Surface composition of ternary cu-ag-au alloys: part i. experimental results
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I.
INTRODUCTION
C O N S I D E R A B L E evidence has accumulated in recent years to support Gibbs' conclusion that at equilibrium, the composition of an interface is generally different from that of the adjacent bulk phases. ]u In the case of solidvapor interfaces that phenomenon has come to be called "surface segregation." Most of the evidence for surface segregation has come from experimental determinations of the equilibrium surface composition of binary metallic solid solutions. Additionally, a reasonable theoretical basis has been developed for the interpretation of experimental results. However, there remain important areas for both experimental and theoretical study. Most of the previous experimental studies of surface segregation phenomena have been confined to binary alloys. Many of these studies have mentioned, in passing, that strongly segregating trace impurities can have profound effects on the segregation behavior of the nominally binary alloys, and it is common for much effort to be devoted to the removal of these undesirable impurities from the alloys. In contrast, however, only little effort has been expended on the systematic study of those ternary effects in the surface segregation of alloys, and the work that has been done has tended to focus on the complex problem of the kinetics of surface segregation. ]2,3] Some measurements of composition at ternary surfaces t4,5] have been reported, but these have not addressed the issue of the temperature dependence of surface segregation. The majority of segregation studies in ternary systems have investigated the composition of grain boundaries, t6,7.s] where interest has focused on grain boundary embrittlement phenomena. In general, interesting "ternary effects" on surface segregation arise in ternary solid solutions which contain two surface-active solutes. Under those conditions, two types of phenomena distinguish surface segregation behavior in ternary solutions from those in their binary alloy
M.A. HOFFMANN, Graduate Student, and P. WYNBLATT, Professor, are with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted August 17, 1990. METALLURGICAL TRANSACTIONS A
counterparts. [9A~ The first ternary effect has been termed "cosegregation" and the second "site competition. "[9,~~ Cosegregation arises when there is a strong attractive interaction between the two surface-active solute species and leads to an enhancement in the surface concentration of the surface-active solute species over that observed in binary alloys of similar solute content, when the total amount of solute at the surface exceeds about half a monolayer. These enhancements are most noticeable in the more weakly segregating of the two species. In cases where a strong repulsive interaction prevails, the converse is true, and the surface concentration of the segregating species tends to be supressed with respect to that in the binary solutions. As the surface concentration of segregants continues to inc
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