An experimental investigation on the kinetics of solute driven remelting
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I. INTRODUCTION
THE melting of a metal or a metal alloy is an important phenomenon preceding any solidification process. However, while there has been an extensive amount of theoretical and experimental work on solidification, the research on melting has attracted limited attention, mainly due to the fact that the microstructure of a material forms during solidification. Recently, however, melting has gained more interest, since in some technical applications, local remelting of partly solidified structures has been observed. Often, during a cooling process, a liquid of high-solute concentration or of higher temperature is transported by convection and brought into contact with a solid. This creates an out-of-equilibrium situation and can result in the remelting of the solid. Such remelting can occur in the mushy zone of a solidifying alloy, or during the joining of dissimilar metals and alloys. Local remelting has been identified as one of the main mechanisms for the fragmentation of dendrite arms[1] and for freckle formation in directionally solidified nickel-base superalloys.[2] It also plays an important role in the formation of inverse segregation (exudation) in continuous castings.[3] Understanding these phenomena requires a thorough understanding of remelting processes. Some researchers have treated melting as the inverse process of solidification. Following the work on directional solidification and on accompanying phenomena such as constitutional undercooling or, in general, interface stability, theoretical studies on directional melting were carried out. These studies involved solving the diffusion equation in the solid instead of in the liquid.[4,5] The theoretical studies
B. DUTTA, Research Associate, and M. RETTENMAYR, Senior Research Scientist, are with the Department of Materials Science, Technical University of Darmstadt, Petersenstrsse 23, Darmstadt 64287 Germany. Manuscript submitted June 15, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
were accompanied by experiments in Sn-Sb[6] and in SnBi[7] alloys. Han and Hellawell[8] have qualitatively discussed the two ranges of melting-interface velocities that result from two different driving forces, i.e., solute driven dissolution and thermally controlled remelting. For both types of driving forces, only a few theoretical and experimental studies have been carried out. Thermally controlled melting has been investigated by subjecting Sn-Bi particles[9] or Nb-Ti wires[10] to rapid heating. It appears that in these alloys a high level of superheating (or supersaturation) is achievable, while in pure materials, superheating is difficult to attain.[11] Some results are available for the isothermal melting of succino nitrile-water alloys.[12] The effects of convection and exothermic heat of mixing on the dissolution of ice in sulfuric acid and silicon in high-carbon steels have also been reported.[13] However, the present authors are not aware of any systematic study done on solute-driven melting. An experimental method in which a pure solid and a high
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