Banded microstructure formation in off-eutectic alloys
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I. INTRODUCTION
EUTECTIC growth has been studied extensively, because of its unique properties and possible applications in many fields. The characteristics of eutectic alloys are that they have lower melting temperatures than pure elements and a zero freezing range, which result in excellent flow properties that are valuable in casting, welding, and soldering processes. In addition, eutectic microstructures are much finer than those of dendrites, so that improved mechanical properties can be attained. The possibility of forming insitu composites that have the desired phases growing simultaneously has also attracted much interest in eutectic growth. Two critical microstructural aspects are the eutectic spacing, which is controlled by the growth rate, and the relative volume fraction of the two phases, which is controlled by the alloy composition and phase diagram. In order to control the volume fractions, the conditions that give rise to eutectic microstructures in an off-eutectic alloy need to be properly identified. A proper model requires the study of the stability of the eutectic interface with respect to the single-phase growth, and it is this study that will be critically examined in this article. The stability of eutectic structures against single-phase formation is generally described by a coupled zone, which represents a regime of composition and undercooling (or velocity) conditions, in which a eutectic microstructure can be formed. The basic model for the coupled zone was proposed by Tammann and Botschwar,[1] who characterized the coupled zone as the region in which the eutectic phase grew S.H. HAN, formerly Graduate Student, Department of Materials Science and Engineering, Iowa State University, is Senior Engineer, Samsung Electronic, Seoul, Korea. R. TRIVEDI, Professor, is with Ames Laboratory, United States Department of Energy, and the Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011. Manuscript-submitted August 17, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
faster than the primary phases for a fixed given undercooling in an undercooled alloy melt. This approach is known as the competitive growth model. The velocities of different microstructures, for comparison, were determined from the steady-state growth model for each possible morphology, and the regime of composition and undercooling where the eutectic structure would be selected was plotted on a phase diagram. Kofler[2] extended this work in binary organic systems and described three different types of coupled zones: a regular zone around the eutectic composition, a skew zone, and a mixed-mode zone. A more quantitative approach was developed subsequently by Hunt and Jackson[3] for the selection of a eutectic microstructure under directional solidification conditions. In this case, the stability of the eutectic structure was based on the steady-state interface temperatures, with the microstructure having a larger interface temperature being the one selected by the system. Later, Jackson[4] modified the
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