Effect of cooling rate on the formation pathway of medium-range FCC nanocrystals: the role of MRCO in structural evolution

During the undercooled solidification of monodisperse metallic systems, although the impact of cooling rate on the evolution of local structures has been widely observed, its underlying physical mechanism remains insufficiently understood. In this study, we employ molecular dynamics simulations combined with free energy analysis to investigate the nucleation pathways of medium-range FCC nanocrystals under different cooling rates, focusing on the role of medium-range crystalline order (MRCO) in the structural evolution process. MRCO structures are formed under all cooling conditions, yet their conversion into FCC-like nanoclusters becomes markedly more efficient as the cooling rate decreases. Under slow cooling, frequent structural transitions between MRCO and other locally ordered structures (<i>e.g.</i>, HCP and BCC) occur, significantly increasing the free energy barrier for forming nanocrystals of the same scale. In contrast, fast cooling tends to freeze MRCO clusters before full transformation. This study provides direct evidence supporting the non-classical multi-step nucleation mechanism and, based on Tanaka's two-order-parameter model, offers a microscopic foundation for microstructural control in metallic glasses and a unified theoretical framework for understanding the structural evolution of glasses and crystals.