Macrosegregation caused by thermosolutal convection during directional solidification of Pb-Sb alloys
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I. INTRODUCTION
MACROSEGREGATION during directional solidification with a dendritic liquid-solid interface morphology has been extensively investigated because of the commercial importance of superalloy components. Macrosegregation, i.e., solutal inhomogeneity over length scales larger than primary dendrite spacing, results from convection in the mushy zone. Convection can be caused by solidification shrinkage, sinking or floating of broken-off dendrite segments, and the temperature- or composition-induced melt density inversion. It can produce localized segregation (“channel segregates or freckles”)[1,2,3] clustering or steepling of dendrites across the liquid-solid interface,[4] or a systematic variation in solute content along the length of the directionally solidified ingot.[5,6,7] We had earlier examined macrosegregation along the length of the directionally solidified Pb-Sn alloys as part of a program to study the influence of convection on the dendritic and mushy-zone morphology.[5,6,7] In this article, we examine the macrosegregation caused by solutal buildup at the dendrite tips under thermally stable and solutally unstable directional solidification conditions for two hypoeutectic Pb-Sb alloys containing 2.2 and 5.8 wt pct Sb, respectively. Solute content of the first alloy is less than the terminal solid solubility limit of antimony in lead (3.5 wt pct Sb), whereas that of the second alloy is more. The second alloy, containing 5.8 wt pct Sb, is therefore expected to have a more permeable mushy zone compared to that of the alloy containing 2.2 wt pct Sb. II. EXPERIMENTAL PROCEDURE Approximately 24- to 30-cm-long Pb-Sb feedstock samples were obtained by induction melting a charge (lead, 99.99 pct purity and antimony, 99.999 pct purity) under an argon atmosphere in a graphite crucible and pushing the S.N. OJHA, formerly Visiting Professor, Chemical Engineering Department, Cleveland State University, is Professor, Department of Metallurgical Engineering, Banaras Hindu University, Varanasi, India 221-005. G. DING, Research Associate, Y. LU, Graduate Student, J. REYE, Undergraduate Student, and S.N. TEWARI, Professor, are with the Chemical Engineering Department, Cleveland State University, Cleveland, OH 44115. Manuscript submitted September 9, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
melt into evacuated quartz tubes (0.7-cm i.d.) with the help of argon pressure. The cast Pb-2.2 wt pct Sb and Pb-5.8 wt pct Sb feedstock cylinders were extracted and placed into the quartz directional solidification ampoule (0.7-cm i.d., 70-cm long). The quartz ampoule contained two ChromelAlumel thermocouples (0.01-cm-diameter wires kept in closed-end silica capillaries, of 0.06-cm o.d.) with their tips separated by about 3 cm along the length. Cylindrical PbSb alloy plugs were placed at the bottom end of the quartz ampoules. These plugs had axial holes through which the silica capillaries containing the thermocouples were inserted into the quartz ampoule. The quartz ampoule was then sealed at the bottom. It was evacua
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