【作者】 吕勇军；

【导师】 魏炳波；

【作者基本信息】 西北工业大学 ， 材料物理与化学， 2002， 硕士

【摘要】 深过冷液态金属与非金属的无容器快速凝固研究是当前空间材料科学领域研究的重要课题。本文采用声悬浮无容器处理技术系统地研究了大体积水及Pb-Sn二元共晶合金在深过冷条件下的快速凝固规律，讨论了声场对过冷度及凝固后的微观组织形貌的影响。同时，采用落管无容器处理技术，研究了Al-32.7wt％Cu二元共晶合金的深过冷及快速凝固行为。主要取得了以下几方面的研究成果： 研究了φ5～8mm的大体积水在声悬浮无容器条件下的快速凝固行为。获得了最大为24K的过冷度。讨论了空化效应对水滴过冷度的影响，认为空化效应诱发水滴提前形核，抑制了大过冷度的获得。实验测定了不同过冷度下冰枝晶的生长速度，并且改进了冰枝晶的生长理论，使之更符合大过冷度时的枝晶生长情况。 采用声悬浮无容器处理技术研究了Pb-Sn共晶合金在深过冷条件下的凝固规律。发现在小过冷范围内，由于超声波的搅拌作用层片共晶组织发生碎断现象；在大过冷情况下，超声波可以有效地抑制宏观偏析的发生。分析了声场对过冷度的影响，认为声波引起的样品表面振动以及试样的绕轴自转都会抑制过冷度的进一步提高，不利于深过冷条件的获得。 采用落管无容器处理技术研究了Al-32.7wt％Cu二元共晶合金在深过冷条件下的快速凝固规律。发现随着过冷度的增大，Al-Cu共晶合金发生“规则共晶—不规则共晶”的组织形貌转变。实验中获得了间距为65nm的精细层片共晶组织。利用TMK模型理论描述了Al-Cu共晶合金的快速生长规律，确定了“规则共晶—不规则共晶”转变的过冷度阈值为199K，确定了粒子直径与过冷度呈负指数关系。更多还原

【Abstract】 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.更多还原