Possible origin of $\beta$-relaxation in amorphous metal alloys from atomic-mass differences of the constituents

TL;DR

This study combines theory, simulation, and experimental data to identify atomic-mass differences as a key factor in the origin of $eta$-relaxation in metallic glasses, suggesting a unified understanding of secondary relaxations.

Contribution

It introduces an atomic-scale theoretical approach linking mass asymmetry to $eta$-relaxation, supported by simulation and experimental evidence in metallic glasses.

Findings

Mass asymmetry causes relaxation time separation.

Lightest atoms control Johari-Goldstein relaxation.

Vibrational density of states influences mechanical response.

Abstract

We employ an atomic-scale theory within the framework of nonaffine lattice dynamics to uncover the origin of the Johari-Goldstein (JG) $\beta$-relaxation in metallic glasses (MGs). Combining simulation and experimental data with our theoretical approach, we reveal that the large mass asymmetry between the elements in a La$_{60}$Ni$_{15}$Al$_{25}$ MG leads to a clear separation in the respective relaxation time scales, giving strong evidence that JG relaxation is controlled by the lightest atomic species present. Moreover, we show that only qualitative features of the vibrational density of states determine the overall observed mechanical response of the glass, paving the way for a possible unified theory of secondary relaxations in glasses.