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, remaining 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 increase 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,x03BB;21fE(CE−Ct)}, where A, is the primary dendrite spacing,fEis the volume fraction of the interdendritic melt, andCEandCtare the eutectic composition and the melt composition ahead of the dendrite tips, respectively.