Sn-Bi系合金鋳塊の凝固時の結晶移動に起因したマクロ偏析の生成機構について

Abstract

The formation mechanisms in the macrosegregation of some Sn-Bi and Bi-Sn alloy ingots with equiaxed or angular crystal zones were studied using the unidirectional solidification technique. The solidification phenomena during horizontal unidirectional cooling were observed directly on the molten surface. The solute concentration was determined at various points in each ingot.In Sn base Bi alloys, the formation and the separation of granular crystals on the cooling end were recognized at the initial stage of solidification. Separate crystals moved toward the hotter zone, and formed an equiaxed zone there. Then columnar dendrites grew from the cooling end and extended to the equiaxed zone. The columnar dendrite zone showed a normal segregation, but the equiaxed zone showed a negative segregation.In Bi base Sn alloys, angular primary crystals floated up to the molten surface at a great distance from the cooling end, and then these crystals moved to the cooling end by thermal convection and were trapped there. Equiaxed eutectic grains were formed at the last stage of solidification. The solute concentration in these alloys was low near the cooling end and high at the equiaxed eutectic zone. These ingots showed a normal segregation.