Observation of two smooth-to-rough facet edge transitions at the {111} facets of a Pb alloy

Two smooth-to-sharp facet edge transitions have been observed in a Pb-5at%Bi-0.04at%Ni alloy. At 260 degrees C, equilibrated crystallites of the alloy are generally spherical with small facets at {111} and {100} orientations, and the rounded parts of the equilibrium form are smoothly connected to the facets. Upon heating to a temperature 10 degrees C below the solidus (or bulk melting) temperature, the edges of the facets become sharp, as has been observed in pure Pb, and this smooth-to-sharp facet edge transition is interpreted as being due to surface premelting of the rounded parts of the equilibrium form. Upon cooling from 260 to 250 degrees C and reequilibrating, a second smooth-to-sharp facet edge transition tales place at (111) facets. This transition is not observed in pure Pb, and is accompanied in the alloy by an abrupt increase in the levels of segregation of Bi and Ni at the surface. The paper reports results on the sharp compositional phase transition at the surface of the {111} facets, and provides an interpretation of the second smooth-to-sharp edge transition based on the lowering of the surface excess free energy of {111} orientations which results from the compositional surface phase transition.