Metal alloy solidification in thin capillaries - a study of the solid-liquid interfacial energy anisotropy using a novel approach

固液界面能的各向异性是金属凝固过程各方面的中心，对凝固组织有重要影响。然而，将各向异性效应与其他原因（如熔体对流引起的形态变化）的实验分离是极具挑战性的。现有的测量各向异性参数的方法需要大量的实验工作，并且导致测量值的大量分散。本项目旨在研究金属合金在直径为1µm数量级的细毛细管中的凝固。先前文献中记录的金属合金实验没有达到直径低于200微米。在本项目中使用的毛细管中，对流效应被抑制到实验可达到的最小量，从而允许研究准无波动的固液界面形状。界面是在定向凝固装置中创建的，其参数选择以强制形态不稳定。通过在淬火过程中用二次相修饰界面，在扫描电子显微镜下可以观察到其形状。结合聚焦离子束层析成像和电子背散射衍射，研究了三维界面形状与晶体生长方向的关系。数值研究将使用我们最近开发的“无网格前跟踪”（MFT）方法进行相变模拟，该方法将扩展到毛细管中凝固的条件。与已有的模拟方法相比，MFT方法不受空间离散化对模拟结果的各向异性影响，特别适合于各向异性效应的研究。用该方法对界面能各向异性和晶体生长取向的影响进行了数值研究。通过实验与仿真相结合，确定了各向异性参数。因此，本项目既是对一种新型测量方法的概念验证，也是对固液界面能各向异性对凝固影响的研究。

The anisotropy of the interfacial energy of the solid-liquid phase boundary is central with respect to various aspects of metal solidification processes and exerts a crucial influence on the solidified microstructure. The experimental separation of the anisotropy effects from other causes, e.g., morphology changes through melt convection is, however, extremely challenging. Existing methods for the measurement of the anisotropy parameters require considerable experimental effort and lead to substantial scatter of measured values.The present project aims to study the solidification of metallic alloys in thin capillaries with diameters in the order of magnitude of 1µm. Experiments with metal alloys previously documented in the literature did not achieve diameters below 200µm. In the capillaries used in the present project convection effects are suppressed down to the minimum experimental attainable amount which allows the investigation of the shape of the solid-liquid interface quasi free of fluctuations. The interface is crated in a setup for directional solidification with parameters chosen to force morphological instability. By decorating the interface with a secondary phase during quenching its shape remains observable post mortem by scanning electron microscopy. Combining focused ion beam tomography and electron backscatter diffraction, the three dimensional interface shape will be investigated in dependence of its crystallographic growth orientation.Numerical investigations will be conducted using our recently developed "Meshless Front Tracking" (MFT) method for the simulation of phase transformations, which will be extended to the conditions of the solidification in capillaries. In contrast to the established simulation methods, the MFT method is free of any anisotropic influence of the spatial discretization on the simulation results, which renders it especially suited for the examination of anisotropy effects. With this method the influence of both the interfacial energy anisotropy and the crystallographic growth orientation will be numerically investigated. Furthermore, by combining experiments and simulation the anisotropy parameters can be determined. The present project is therefore both a proof of concept for a novel measuring method and a study of the influence of the solid-liquid interfacial energy anisotropy on solidification.