Is icosahedral short-range order presented in supercooled transition metals?

Abstract

Supercooled transition metals are characterized by the absence of long-range order and the presence of a specific short-range order in the arrangement of atoms. So, the presence of shoulders and broadenings at the second maximum in the experimentally measured quantity—in the static structure factor S(k)—is usually interpreted as a manifestation of the icosahedral (ideal or distorted) short-range order (ISRO—Icosahedral Short-Range Order). Icosahedral short-range order is a structure with fivefold symmetry in the arrangement of atoms, which can lead to the possibility of achieving deep supercooling. In this work, we study the local structural features of equilibrium and supercooled nickel melts under various cooling protocols ( K s−1) in order to clarify the mechanism of formation of the icosahedral short-range order in pure transition metals. Comprehensive studies of the properties of nickel melts were carried out using experiments on x-ray diffraction, large-scale molecular dynamics (MD) simulations with subsequent structural and cluster analysis. A good agreement was found between the results of x-ray diffraction data and the MD simulations results for an equilibrium nickel melt. It was found that the nickel melt is characterized by a short-range order, formed by fragments of icosahedra and distorted icosahedral clusters. It was revealed that the ‘liquid-crystal’ phase transition in nickel is accompanied by the transformation of distorted icosahedral clusters into clusters with the fcc/hcp-symmetry. It is shown that, in contrast to the Voronoi tessellation method, the cluster analysis method based on the () rotational invariants does not allow sufficiently correct identification of the distorted icosahedral short-range order in metal melts.