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 (k _ε), 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, {λ ²₁ ƒ_E(C _E -C _t)}, where λ₁, is the primary dendrite spacing,f _E is the volume fraction of the interdendritic melt, andC _E andC _t, are the eutectic composition and the melt composition ahead of the dendrite tips, respectively.