount of segregation and the lateral deformation of the film both vary in inverse proportion to the film thickness. The size effect of segregation is explained by the elastic deformation of the film in response to the surface stress. It is predicted that the surface free energy decreases and the surface stress increases as the film becomes thinner. These effects are not specific to free-standing films but are also relevant to films on substrates, multilayers, new phase nuclei, and similar situations.