超声场中三元共晶型合金的动态凝固机制研究

This project focuses on the dynamic solidification mechanism of ternary eutectic alloys within ultrasonic field. The liquid to solid transformations of Ag-Cu-Ge and Al-Cu-Si ternary eutectic alloys will be accomplished by employing ultrasonic fields to set up dynamic solidification conditions. A theoretical model on wave propagation with the alloy melts will be proposed to calculate the acoustic field and flow field within the solidifying alloy melt, and to investigate the effect of ultrasound on the heat and mass transportation during solidification. The undercoolability and the competition nucleation mechanism of the three eutectic phases of the ternary eutectic alloys within ultrasonic field, their solidification path versus sound intensity, and the growth relationship among primary phase, pseudobinary eutectics and ternary eutectic will be studied. Furthermore, the microstructural characteristics of pseudobinary eutectics and ternary eutectics will be discussed. On the basis of the calculated results of acoustic field and flow field, the temperature and solute distribution around the solid-liquid interface will be analyzed to explore the dynamic conditions of the regular eutectic to anomalous eutectic transition. The characteristics of structural morphology, solute distribution and growth orientation for primary (Ge) and (Si) phases in response to the ultrasonic field will be studied to reveal their faceted to nonfaceted growth mode transformation. The results of this project will provide an in-depth insight into the nucleation and growth mechanisms of ternary eutectic alloys within ultrasonic field.

本项目采用超声场建立动态凝固条件，实现Ag-Cu-Ge和Al-Cu-Si三元共晶型合金在超声作用下的定向凝固和等温凝固。建立合金熔体内部声波传播模型，数值计算相应的声场和流场分布，探索超声场对于凝固过程传热传质的影响规律。揭示超声空化效应作用下液态合金的过冷能力变化特征以及三个共晶相的竞争形核机制，确定不同声强下的合金凝固路径，探讨初生相、二相共晶和三元共晶之间的相互依存关系。分析二相共晶和三元共晶生长方式对超声场的动态响应，根据声场和流场分布获取凝固过程中温度场和浓度场的演变信息，确定空化和声流作用下共晶组织发生"规则共晶-不规则共晶"转变的动力学条件。研究初生(Ge)和(Si)相的组织形态、溶质分布和生长取向与超声场的相关特性，揭示其"小平面相-非小平面相"的转变机制。在实验研究和理论分析的基础上，阐明超声场中空化效应和声流作用下的三元共晶型合金动态凝固规律。

超声场中的合金凝固规律研究可以追溯到上个世纪30年代。但是，目前国内外的大多数研究集中于以枝晶生长方式凝固的Al/Mg基单相合金，而对于其它类型的复相合金在超声作用下的凝固机制研究较少。. 本项目采用一维高强超声，实现了共晶和包晶这两类复相合金的在超声场中的动态凝固过程。主要开展了以下四个方面实验和理论研究：一是自主研制超声场中的定向凝固实验装置，该装置能够实现不同类型中低温合金在一维超声作用下的定向或自由凝固过程；二是重点研究了三元Ag-Cu-Ge/Sb和Al-Cu-Si共晶合金在超声场中的自由和定向凝固过程，揭示了超声作用下三元共晶竞争形核和离异生长的共性特征；三是阐明了高强超声作用下包晶合金初生相和包晶相的竞争形核以及包晶转变机理；四是采用DSC方法，系统测定了多种不同类型复相合金的液相线温度和斜率，熔化焓和熔化熵以及过冷能力等热力学性质。. 本项目的研究丰富了超声作用下的合金凝固理论，并为生产实践中利用超声场改善合金凝固组织形态、优化物理性能提供了指导依据。本项目共计发表SCI收录论文16篇，培养在读博士研究生1人、在读硕士生研究2人，项目主持人翟薇副教授入选了2016年度陕西省青年“科技新星”。