System size effect on crystal nuclei morphology in supercooled metallic melt

Abstract

We present a molecular dynamics (MD) study of size effect on the crystal nuclei geometry, formation and growth in supercooled tantalum film. The process is studied in a set of MD trajectories that are obtained by ultrafast cooling from the stable liquid phase to the temperature below the glass transition temperature. We describe the nucleation process by two morphological parameters. Along with the nucleus size, we analyze the asphericity that is a measure of deviation of crystal nuclei from idealized spherical form. This method allows to demonstrate that there are two paths for crystal nucleus shape and size evolution. The first path is crystal growth through high asphericity values. We show that this is caused by coalescence of the crystals. This mechanism is not affected by the size of the system. The second path is formation of long-lived crystal clusters that do not lead to the crystallization of the whole system on the MD simulation timescale. We demonstrate that such clusters have common geometric features which strongly depend on the system size.