Thermodynamic approach for the understanding of the kinetics of heat effects induced by structural relaxation of metallic glasses.

We present a novel approach to the understanding of heat effects induced by structural relaxation of metallic glasses. The key idea consists in the application of a general thermodynamic equation for the entropy change due to the evolution of a non-equilibrium part of a complex system. This non-equilibrium part is considered as a defect subsystem of glass and its evolution is governed by local thermoactivated rearrangements with a Gibbs free energy barrier proportional to the high-frequency shear modulus. The only assumption on the nature of the defects is that they should provide a reduction of the shear modulus-a diaelastic effect. This approach allows to determine glass entropy change upon relaxation. On this basis, the kinetics of the heat effects controlled by defect-induced structural relaxation is calculated. A very good agreement between the calculation and specially performed calorimetric and shear modulus measurements on three metallic glasses is found.

Relation of the fragility and heat capacity jump in the supercooled liquid region with the shear modulus relaxation in metallic glasses.

Fragility constitutes a major parameter of supercooled liquids. The phenomenological definition of this quantity is related to the rate of a change of the shear viscosityηat the glass transition temperature. Although a large number of correlations of the fragility with different properties of metallic glasses were reported, an adequate understanding of its physical nature is still lacking. Attempting to uncover this nature, we performed the calculation of the fragility within the framework of the interstitialcy theory (IT) combined with the elastic shoving model. We derived an analytical expression for the fragility, which shows its relation with the high-frequency shear modulusGin the supercooled liquid state. To verify this result, specially designed measurements ofηandGwere performed on seven Zr-, Cu- and Pd-based metallic glasses. It was found that the fragility calculated from shear modulus relaxation data is in excellent agreement with the fragility derived directly from shear viscosity measurements. We also calculated the heat capacity jump ΔCsqlat the glass transition and showed that it is related to the fragility and, consequently, to shear modulus relaxation. The ΔCsql-value thus derived is in a good agreement with experimental data. It is concluded that the fragility and heat capacity jump in the supercooled liquid state can be determined by the evolution of the system of interstitial-type defects frozen-in from the melt upon glass production, as suggested by the IT. This connection is mediated by the high-frequency shear modulus.