Determination of the glass transition temperature of metallic glasses by low-frequency internal friction measurements

Abstract

Based on the “isoviscous” definition of the glass transition (η(Tg) = 1012 Ns/m2), it is shown that the glass transition temperature Tg can be determined by low-frequency internal friction measurements. By comparing the anelastic and viscoelastic contributions to internal friction, an upper frequency limit for this kind of Tg determination is derived; for metallic glasses, a rough estimate of 0.2 Hz is given. In corresponding experiments at 0.1 Hz on amorphous Pd40Ni40P20, a cross-over of the internal friction Q−1 vs temperature is observed for different heating rates Ṫ: whereas below the glass transition range Q−1 is lower for lower Ṫ due to structural relaxation, the opposite is true for higher temperatures due to the shift in Tg with Ṫ. This shift is found to be linear in log Ṫ both for Tg values obtained by internal friction and by DSC. However, a single straight line of Tg (log Ṫ) for both methods is obtained from 0.03 to 100 K/min only if in DSC Tg is defined at the end of the corresponding increase in specific heat. These findings are discussed with respect to the compatibility of the different definitions of Tg. There seems to be some independent evidence that the results obtained on Pd40Ni40P20 might be of more general importance.