A Study on Prenucleation and Heterogeneous Nucleation in Liquid Pb on Solid Al using Molecular Dynamics Simulations

In this paper, we investigate prenucleation and heterogeneous nucleation in the liquid Pb/solid Al system as an example of systems with large lattice misfit, using molecular dynamics simulation. Solid Pb and Al have a large positive lattice misfit ( f) of 18.2% along the densely packed [110] direction. This study reveals that prenucleation occurs at 600 K (an undercooling of 15 K), and a 2-dimensional (2D) ordered structure forms at the interface with a coincidence site lattice (CSL) between the 1st Pb and 1st Al layers. The CSL accommodates the major part of the f, and only a small residual lattice misfit ( fr) of 1.9% remains. The formation of the CSL transforms original substrate into a considerably potent nucleant, where the 1st Pb layer becomes the new surface layer of the substrate. At an undercooling of about 22 K, nucleation proceeds by merging 2D ordered structure through structural templating: the 2nd Pb layer is epitaxial to the CSL Pb layer, the 3rd Pb layer largely accommodates the fr, and the 4th Pb layer is a nearly perfect crystalline plane. Further analysis indicates that the interface with the CSL has a lower interfacial energy than with a cube-to-cube orientation relationship. For the first time, we established that the CSL was an effective mechanism to accommodate the f for systems with large positive misfit. Heterogeneous nucleation is governed not by a single mechanism, instead by various mechanisms depending on f. This study sheds new light on the atomistic mechanism of heterogeneous nucleation.