Macrosegregation during steady-state arrayed Growth of Dendrites in Directionally Solidified Pb-Sn Alloys

Abstract

Macrosegregation along the length of the directionally solidified samples is produced when Pb-Sn alloys (10 to 58 wt pct Sn) are directionally solidified in a positive thermal gradient (melt on top, solid below, and gravity pointing down) with steady-state dendritic arrayed morphology (the length of the mushy zone, much smaller than the initial length of the melt column, re- maining nearly constant during growth). The extent of the macrosegregation increases with increasing tin content, becomes maximum for 33.3 wt pct Sn, and decreases with further in- crease in tin content. The intensity of the interdendritic thermosolutal convection responsible for the longitudinal macrosegregation can be represented by the effective partition coefficient (ke), anempirical parameter obtained from the dependence of the longitudinal macrosegregation on fraction distance solidified. The extent of the macrosegregation appears to be related to a parameter, {λ12ƒE(CE -Ct)}, where λ1, is the primary dendrite spacing,fE is the volume fraction of the interdendritic melt, andCE andCt, are the eutectic composition and the melt composition ahead of the dendrite tips, respectively.