Solidification of highly undercooled Sn- Pb alloy droplets
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INTRODUCTION
R A P I D solidification has marked potential advantages over conventional casting techniques. As the solidification rate increases, the dendrite arm spacing is reduced, supersaturation of the primary phase can occur, and metastable phases including metallic glasses, can be formed. These structural changes are associated with increasing undercooling of the liquid, either before nucleation or in front of the growing solid interface. In the more rapid solidification processes, the liquid is sometimes greatly undercooled before any solid is formed, but in these processes it is difficult or impossible to make direct measurements of undercooling before nucleation. Controlled studies on highly undercooled metals and alloys where rate of bulk heat extraction is low provide the opportunity for direct and simple thermal measurements, the results of which can be related directly to structures obtained. Such studies provide useful insight into the fundamentals of conventional rapid solidification processes and might also lead to new practical techniques for high undercooling and rapid solidification (e.g., techniques of emulsification; i.e., "atomization" in a liquid rather than in a gas or vacuum). Heterogeneous nucleation catalysts are present in metals, and the most potent of these limit the amount of undercooling that can be achieved. By dispersing a liquid metal into many fine particles, these nucleation catalysts are trapped within a small fraction of the particles; the majority are free from impurities and can be highly undercooled. In some systems, a subsidiary advantage is thought to result from purification of the droplet by the surrounding medium. Turnbull and co-workers ~'2'3initially observed directly the solidification behavior of small metal droplets, and later prepared and studied solidification of emulsions of mercuryJ The emulsion technique has been recently extended M.G. CHU, formerly Graduate Research Assistant and Postdoctoral Associate in the Department of Materials Science and Engineering, Massachusetts Institute of Technology, is now Senior Scientist with Alcoa Laboratories, Alcoa Center, PA 15069. Y. SHIOHARA is a Research Associate in the Materials Processing Center, Massachusetts Institute of Technology, and Special Postdoctoral Research Fellow at the Castings Research Laboratory of Waseda University, Tokyo, Japan. M.C. FLEMINGS is Toyota Professor of Materials Processing and Head of the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted August 2, 1983. METALLURGICAL TRANSACTIONS A
by Rasmussen, Loper, Perepezko, and co-workers 5-~3 for many alloy systems that melt below 725 K. The average maximum undercoolings observed by Turnbull for a variety of metals was about 0.18 Tin, where Tm is the melting temperature in degrees Kelvin. He achieved a maximum of 0.3 T,, for mercury. The undercoolings achieved by Perepezko et al. have been greater than any previous measurement, exceeding 0.4 7",,. Both pure metal
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