The nucleation process and the roles of structure and density fluctuations in supercooled liquid Fe

Abstract

We observed homogeneous nucleation process of supercooled liquid Fe by molecular dynamics simulations. Using bond-orientational order parameters together with Voronoi polyhedron method, we characterized local structure, calculated the volume of Voronoi polyhedra of atoms and identified the structure and density fluctuations. We monitored the formation of nucleus and analyzed its inner structure. The birth and growth of the pre-nucleus and nucleus are accompanied with aggregating and disaggregating processes in the time scale of femtosecond. Only the initial solid-like clusters (ISLC), ranging from 1 to 7 atoms, pop up directly from liquid. The relation between the logarithm of number of clusters and the cluster size was found to be linear for ISLCs and was observed to be parabolic for all solid-like clusters (SLC) due to aggregating and disaggregating effects. The nucleus and pre-nuclei mainly consist of body centered cubic (BCC) and hexagonal close packed atoms, while the BCC atoms tend to be located at the surface. Medium-range structure fluctuations induce the birth of ISLCs, benefit the aggregation of embryos and remarkably promote the nucleation. But density fluctuations contribute little to nucleation. The lifetime of most icosahedral-like atoms (ICO) is shorter than 0.7 ps. No obvious relationship was found between structure/density fluctuations and the appearance of ICO atoms.

Comparative study of crystallization process in metallic melts usingab initiomolecular dynamics simulations

“…local density) and orientational (bond orientational ordering) order. Recently, the role of density fluctuation and development of bond orientational ordering on the crystallization process is investigated for bodycenteredcubic (bcc) refractory metals Nb [4] and V [5] using ab initio molecular dynamics, and Fe [6] using classical molecular dynamics; and concluded that the early stage of nucleation is mainly driven by the fluctuation of bond orientational ordering rather than density fluctuation which is consistent with the proposed nucleation scenario by Tanaka and coworkers for colloidal systems [7][8][9].…”

Identification of critical nuclei in the rapid solidification via configuration heredity

“…One can see that an initial nucleation occurs at T n = 0.38T cal m and the maximum I max related to a critical temperature of nucleation [24] emerges at T n = 0.45T cal m , but both of which are far away from the thermodynamic under-cooling T m = 0.42T cal m . These results clearly indicate nucleation as a dynamic behavior cannot be directly inferred from an abrupt change in thermodynamic parameters such as E (T), V (T) and q (T) [56]. Nevertheless, from a statistical point of view, a good agreement between thermodynamic events and dynamic behaviors can be observed.…”

Polymorphism in glassy silicon: Inherited from liquid-liquid phase transition in supercooled liquid

“…Voronoi polyhedral analyses is a geometrical spatial method and, can identify the unknown structures and give some statistics information about individual atoms such as the atomic coordination number and 3-D space shared by the central atom and all its neighbors. The Voronoi polyhedron method has been proven to be effective for the analyses of local atomic environment 31 and the volume of cluster 32 in liquids and glasses. The Voronoi index is expressed as < n 1 ,n 2 ,n 3 ,n 4 >, where n i denotes the number of i -edged faces of the Voronoi polyhedrons 33 34 35 .…”