Ab initio study of local structures during cooling of liquid Fe-C and Fe-Cr-C alloys

The cooling process of liquid Fe-C and Fe-Cr-C systems from 2100 K to 1100 K was studied by ab initio molecular dynamics simulations. The evolution of local atomic structure was statistically analyzed by the pair correlation
function, structure factor, coordination number, bond angle distribution, and the Honeycutt-Anderson (HA) index as well as the Voronoi index. We found that C atoms tend to be surrounded by metal atoms and the strong
chemical bonds of Fe-C and Cr-C make the C-centered clusters more stable than Fe-centered clusters. The two alloy systems are dominated by icosahedral order in the liquid state but show different evolutionary trends of
atomic structures during the cooling process. The Fe-C system undergoes a bcc-like crystalline phase transition at 1500 K, while the Fe-Cr-C system can be supercooled to an amorphous state with decreasing disclination density in the polytetrahedral packing. The analyses of clusters indicated that doping Cr can introduce new local topological and chemical orderings and thus renders a better glass-forming ability. Furthermore, to understand the mechanism of carbides precipitation from undercooled liquid of high-carbon chromium bearing steel, we simulated the local structures of several typical Fe and Cr carbides. We identified a close connection in the shortrange order between the carbides and metallic liquids, which provides a theoretical basis for the formation of primary carbides.