Heterogeneous nucleation of solid Al from the melt byTiB2andAl3Ti: Anab initiomolecular …

The nucleation of solid Al from the melt by ${\text{TiB}}_{2}$ is well established and is believed to involve the formation of ${\text{Al}}_{3}\text{Ti}$. Since the atomic-scale mechanisms involved are not fully understood, we look to computer simulation to provide insight. As there is an absence of suitable potentials for all of this complex system we have performed large-scale density-functional-theory molecular dynamics simulations of the nucleation of solid Al from the melt on ${\text{TiB}}_{2}$ and ${\text{Al}}_{3}\text{Ti}$ substrates at undercoolings of around 2 K. Using periodic boundary conditions, we find limited ordering and no signs of incipient growth in the liquid Al close to the B-terminated surface of ${\text{TiB}}_{2}$. By contrast, we see fcc-like ordering near the Ti-terminated surface, with growth being frustrated by the lattice mismatch between bulk Al and the ${\text{TiB}}_{2}$ substrate. The Al interatomic distances at the Ti-terminated surface are similar to distances found in ${\text{Al}}_{3}\text{Ti}$; we suggest that the layer encasing ${\text{TiB}}_{2}$ observed experimentally may be strained Al on a Ti-terminated surface rather than ${\text{Al}}_{3}\text{Ti}$. For the ${\text{Al}}_{3}\text{Ti}$ substrate, fcc-like structures are observed on both sides which extend rapidly into the melt. Periodic boundaries introduce unphysical stresses which we removed by introducing a vacuum region to separate the liquid from the solid at one of the interfaces. We see ordering in the Al on both the B-terminated (0001) surface of ${\text{TiB}}_{2}$, and on ${\text{Al}}_{3}\text{Ti}(112)$, with the ordering able to be stronger on the ${\text{Al}}_{3}\text{Ti}$ substrate. However, we cannot draw strong conclusions as these simulations need more time to allow long-ranged fluctuations in the liquid Al to dampen out. The huge computational cost restricted the range and duration of simulations that was possible.