Modelling & Simulation


Atomistic simulations of nucleation during solidification in nickel

Grisell Diaz Leines, ICAMS, Ruhr Universität Bochum, Bochum, Germany
Ralf Drautz, ICAMS, Ruhr Universität Bochum, Bochum, Germany
Jutta Rogal , ICAMS, Ruhr Universität Bochum, Bochum, Germany

Fundamental insight into solidification in metals can significantly contribute to improve our understanding of their behaviour during processing and under operating conditions. Molecular dynamics simulations can provide relevant atomistic insight into solid-liquid transformations, but the modelling of the initial nucleation during solidification remains challenging due to the extended timescales of the process.

In this work, we employ transition path sampling (TPS), an advanced computational method, to investigate the nucleation mechanism during solidification in nickel. We initially focus on homogeneous nucleation as a function of undercooling. Here, a comparison of the temperature dependence of the free energy barriers with the predictions of classical nucleation theory and experiments is discussed. As a second step towards more complex materials, we extend our study by including small Ni-clusters as seeds during heterogeneous nucleation. 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. Furthermore, the TPE shows predominantly nonspherical shapes of the nuclei at different undercoolings.

Our simulations also allow a direct evaluation of the transition state ensemble, from which we obtain atomistic insight into the structure and size of the critical nuclei. Moreover, optimal candidates for reaction coordinates can be identified based on local structural parameters and the nucleus shape using maximum likelihood analysis of the committor function. These results provide fundamental understanding of the nucleation mechanisms that can help to validate and improve existing thermodynamic models describing nucleation in metals.

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