Controlling polymorph selection during nucleation by tuning the structure of metallic melts.

Abstract

Controlling the polymorphism of crystals is crucial to the design of novel metallic materials with specific properties; however, the atomistic mechanism underlying polymorph selection during crystallization remains unclear. In this work, molecular dynamics simulations combined with well-tempered metadynamics simulations are employed to explore the atomic mechanisms of polymorph selection during the nucleation process of FCC aluminum and copper. Simulation results suggest that the distinct nucleation pathways of both FCC metals originate from different free-energy surfaces of nucleation processes and diverse symmetries of nucleation precursors. The initially forming phase from undercooled melts is most likely to be the one that has the symmetry closest to the precursors. Besides, tiny seeds with diverse crystal symmetries could induce the formation of preordered precursors for nucleation around the seed, leading to the reduction of free-energy barrier and thus the promotion of nucleation. Controlling polymorph selection with tiny seeds is realized by tuning the symmetry of precursors. Our findings not only shed significant light on understanding polymorph selection, but also provide theoretical guidance for better controlling the nucleation pathway in practice.