Surface-stress effects on elastic properties. II. Metallic multilayers

In the adjoining paper we presented a detailed model describing the effects of surface stress on the equilibrium spacing and biaxial modulus of thin metal films. We extend the model to describe the effects of interface stress on metallic multilayers. The model predicts that very thin layers will equilibrate to a spacing in the plane smaller than the bulk spacing for the material, and that this effect will vanish as the reciprocal of the layer thickness. The model predicts enhancements in the biaxial modulus of metallic multilayers which scale with the reciprocal of the layer thickness. The magnitude of both the strain and the resulting change in biaxial modulus are proportional to the magnitude of the interface stress. In order to verify the predictions of the interface stress model, we performed molecular-dynamics computer simulations of metallic multilayers using the ‘‘universal’’ form of the embedded-atom-method (EAM) potentials, an analytic form of the EAM potential, and a slightly modified version of the analytic form. The model was found to predict accurately the equilibrium properties of metallic multilayers. We discuss the limitations of the EAM potentials and the implications of this work for the supermodulus effect.