COMPUTER-SIMULATION OF PHASE-TRANSITIONS ASSOCIATED WITH SURFACE MISCIBILITY GAPS

The Embedded Atom Method has been used in conjunction with Monte Carlo modeling to simulate phase transitions associated with surface miscibility gaps in dilute Cu-Ag alloys. The simulations show that the (111) surface undergoes an abrupt transition from a Cu-rich to a Ag-rich surface phase at 750 K over a very small increase in bulk Ag concentration. No comparable transition is found on the (100) surface down to temperatures as low as 400 K. The Cu-rich phase on the (111) surface shows no significant imperfections. In contrast, the Ag-rich phase is quite disordered, and the high concentration of the larger silver atoms can only be accommodated if some of those atoms are squeezed out into previously unoccupied adatom sites. The results of the simulation are compared with predictions of a regular solution model of surface phase transitions. While the general trends predicted by the two approaches are similar, significant quantitative differences are apparent between both the predicted phase boundaries and the critical temperatures associated with the surface miscibility gap.