Density power law and structures of metallic glasses

Abstract

The existence of a universal power law relating the position of the first sharp diffraction peak (q, FSDP) to the density (ρ or the volume V) with a constant exponent <3 has been debated in the last decade. A constant dimensionality is important because it reflects the fractal topology of the glass structures. In this study, the validity of the Ehrenfest equation applied to multi-component metallic glasses is examined using first-principles molecular dynamics calculations. The results show that the Ehrenfest coefficient depends on the local structures of the glasses and is not a constant for all glasses. Moreover, since the diffraction pattern is determined by the scattering between atom pairs, in a multi-component glass, the X-ray diffraction FSDP is only sensitive to the heavy atoms, and the observed P-q relationship does not necessary correspond to the PV equation of state of the bulk material and is not always a suitable indicator for monitoring structural phase transitions or volume changes. On the other hand, for suitable systems, neutron diffraction is a reliable method to determine the structural features of both heavy and light atoms. In this study, the simulated neutron diffraction patterns of Ca72·7Al27.3 metallic glasses show a clear splitting of the FSDP at the pressure where the pressure-induced polyamorphism transition occurs. From the presented results, there is no justification for expecting the existence of a universal power scaling law with a constant exponent for all glasses.