Simulation of surface segregation free energies

The effects of different assumptions made in the simulation of surface segregation free energies are considered. Thirty face-centered-cubic dilute binary alloys are investigated using embedded-atom method potentials. First, it is demonstrated that the inclusion of local atomic relaxations in lattice statics simulations strongly affects the segregation free energy at (111) free surfaces in over half of the alloys. Second, a Monte Carlo technique, namely, the overlapping distributions method (ODMC), is used to determine the effect of the vibrational entropy term on the surface segregation Helmholtz free energy for simulations at elevated temperatures (1000 K). It is determined that the vibrational entropy term is important for a quarter of the alloys. We conclude that for accurate calculations of surface segregation free energies, both local atomic relaxations and the vibrational entropy must be included in nearly all cases. Since the ODMC method is very computer-time intensive, a technique based on a simplification of the quasiharmonic approximation for calculating free energies is investigated. Results from this free-energy minimization (FEM) method using the local harmonic approximation are compared to results from the ODMC method. It is found that the FEM method calculates the segregation free energies at (111) free surfaces accurately for most of the 30 alloys. Segregation free energy profiles are also calculated as a function of distance from (111) free surfaces employing both methodologies. The agreement is found to be poor for alloys that have a large solvent atom and a small solute atom. Other problems and sources of error with the FEM method are also discussed.