Effect of physical aging on Johari-Goldstein relaxation in La-based bulk metallic glass.

The influence of physical aging on the ß relaxation in La60Ni15Al25 bulk metallic glass has been investigated by mechanical spectroscopy. The amplitude of the ß relaxation (ΔG″) decreases while its relaxation time (τ(ß)) increases during aging. We find that, as in organic glasses, the changes of ln (τ(ß)) and ln (ΔG(max) ) are linearly correlated with ln (τ(ß)) = b - a ln (G(max)″). This behavior is discussed in term of the asymmetric double-well potential (ADWP) model, with U and Δ the energies characterizing the ADWP. It is suggested that during aging the ratio U/Δ remains approximately constant, with a value close to the coefficient describing the linear correlation between ln (τ(ß)) and ln (G(max)″)(U/Δ ~ a). Moreover, the evolution versus aging time of ΔG(max) can be described by a simple stretched exponential equation giving values of τ(aging) consistent with tan(δ) measurements during aging. The very similar behavior of the ß relaxation during aging in metallic glasses and organic material strongly suggests a common nature for this relaxation.