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 alpha relaxation determined at different combinations of temperature T and pressure P while maintaining its relaxation time tau(alpha)(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) beta 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 , the non-Gaussian parameter alpha(2)(t) and the self-intermediate scattering function F-s(q,t) at various combinations of T and P were obtained over broad time range covering the three processes. Isochronal superpositioning of , alpha(2)(t), and F-s(q, t) was observed over the entire time range, verifying that the property holds not only for the alpha relaxation but also for the caged dynamics and the JG beta relaxation. Moreover, we successfully performed density rho scaling of the time tau(alpha 2,max)(T, P) at the peak of alpha(2)(t) and the diffusion coefficient D(T, P) to show both are functions of rho(gamma)/T with the same gamma. It follows that the JG beta relaxation time tau(beta)(T, P) is also a function of rho(gamma)/T since tau alpha(2,max)(T, P) corresponds to tau(beta)(T, P).

Automated summary

This study investigated the superposition of frequency dispersion of structural alpha relaxation at different temperature and pressure combinations, as well as the time dependence of faster processes like caged molecule dynamics and JG beta relaxation. Through molecular dynamics simulations, it was confirmed that this property holds for not only alpha relaxation but also caged dynamics and JG beta relaxation.