Eutectic interface configurations during melting
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I. INTRODUCTION
EXTENSIVE research has been performed in the field of solidification, including a considerable amount on eutectic solidification. In comparison, very little research has been conducted on melting at the microstructural level, with no analytical work on eutectic melting. With the widespread use of eutectic alloys as solders and brazes, the melting characteristics can become an important issue. The melting structure can also play a crucial role in the soldering, welding, or laser processing of materials. For example, during the remelting process in welding, significantly different dissolution rates of the two lamellae can lead to spherodization of the leading phase and give rise to an undesirable microstructure during the solidification of welds. For eutectic dissolution, several critical questions need to be investigated. First, to determine the conditions for coupling of the diffusion field between the two phases during melting, and second, to investigate whether this coupling leads to a melting interface that is macroscopically planar or nonplanar. Also, because the spacing is fixed during eutectic growth, it is important to establish the characteristic that the system can adjust to achieve stable steady-state melting. In order to examine these ideas, experiments were conducted using a transparent organic eutectic alloy in which the solidliquid interface could be observed in situ. The microstructure was characterized for various dissolution velocities and compositions near the eutectic. The melting microstructure was seen to differ significantly from a solidifying microstructure. Further, several unique dynamic processes and unstable interfaces have been observed under melting conditions. Eutectic melting is analyzed by solving the steady-state diffusion equation in the liquid. The interfacial temperature and morphology are determined by examining the role of solute diffusion and surface energy. Because the lamellar spacing is determined by the prior solidification process, the model lacks a mechanism to select stable diffusive coupling. C.A. NORLUND, formerly with the Department of Materials Science and Engineering, Iowa State University, is with Oregon Metallurgical, Albany, OR 97321. R. TRIVEDI is Professor, Department of Materials Science and Engineering, Iowa State University, Ames, IA, and Senior Scientist, Ames Laboratory, United States Department of Energy, Ames, IA 50011. Manuscript submitted October 19, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
However, interface shapes can be adjusted, and we shall show that the shapes adjust such that the average position of the interface remains isothermal to allow for changes in melt velocity and composition. The predicted interface morphology is then shown to match with experimental observations. The regime of coupled melting is also evaluated with respect to constitutional supercooling, and the general behavior predicted by constitutional supercooling is then compared with experimental observations of interfacial instability. II. EXPERIMENT
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