Prenucleation Induced by Crystalline Substrates

For one to control solidification, it is crucial that first they comprehend the dynamics of heterogeneous nucleation. This type of nucleation depends on the nature of particles, as in the case of water. Alternatively, prenucleation refers to the phenomenon of atomic ordering in the liquid adjacent to the substrate/liquid interface at temperatures above the liquidus. Recent experimental observations have confirmed that substrates can induce atomic ordering in the liquid at the interface. Moreover, it has been reported that atomic ordering at the interface is closely related to the chemical and structural characteristics of the substrate. Therefore, it is highly desirable that the chemical and structural effects on the atomic arrangements be investigated, using atomistic simulations as they offer direct access to microscopic details of the liquid/solid interface. So far, results based on atomistic simulations have revealed that the atomic ordering at the interface is distinct from that in the bulk liquid, and is highly dependent on the structural and chemical properties of the substrate for a given liquid. Unfortunately, it has proven to be quite daunting when it comes to determining the atomic structure at the interface, primarily due to the difficulty in identifying whether an atom is in the solid or liquid state, hence more comprehensive work is desirable.

In a recent research paper published in the research journal, Metallurgical and Materials Transactions A, Brunel University London researchers: Dr. Hua Men and Professor Zhongyun Fan from Brunel Centre for Advanced Solidification Technology (BCAST) systematically investigated and holistically quantified the prenucleation phenomenon as a function of temperature and the lattice misfit between the substrate and the solid, using molecular dynamics (MD) simulations. Specifically, they combined various techniques and carried out a systematic investigation of the effect of the lattice misfit and temperature on the atomic ordering at the interface using classical MD simulation.

In brief, the research method employed involved carrying out MD simulations for systems consisting of liquid Al and substrates with a face-centered cubic crystal lattice having a {111} surface orientation, with the z axis being normal to the {111} plane. In particular, the researchers focused on assessing the effects of substrate chemistry on the atomic ordering at the interface for systems with zero lattice misfit.

The authors observed that atomic layering was independent of the crystal structure and surface orientation of the substrate and was only slightly enhanced by reducing temperature. In addition, they noted that the ordered regions in the 1st layer had either a face-centered cubic or a hexagonal close-packed stacking sequence relative to the lattice of the substrate, and Shockley partial dislocations with a dominant edge component were generated between the face-centered cubic stacking regions and the neighboring hexagonal close-packed stacking regions, forming a dislocation network.

In summary, Professor Zhongyun Fan and Dr. Hua Men methodically investigated and quantified the effects of both lattice misfit and temperature on the atomic ordering in the liquid induced by the substrate at temperatures above the liquidus, using a set-up of fixing the substrate for MD simulations. In general, their work provided evidence of prenucleation. They successfully demonstrated that prenucleation could either enhance or impede the subsequent heterogeneous nucleation process at the nucleation temperature depending on whether the prenucleation decreased or increased the initial lattice misfit.