Materials Science and Engineering A 375–377 (2004) 473–478 Microstructure of under-cooled Sn–Bi and Al–Si alloys Alessandro F. da Silveira a , Walman B. de Castro a,∗ , Benedito A. Luciano b , Claudio S. Kiminami c a UFPB – Department of Mechanical Engineering, P.O. Box 10069 58109-970 Campina Grande, PB, Brazil b UFPB – Department of Electrical Engineering, P.O. Box 10069, 58109-970 Campina Grande, PB, Brazil c UFSCar – Department of Materials Engineering, P.O. Box 676 13565-905, São Carlos, SP, Brazil Abstract Rapid Solidification Processing (RSP), of metals and alloys, is established by increasing the under-cooling by applying high cooling rates (10 2 to 10 6 K/s) or by reducing the nucleation sites using low cooling rates (1 K/s). Melt under-cooling opens new solidification pathways for new non-equilibrium phases and unusual microstructures. Several techniques have been developed to reduce the nucleation sites and produce increased under-cooling in metals and alloys including the fluxing technique. The aim of this paper is to study the influence of the under-cooling level on microstructures of Sn–Bi and Al–Si alloys by using the fluxing technique. Increasing the under-cooling of the molten Sn–Bi alloys in the range of 11K, lead to a refinement of the primary phase and eutectic constituent. However, for the molten Al–Si alloys, the increase of under-cooling, in the range of 25 K, led to no microstructure change. This is probably because sufficient under-cooling values were not obtained to produce rapid solidification in Al–Si alloys. © 2003 Published by Elsevier B.V. Keywords: Under-cooling; Sn–Bi alloys; Al–Si alloys; Fluxing technique 1. Introduction Rapid solidification has been well known to produce non-equilibrium microstructures and some distinct results are structural refinement, novel crystalline or amorphous phases and solid–solubility extension [1]. Traditionally, rapid solidification is achieved by employing rapid quench- ing techniques, such as splat quenching, melt spinning, laser surface melting, and gas atomization, etc. The main advan- tage of these methods is the rapid removal of accumulated latent heat while the main disadvantage is the fact that the specimen is usually small in one-dimension, leading to a difficulty in direct observation of the nucleation and growth phenomena. Therefore, researchers have paid attention to other ways for rapid solidification in recent years [2]. The other ways for rapid solidification are the techniques that are used for high under-cooling under low cooling rates (1 K/s), in order to reduce nucleation and to produce high under-cooling for metals and alloys, for example fluxing technique. In this technique the liquid is immersed in a ∗ Corresponding author. E-mail address: walman@dem.ufpb.br (W.B. de Castro). material that isolates it and prevents it from having contact with the crucible walls and atmosphere, and dissolve im- purities or change structures to make them less active. It suppresses the heterogeneous nucleation, which can lead to an increase in the under-cooling degree [3]. In an under-cooled melt, the thermal gradient at the solid–liquid interface is negative, and it is directly a func- tion of the growth rate [4]. Thus, at a low solidification rate or at a low under-cooling, the microstructure is near to equilibrium. However, at a critical growth rate, the diffusion field becomes shorter (in extent) than the microstructure scale. The diffusion process becomes localized with respect to the microstructure, and several morphological changes might take place. Very few papers predict the phase relationships in modi- fied microstructure of Sn–Bi and Al–Si alloys. Kene et al [5] for the first time reported “splat cooling” technique used for Sn–Bi alloys, while for Sn-rich alloys, a metastable exten- sion of solid solubility of at least 35 at.% Bi was reported. Later Perepezko et al. [6] observed the same metastable extension of solid solubility of to at least 35 at.%Bi by slow cooling droplet technique with under-cooling level of 175 K. The similarity of the solubility extension values obtained with the slow cooling droplet technique and with the rapid 0921-5093/$ – see front matter © 2003 Published by Elsevier B.V. doi:10.1016/j.msea.2003.10.017