Macrosegregation caused by thermosolutal convection during directional solidification of Pb-Sb alloys

Pb-2.2 and 5.8 wt pet Sb alloys were directionally solidified with a positi ve thermal gradient of 140 K cm(-1) at growth speeds ranging from 0.8 to 30 mu m s(-1), and then quenched to retain the mushy-zone morphology. Chemica l analysis along the length of the directionally solidified portion and in the quenched melt ahead of the dendritic array showed extensive longitudina l macrosegregation, Cellular morphologies growing at smaller growth speeds are associated with larger amounts of macrosegregation as compared with the dendrites growing at higher growth speeds. Convection is caused, mainly, b y the density inversion in the overlying melt ahead of the cellular/dendrit ic array because of the antimony enrichment at the array tip. Mixing of the interdendritic and bulk melt during directional solidification is responsi ble for the observed longitudinal macrosegregation.