Cellular/dendritic array tip morphology during directional solidification of Pb-5.8 Wt Pct Sb alloy
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I. INTRODUCTION
IT is important to understand the development of dendritic microstructures during directional solidification, because these microstructures ultimately control the mechanical properties of directionally solidified components. The cellular/dendritic arrayed growth models[1,2,3] have generally assumed that for a given processing condition, i.e., thermal gradient (Gl), growth speed (V ), and solute content of a binary alloy melt (C0), the array attains a unique morphology, i.e., unique cell/dendrite tip radius and primary spacing. However, experiments, both on transparent alloys[4] and metallic alloys[5,6,7] have shown that a range of primary spacings is stable under a given growth condition. Recent theoretical analyses[8,9,10] also support this view. This would suggest that a range of tip radii may also exist during the steady-state growth of a cellular/dendritic array. The purpose of this research was to investigate this possibility and obtain a quantitative relationship between the primary spacings and the corresponding tip radii during directional solidification of binary metallic alloys. Three-dimensional arrayed growth during directional solidification of binary alloys with the melt on top and the solid at the bottom can be carried out under two conditions: solutally stable (for example, hypoeutectic Al-Cu alloy) and solutally unstable (for example, hypoeutectic Pb-Sn). One would expect that there would be no density inversion in the mushy zone and in the bulk melt ahead of the array tips during directional solidification of hypoeutectic Al-Cu alloys, because copper enrichment increases melt density. However, extensive deformation of the cellular/dendritic array tips, attributed to natural convection, has been reported under such growth conditions,[11] which makes it impossible to extract meaningful and accurate tip radii from these experiments. The solutally unstable growth conditions, typified L. YU, Graduate Student, G.L. DING, Research Associate, J. REYE, Undergraduate Student, and S.N. TEWARI, Professor, are with the Chemical Engineering Department, Cleveland State University, Cleveland, OH 44115. S.N. OJHA, formerly Visiting Professor, Chemical Engineering Department, Cleveland State University, is Professor, Metallurgical Engineering Department, Banaras Hindu University, Varanasi, India. Manuscript submitted November 19, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
by hypoeutectic Pb-Sn alloys, do cause extensive convection, but they also produce cellular and dendritic arrays with uniform morphology if the formation of “channel segregation” can be avoided.[5] This would allow simultaneous measurements of tip radius and primary spacing for the individual cells and dendrites in the array. Therefore, for this study, we selected Pb-5.8 wt pct Sb alloy, which has a solidification behavior similar to hypoeutectic Pb-Sn. The metallic alloys are opaque; hence, the tip radii and the corresponding primary arm spacings were determined by metallographic examination of the quenched mushy zone near t
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