Nucleation kinetics in a supercooled metallic glass former

We use molecular dynamics simulations to shed light on the mechanism underlying crystal nucleation in a supercooled metallic glass former characterized by a concurring crystal and amorphous local order based on icosahedral symmetry. At a crossover temperature, well below the melting point, we find that the supercooled phase exhibits glassy dynamics which includes a breakdown of the Stokes-Einstein relation and the emergence of spatially heterogeneous dynamics. In addition, we show that the origin of these phenomena can be related to a structural heterogeneity caused by the increase of icosahedral symmetry upon cooling. We also reveal that crystal nucleation occurs close to the glass transition and takes place in regions of high icosahedral symmetry, which can be interpreted by a strong reduction of the crystal-liquid interfacial energy. This scenario provides a framework for testing the estimation of the nucleation time according to the classical nucleation theory. More specifically, our findings allow to quantify how the heterogeneous character of the supercooled phase affects the predictions of this theory. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.