Modeling of irregular eutectic microstructures in solidification of Al-Si alloys
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1/4/04
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Modeling of Irregular Eutectic Microstructures in Solidification of Al-Si Alloys M.F. ZHU and C.P. HONG A modified cellular automaton (MCA) model was developed and applied to simulate the evolution of solidification microstructures of both eutectic and hypoeutectic Al-Si alloys. The present MCA model considers the equilibrium and metastable equilibrium solidification processes in a multiphase system. It accounts for the aspects including the nucleation of a new phase, the growth of primary dendrites and two eutectic solid phases from a single liquid phase, as well as the coupling between the phase transformation and solute redistribution in liquid. The effects of alloy composition and eutectic undercooling on eutectic morphology and eutectic nucleation mode were investigated. The simulated results were compared with those obtained experimentally. I. INTRODUCTION
EVOLUTION of solidification microstructures can be the strategic link between materials processing and materials behavior.[1] The eutectic structure is the basis of most commercial casting alloys, and thus, the properties of these alloys strongly depend on the amount and morphology of the eutectic phases, which, in turn, are affected by various variables, including cooling rate, modification, and faceted or nonfaceted nature of the constituent phases.[2–5] So-called nonfaceted-faceted (nf-f) eutectics, such as Al-Si and Fe-C alloys, make up the main bulk of cast alloys and are thus of great practical importance. Hypoeutectic Al-Si alloys are the most widely used aluminum alloys in industry today due to the excellent combination of good castability and desirable mechanical properties.[6–10] Depending on the alloy composition, the principal microstructure features of these alloys are the primary -phase dendrites with interdendritic lamellar or fibrous eutectics. Better understanding of irregular eutectic growth behavior would lead to a better understanding of how to control the microstructures in these alloys. Particularly, since the eutectic formation is usually in the final stage of solidification, the interdendritic eutectic solidification behavior as well as its volume fraction, morphology, and distribution have been shown to influence markedly castability, casting defects, and mechanical properties.[11] However, the knowledge and quantitative understanding of this crucial stage in solidification has still remained very limited.[12] A time-dependent description of eutectic growth is required in order to quantitatively understand the mechanisms of eutectic formation and thus to better control the solidified microstructures of these important practical eutectic casting alloys. Over the last decade, significant advances have been made in the development of numerical modeling for quantitatively understanding the time-dependent structure evolution during M.F. ZHU, Professor, formerly with the Center for Computer-Aided Materials Processing (CAMP), Department of Metallurgical Engineering, Yonsei University, is with the Department of
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