Microstructure Formation in a Gas-Atomized Drop of Al-Pb-Sn Immiscible Alloy
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DUCTION
ALLOYS with a miscibility gap in the liquid state include a broad range of materials. Many of them are excellent candidates to be used in industry. These alloys, however, have an essential drawback in that the miscibility gap poses problems during solidification. When an alloy is cooled into the miscibility gap, it develops into two liquids. Generally, the liquid–liquid phase transformation leads to a microstructure with a serious phase segregation under conventional solidification conditions. It has been demonstrated that under the conditions of rapid quenching or fast cooling, thus passing the miscibility gap quickly, hypermonotectic alloys with well-dispersed microstructure can be obtained. The rapid solidification technique is very promising in the manufacturing of immiscible alloys. Many experiments[1–5] and simulations[6–10] have been carried out to investigate the rapid solidification behavior of immiscible alloys. It has been indicated that the microstructure evolution during cooling an immiscible alloy in the miscibility gap is a result of the concurrent actions of the nucleation, diffusional growth, spatial motions, and collision and coagulation of the minorityphase droplets. It is a very complex process. Great progress has been made on the modeling and simulation of the microstructure formation during the cooling of an alloy in the miscibility gap in the last decades. The models presented recently describe the microstructure LEI ZHAO, Ph.D. Candidate, and JIUZHOU ZHAO, Professor, are with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang, Liaoning 110016, P.R. China. Contact e-mail: zhaolei@imr. ac.cn Manuscript submitted November 2, 2011. Article published online July 19, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
development in a binary immiscible alloy well.[10] But to date, no literature can be found on the modeling and simulation of the solidification process of a ternary immiscible alloy. The microstructure evolution during cooling a ternary or a multicomponent immiscible alloy remains a scientific problem to be resolved. A model to describe the microstructure formation in a ternary immiscible alloy solidified under the rapid cooling conditions of high-pressure gas atomization is developed in this article. Experiments are carried out with Al-Pb-Sn alloys, which are of technical importance as a bearing material. The model is first verified by comparing with the experimental results and then applied to investigate the kinetic details of the microstructure evolution in the rapid solidified Al-Pb-Sn alloy.
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THEORETICAL MODEL
A. Nucleation The nucleation of the minority-phase droplets can be described with the classical nucleation theory. The stationary homogeneous nucleation rate is given by[11] DGC I ¼ N0 O C Z exp ½1 kB T where N0 ¼ ðxA XA þ xB XB þ xC XC Þ1 is the number density of atoms in the liquid. XA ; XB , and XC are the atomic volumes of components A, B, and C, respectively. xA ; xB , and xC are the mole fractions of 2=3 components A, B, and C, res
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