Solidification of undercooled Sn-Sb peritectic alloys: Part II. Heterogeneous nucleation
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I.
INTRODUCTION
IN spite of the fact that peritectic reactions are about as common as eutectics, the number of reported studies of the latter considerably exceeds that of the former. According to the classical description, normal solidification of a peritectic alloy begins with precipitation of a primary solid phase, and during cooling, a secondary phase forms by a "reaction" between the primary solid and liquid. Since the primary solid and liquid become separated by a layer of the solid phase product once the peritectic reaction occurs, continued growth of the secondary phase is dependent upon solid-state diffusion. Therefore, a distinction is often made between the peritectic reaction in which all three phases are in contact with one another and the peritectic transformation in which the liquid and primary phases are separated by a layer of the growing product phase, ttl The peritectic transformation occurs below the peritectic temperature (following the peritectic reaction) and can be described by three separate but cooperative reactions, t21 These reactions are seldom given sufficient time to go to completion during conventional solidification processing, so that the crystallization of alloys with a bulk composition that intersects the peritectic horizontal usually results in a highly segregated structure consisting of primary phase dendrites in a matrix of secondary phase, t3] In more recent work using the entrained droplet technique, t41 however, it has been demonstrated that the peritectic "reaction" actually involves heterogeneous nucleation of secondary phase on the primary substrate at a finite level of undercooling below the peritectic temperature.
W.P. ALLEN, formerly Graduate Student, Department of Materials Science and Engineering, University of Wisconsin-Madison, is Associate Research Engineer with United Technologies Research Center, East Hartford, CT 06108. J.H. PEREPEZKO, Professor, is with the Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706. Manuscript submitted August 24, 1990. METALLURGICAL TRANSACTIONS A
The Sn-Sb system is somewhat unique in that it is one of the few binary systems in which a low melting point (Tm < 350 ~ terminal solid solution phase (exhibiting significant solubility) forms by a peritectic reaction. The low melting point of Sn-rich, Sn-Sb alloys permits application of the droplet emulsion technique to study undercooling and microstructural evolution in a controlled manner, tS1In the present investigation, the droplet technique was applied to elucidate aspects of heterogeneous nucleation, with the ultimate goal of generalizing the results to guide interpretation of solidification pathways in peritectic alloy systems. II.
EXPERIMENTAL APPROACH
Within the framework of the droplet emulsion technique, the identity of possible heterogeneous sites can be examined in a controlled nucleation catalysis experiment. Itj The approach which has been developed to permit the evaluation of alloy phase catalysis is outlined in Figure
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