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I. INTRODUCTION

IN earlier work, we developed a synchrotron microradiography technique to observe dendritic solidification in real alloy in real time[1] and applied this technique to the study of dendrite coarsening in Sn-13 wt pct Bi alloy. Coarsening mechanisms, such as dendrite remelting and dendrite coalescence, were observed in real time during solidification and during isothermal holding in the solidification range under conditions where a roughly zero temperature gradient was imposed on the sample. The objective of this work was to determine whether the same or similar coarsening mechanisms are also predominant in directional solidification of Sn-13 wt pct Bi alloy, where a temperature gradient is imposed during solidification and during holding. In the preliminary studies of real-time observations of dendritic solidification using a microfocal X-ray source,[2] local remelting and solidification of the dendrite arms was observed when a Sn-13 wt pct Bi sample was held at a constant temperature gradient, without cooling. The dendrites appeared to be moving in the direction of the temperature gradient, from the cold end to the hot end (from high solute concentration to low solute concentration). The mechanism for this scenario is called temperature gradient zone melting (TGZM).[3] The series of experiments in this paper were designed to see whether TGZM became the dominant mechanism in determining dendrite morphology when the imposed temperature gradient during solidification was appreciable (from 7 to 25 °C/cm). The concept of TGZM was first proposed by Pfann.[3] The idea is that when a liquid zone, sandwiched by two solid zones, reaches equilibrium under a temperature gradient, the liquid compositions at the two solid/liquid interfaces are determined by the liquidus line in the equilibrium phase diagram. Hence, a composition gradient is established across the liquid zone. As a result of diffusion of the solute across the molten zone, solidification on the cold side and remelting on the hot side of the liquid zone are required to maintain equilibrium at the interfaces. The concurrent solidification on the cold side and remelting on the hot side of the liquid zone drives the liquid march up the temperature gradient. B. LI, Postdoctor, and H.D. BRODY, Professor, are with the Department of Metallurgy and Materials Engineering, University of Connecticut, Storrs, CT 06269-3136. Contact e-mail: [email protected] A. KAZIMIROV, Beamline Staff, is with Cornell High Energy Synchrotron Source, Ithaca, NY 14853. Manuscript submitted May 24, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A

Allen and Hunt[4,5] first observed TGZM in dendritic solidification of succinonitrile-6 wt pct camphor solution, a transparent, organic alloy analog, at a growth rate R  3 m/sec, temperature gradient G  50 °C/cm. They found, surprisingly, that during steady-state dendrite growth in the directional solidification, all the secondary arms moved two or three secondary arm spacings up the temperature gradient, indicating that a signif