Rapid Solidification Of Undercooled Water And Alloy During Acoustic Levitation And Free Fall

The rapid solidification of undercooled alloy and water is an important research subject in the field of materials science. Containerless processing is a promising way to study rapid solidification. It can efficiently eliminates heterogeneous nucleation induced by container wall so that large undercoolings can be achieved prior to solidification. The present thesis has carried out detailed investigation on the rapid solidification of undercooled bulk water and Pb-Sn binary eutectic alloy with acoustic levitation, and the various factors influencing the achievement of undercooling in sound field have been analyzed. Meanwhile, the containerless rapid solidification of Al-32.7wt%Cu eutectic alloy in drop tube is also investigated and its nucleation and growth characteristics are explored.Water drops with diameter ranging from 4 to 8mm are highly undercooled by up to 24K with acoustic levitation technique. Compared with the case in tube, acoustic levitation can efficiently avoid the heterogeneous nucleation from container walls and increase the undercooling level. However the cavitation effect induced by ultrasound may prematurely catalyze nucleation, which hinders the further achievement of bulk undercooling. The measured growth velocity of ice dendrite is 0.17m/s corresponding to the maximum undercooling of 24K. Considering the fact that the growth velocity exceeds the threshold value (0.01m/s) of rapid dendrite growth, it is concluded that the rapid solidification of ice occurs at this undercooling. The current dendritic growth theory is modified so as to be applicable to ice dendritic growth under high undercooling conditions. Experimental measurements are in agreement with the theoretical prediction at small undercoolings but exhibit some deviations at large undercoolings due to systemic errors.The rapid solidification process of Pb-Sn eutectic alloy is investigated with acoustic levitation and the maximum undercooling of 38K has been achieved. At small undercoolings that lie inside the coupled zone, a microstructure consisting ofbroken lamellae has been observed, which resulted from the ultrasonic vibration inside drops. When the undercooling goes beyond the coupled zone, it is found that the (Pb) dendrites in the acoustically levitated drops can also appear in the top of the drops and the gravitational macrosegregation is effectively suppressed, which may be caused by the ultrasound and complicated fluid flow inside the drops. The structure looking like spreading ripples on water surface is observed, which is considered as the results of the capillary waves excited by the surface vibration. The influences of the rotation of drops forced by orthogonal acoustic waves and the surface vibration on undercooling are analyzed and the conclusion that ultrasound can suppress the undercooling level has been drawn.The containerless rapid solidification of Al-32.7wt%Cu eutectic alloy in drop tube is investigated. The "lamellar eutectic-anomalous eutectic" microstructural morphology transition occurs with the decrease of droplet size. The theoretical calculation based on JH and TMK models is accomplished and predicts that the maximum undercooling for cooperative eutectic growth is 199K. The experimentally measured minimum lamellar spacing is 65nm, which is larger than that of 7.4nm predicted by TMK model. The coupled zone of Al-Cu eutectic alloy is calculated on basis of TMK and LKT/BCT models, which provides a further interpretation for the formation of anomalous eutectic.