The Role of Inverse Segregation and Redistribution of Solute Atoms in the Freezing of Hypoeutectic Lead‐Antimony Alloys

Abstract

Inverse segregation has been found to occur in the lead‐antimony alloys of the range of concentration used for battery grids. Because of the possible harmful effects of such large concentrations of antimony at the surface of the battery grid a study has been made as to the cause of this phenomenon. The extent of the antimony dispersion in the surface layer has been found to be larger than can be explained by any one of the existing theories of inverse segregation. The effect appears to be caused by an interdendritic flow of still molten alloy of near eutectic composition into the gap left between the semisolid crust and the mold face during the solidification contraction. The difficulty of inducing the nucleation of antimony produces a condition of supersaturation for the β phase in this layer while the lead continues to crystallize out at temperatures below the normal eutectic. The continually increasing concentration of antimony added to the decrease in temperature eventually brings a limit to supersaturation, whether or not nucleation is promoted by the mold face. The antimony present in excess of the eutectic concentration then forms primary dendritic crystals which grow until the eutectic composition is again reached, at which point eutectic crystallization occurs. The result is a surface film of antimony far in excess of the distribution found within the ingot.