Relaxation of Bulk Metallic Glasses Studied by Mechanical Spectroscopy

The relaxational dynamics in metallic glasses (MGs) is investigated by using mechanical spectroscopy. The spectra show that in MGs there are two relaxations: (i) the alpha relaxation, linked to the glass transition, as observed in other classes of amorphous materials; and (ii) the beta relaxation, well observed below the glass transition, with an intensity strongly dependent on the MG composition, the nature of which has been linked to the local microstructure of MGs. For the investigated MGs we find that the intensity and relaxation time of the beta relaxation depends, in a reproducible fashion, on the thermal history of the samples. During aging experiments, the intensity decreases (as well as the tau(beta)) with a time dependence described well by a stretched exponential, with an exponent beta(aging) independent of the driving frequency. Moreover, we find that the activation energy U-beta and the peak temperature T-beta p of the beta relaxation follow the approximate relationship: U-beta approximate to 31.5RT(beta p) (for driving frequency 1 Hz), indicating that the high temperature limit of the peak frequency is approximately the same for all the MGs investigated. Finally, the frequency separation of the alpha and beta processes in the mechanical loss spectra for La-and Pd-based metallic glasses is tested against the prediction of the Coupling Model.