Invariance of the relation between α relaxation and β relaxation in metallic glasses to variations of pressure and temperature

Dielectric relaxation experiments performed at ambient and elevated pressures P in molecular, ionic, and polymeric glass formers have established that the relation of the Johari-Goldstein (JG) β-relaxation time τβ(T,P) to the α-relaxation time τα(T,P) is invariant to changes of T and P while the latter is kept constant. This property of the JG β relaxation is remarkable despite the fact that the invariance of the ratio τβ(T,P)/τα(T,P) from experiment is sometimes approximate because the β relaxation is composed of a distribution of processes, and the τβ(T,P) determined can be arbitrary. The property indicates the fundamental importance of the JG β relaxation and it cannot be neglected whenever the α relaxation is considered. Notwithstanding, the property has not been checked on whether it applies to metallic glasses. Conventional experiment techniques cannot fulfill the task, and the alternative is molecular dynamics simulations. In this paper we report the results of molecular dynamics simulations of dynamical mechanical spectroscopy performed on two very different metallic glasses, Zr50Cu50 and Ni80P20, at different pressures P. The JG β relaxation appears as an excess wing on the low-temperature side of the α loss peak at Tα,P in the isochronal loss modulus spectra EP″(T). On the other hand, the isochronal non-Gaussian parameter α2P(T) peaks at the temperature Tα2,P different from Tα,P of E″P(T). From the fact that Tα2,P is significantly lower than Tα,P, we identified the peak temperature Tα2,P of α2P(T) with the JG β relaxation, and hence the JG β relaxation is fully resolved by studying the isochronal non-Gaussian parameter α2P(T). After scaling temperature by Tα,P, the normalized EP″(T/Tα,P) and α2P(T/Tα,P) both show superposition of data taken at various pressures for all T/Tα,P covering the JG β relaxation and the α relaxation. Moreover the ratio Tα2,P/Tα,P is invariant to changes of T and P while τα(T,P) is maintained constant. Thus we have verified for two different metallic glasses, Zr50Cu50 and Ni80P20, that τα(T,P)/τβ(T,P) is invariant to changes of T and P at constant τα(T,P), as found in soft matter. © 2020 American Physical Society.