快速凝固块体纳米规则共晶组织形成机理研究

Bulk eutectic alloys with nanoscale regular eutectic structures have a great potenital in a varity of applications, because this type of materials have excellent properties and can be fabricated by rapid solidification directly. However, the mechanism of formation of the nanoscale regular eutectic structures under the conditions of rapid solidification, which is, although, very important in fabrication of these alloys, has not been fully understood. This project aims to provide a quantitative illumination on this topic by a combination of experimental investigations and theorectical studies. Three binary eutectic systems, i.e. Ag-Cu, Al-Cu, and Ni-Sn, which exhibit greatly different solidification behaviors regarding the nanostructural formation will be selected as model systems. Rapid solidification of the alloy melts will be achieved by means of bulk melt undercooling techniques combining with melt qunching methods. Through a systematic experimental investigation on the solidification behaviors and the as-solidified microstructures, evolution of the microstructural characteristics as functions of undercooling, solidification velocity, and cooling rate will be presented. A criterion on a critical undercooling/solidifcation velocity corresponding to decoupled growth of the undercooled eutectic alloy melts will be derived by studying the compititive growth between coupled eutectic growth and decoupled dendrite growth. Meanwhile, a characteristic cooling rate corresponding to the suppression of the remelting of primary regular eutectic structure will be established by studying the heat balance between crystallization latent heat releasing and heat conducting to ambiance. Based on the theory of eutectic solidification in bulk undercooled melts, solidification parameters, physical parameters of alloys, and characteristic length paramemters of the as-solidified microstructures will be quantitatively correlated and modeled. Accordingly, the mechanism of formation of the nanoscale regular eutectic structures under the conditions of rapid solidification will be demonstrated and the key solidification parameters and physical parameters in controlling the nanostructure formation will be illuminated. In the end, a criterion in elements selection of the base binary alloy and strategies in controlling solidification conditions for fabrication of bulk nanostructured eutectic alloys will be proposed. The achievements by carrying out this project will be expected to provide a theorectial basis in alloy design and structure control of the rapid solidified bulk nanostructured eutectic alloys and also to be an important contribution to the theory of non-equilibrium eutectic solidification.

块体纳米规则共晶合金拥有极为优越的性能，可通过快速凝固技术直接制备，是一类极具应用前景的先进材料。但这类合金在快速凝固条件下的纳米规则共晶组织的形成机理仍未得到清晰阐述。本项目拟通过典型的二元共晶合金，采用实验和理论分析相结合的方法，深入研究块体纳米规则共晶合金的凝固过程；分析凝固条件及合金自身性质对规则共晶两相间距纳米化的影响；考察共晶两相耦合生长与解耦合生长的竞争关系及抑制初始规则共晶组织重熔的临界条件；建立凝固参量、物性参数及凝固组织特征尺度间的关系模型，对快速凝固块体纳米规则共晶的组织形成做出模型化描述，定量的阐明其机理；以此为基础，确定出这类组织形成的关键凝固条件和物性参数；并提出这类合金设计的二元基础合金的组分选择准则及组织形成的凝固条件控制方法。项目如获突破，不仅可为快速凝固块体纳米规则共晶合金的设计及组织控制提供可靠的理论依据，还将对共晶合金的非平衡凝固理论产生重要贡献。

块体纳米规则共晶合金拥有极为优越的性能。这类材料可通过过冷熔体快速凝固技术直接制备，是一类极具应用前景的先进材料。但这类合金在快速凝固条件下的纳米规则共晶组织的形成机理仍不明晰，这制约了此类合金的成分及制备工艺设计，也成为这类材料规模化制备及应用的瓶颈。为解决该问题，本项目以典型二元共晶合金为对象，采用实验和理论建模相结合的方法，深入研究了块体共晶合金纳米规则组织的形成过程，分析了控制规则共晶层片组织形成及细化的关键凝固条件，进而阐明了过冷熔体快速凝固条件下规则共晶组织的形成机理，并在前期研究的基础上，将部分理论成果拓展至共晶Al-Si铸造合进行微观组织及力学性能的调控，取得了以下主要结果：（1）建立了考虑界面非平衡动力学效应的共晶耦合生长模型，更为准确的描述了规则层片共晶的耦合生长行为；（2）揭示了快速凝固规则层片共晶在再辉快速热效应下的失稳、球化的动力学机制，并建立模型定量描述了层片失稳的动力学过程；（3）依据所提出的层片失稳模型，提出了纳米共晶层片组织形成的理论判据；（4）提出了共晶Al-Si铸造合金的非平衡凝固与固态相变一体化的处理思路，大幅度提高了材料的固溶和时效效率，同时提高了材料的强度和塑性。以上成果在Scripta Materialia、Journal of Alloys and Compounds等期刊发表SCI论文11篇，其中TOP期刊4篇，申报国家发明专利2项。研究成果不仅对纳米规则共晶合金及相关工程共晶合金的可控性制备有指导作用，还推动了共晶合金非平衡凝固理论的发展。