Solidification Of Undercooled Co-Sn Eutectic Alloy And That Containing A Third Element

Bulk Co-24at%Sn eutectic alloy, Co-20at%Sn hypo-eutectic alloy, Co-28at%Sn hyper-eutectic alloy and the Co-24at%Sn alloy with a small addition of Mn and Sb were undercooled to different degrees below the equilibrium liquidus temperature by the glass fluxing technique in combination with cyclical superheating, and a series of samples with different undercoolings for each alloy were solidified. The temperature change in melting and solidification was monitored by a high-speed infrared pyrometer. The crystal growth velocity was measured by two infrared pyrometers set along the sample axis (the dual-probe method). The solidification structure was observed by an optical microscope (OM) and a scanning electron microscope (SEM). The grain orientations of the?-Co and?-Co₃Sn₂ phases in the anomalous eutectics were identified by the electron back scattered diffraction (EBSD) technology. Assmuing that the eutectic solidification interface advances in a dendritic form with a tip of paraboloid of revolution in the undercooled melt, and the small addition of a third element does not induce a new phase, a theoretical model was estabilished for the solidification of undercooled eutectic alloys containing a third element. The solidification interface morphology and the solidification structure formation of the undercooled alloys were comprehensively analyzed.As the content of the third element increases, the eutectic dendrite tip radius reduces. When the content of the third element is less than a critical value, the eutectic growth velocity becomes larger at low undercoolings but smaller at high undercoolings. Once the content of the third element exceeds the critical value, the eutectic growth velocity is always reduced. After the addition of the third element, the eutectic lamellar (rod) spacing changes in an opposite way of the growth velocity. When the partitioning coefficients of the third element in two eutectic phases differ greatly from each other, the effects of the third element on the eutectic growth become remarkable.The solidification structure of Co-24at%Sn alloy is composed of lamellar eutectics at undercoolings below 20 K, and lamellar eutectics plus anomalous eutectics at larger undercoolings. The fraction of anomalous etuectics in the solidification structure increases with undercooling. The grains of two eutectic phases in the anomalous eutectics orient randomly whether the undercooling is how large, indicating that coupled eutectic growth takes place in the rapid solidification stage. However, the primarily formed lamellar eutectics are subjected to superheating and partially remelting during the temperature recalescence and ripening in the subsequent solidification, and therefore evolved into anomalous eutectics.The eutectic solidification interface in the Co-24at%Sn alloy melt grows in seaweed modes at all experimental undercooling. The reason is that the solid/liquid interface energy anisotropies of the two eutectic phases are relatively low during coupled growth, and the eutectic interface tip radius after tip-splitting is much larger than the eutectic lamellar spacing, which leads to an extremely low interface energy anisotropy of the eutectic solidification interface. When undercooling is not too large, the eutectic solidification interface of Co-24at%Sn is of fractal seaweed pattern. Once undercooling exceeds 172 K, the interface morphology transforms to a compact seaweed pattern, accompanyied with a fast increase of growth velocity.Small additions of Mn and Sb remarkablely change the solidification behavior of undercooled Co-24at%Sn eutectic alloy. The addition of Mn increases the interface energy and its anisotropy, making the eutectic solidification interface change from a seaweed into dendrite form at low undercooling, and the critical undercooing for the eutectic interface to transit from a fractal seaweed at moderate undercooling to a compact seaweed at large undercooling increases to 182 K. Different from Mn, the addition of Sb mainly influences the eutectic growth velocity, i.e. the addition of Sb increases the growth velocity at lower undercoolings, and decreases the growth velocity at larger undercoolings.The eutectic growth velocity in the undercooled Co-24at%Sn eutectic melt was calculated using related theoretical models. It is found that when the critical stability parameter is let to be 0.025 according to the interface critical stability theory, the calculated growth velocities are obviously different from the experimental values. If the“microscopic solvabiltycriterion”is adopted and the critical stability parameter is let to be 0.001, the calculation results fit the experimental growth velocities well even if the eutectic growth interface is of fractal seaweed pattern.