强磁场下深过冷Cu-Co、Al-Bi难混溶合金的形核、生长及凝固组织演变行为研究

Investigation of the immiscible alloys becomes one of the hottest topics in the field of material due to its great potential application in the engineering. The high magnetic field as an extreme condition can adjust the microstructure during solidification and promote the related properties of the materials. The present research will choose the Cu-Co (with a metastable miscibility gap) and Al-Bi (with a stable miscibility gap) immiscible alloys as modal alloys, the nucleation kinetics and related parameters of the two alloy systems will be systematically investigated by using a combination of a superconducting high magnetic field and glass fluxing methods. The crystallization process of undercooled Cu-Co and Al-Bi alloys will be in-situ observed using a high-speed camera. The microstructure of alloys as well as the particle size distribution, composition and substructure will be systematically studied by means of SEM, EDS and EBSD methods. A quantitative relationship between magnetic fields, nucleation kinetics and related parameters, growth kinetics, and droplet size distribution and composition will be established based on the experimental results and related theory. The inherent correlation between nucleation, growth and phase evoluation in immiscible alloys during solidification will be clarified. The present research may provide some new data and methods to promotion of microstructure and related property modulation for industrial application of immiscible alloys.

由于难混溶合金在工业上具有非常重要的应用价值而成为材料科学领域的研究热点之一，同时强磁场作为一种极端手段可有效地调控材料在凝固过程中的组织结构进而提高其相关性能。本项目拟选择具有亚稳溶解度间隙的Cu-Co和稳定溶解度间隙的Al-Bi难混溶合金为模型合金，采用超导强磁场和深过冷快速凝固相结合对两种合金的形核动力学及相关参数进行系统研究；在此基础上，结合高速摄影技术对合金过冷熔体的枝晶生长速率进行原位观测；并利用SEM/EDS/EBSD等对合金及弥散相液滴的微观组织进行表征；最终建立磁场强度-形核动力学及相关参数-生长动力学-弥散相液滴尺寸、成分分布之间的定量关系。旨在揭示强磁场下难混溶合金的形核、生长过程与最终的凝固组织之间的内在关联，为难混溶合金的组织优化及性能调控到工业应用提供新的数据和思路。

本项目利用传统DSC和快速DSC通过多次反复熔化-过冷-凝固循环过冷实验功地使得Bi-Ga难混溶合金、纯金属Ag和纯金属Sn获得了深过冷，并结合经典形核理论、概率存活函数（survivorship function）及泊松分布（Poisson distribution）对其形核动力学及形核机制进行了系统的研究。结果表明，Bi40Ga60难混溶合金在冷却速率达到超过1000 K/s 时，发生了不经过液-液相分离直接从过冷熔体中凝固的现象，在冷却速率1000 -2000 K/s时，形核机制从异质形核转变为均质形核；纯Ag过冷熔体中仅存在一种形核机制和质量无关，其形核机制为均质形核；纯Sn过冷熔体的形核是由异质形核控制的。