  Although the equilibrium composition of many alloy surfaces is well
understood, the rate of transient surface segregation during annealing is not
known, despite its crucial effect on alloy corrosion and catalytic reactions
occurring on overlapping timescales. In this work, CuNi bimetallic alloys
representing (100) surface facets are annealed in vacuum using atomistic
simulations to observe the effect of vacancy diffusion on surface separation.
We employ multi-timescale methods to sample the early transient, intermediate,
and equilibrium states of slab surfaces during the separation process,
including standard MD as well as three methods to perform atomistic, long-time
dynamics: parallel trajectory splicing (ParSplice), adaptive kinetic Monte
Carlo (AKMC), and kinetic Monte Carlo (KMC). From nanosecond (ns) to second
timescales, our multiscale computational methodology can observe rare
stochastic events not typically seen with standard MD, closing the gap between
computational and experimental timescales for surface segregation. Rapid
diffusion of a vacancy to the slab is resolved by all four methods in tens of
ns. Stochastic re-entry of vacancies into the subsurface, however, is only seen
on the microsecond timescale in the two KMC methods. Kinetic vacancy trapping
on the surface and its effect on the segregation rate are discussed. The
equilibrium composition profile of CuNi after segregation during annealing is
estimated to occur on a timescale of seconds as determined by KMC, a result
directly comparable to nanoscale experiments.
