Effect Of Liquid-Liquid Structure Transition On Solidification Process Of Pb-Sn-Bi Ternary Alloys

Metallic materials used in science researching and industry manufacturing, are mostly multi-component alloys. As one of the simplest multi-component alloys, the Pb-Sn-Bi ternary alloy is studied for exploring other complex multi-component alloys. The melting point of Pb-Sn-Bi ternary alloy is low, which widely used in mechanism, aviation, automobile, wiring and meter, light industry and atomic energy industry, and so on. It is important to research the forming and controlling of its solidification micro-structure for developing theory and practice. The present thesis carries out detailed investigation on the influence of solidification process, experimental conditions and liquid structure transition on the solidification micro-structure and performance of Pb-Sn-Bi ternary alloys.The solidification process and micro-structure of Pb-42Sn-32Bi ternary alloy solidified in room temperature was analyzed, and their phase composing and freezing mechanism were researched. The growth of primary dendrite was mostly effected by solute reallocate, and element content in liquid was increased during the solidification process, the end of primary phase was enriched solute atoms. At different cooling rate of Pb-30.95Sn-50Bi ternary alloys, the appearances of micro-structure were distinction. Cooled at room temperature, the growth of primary dendrite was inadequate, and some branches were broken, while at speediness solidification, primary dendrite and eutectic grains was refined. Combined thermodynamics and dynamics of dendrite growth, the model of ternary alloys dendrite growth was built.The effects of liquid structure transition on Pb-55.71Sn-10Bi and Pb-6.19Sn-90Bi ternary alloys during solidification were investigated by heat-analysis and micro-structure. Before the liquid structure change, the degree of super-cooling was little and microstructure was large, the diffuse coefficient was augmented with the temperature increasing and the stability of solid-liquid interface was strengthened. While after liquid structure transition, the primary crystal was refined, as the average radius of atomic cluster in melts was diminished, the stability of solid-liquid interface was reduced, which caused by the degree of super-cooling increasing. Effects of the Pb-18.57Sn-70Bi and Pb-55.71Sn-10Bi ternary alloys liquid structure transition on the mechanism of solidified micro structure evolvement were researched. The number of primary crystal was little before the liquid structure change, which was formed in large and existed converging. As the barrier energy of atomic jumping from liquid-solid interface was increased, which induced the rate of crystal growth decreasing, micro-structure was refiner than those solidified before liquid structure transition. The largest variation of micro-hardness was the melt heated to the point of before and after liquid structure transition.