Different metastable equilibrium states in metallic glasses occurring far below and near the glass transition

Abstract Precise measurements of the high-frequency shear modulus of two bulk Zr- and Pd-based glasses in the initial and strongly preannealed states in a wide temperature range are performed. Isothermal structural relaxation of preannealed samples leads to either decrease or increase of the shear modulus. At temperatures below particular temperature Tpc, one observes cross-over relaxation (decrease→increase) of the shear modulus. The minimal shear modulus attained upon relaxation corresponds to the quasi-metastable equilibrium, which depends both on temperature and thermal prehistory. At higher temperatures T > Tpc, the relaxation always leads to shear modulus decrease towards the “true” metastable equilibrium, which depends only on temperature. The temperature Tpc for both glasses is by ≈ 25 K smaller than the calorimetric glass transition temperature. It is shown that this temperature constitutes the low temperature limit of the linear extrapolation of the shear modulus from the supercooled liquid state. By definition, the temperature Tpc corresponds to the pseudocritical temperature introduced by Kauzmann in 1948 (W. Kauzmann, Chem. Rev. 43 (1948) 219). A detailed discussion of the microscopic relaxation mechanism below and above Tpc based on the Interstitialcy theory is presented.