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Abstract

A crucial stage during solid-liquid phase transformations is the initial nucleation of a stable phase within a metastable medium. Molecular dynamics simulations provide an atomistic picture of solidification, but the modelling of the initial nucleation is hampered by the extended timescale of the process. In this work we employ an advanced computational method, transition path sampling (TPS), to enable the investigation of nucleation in elemental nickel on the atomistic level. We initially focus on homogeneous nucleation. Here, a comparison of the temperature dependence of the free energy barriers to the predictions of classical nucleation theory is discussed. As a second step, we extend our study by including small Ni-clusters as seeds during heterogeneous nucleation. The analysis of the transition path ensemble (TPE) obtained from our simulations indicates the presence of fcc and hcp crystalline structures and nonspherical shapes of the clusters. Furthermore, critical nuclei sizes, free energy barriers and rates, as well as optimal candidates for reaction coordinates based on local structural parameters are identified in the TPE. These results provide fundamental understanding of the nucleation mechanisms and can help to validate and improve existing thermodynamic models describing nucleation in metals.