COUPLED COMPOSITIONAL AND ROUGHENING PHASE-TRANSITIONS AT THE SURFACE OF A PB-BI-NI ALLOY

The co-segregation of Bi-Ni to the surfaces of small crystals of a dilute Pb-Bi-Ni alloy changes the crystalline equilibrium form in relation to that of pure Pb and leads to the reversible formation of {112} and {110} facets at roughening transition temperatures of 515 and 510 K, respectively. These roughening transition temperatures are higher than those in pure Pb, thus increasing the kinetics of equilibration to the point that the observation of the appearance and disappearance of facets at those orientations becomes possible. Most interesting is the fact that the appearance and disappearance of these new facets is coupled with a compositional surface phase transition which manifests itself as an abrupt change in the surface concentrations of the Bi and Ni solutes.