Microstructure of one of the ternary eutectic alloys in the Bi-In-Sn system
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INTRODUCTION
W H I L E considerable work has been done on binary eutectic alloys to allow predictions of the microstructure based upon the known phase diagrams and the properties of the elements involved, the same cannot be said for ternary systems. During the last several years, however, the volume of work on ternaries has been increasing and the structure of several ternary eutectics is now known; see, for example, Kabassis et al., ~'2 Kerr et al., 3 Cooksey and Hellawell,4 and McCartney et al. 5 The current experiments were undertaken in order to investigate the possibility of having a fibrous structure of two minor phases in a ternary, even though the combined volume fraction of fibrous phases is larger than 30 pct, the maximum normally achievable in binary systems.
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BACKGROUND INFORMATION
Figure 1 shows the Bi-In-Sn phase diagram as reported by Stel'makh et al. 6 Two ternary eutectic points are shown on the diagram, as well as several binary eutectics. To understand the diagram and those features of it that are still uncertain, it is helpful to refer to the binary systems associated with this ternary system. The In-Sn system, according to Metals Handbook,7 has one eutectic point at 48 wt pct Sn and 390 K involving the In-rich/3 phase and the Sn-rich y phase, which is formed peritectically at 497 K. The solid solubility of In in Sn is roughly 5 wt pct In at 373 K, and that of Sn in In is 12 wt pct Sn at 373 K. The Bi-ln system, according to Evans and Prince, 8 contains four compounds, Biln, Bi3Ins, Biln2, and a phase designated as e with a composition of approximately 85 wt pct In.Three eutectic points exist. One is located at 32.6 wt pct In and 382.8 K between Bi and Biln, another at 50.1 wt pct In and 361.8 K between the Bi3In5 and Biln2, and the third at 66.7 wt pct In and 345.8 K between Biln2 and e. The e phase, which forms peritectically at H. KABASSIS, Research Associate, and J.W. RUTTER and W.C. WINEGARD, Professors, are with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, Ontario M5S 1A4, Canada. Manuscript submitted February 7, 1983. METALLURGICALTRANSACTIONS A
In
In2Bi
InBi
Bi
Fig. 1- - T h e Bi-ln-Sn phase diagram according to Stel'makh et al.
366.5 K, decomposes eutectoidally at 322 K. There is no solid solubility of In in Bi, and the solid solubility of Bi in In is estimated as 7 wt pct Bi at room temperature. The lnBi-Sn system, a pseudobinary system that divides the ternary Bi-In-Sn system, was studied by Dooley and Peretti;9 it has a eutectic point at 16.3 wt pct Sn and 354 K. There is no solid solubility of Sn in the InBi compound, and the solid solubility of lnBi in Sn is large, 58 wt pct at the eutectic temperature and 20 wt pct at 344 K. The Bi-ln-Sn system is shown in Figure l, according to Stel'makh e t al.; 6 a ternary eutectic point exists at 31.6 wt pct Bi, 48.8 wt pct In, 20.6 wt pct Sn and 332 K (note that the figures given by Stel'makh et al. 6 add to 101 wt pct). The other ternary eutectic point determined by Scherpereel a
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