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

摘要

Directional solidification experiments on Pb-5.8 wt% Sb alloy have been carried out over growth rates varying from 0.8 to 20 sm 5 in a positive temperature gradient of 140 Kcm{sup}(-1). The microstructural examination of this alloy composition indicated cellular to dendritic transition at a growth rate of 1.5 μm s{sup}(-1). The primary arm spacing was observed to decrease with growth rate in dendritic solidification regime. The maximum primary arm spacing occurred at the growth rate corresponding to the cellular to dendritic transition. Chemical analysis data revealed large amounts of macrosegregation in the longitudinal section of the alloy. A decrease in growth rate resulted in an increased degree of macrosegregation. These effects are discussed in light of thermo-solutal convection in the melt ahead of the solid-liquid interface during directional solidification of this alloy. A parameter k{sub}e is used to represent the extent of convection in the melt and the consequent degree of macrosegregation of the alloy with varying growth rates for solidification with a cellular-dendritic interface in the same way as it has been earlier used for the planar-front solidification condition.