微/纳米颗粒对复杂相分离体系凝固组织的调控机制

The preparation of homogenous immiscible alloy without the formation of macroscopic segregation is quite a challenge due to the occurrence of liquid-liquid phase separation when the alloy melt falls into the liquid immiscibility gap. In this project, we propose a novel approach to obtain homogenous microstructure by tuning the liquid phase separation and microstructural evolution through introducing micro/nano-sized solid particles into the alloy melt. We will study the phase separation and solidification of three types of immiscible systems: Al-Pb monotectic alloy, metastable immiscible Co-Ni-Cu alloy and transparent H2O-SCN system. The introduced solid particles include SiC, Al2O3 and SiO2 particles. The techniques of drop tube, melt spinning and glass fluxing will be applied to obtain different physical conditions for solidification, such as different cooling rate and temperature gradient. The wettability of particles with the alloy melt will be studied. How the particles influence the temperature and solute fields and induce the nucleation of solid phase and L2 droplet will be clarified. The solidification rate versus undercooling will be measured experimentally. The microstructural evolution and the redistribution of particles will be researched by microstructure analysis of the final solidified samples. To get better understanding of phase separation with the presence solid particles, we will investigate the emergence of the secondary L2 droplets and the kinetics of their growth, and the propagation of solidification front via in-situ microscope observation of transparent immiscible system. The mechanisms of how the solid particles affect the migration velocity of solidification front and change the route of liquid phase separation will be clarified. Meanwhile, we will use phase field simulation to investigate the microstructural evolution with the presence of solid particles. The interaction between solid particles and solid/liquid and liquid/liquid interfaces and its dependence on the particle properties, such as the shape,size and wettability, will be elucidated. The relation between the particle/interface interaction and the liquid phase separation, and microstructural evolution will be established, thus shedding light on the preparation of immiscible alloys with homogenous microstructures.

通过在稳定、亚稳和模拟合金三类相分离体系中主动引入微/纳米尺度的固体颗粒，探索颗粒与合金熔体的浸润性，研究颗粒对合金熔体温度场、溶质浓度场的影响规律，揭示颗粒对初生固相、第二相液滴的诱发形核机制。利用落管、单辊急冷和熔融玻璃等实验手段实现合金熔体在不同物理条件下的相分离与凝固，研究固体颗粒、溶质分布规律以及两者之间的竞争机制，阐明合金凝固组织演变规律，揭示固体颗粒性质、颗粒引入方式等因素对凝固组织的影响机制。利用透明模拟合金进行实时观测，研究固体颗粒对固/液界面形态及其迁移速度的调控作用，探索其在液/液界面的吸附机理。同时结合相场模拟，深刻揭示固体颗粒与固/液、液/液界面的相互作用机制，建立"颗粒/界面相互作用－液相分离－凝固组织演变"的内在关系，最终阐明固体颗粒对复杂难混溶合金体系液相分离与凝固组织的调控规律，为该类合金的均质化制备提供新的思路。

微/纳米颗粒对材料相变具有重要作用，特别是对不混溶体系的相分离热力学和动力学将产生显著影响。微/纳米颗粒如何诱发相变时的形核及长大，如何影响液-液界面动力学以及相分离的形貌演化，这些问题是材料和凝聚态物理领域热切关心且悬而未决的问题。在此背景下，本项目研究了微/纳米结构对材料润湿性的影响，探索了微/纳米颗粒对液滴静态性质及其动态行为的调控规律，开展了微/纳米颗粒作用下的液固相变和相分离的研究。取得了以下主要结果：. 1. 研究了不同形貌的微米结构和微/纳米复合结构的润湿性，发现两类结构均可实现材料的超疏水。但微/纳米复合结构的疏水性质更为稳定。这项研究揭示了纳米结构对抑制Cassie-Wenzel润湿性转变的重要贡献。. 2. 微/纳米颗粒在液滴表面的吸附显著改变了其有效表/界面张力，缩短了液滴撞击时的接触时间和振荡周期。颗粒的吸附也影响声悬浮液滴的扇谐振荡动力学过程，本项研究建立了液滴表面颗粒层力学性质与扇谐振荡频率的理论关系。. 3. 研究了胶体液滴蒸发过程中的相变。首次发现了胶体颗粒在毛细作用驱动下聚集所形成的辐射状波纹，最终导致形成类似的蒸发裂纹图案。由于盐离子对胶体颗粒间作用的调节，加入无机盐后，胶体液滴蒸发形成了有趣的枝晶形貌。研究还发现，蒸发产物的形貌与颗粒的润湿性及蒸发速度密切相关。. 4. 研究了纳米颗粒包覆的水滴的结冰，发现水滴结冰的最终形态随颗粒的润湿性而变化，这充分证明了微/纳米颗粒的异质形核作用。颗粒在液-液界面的吸附，有效增强了液滴的稳定性从而抑制相分离。进一步采用相场方法对相分离的形态演化进行了数值模拟，发现了粘度的增加或热扩散的减小均有利于获得较细的层状结构。超声波的引入及其频率的提高，可加速液滴的凝并，从而提高相分离效率。. 该项目揭示了微/纳米颗粒在气-液界面及液-液界面的吸附机理，发现了颗粒对液滴动力学的调控规律，阐明了固体颗粒与流体界面的相互作用机制，为新材料的均质化制备和组织控制提供了借鉴。