Heterogeneous nucleation of Pb
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INTRODUCTION
N U C L E A T I O N during solidification plays an important role in controlling many microstructural features, such as the grain size, shape, and distribution of the secondphase particles, which affect the final properties of the solidified materials. There are inherent difficulties in carrying out successfully controlled nucleation experiments during solidification because the nuclei are too small to be viewed microscopically and are highly sensitive to impurities. The idea of subdividing a given melt volume to avoid the effect of impurities during the nucleation was first implemented b~, Turnbull. t~} He used organic emulsifying agents to divide liquid Hg into a large number of the droplets. Nucleation of individual droplets has also been studied in contact with a clean substrate [2-5] or without a substrate, t6'7'8] Most of the experimental results from these nucleation studies show that the undercooling in the liquid before the onset of solidification is not reproducible. This emphasizes the fact that there are experimental difficulties in avoiding undesirable catalytic effects due to the presence of impurities. The present work uses an elegant technique first devised by Wang and Smith 19] and applies this technique to study the nucleation behavior of Pb droplets entrapped within a Zn matrix. The original technique was later adopted by Southin and Chadwick [t°] to study heterogeneous nucleation of solidification and was then further improved by Moore et al.l~l by using rapid solidification to obtain monotectic binary alloy samples. Rapid solidification of a monotonic binary alloy containing a large miscibility gap produces a very fine dispersion of a large number of nanometer-sized low melting point secondphase particles embedded in a high melting point matrix. This ensures the segregation of impurities into an insigR. GOSWAMI, Research Fellow, and K. CHATTOPADHYAY, Associate Professor, are with the Centre for Advanced Study, Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India. W.T. KIM, Lecturer, is with the Department of Physics, Chongju University, Chongju-shi, South Korea. B. CANTOR, Director, is with the Oxford Centre for Advanced Materials and Composites, Department of Materials, University of Oxford, Oxford, OX1 3PH, United Kingdom. Manuscript submitted November 25, 1991. METALLURGICAL TRANSACTIONS A
nificant number of the second-phase particles and thus provides an ideal environment for heterogeneous nucleation studies during subsequent heating and cooling to melt and solidify the particles. Several such studies with low melting point second-phase particles embedded in an A1 matrix have recently been reported, t~2-~5} The Zn-Pb alloy system shows a large miscibility gap in the liquid state Ij6] and contains monotectic and eutectic points at temperatures of 694 and 591.2 K, respectively. The present article deals with the crystallography of Pb particles embedded in a Zn matrix and heterogeneous nucleation of the Pb particles during solidification by the surro
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