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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 can provide relevant atom- istic insight into solid-liquid transformations, but the modelling of the initial nucleation during solidification is hampered by the extended timescales of the process. In this work we employ an advanced computational method, transition path sampling (TPS), to investigate the nucleation mechanism during solidification in nickel. We initially focus on homogeneous nucleation as a func- tion of undercooling. Here, a comparison of the temperature dependence of the free energy barriers to the predictions of classical nucleation theory and experiments is discussed. The analysis of the transition path ensemble (TPE) reveals the presence of clusters that consist of fcc coordinated atoms in the core surrounded by a cloud of hcp coordinated atoms and prestructured liquid. The TPE also shows predominantly nonspherical shapes of the nuclei at different undercoolings. As a second step towards more complex materials, we extend our study by including small Ni-clusters as seeds during heterogeneous nucleation. A comparison between heterogeneous and homogenous nucleation mechanisms shows that the nucleation rate is sensitive to the crystal structure of the seed. In particular, small fcc seeds largely decrease the nucleation barrier, while hcp seeds do not influence the overall nucleation rate. These results provide fundamental understanding of the nucleation mechanisms that can help to validate and improve existing thermodynamic models describing nucleation in metals.