Abstract
Heterogeneous nucleation is a widespread phenomenon in both nature and technology. However, our current understanding is largely confined to the classical nucleation theory (CNT) postulated over a century ago, in which heterogeneous nucleation occurs stochastically to form a spherical cap facilitated by a substrate. In this paper, we show that heterogeneous nucleation in systems with negative lattice misfit completes deterministically within three atomic layers by structural templating to form a two-dimentional template from which the new phase can grow. Using molecular dynamics (MD) simulations of a generic system containing metallic liquid (Al) and a substrate of variable lattice misfit (fcc lattice with fixed Al atoms), we found that heterogeneous nucleation proceeds layer-by-layer: the first layer accommodates misfit through a partial edge dislocation network; the second layer twists an angle through a partial screw dislocation network to reduce lattice distortion; and the third layer creates a crystal plane of the solid (the 2D nucleus) that templates further growth. The twist angle of the solid relative to the substrate as a signature of heterogeneous nucleation in the systems with negative lattice misfit has been validated by high resolution transmission electron microscopic (HRTEM) examination of TiB2/Al and TiB2/α-Al15(Fe, Mn)3Si2 interfaces in two different Al-alloys.
Highlights
Nucleation of crystals in liquids is one of the most ubiquitous phenomena in both natural and industrial processes [1,2]
This is in clear contrast to the classical nucleation theory (CNT), in which heterogeneous nucleation only occurs when an energy barrier is overcome by structural fluctuation in the disordered liquid at the interface
We have investigated systematically the heterogeneous nucleation mechanism in generic systems with negative misfits using molecular dynamics simulations
Summary
Nucleation of crystals in liquids is one of the most ubiquitous phenomena in both natural and industrial processes [1,2]. Our current understanding of nucleation is far from complete; and nucleation research has been dominated by the classical nucleation theory (CNT) for more than a century. There is a massive gap of 1010 ·m−3 s−1 in the homogeneous ice nucleation rate between CNT-based computer simulations and experimental measurements that has triggered an intense debate between theoreticians and experimentalists [4]. The CNT provides little guidance to nucleation control in important industrial processes. The TiB2 based grain refiner has been used in metallurgical industry for over 70 years [8], it was mainly developed by trial-and-error with little help from the CNT
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