Solidification Of Undercooled Ag-28.1Cu-xSb Eutectic Alloys

Deep undercooling is the only way of achieving rapid solidification of bulk liquid metals, and has been widely used in investigating the non-equilibrium solidification theory and preparing of bulk non-equilibrium materials. While studying the relationship between the solidification structure of eutectic alloys and the melt undercooling, it has been discovered that eutectic phases grow in a coupled mode at small undercoolings but in a decoupled mode at large undercoolings. The solidification far from equilibrium results in a transition from regular lamellar eutectics to anomalous eutectics. So far there are furious controversies among researchers on the causes of such solidification behaviors.Ag-28.1wt.% Cu eutectic alloy is one of few eutectic alloy systems composed of terminal solid solutions. The large difference in composition between the two eutectic phases leads to a considerable lateral diffusion of solute ahead of the solidification interface. Novel solidification behaviors are expected to be revealed as well as the mentioned problems can be understood deeply, if the Ag-28.1wt.% Cu eutectic is considered as a research object. Therefore, in this dissertation,the Ag-28.1wt.% Cu (x=0) eutectic alloy was undercooled by the glass fluxing technique in combination with cyclical superheating and the solidification behaviors and formation mechanism of structures were investigated. Furthermore, 0.5 and 1 wt.% Sb (x=0.5, 1) is added to the Ag-28.1wt.% Cu eutectic alloy respectively. The evolution of eutectic interface morphology with the Sb addition, and the other solidification behaviors were studied. The main research achievements are as follows:The effect of solute trapping on the eutectic growth of undercooled alloy melts has been investigated on the base of the LZ (Li-Zhou) eutectic growth model. It is revealed that solute trapping makes growth rate increase, and lamellar spacing and dendrite tip radius decrease. And the increase of growth rate becomes more remarkable with the increasing undercooling. By measuring the eutectic growth rate at different undercoolings, it is verified that the LZ theoretical models including solute trapping can predict the eutectic grow very well, and the solute trapping should be taken into account while analyzing eutectic growth if the equilibrium solute partition coefficients are small.The Ag-28.1wt.% Cu eutectic alloy melt solidifys in coupled eutectic lamellae during rapid solidification up to the largest experimental undercooling 100 K. However, when undercooling is less than a critical value of 76 K, cellular growth of lamellar eutectics from the nucleation site takes place because of the large difference in composition between two eutectic phases and the very large thermal diffusion coefficient of the liquid. Three regions of microstructures are observed in the sample. They are the anomalous eutectic region near the nucleation site that generally located at the sample surface, cellular eutectic region in the middle, and equiaxed lamellar eutectic region at the end. The primary lamellar eutectics near the nucleation site solidify under conditions far from equilibrium, and therefore are supersaturated with more solute, and then partially remelted and ripened into anomalous eutectics. With the distance along the growth direction increasing, the finest lamellar spacing across the cellular eutectics rises, indicating a gradually decreasing growth velocity of the primary eutectics. This means that the eutectic growth during rapid solidification is unsteady. Such an argument is also supported by the appearance of the three regions of microstructures.When undercooling is equal to or higher than 76 K, lamellar eutectics grow in a dendritic form during rapid solidification. There are no longer regional characteristics in the solidification microstructures. The significant remelting and ripening of the primary eutectic dendrites result in appearance of anomalous eutectics inside the sample. But the eutectic dendrites on the sample surfaces survive because of the better heat dissipation conditions. The transition from the unsteady cellular growth to steady eutectic dendrite growth leads to a sudden increase in recalescence rate at the critical undercooling 76 K.The addition of a third element Sb into the Ag-28.1wt.% Cu eutectic alloy creates an additional constitutional undercooling ahead of the solid/liquid interface, but does not change the unsteady eutectic growth at low undercoolings (<76 K), and the steady eutectic dendrite growth at high undercoolings (76 K ~ 100 K). The solidification morphology changes from a cellular into a cellular dendritic and then an undeveloped dendritic form with the increasing addition of Sb at low undercoolings. After the addition of Sb, the cell or dendrite tip of the Ag-28.1wt.% Cu eutectic is sharpened. As a result, the eutectic growth is accelerated and recalescence rate is increased. Meanwhile, the volume fraction of anomalous eutectics in the microstructure and the size of the particles in the anomalous eutectics increase because of the increasing recalescence degree and recalescence rate.