Study of crystallization pathway and heterogeneous dynamics in supercooled liquid and amorphous iron

Abstract

Supercooled liquid and amorphous iron (Fe) was investigated by means of molecular dynamics (MD) simulation. The crystallization was analyzed through pair radial distribution function, bond angle distribution, coordination number and transition to different atom types. Amorphous Fe possesses a large number of icosahedron type atoms which play a role in preventing of transformation into the bcc phase. The structure of amorphous Fe slightly changes during the relaxation time. The crystallization occurred when Fe was annealed at 950 K for 1.6 × 10⁷ steps. It is found that transitions to bcc-type do not happen arbitrarily at any location in the system, but instead, they are concentrated in a non-equilibrium region. Moreover, the crystallization pathway comprises intermediate states between amorphous and crystalline ones. At the early stage, a large cluster of crystal atom formed is located in system. Then, this cluster grows up rapidly. At the final stage, the cluster of crystal atom is located in a well-equilibrium region covered a major part of the system. We found that unlike amorphous Fe, the structure of crystalline Fe is strongly heterogeneous and consists of separate regions with different local microstructure. Heterogeneous dynamics (HD) in the supercooled liquid and amorphous Fe was also examined through the distribution of mobile and immobile atoms. It is found that there is a connection between local structure, crystallization pathway and HD in the system. Mobile and immobile atoms have a tendency to segregate into separate regions.

Graphical abstract