Competing pathways and polymorph selection during the crystallization of molten lead

The competitive growth of polymorphs during solidification is a fundamental problem governing microstructural evolution and the resulting properties of metals. In this study, we investigate nucleation, competitive growth, and transformation mechanisms among the BCC, FCC, and HCP phases of lead melts using molecular dynamics (MD) simulations combined with well-tempered metadynamics (WTMetaD). Two dominant thermodynamic pathways are unveiled: a one-step liquid-to-FCC transition and a two-step liquid-to-BCC-to-FCC transition. In the two-step pathway, the FCC phase exhibits the fastest crystal growth rate, followed by the BCC phase and then the HCP phase. A substantial population of BCC-like precursors preferentially accumulates at the solid-liquid interface, and the population of BCC-like precursors reaches a maximum during the mid-growth stage. These characteristics of the BCC-like precursors make the BCC phase the second most abundant, surpassed only by the thermodynamically most stable FCC phase during the early solidification process. The thermodynamic analysis indicates that the complex free energy surface provides multiple pathways for solidification, dominating the competition of different polymorphs. These findings provide atomistic insights into the competitive mechanisms among polymorphs during the solidification of FCC metallic systems, and offer a theoretical basis for understanding and controlling polymorph selection during solidification.