Directional Solidification and Microstructural Refinement of Immiscible Alloys
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systems, such as Al-Bi, Al-Pb, and Cu-Pb, are a kind of extensive alloys that are characterized by a miscibility gap in the liquid state.[1] The alloys have potential for practical application because of their good physical and chemical properties.[2–4] For instance, it is a good self-lubricating material used as a bearing alloy for automobiles if the soft phase Bi-rich or Pb-rich spheres are dispersedly distributed in the hard matrix Al or Cu. The phase segregation, however, easily takes place due to the density difference between the liquid matrix and the sphere. Many investigations on the effect of the gravity level and the wet behavior were performed for the immiscible alloys.[5–9] The phase separation caused by the multifactor reaction is a very complex process during the liquid-liquid decomposition L fi L1 + L2. In general, the liquid-liquid phase transformation begins with the nucleation of the second-phase L2 in the form of the liquid sphere. The nucleated small spheres grow and coalesce in the supersaturated matrix liquid. The sediment or floating of the second phase will occur during the solidification on earth. Many methods were attempted to fabricate the homogeneous immiscible alloys in the last decade.[10] The desired microstructure, however, was difficult to obtain by the normal solidification technology. Recently, the numerical simulation results[11–13] indicated that the melt flow, the sphere’s spatial movement, JIE HE, Assistant Professor, JIUZHOU ZHAO, Professor, and HAILI LI, XIANFEI ZHANG, and QINXIA ZHANG, Graduate Students, are with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. Contact e-mail: [email protected] Manuscript submitted August 13, 2007. Article published online March 26, 2008 1174—VOLUME 39A, MAY 2008
and the interaction between the solid/liquid interface and the spheres are main factors that result in phase segregation. In order to explore the microstructural refinement, in the present study, the directional solidification has been performed for Al-9, 12Bi, and Al-3.4Bi-2.5Si alloys. The transverse magnetic field was applied to suppress the flow during directional solidification. The effects of the MS treatment and the magnetic field on the solidification microstructure have been originally analyzed. The volume fraction of the Bi-rich spheres near the solidifying front was determined. The morphology of the solid/liquid interface and the interaction between the interface and the spheres were discussed for the immiscible alloys with different MS temperatures. The influences of the transverse magnetic field on the spheres’ nucleation rate, the movement velocity, the collision between the spheres, and the formation mechanism of the alignment structure have been discussed.
II.
EXPERIMENTAL PROCEDURE
The alloy samples were prepared by melting pure Al, Bi, and Si (99.99 wt pct) in a Bridgman apparatus with a transversal static magnetic field. After being completely melted in the graphite crucible under argon atmosphere, the sample was driven downward out of the r
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