A Molecular Dynamics Simulation On Structure And Crystal Nucleation Of Supercooled Liquids

Crystal nucleation is an important phenomenon in material science and technology. Liquid structure and its evolution are commonly believed to determine the solidification structure and performance of the products. With molecular dynamics (MD) simulation method, liquid structure and its evolution during nucleation in Lennard-Jones (LJ) supercooled liquids are studied in the present thesis.Three widely used methods for analyzing atomic structures are evaluated in crystalline solids and supercooled liquids. The local order parameter approach based on theoritical values of perfect crystals fails in some perturbed body-centered-cubic (bcc) environments, while the pair analysis method behaves approximate depending on the definition of the pairs. An improved procedure is designed to eliminate distorted Voronoi faces and edges in the Voronoi analysis method.Structures of the LJ liquids at different temperatures are simulated in isothermal- isobaric (NPT) ensembles. In the simulated states, the metastable structures are independent of cooling rates, and the inherent structures keep identical at different temperatures, thus the temperature effect of liquid structure seems not to alter the intrinsic nucleation rules. The icosaheral (ico) structures already exist in the liquids upon the melting points, and increase with the decrease of the temperature.The structure evolution during nucleation is traced in the proceeding studies. The ico structures compete with the crystalline clusters in space, and must be rearranged before they nucleated, therefore obstruct LJ liquid from crystal nucleation. The initial nucleated crystal is surrounded by heavy interface, and contains more face-centered-cubic (fcc) and hexagonal-close-packed (hcp) particles than bcc. However, the bcc structures grow during an intermediate stage and form an incomplete meso-layer in the clusters. This indicates a complicated dynamics in which many structures may grow simultaneously because of the comparable energy barriers between nucleation of these structures, and connects the differences between nucleation at lower and deeper undercoolings. The nucleated crystal forms multiply twinned particles with fivefold axes, bounded by hcp planes. The structures of thefivefold axes are characterized, and the discussion of their formation mechanism is also included.