【作者】 洪振宇；

【导师】 魏炳波；

【作者基本信息】 西北工业大学 ， 凝聚态物理， 2004， 硕士

【摘要】 本文利用自行研制的单轴式声悬浮无容器处理装置对Bi-Ga过偏晶型合金和Bi-In共晶型合金在声悬浮条件下和常规条件下的凝固过程进行了对比研究，主要取得以下研究结果。 研究发现Bi-Ga过偏晶合金在常规条件下凝固时，第二相有明显的向上运动的特征。在低冷速时，宏观偏析严重，分析表明Stokes运动是导致宏观偏析的主要原因。在声悬浮条件下，第二相没有向上运动的特征，可以得到均匀弥散分布的组织，第二相平均尺寸较大。讨论分析了声悬浮条件对液相分离的影响，认为悬浮液滴表面受迫振动可能有抑制Stokes运动的作用，有助于获得第二相均匀弥散分布的结构，其它振动方式将促进碰撞和凝并的发生，使第二相的平均尺寸变大。此外，在声悬浮条件下，第二相主要以球状形式存在，第二相的偏聚程度受加热温度和过冷度的影响较大，第二相的平均尺寸随过冷度的增大而显著减小。 对于Bi-In亚共晶合金，发现在常规条件下初生相BiIn₂主要以枝晶方式生长，而在声悬浮条件下倾向于以顶端分裂方式生长。分析认为超声振动引起凝固前沿浓度场的扰动，使凝固前沿变得不稳定，从而导致顶端分裂方式生长。发现BiIn₂相的顶端分裂方式生长与枝晶碎断有密切的关系，顶端分裂方式生长可能是导致晶粒细化的内在原因。另外，还讨论了声悬浮条件对过冷度及Bi-In过共晶凝固组织的影响，认为超声波的存在不利于大过冷度的获得，超声振动有促使枝晶碎断及粗化的作用。更多还原

【Abstract】 Bi-Ga hypermonotectic and Bi-In eutectic alloys have been successfully processed by a single-axis acoustic levitator. The solidification process under acoustic levitation conditions has been investigated systematically in comparison with static conditions. The main results are obtained as follows.Under static conditions, the second phase (Ga) in the Bi-Ga hypermonotectic alloy has prominent upward floating tendency, leading to serious macrosegeration at low cooling rate. Calculation indicates that Stokes motion is the main influence factor. But under acoustic conditions, the second phase (Ga) has no upward floating tendency and the average size is larger. The second phase (Ga) distributes homogeneously in the matrix (Bi). Analyses indicate that surface vibration and shape oscillation of the levitated droplets may have the effect of preventing Stokes motion, reducing macrosegeration, and other unstable factors promote collision and consolidation, increasing the average size of the second phase. Moreover, the content of Ga, heating temperature and cooling rate can affect significantly the final microstructures. With the increase of undercooling, the average size of the second phase decreases obviously.Based on the experimental observation on the solidification microstructures of Bi-60wt%In hypoeutectic alloy, it can be found that under static conditions, the primary phase BiIn2 is characterized by dendrite structures, but under acoustic levitation conditions, the primary phase is characterized by tip-splitting structures. Analyses indicate that the instability of solid-liquid interface aroused by ultrasound may cause a morphological transition of primary phase BiIn2 from dendrite growth to tip-splitting growth. It has also been found that dendrite breaking has close relationship with tip-splitting structures, which may be the substantial reason resulting in spontaneous grain refinement. Microstructures of Bi-72wt%In hypereutectic alloy indicate that the vibration aroused by ultrasound can break the dendrites of s primary phase and make them coarser. Moreover, the ultrasound tends to increase the nucleation temperature.更多还原