JOURNAL OF MATERIALS SCIENCE LETTERS 18 (1999) 487±488 Heterogeneous nucleation behavior in undercooled Sn±Bi alloys W. B. DE CASTRO UFPb, CCT, Federal University of Paraõ Âba, Department of Mechanics Engineering, C.P. 10069, 58109-970, Campina Grande, ParaõÂba, Brazil C. S. KIMINAMI UFSCar, Federal University of Sa Ä o Carlos, Department of Materials Engineering, Sa Ä o Carlos, SP, Brazil Usually, during the solidi®cation of the off-eutectic alloys, two nucleation events may occur before the ®nal microstructure forms. The ®rst is the nucleation of the primary phase, which occurs at some under- cooling below the liquidus temperature. The second nucleation event, i.e., the nucleation of the eutectic phase, can be either promoted or hindered depending on the capacity of the primary phase to serve as a nucleating agent. The ability of one solid to nucleate heterogeneously on another solid has been the subject of a number of investigations, particularly in eutectic systems. Most studies have indicated that eutectic alloys exhibit non-reciprocal nucleating characteristics, that is, one primary phase will act as an effective heterogeneous nucleation site for the other phase, although the reverse is not true [1±4]. For example, Sundquist and Mondolfo [2] found that for the Pb±Sn system, primary Sn can readily promote the nucleation of Pb at undercoolings between 0 and 0.5 K, while primary Pb has no apparent effect on the nucleation of Sn, resulting in an undercooling as high as 55 K for Sn nucleation. For some eutectic alloys, a primary phase may be such a poor nucleant that the surrounding melt becomes severely undercooled and solute enriched. In this letter, we report the ®rst observation of non- reciprocal nucleation in off-eutectic Sn±Bi alloys using the ¯ux technique. The Sn±Bi alloys with compositions of Sn± 30 wt % Bi, Sn±47 wt % Bi, Sn±65 wt % Bi, Sn± 82 wt % Bi, Sn±95 wt % Bi were prepared by using 99.998% purity Sn and Bi. Quartz ampoules containing these metals were purged with argon and sealed under a vacuum of 1:33 3 10 3 Pa. The materials were alloyed in a rocking furnace at 773 K for 10 h. The ingots were cut into several pieces approximately 0.4 cm 3 in volume (3 g). The sample and ¯ux were loaded in a quartz crucible (7 cm long and 2 cm in diameter), purged with high-purity argon and evacuated to 0:13 Pa. The melting took place in an electric resistance furnace with linear movement, which allowed heating and cooling cycles of the sample. Fig. 1 shows the special furnace con®guration used to avoid sample vibration. In this system, the sample was independently held from the heating unit. By using such an experimental set-up, the heat unit was easily able to move along the quartz crucible, which allows one to obtain rapid heating and cooling of the sample. The heating rate was about 0.6 K=s and the cooling rate was near 0.5 K=s. The temperature measurements were per- formed using a mineral insulation J-type thermo- couple 1.5 mm in diameter. This thermocouple was immersed in the melted sample for accuracy. The nucleation temperature was detected by ®nding the in¯ection point in the temperature versus the time- cooling curve. Cooling curves were recorded using a computerized data acquisition system. The proeutectic and eutectic nucleation tempera- tures measured by the ¯ux technique at a cooling rate of 1 K=s are shown in Table I and in Fig. 2. The liquidus and eutectic lines, of the equilibrium phase diagram [5], are also drawn on this plot. Fig. 2 clearly shows that non-reciprocal nucleation behavior is observed for the Sn±Bi system. For alloys in which Sn is the proeutectic phase, an average undercooling of 20 to 30 degrees below the eutectic temperature (412 K) is required for Bi nucleation, and hence eutectic formation. For hypereutectic alloys in which Bi is the proeutectic phase, an average undercooling of 5 degrees is necessary for eutectic nucleation. The mechanism for the different nucleation abilities for each primary phase is still not clear, but previously it has been suggested that it is related to the interfacial energy [2, 6]. From embryo formation on an existing substrate and associated interfacial energies, it is seen that the solid±liquid interfacial energy of a nucleation catalyst (ó CL ) can be considered to be part of the driving force for solidi®cation, while the 0261-8028 # 1999 Kluwer Academic Publishers Argon out Vacuum Crucible support Thermocouple in the sample Crucible Flux Sample Furnace with vertical moving Argon in Data acquisition system Thermocouple Temperature control Figure 1 Details of the experimental apparatus. 487