Surface energy, adsorption, and wetting transitions in ternary liquid alloys
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I. INTRODUCTION
THERE is a large body of literature on sessile drop measurements of the surface energy of liquid metals. However, only one previous study has been performed under ultrahigh vacuum (UHV) conditions where surfaces can be kept free of adsorbed gases.[1] Furthermore, whereas several studies of the surface energy of binary liquid alloys have been performed previously (under conventional, non-UHV conditions), there have been no systematic studies of surface energy in liquid ternary alloys. The surface energy of ternary alloys containing two surface active solutes is interesting because it depends not only on the nature of the solutes but also on their mutual interactions. The purpose of the present article is to investigate the effects of Sn and Tl additions on the previously studied surface energy of binary Ga-Pb alloys.[1] In addition, the nature of adsorption at liquid metal surfaces can provide information on wetting transitions in two-phase liquid alloys. This issue will also be addressed. In this article, we present surface energy measurements performed on the Ga-Sn and Ga-Pb-Sn systems. Together with data obtained previously on Ga-Pb,[1] it is possible to gain a clear understanding of the behavior of the ternary. We also report less detailed measurements on Ga-Pb-Tl alloys. In addition, we extend a previous regular solution, multilayer model of the surface energy, and surface composition in binary liquid alloys to the case of ternaries. Although this type of model is not expected to yield highly quantitative predictions, it can be used to provide qualitative guidance for experiments and can serve as a tool for the interpretation of results. Tin was selected as a third element addition to Ga-Pb because it possesses a surface energy that lies below that of Ga but above that of Pb (Table 1). Thus, in the absence of Pb, Sn is expected to adsorb to the Ga surface. As Pb is CRISTOPHE SERRE, Engineer, DOMINIQUE CHATAIN, Director of Research, M. MURIS, Postdoctoral Student, and MICHEL BIENFAIT, Professor, are with CRMC2-CNRS, Campus de Luminy, 13288 Marseille, Cedex 9, France. PAUL WYNBLATT, Professor, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted June 27, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
added to the alloy, it will tend to segregate more strongly than Sn and eventually displace the Sn from the surface. In order to fully understand the effects of Sn additions to GaPb, the other underlying binary system, Ga-Sn, was also studied in detail. The effects of Tl additions to Ga-Pb are expected to be different from those of Sn. Thallium has a lower surface energy than either Ga or Pb. It will therefore segregate to the surface more strongly than Pb as long as its concentration in the alloy is higher than that of Pb. However, as the concentration of Pb exceeds that of Tl, Tl at the surface will also be replaced by Pb. II. EXPERIMENTAL APPROACH AND RESULTS Surface energy measurements were performed by the sessile
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