Invariance of the relation between a relaxation and beta relaxation in metallic glasses to variations of pressure and temperature
B Wang and ZY Zhou and PF Guan and HB Yu and WH Wang and KL Ngai, PHYSICAL REVIEW B, 102, 094205 (2020).
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) beta- relaxation time tp(T, P) to the alpha-relaxation time tau(alpha)(T, P) is invariant to changes of T and P while the latter is kept constant. This property of the JG f relaxation is remarkable despite the fact that the invariance of the ratio tau(beta)(T, P)/tau(alpha)(T, P) from experiment is sometimes approximate because the beta relaxation is composed of a distribution of processes, and the tau(beta)(T, P) determined can be arbitrary. The property indicates the fundamental importance of the JG beta relaxation and it cannot be neglected whenever the alpha 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 beta relaxation appears as an excess wing on the low-temperature side of the alpha loss peak at T-alpha,T-P in the isochronal loss modulus spectra Ep ''(T). On the other hand, the isochronal non-Gaussian parameter alpha 2(P)(T) peaks at the temperature Tap different from T-alpha,T-P of E '' p(T). From the fact that T-alpha 2,T-P is significantly lower than T-alpha,T-P we identified the peak temperature T-alpha,T-P of alpha 2(P) (T) with the JG beta relaxation, and hence the JG beta relaxation is fully resolved by studying the isochronal non-Gaussian parameter alpha 2(P)(T). After scaling temperature by T-alpha,T-P, the normalized Ep ''(T/T-alpha,T-P) and alpha P-2(T/T-alpha,T-P) both show superposition of data taken at various pressures for all T 'Tex covering the JG beta relaxation and the a relaxation. Moreover the ratio T-alpha 2,T-P/T-alpha,T-P is invariant to changes of T and P while tau(alpha)(T, P) is maintained constant. Thus we have verified for two different metallic glasses, Zr50Cu50 and Ni80P20, that tau(alpha)(T, P)/tau(beta)(T, P) is invariant to changes of T and P at constant tau(alpha)(T, P), as found in soft matter.
Return to Publications page