Nanostructure of Metallic Amorphous Alloys —Characterization of Medium-Range Structure and Dynamics by Pulsed Neutron Scattering—

Abstract

Atoms in metallic amorphous alloys are preferably combined into a configuration of minimizing the local energy in the short-range structure, while the total energy minimum is realized in the periodic long-range structure of crystalline metals. The topological mechanism of crystal-to-amorphous solid state structure transition is demonstrated as a conversion of octahedral structure units into tetrahedral structure units. However, the short-range structure of metallic amorphous alloys often shows similarity to that of their crystalline counterparts. Survival of the local coordination in metallic amorphous alloys is also confirmed by characterizing the dynamic structure. The low-energy excitation often called the Boson peak in inelastic neutron scattering is observed in an energy range of I to 3 meV for metallic amorphous alloys, corresponding to an excess specific heat in addition to the Debye-type harmonic vibration in a temperature range of 10 to 30 K. The medium-range structure of metallic amorphous alloys has a quite unique nature in contrast to the crystalline alloys. In case of metal-metalloid (Pd-Si and Pd-Ge) amorphous alloys it is discussed that the low-energy excitation is correlated to a locally collective atomic motion of hinge-like movement between trigonal prismatic structure units.