Surface Segregation Of Binary Alloys With Computer Simulation

The equilibrium compositions of the free surface of alloys, which consist of two or more components, are generally different from that of the bulk. This phenomenon is known as the surface segregation. The earliest motivations for studying the surface segregation occurred in the 1970's as a result of growing needs for the better understanding of catalysis-related phenomena. Recently, continuous expansion of interest in surface segregation was fueled by the recognition of the important role which plays in many other types of materials behavior, such as corrosion, surface energy and its anisotropy, compositional surface phase transitions, and wetting, as well as certain aspects of materials processing, such as crystal growth. Over the past several years, the topic of surface segregation has received much attention from both experimental and theoretical perspectives. Up to now various theories have been developed in order to explain the enrichment at the alloy surface. The first-principle approach seems be to highly accurate and informative, while the intensive calculations place practical limits on the complexity of the alloy systems. The empirical simulation approach, based on thermodynamics theory, can predict the segregation trend of binary alloys, but is too simple to calculate the surface compositions and composition depth profiles of binary alloy systems. Based on density function theory and including the pair potential between atoms, the embedded atom method (EAM) originally presented by Daw and Baskes is one of the most successful semi-empirical simulation methods and is much more computationally efficient than the first-principle method. Based on Johnson's analytic EAM model, a modified analytic EAM (MAEAM) is presented in this paper. The model parameters are determined by fitting the properties of metals, such as lattice constants, cohesive energy, monovacancy formation energy and elastic constants. Using only the physical parameters of pure elements, the properties of alloy can be calculated by constructing an analytic alloy potential, which shows that the NIAEAM has some universality. The surface energies of elements, the formation enthalpies of binary disordered solid solutions, and the surface segregation energies of dilute alloys are calculated with the MAEAM, and the surface segregation trends of binary alloys are predicted qualitatively in this paper. Combining the Monte Carlo (MC) simulation method with the Metropolis algorithm, the surface compositions and composition depth profiles of ten binary alloy systems with different structures are studied systematically. Here are the main points: 1. The surface segregation of six fcc-type binary alloys, whose constitutional elements are with fcc structure, is simulated with MAEAM potential and MC technology. The results indicate that (1) Cu segregates to the topmost layer over the whole range of compositions in Cu-Ni alloys at 1000K, and a monotonic approach of the equilibrium segregation profile to the bulk composition is found. More Cu enrichment is found on the (100) surface than that on the (111) surface. And it is not observed that Ni enrich on the surface for Cu-rich alloys. (2) As for Pt-Rh alloy at 1000K, the top-surface is enriched with Pt, the sub-surface layer is depleted of Pt, and the Pt concentration oscillates toward the bulk value. The anisotropy of the segregation for different low indeK faces is appreciable. A decreasing Pt surface segregation with increasing temperature for T> 12...