Convective effect on the solidification of hypermonotectic alloys
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Jiuzhou Zhaoa) Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, China (Received 18 September 2010; accepted 10 January 2011)
A model is developed to analyze the microstructure evolution in a continuously solidified hypermonotectic alloy. The model takes into account the common actions of the nucleation and diffusional growth/shrinkage of the minority phase droplets, the spatial phase segregation, and the convections of the melt. The microstructure formation in a continuously solidified hypermonotectic alloy is calculated. The numerical results demonstrate that the convections have great effect on the microstructure formation. The convective flow against the solidification direction causes an increase in the nucleation rate while the convective flow along the solidification direction causes a decrease in the nucleation rate of the minority phase droplets. The convections lead to a more nonuniform distribution of the minority phase droplets in the melt. It causes an increase in the size of the largest minority phase droplets and is against the obtaining of the hypermonotectic alloys with a well-dispersed microstructure.
I. INTRODUCTION
II. EXPERIMENTS AND RESULTS
A hypermonotectic alloy decomposes into two liquids when cooled in the miscibility gap. The liquid–liquid decomposition begins with the nucleation of the minority phase droplets. These droplets then grow by the diffusion of solute in the matrix. They can also move due to gravity, temperature gradient, or concentration gradient. The microstructure evolution during the liquid–liquid phase transformation is the result of the common actions of the nucleation and the diffusional growth/shrinkage of the minority phase droplets, the motions, and the spatial separation of phases. It is very complex. In particular, the motions of the minority phase droplets and the temperature difference inside the melt induce a strong flow field. The convections mix up the effects of all factors and make it extremely difficult to investigate the microstructure evolution in hypermonotectic alloys.1 Efforts have been made to investigate the kinetics of the liquid– liquid demixing,2–10 but the microstructure evolution and the convective effect remain an unsolved scientific problem. To clarify the mechanisms of the convective effect on the microstructure formation, directional solidification experiments were carried out with Al–Pb alloys. A model is presented describing the microstructure evolution during the liquid–liquid decomposition. The convective effects on the microstructure formation in hypermonotectic alloys are scrutinized.
Al–Pb alloys were solidified by using a Bridgman-type solidification setup.11 A graphite crucible was used as the sample container. The alloy was prepared by melting pure Al (99.99 wt.%), Pb (99.99 wt.%) in the crucible, heated to 950 °C, and the temperature held for 30 min to form a single phase liquid. The crucible was then withdrawn at different rates into a Ga–In–Sn liquid alloy bath. The samples have a cylindrical shape. The diameter a
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