Institut für Materialphysik im Weltraum, Deutsches Zentrum für Luft- und Raumfahrt (DLR), 51170 Köln, Germany.
Institut für Physik, Johannes Gutenberg Universität Mainz, 55099 Mainz, Germany.
J Chem Phys. 2016 Dec 7;145(21):211703. doi: 10.1063/1.4963684.
Crystallization is one of the most important phase transformations of first order. In the case of metals and alloys, the liquid phase is the parent phase of materials production. The conditions of the crystallization process control the as-solidified material in its chemical and physical properties. Nucleation initiates the crystallization of a liquid. It selects the crystallographic phase, stable or meta-stable. Its detailed knowledge is therefore mandatory for the design of materials. We present techniques of containerless processing for nucleation studies of metals and alloys. Experimental results demonstrate the power of these methods not only for crystal nucleation of stable solids but in particular also for investigations of crystal nucleation of metastable solids at extreme undercooling. This concerns the physical nature of heterogeneous versus homogeneous nucleation and nucleation of phases nucleated under non-equilibrium conditions. The results are analyzed within classical nucleation theory that defines the activation energy of homogeneous nucleation in terms of the interfacial energy and the difference of Gibbs free energies of solid and liquid. The interfacial energy acts as barrier for the nucleation process. Its experimental determination is difficult in the case of metals. In the second part of this work we therefore explore the potential of colloidal suspensions as model systems for the crystallization process. The nucleation process of colloids is observed in situ by optical observation and ultra-small angle X-ray diffraction using high intensity synchrotron radiation. It allows an unambiguous discrimination of homogeneous and heterogeneous nucleation as well as the determination of the interfacial free energy of the solid-liquid interface. Our results are used to construct Turnbull plots of colloids, which are discussed in relation to Turnbull plots of metals and support the hypothesis that colloids are useful model systems to investigate crystal nucleation.
结晶是一级相变中最重要的相变之一。在金属和合金的情况下，液相是材料生产的母相。结晶过程的条件控制着凝固材料的化学和物理性质。成核引发液体的结晶。它选择结晶相，稳定或亚稳定。因此，详细了解成核对于材料设计是强制性的。我们介绍了用于金属和合金成核研究的无容器处理技术。实验结果证明了这些方法的强大功能，不仅适用于稳定固体的晶体成核，而且特别适用于在极端过冷条件下研究亚稳固体的晶体成核。这涉及非均匀成核与均匀成核以及在非平衡条件下成核的相的物理性质。结果在经典成核理论的范围内进行了分析，该理论根据界面能和固相与液相的吉布斯自由能差来定义均匀成核的活化能。界面能作为成核过程的障碍。在金属的情况下，其实验测定是困难的。因此，在这项工作的第二部分，我们探索了胶体悬浮液作为结晶过程模型系统的潜力。通过使用高强度同步辐射的光学观察和超小角 X 射线衍射原位观察胶体的成核过程。它允许明确区分均匀成核和非均匀成核，并确定固液界面的界面自由能。我们的结果用于构建胶体的图，讨论与金属的图的关系，并支持胶体是研究晶体成核的有用模型系统的假设。
