Modeling of Microstructure Evolution in Nb-Si Eutectic Alloy Using Cellular Automaton Method
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1128-U07-02
Modeling of Microstructure Evolution in Nb-Si Eutectic Alloy Using Cellular Automaton Method Kenichi Ohsasa1 and Seiji Miura1 Graduate School of Engineering, Hokkaido University N13, W8, Kita-ku, Sapporo 060-8628, Japan ABSTRACT A numerical model was developed for the simulation of microstructure evolution during the solidification of Nb-Si eutectic alloy. In this model, the cellular automaton method was used to simulate the eutectic growth of Nb solid solution and Nb3Si intermetallics. Diffusion in liquid, mass conservation at the solid/liquid interface and local equilibrium at the solid/liquid interface with consideration of curvature undercooling were solved to determine the positions of the Nb/liquid and Nb3Si/liquid interfaces. In the alloy with eutectic composition of 17.5 at%Si, irregular eutectic growth morphology was observed in relatively low undercooling region. On the other hand, in high undercooling region over 50 K, dendrite morphology of Nb3Si was observed. In the simulation for the alloy with hypo-eutectic composition of 16.0 at%Si, Nb solid solution grew with cell morphology in low undercooling region, while coupled eutectic morphology was formed in higher undercooling region over 8 K. The growth velocity of the coupled growth increased with increase in the degree of undercooling. In the alloy with hypoeutectic composition of 12.0 at%Si, cell and dendrite morphology of Nb was formed in whole undercooling region, INTRODUCTION Eutectic alloys are widely used as industrial materials and the eutectic materials have remarkable feature as in-situ composite. Namely, the minor phase covers some functions and the major phase keeps the entire strength of the material, compensating for the lack of strength of the minor part. In order to develop a new functional structure of a eutectic alloy as a composite material, control of the eutectic solidification structure would be very important. Nb-Si eutectic alloy is a candidate for high temperature materials. In order to predict the optimum conditions to obtain a desired eutectic structure of Nb-Si alloy, it is important to clarify the effects of the process parameters on the eutectic solidification structure of the Nb-Si alloy. However, many experimental works would be required to obtain optimum conditions. Hence, it is desirable to develop a mathematical model to simulate eutectic structure formation. In the past two decades, several models have been developed for predicting the evolution of micro and macrostructures of alloys during solidification. Among those models, the phase-field method (PF) is a very powerful tool for simulating the evolution of a dendrite structure. However, a drawback of the PF method is a very small computational domain, and the use of the PF method for the simulation of eutectic formation in a real casting will be difficult. The cellular automaton (CA) method1,2) is another way to simulate the evolution of solidification structures. The CA method has the advantage of relatively small computational load in comparison with th
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