Segregation and ordering at Fe1−xAlx(1 0 0) surfaces – a model case for binary alloys

The geometrical structure, chemical order and composition of (1 0 0) oriented surfaces of the binary alloy system Fe1−xAlx were investigated in the Fe-rich regime (x=0.03, 0.15, and 0.30) using quantitative low-energy electron diffraction. Low-energy He+ ion scattering and scanning tunneling microscopy were additionally employed to characterize the x=0.15 sample. The equilibrium structures developing with increasing bulk Al content can be consistently explained by the interplay between Al surface segregation and ordering processes which are controlled by atomic interactions similar to those in the bulk. These interactions divide the process of Al segregation to the very surface into two steps whereby Al atoms occupy sites of two different sublattices of c(2×2) periodicity with different probability. Whilst one sublattice is already completely filled at low bulk Al concentration, the other sublattice fills only gradually with increasing bulk Al content. The local order in deeper layers is consistent with the bulk phase diagram.