Effect of growth rate on macrosegregation during directional solidification of a Pb-Sb alloy

Directional solidification experiments on Pb-5.8 wt% Sb alloy have been car ried out over growth rates varying from 0.8 to 20 mum s(-1) in a positive t emperature gradient of 140 Kcm(-1). The microstructural examination of this alloy composition indicated cellular to dendritic transition at a growth r ate of 1.5 mum s(-1). The primary arm spacing was observed to decrease with growth rate in dendritic solidification regime. The maximum primary arm sp acing occurred at the growth rate corresponding to the cellular to dendriti c transition. Chemical analysis data revealed large amounts of macrosegrega tion in the longitudinal section of the alloy. A decrease in growth rate re sulted in an increased degree of macrosegregation. These effects are discus sed in light of thermo-solutal convection in the melt ahead of the solid-li quid interface during directional solidification of this alloy. A parameter k(e) is used to represent the extent of convection in the melt and the con sequent degree of macrosegregation of the alloy with varying growth rates f or solidification with a cellular-dendritic interface in the same way as it has been earlier used for the planar-front solidification condition.