A comparative study of the microstructures observed in statically cast and continuously cast Bi-In-Sn ternary eutectic a
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I. INTRODUCTION
THE development of the Ohno Continuous Casting (OCC) process has allowed the casting of alloy wires ,1.7 mm in diameter, which are uniform in composition and microstructure, for use in vapor deposition applications.[1] This technology has the potential to provide an alternative route to conventional methods such as extruding and rolling for materials that are otherwise difficult to fabricate due to limitations in their physical properties. This casting technique is a continuous unidirectional solidification method in which a mold is heated above the solidification temperature of the metals to be cast, and a cooling device is located a short distance away from the mold exit. Molten metal is introduced continuously into the mold and exits into the cooling zone solidifying just before entering the cooling water, resulting in a unidirectional cast structure. Pure bismuth wire 0.5 to 2 mm in diameter has been cast recently by this continuous casting process.[2] The result of this investigation suggested that Bi-bearing alloy or lead-free low temperature solder alloy wires could also be produced by this method. For solder alloys, good microstructural control of cast products is essential because it ensures uniform composition. In fact, some solder alloys are known to have nonuniform microstructures due to gravity segregation during casting, resulting in nonuniform chemical composition of the extruded products. In the present work, experiments were conducted with the aim of producing ternary alloy wires containing components of solder alloys. The material used in this study has the ternary eutectic composition 57.2 S. SENGUPTA, Graduate Student, H. SODA, Senior Research Associate, and A. McLEAN and J.W. RUTTER, Professors, are with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON, Canada M5S 3E4. Manuscript submitted October 13, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
pct Bi, 24.8 pct In, and 18 pct Sn (all in wt pct) with a melting temperature of 77.5 8C (350.5 K).[3] This was chosen because the microstructure of this particular alloy was studied by Ruggiero and Rutter[4] by solidifying the specimen unidirectionally at the very slow speeds of 0.74 to 53 mm day21. Thus, the microstructures obtained by the OCC process can be examined and compared with those described in the literature. Material of the same composition was also produced by a static method in which the specimens were melted and solidified within the furnace, and the microstructures compared with those of OCC specimens. This information may help in assessing the suitability of the OCC process as an alternative method for producing low-temperature solder alloy wires. II. EXPERIMENTAL ASPECTS Figure 1(a) shows a schematic diagram of the melting and casting arrangement designed to apply the principle of the OCC process[5] to the production of small wires of 1.5 to 2-mm diameter. It consists of a melting apparatus with a rectangular open cavity constructed out of a 37-mm-long and 30-mm-diameter grap
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