Gravitational macrosegregation in unidirectionally solidified Lead-tin alloy
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I.
INTRODUCTION
IT is
now generally accepted that macrosegregation in unidirectionally solidified ingots is a result of interdendritic fluid flow during solidification. I~-25j In a binary alloy such as the Pb-15 wt pct Sn alloy used in this study, the segregating solute, tin, is lighter than the solvent, Pb. Hence, during upward solidification the Sn-enriched interdendritic liquid will tend to rise because of buoyancy effects. The upward flotation of tin will increase the average composition of the solid in the upper portions of the ingot above the nominal composition. This is usually referred to as 'normal' macrosegregation and some examples of this type of segregation have been reported in binary lead-tin and aluminum-magnesium alloys. [17' 18' 191 In th e t"i n -r i"c h allo y, Sn-15 wt pct Pb, on the other hand, the segregating solute, Pb, increases the density of the interdendritic liquid. This should lead to a stable stratification within the liquid phase, during upward solidification, and hence minimal convection in the interdendritic regions due to buoyancy effects in the Sn-rich alloy. Buoyancy effects are, however, not the only contributing factor to interdendritic fluid flow during solidification. In many alloys the density of the solid phase is greater than the density of the liquid phase. In both the Pb-rich and Snrich alloys used in this study, the solid phase is heavier than the liquid phase of the same composition. For example, the eutectic solid is denser than the eutectic liquid, t2~ This
L. WANG, Graduate Assistant, V. LAXMANAN, Senior Research Associate, and J.F. WALLACE, Professor, are with the Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106. This paper is based on a presentation made in the symposium "Experimental Methods for Microgravity Materials Science Research" presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25-29, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee. METALLURGICALTRANSACTIONS A
leads to shrinkage and hence flow of solute-enriched liquid (partition ratio k < 1) into areas in a more advanced stage of solidification. If shrinkage effects are dominant, we usually find that the part of the ingot which solidified first is richer amin solute than the portions of the ingot which freeze last. This is called 'inverse' segregation. Examples of inverse segregation of copper in binary A1-Cu alloys have been reported by a number of investigators.[l~ Similarly, inverse segregation of lead in binary Sn-Pb alloys has also been observed. [19'24] It is interesting to note that in the examples cited here the segregating species, Cu or Pb as the case may be, increases the density of the interdendritic liquid. There is no known example of inverse segregation of a lighter solute, say Sn in a Pb-rich alloy. In actual solidification processes both buoyancy effects and phase-change effects (shrinkage or expansion) will act simultaneously. Buoyancy effects may
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