Dendritic solidification in binary alloys
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I.
INTRODUCTION
EQUIAXED dendrites found in alloy castings grow under conditions similar to those for free dendritic growth; that is, growth proceeds free of any imposed gradients under a driving force arising purely from supercooling or supersaturation. In the case of the solidification of an alloy, there exists a thermal diffusion field ahead of the solid-liquid interface, from the release of latent heat of fusion, and a chemical diffusion field from the release of solute. It is believed that for equiaxed dendritic growth both the temperature and concentration gradients must be negative; i.e., temperature and concentration decrease with distance from the interface. This should be contrasted with the conditions required for columnar growth or directional solidification, where a positive temperature gradient is imposed. Kinetic and morphological data have been obtained on succinonitrile-acetone alloy dendrites growing unconstrained in a supercooled melt. This paper also describes the first attempt at defining the SCN-acetone system for solidification studies. The reasons for using SCN-acetone are, as shown previously, 1~-51 that SCN has well-established properties, freezes like a cubic metal and, specifically, SCN-acetone provides a system in which the phase diagram exhibits linear solidus and liquidus, yielding a convenient value for the equilibrium distribution coefficient, ko. This study essentially extends the investigation begun by Glicksman, Singh, and Chopra l~j on the quantitative kinetic and morphological measurements of free dendritic growth in the binary alloy system SCN-argon. Small additions of argon to SCN I~1were studied in order to investigate the behavior of dilute alloy systems under conditions conducive for free dendritic growth. The effects of inert gas additions to SCN t~j could, however, be studied only at dilute concentrations, in order to restrict the growth conditions to the region of the SCN-argon phase diagram where the equilibrium distribution coefficient remains fairly constant. This paper extends these studies to acetone additions to succinonitrile providing an alloy system which permits larger additions of solute that preserve the linear nature of the solidus and liq-
M.A. CHOPRA is Research Associate, Cleveland State University, and NASA-Lewis Research Center, Cleveland, OH 44135. M.E. GLICKSMAN is Professor, Rensselaer Polytechnic Institute, Troy, NY 12181. N.B. SINGH is Westinghouse Fellow Engineer in the Crystal Growth and Superconductor Department, Westinghouse R & D Center, Pittsburgh, PA 15235. Manuscript submitted October 23, 1987. METALLURGICAL TRANSACTIONS A
uidus lines on the phase diagram. A partial phase diagram for SCN-acetone was determined over the compositional region of interest. References 1 and 5 have summarized proposed models for alloy dendritic growth. These models all use some version of linear morphological stability for a solid-liquid interface, a concept first advanced for melt solidification by Mullins and Sekerka. Irj Subsequently, Oldfield, fTl Langer
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