Nucleation-controlled microstructures and anomalous eutectic formation in undercooled Co-Sn and Ni-Si eutectic melts
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10/30/03
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Nucleation-Controlled Microstructures and Anomalous Eutectic Formation in Undercooled Co-Sn and Ni-Si Eutectic Melts MINGJUN LI and KAZUHIKO KURIBAYASHI Co-20.5 at. pct Sn and Ni-21.4 at. pct Si eutectic alloys have been levitated and undercooled in an electromagnetic levitator (EML) and then solidified spontaneously at different undercoolings. The original surface and cross-sectional morphologies of these solidified samples consist of separate eutectic colonies regardless of melt undercooling, indicating that microstructures in the free solidification of the eutectic systems are nucleation controlled. Regular lamellae always grow from the periphery of an independent anomalous eutectic grain in each eutectic colony. This typical morphology shows that the basic unit should be a single eutectic colony, when discussing the solidification behavior. Special emphasis is focused on the anomalous eutectic formation after a significant difference in linear kinetic coefficients is recognized for terminal eutectic phases, in particular when a eutectic reaction contains a nonfaceted disordered solid solution and a faceted ordered intermetallic compound as the terminal eutectic phases. It is this remarkable difference in the linear kinetic coefficients that leads to a pronounced difference in kinetic undercoolings. The sluggish kinetics in the interface atomic attachment of the intermetallic compound originates the occurrence of the decoupled growth of two eutectic phases. Hence, the current eutectic models are modified to incorporate kinetic undercooling, in order to account for the competitive growth behavior of eutectic phases in a single eutectic colony. The critical condition for generating the decoupled growth of eutectic phases is proposed. Further analysis reveals that a dimensionless critical undercooling may be appropriate to show the tendency for the anomalous eutectic-forming ability when considering the difference in linear kinetic coefficients of terminal eutectic phases. This qualitative criterion, albeit crude with several approximations and assumptions, can elucidate most of the published experimental results with the correct order of magnitude. Solidification modes in some eutectic alloys are predicted on the basis of the present criterion. Future work that may result in some probable errors is briefly directed to improve the model.
I. INTRODUCTION
RECALESCENCE from an undercooled matallic melt is ascribed to crystal growth and forms an image of the solidification front that is considered to be a solid/liquid interface, from which the crystal growth velocity is obtained by dividing the propagation distance covered within a specific recalescence interval in a bulk sample.[1] Based on this principle, growth velocities vs melt undercooling in Fe-Ni[2] and Cu-Ni[3] alloys have been measured, and they show good agreement with the LKT[4] or BCT[5] model in dendrite regimes. Moreover, solidified microstructures at certain undercoolings exhibit a directional feature, starting from the s
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