Characteristics of Johari--Goldstein relaxations in bulk metallic glasses

The dynamics of Pd-based metallic glasses was studied by mechanical spectroscopy and modulated differential scanning calorimetry. The results show the change in composition has a significant effect on the $\alpha $ relaxation dynamics. All Pd-based metallic glasses have similar fragilities, 59 \textless $m$ \textless 67, and Kohlrausch stretched exponents, 0.59 \textless $\beta_{KWW}$ \textless 0.60. The values of $m$ and $\beta _{KWW}$ correlate well with the general relation proposed by B\"ohmer et al. for other glassy materials and the substitution of the Ni with Cu induced a large change in the time constant of the $\beta $ relaxation, $\tau _{\beta }$. The activation energy, U$_{\beta }$, of the $\beta $ relaxation was generally independent of chemical composition. In all cases, 25 \textless U$_{\beta }$/RT \textless 28, the range shows good agreement compared with the results of other glass formers. From the heat capacity and mechanical loss, the number of dynamically correlated units, \textit{Nc }were obtained; significantly larger \textit{Nc} values for these metallic glasses were observed compared with glassy materials.