Abstract
Nucleation from supercooled liquids is the first step in the first-order phase transition from a liquid to solid that governs the final microstructure and thermophysical properties of metallic alloys and glasses. Since microstructures significantly influence the physical properties of materials, understanding this fundamental process is of paramount interest. Classical Nucleation Theory (CNT) has long provided the theoretical framework for describing this process. However, several fundamental assumptions of CNT such as sharp solid-liquid interface, single-atom attachment/detachment, and the neglect of fluid flow under normal processing conditions make the theory inadequate for describing nucleation in complex metallic liquids at deep supercoolings. This dissertation combines containerless processing, structural characterizations, and atomistic simulations in a few high melting temperature metallic alloy liquids to investigate the mechanisms governing crystal nucleation and evaluate the validity of classical and emerging nucleation theories. To eliminate container-induced heterogeneous nucleation and study the intrinsic homogeneous nucleation behaviors of metallic liquids, containerless processing techniques were employed. Ground-based Electrostatic Levitation (ESL) provided a quiescent environment for achieving deep supercooling and conducting in-situ high-energy synchrotron X-ray diffraction experiments to identify the nucleating crystal phases at the very onset of the solidification process. Simultaneously, the Electromagnetic Levitator aboard the International Space Station (ISS-EML) was utilized to study nucleation under controlled fluid flow/stirring to investigate its effects on nucleation kinetics. The system-dependent influence of fluid flow on nucleation was evaluated under both quiescent and stirred conditions. Results from a glass-forming Cu47Zr47Al6 and a eutectic Zr80Pt20 liquid indicate that stirring does not universally enhance diffusion-driven nucleation, as was suggested in the coupled flux model (CFM). Instead, it may influence nucleation by modifying local dynamic heterogeneity, or by disrupting fragile surface catalytic sites, depending on the specific liquid structure. Furthermore, this work provides direct structural evidence that deeply supercooled eutectic metallic liquids do not necessarily crystallize according to equilibrium phase diagrams. In-situ synchrotron diffraction experiments revealed that eutectic systems, such as Zr80Pt20, exhibit multi-step nucleation pathways involving metastable precursors likely associated with the topological compatibility between the liquid's icosahedral (or other types) short-range order (ISRO), chemical short-range order (CSRO), and those of the nucleating phase. This is possibly the first systematic investigation of simultaneous nucleation and phase identification experiments in a series of high melting temperature eutectic and off-eutectic metallic alloy liquids. The most important observation is that the nucleation kinetics in eutectic alloys is much faster than in their off-eutectic counterparts. The detailed mechanisms responsible for this observation are not clear, however. Finally, large-scale Molecular Dynamics (MD) simulations of Cu47Zr47Al6 and Al20Ni60Zr20 liquids revealed that the solid-liquid interfaces of critical nuclei are diffuse, directly contradicting the sharp interface assumption of CNT. Unlike the unphysical negative temperature dependence of the interfacial free energy calculated from sharp interfaces in the CNT, diffuse interfaces in the framework of the Diffuse Interface Theory (DIT) successfully yielded a positive temperature dependence, which is consistent with experiments. Overall, this dissertation demonstrates that crystal nucleation in deeply supercooled metallic liquids cannot be fully explained by Classical Nucleation Theory alone; instead, theoretical frameworks are required that take into account fluid flow, local structures in the liquid and nucleating crystals, and diffuse solid-liquid interfaces.
Committee Chair
Li Yang
Committee Members
Anup Gangopadhyay, Alexander Seidel; Anup Gangopadhyay; Li Yang; Rohan Mishra; Zohar Nussinov
Degree
Doctor of Philosophy (PhD)
Author's Department
Physics
Document Type
Dissertation
Date of Award
8-13-2026
Language
English (en)
DOI
https://doi.org/10.7936/623j-v960
Recommended Citation
Sheng, Yelin, "Nucleation and Crystallization in Metallic Eutectic and Glass-Forming Alloys: A Combined Study using Molecular Dynamics, Ground Based Electrostatic Levitation, and Electromagnetic Levitation under Microgravity on the ISS" (2026). Arts & Sciences Graduate Student Theses and Dissertations. 3834.
The definitive version is available at https://doi.org/10.7936/623j-v960