Isochronal superpositioning of the caged dynamics, the α, and the Johari-Goldstein β relaxations in metallic glasses.

The superposition of the frequency dispersions of the structural α relaxation determined at different combinations of temperature T and pressure P while maintaining its relaxation time τ(T, P) constant (i.e., isochronal superpositioning) has been well established in molecular and polymeric glass-formers. Not known is whether the frequency dispersion or time dependence of the faster processes including the caged molecule dynamics and the Johari-Goldstein (JG) β relaxation possesses the same property. Experimental investigation of this issue is hindered by the lack of an instrument that can cover all three processes. Herein, we report the results from the study of the problem utilizing molecular dynamics simulations of two different glass-forming metallic alloys. The mean square displacement 〈Δrt〉, the non-Gaussian parameter αt, and the self-intermediate scattering function Fq,t at various combinations of T and P were obtained over broad time range covering the three processes. Isochronal superpositioning of 〈Δrt〉, αt, and Fq,t was observed over the entire time range, verifying that the property holds not only for the α relaxation but also for the caged dynamics and the JG β relaxation. Moreover, we successfully performed density ρ scaling of the time τT,P at the peak of αt and the diffusion coefficient D(T, P) to show both are functions of ρ/T with the same γ. It follows that the JG β relaxation time τ(T, P) is also a function of ρ/T since τT,P corresponds to τ(T, P).