A molecular dynamics study of heterogeneous nucleation at grain boundaries during solid-state phase transformations

The study of solid-state nucleation through experiment is often limited by its tiny length range and short time scale. In this study, molecular dynamics (MD) simulations in a quasi 2D geometry are used to study the process of a BCC ferrite phase in pure Fe nucleating at a grain boundary (GB) in an FCC austenite polycrystalline system. In the MD simulations the critical nucleus can be identified, the bulk free energy difference between FCC and BCC is known for the Fe interatomic potential used and all relevant interface and GB energies are computed using a Gibbs–Cahn formulation. For nucleation events that exhibited low energy facets completely contained within the parent FCC phase, the results agreed well with predictions from classical nucleation theory (CNT) in terms of both the size and shape of the critical nucleus. For systems where the emerging nucleus contains facets that cross the GB plane the agreement with classical theory is less convincing and the observed nucleus does not exhibit parallel facets as predicted from the Winterbottom construction. The latter nucleation case involves a so-called pucker mechanism of the FCC grain boundary to accommodate the emerging nucleus and the effect of GB puckering on the incubation time is discussed.