Solute-Driven Melting Kinetics in the Sn-Bi System
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Solute-Driven Melting Kinetics in the Sn-Bi System David Yasensky and Reza Abbaschian Materials Science and Engineering, University of Florida, 100 Rhines Hall, Gainesville, FL, 32611 ABSTRACT The goal of the present study was to gain insight into the mechanisms and behavior of solute driven melting processes. Solute-driven melting refers to melting whose driving force originates from a change in composition, i.e., solute content, rather than a change in temperature. This kind of melting has been known to cause casting defects in superalloys and other metals. An experimental apparatus was designed and a series of experiments were carried out on Sn-Bi alloys. The apparatus involved diffusing Bi into solid Sn cast inside a glass capillary to cause it to melt. The Bi source was an enriched liquid Sn-Bi alloy contained in a reservoir. The Sn in the capillary was kept at a constant temperature below its melting point so that the melting was caused by the increasing Bi composition. The progression of the interface was monitored by quenching the process at various times for the same conditions. The apparatus was successful in delivering data for the displacement of the interface against time. It was found that interface position was approximately proportional to the square root of time, and so the process may be diffusion controlled as conjectured in previous literature. Rough calculations are made relating the temperature and supersaturation to the displacement coefficient A in the equation z = At½ where z is displacement and t is time. By comparing data at different temperatures with the same supersaturation, an activation energy Q of ~60,000J/mol is calculated. This value is between the activations energies for diffusion of Bi through the liquid and the solid. Suggestions for modification of the apparatus to include in-situ interface monitoring are made.
INTRODUCTION This study focuses on melting due to compositional changes known as solutal melting. Several investigators have addressed this topic, however a complete description of the kinetics of melting is lacking from literature [1]. Melting, defined in this paper as a phase change from solid to liquid, is a ubiquitous phenomenon and part of many natural and synthetic processes. From a metallurgical perspective, solidification garners more attention than melting since the microstructure forms during solidification. However, recently there have been studies that expose the need for a more fundamental understanding of melting kinetics and mechanisms. For example, it has been shown that melting is the main cause of fragmentation of dendrite arms in alloy castings [2]. Melting also plays an important role in freckle formation in directionally solidified Ni-based superalloys and exudation in continuous castings [3, 4]. These practical applications along with the desire to further understand this common physical phenomenon have lead to recent interest in solute-driven melting. In the present investigation, a non-intrusive ex-situ technique is used to monitor
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