NUCLEATION OF 2-DIMENSIONAL PHASES ON THE (111) SURFACE OF CU-AG ALLOYS

Experiments have been performed on the (111) surface of dilute Cu-Ag alloys under conditions where the surface segregation of Ag leads to a two-dimensional phase transition in which a 2D Cu-rich surface phase undergoes transformation to 2D Ag-rich phase upon cooling, and vice versa. The results show that the formation sites of the Ag-phase upon cooling are different from those at which the Cu-phase forms on heating. Three possible interpretations are provided: nucleation at points of intersection of high diffusivity paths with the surface, nucleation enhancement by sub-surface compositional fluctuations, and nucleation at rough versus smooth regions of the surface. The first interpretation can be ruled out, but it is not possible to distinguish conclusively between the other two interpretations.