Dendritic growth of undercooled nickel-tin: Part I
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I.
INTRODUCTION
UNDERCOOLING ("supercooling") of metal alloys has been the subject of extensive study for over 30 years. Many researchers have dealt with the homogeneous and heterogeneous nucleation of undercooled melts, beginning with the original work of Hollomon and Turnbull. ~,2 Later studies also considered growth behavior and resulting structures of undercooled melts: notable examples are the work of Walker, 3 Colligan and co-workers, 4 Glicksman and coworkers, 5-9 and Flemings and co-workers. ~~ Interest in solidification behavior of undercooled melts has been heightened in recent years, partly due to the technical and scientific interest in rapid solidification processing. Undercooling plays a major role in determining the structure observed in many rapid solidification processes. However, because of the short times involved in rapid solidification processes and the small size of samples involved, detailed study of thermal behavior in such processes is very difficult. Solidification during recalescence in the relatively larger samples used in this study is expected to be similar to that in rapid solidification processing, except that the effects of heat losses to the surroundings are less. This paper describes a photographic technique for measuring dendrite growth velocity in undercooled metallic melts. A number of studies have been conducted in the past to measure growth velocities. In the early 1950's, Rosenberg and Winegard measured the growth rates of dendrites (up to 0.25 meters per second) on the flat surface of an undercooled pure tin melt using cinematography. 16Later, the growth rates of free dendrites in an undercooled melt of Ni-0.20 pct Cu were measured (up to about 5 meters per second) by Walker. ~7 Colligan and Bayles4 measured den-
Y. WU and T.J. PICCONE are Graduate Research Assistants in the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Y. SHIOHARA is Research Associate in the Materials Processing Center, Massachusetts Institute of Technology, Cambridge, MA 02139, and also a Special 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 March 4, 1986.
METALLURGICALTRANSACTIONS A
dritic growth velocities of undercooled pure nickel, also using high-speed photography of the free surface of the melt. Glicksman and Schaefer 18 used cinematography on free surfaces of Sn and Bi alloys to provide particularly revealing information on dendrite tip shape and dendrite disintegration, as well as tip velocities. In many of the foregoing studies, the dendrites were made visible (in ordinary light) by specular reflections from surface distortions at or near the advancing liquid-solid interface (caused by solidification shrinkage and by surface forces at the juncture of the solid and liquid at the free sur
