On the nature of heat effects and shear modulus softening in metallic glasses: A generalized approach

Abstract

The paper presents a new approach to the nature of heat effects and shear modulus softening in metallic glasses. The approach is based on the assumption that the glass contains quenched-in “defects”—elastic dipoles. Using the nonlinear elastic representation of the internal energy of glass with quenched-in elastic dipoles, we derive a simple analytical law, which connects the heat flow and temperature derivative of the shear modulus. Specially performed experiments confirmed the validity of this law. The exothermal and endothermal heat processes in glass reveal through the relaxation of the shear modulus confirming it as a key parameter for the understanding the relaxation processes in glasses.

Understanding of the Structural Relaxation of Metallic Glasses within the Framework of the Interstitialcy Theory

“…In the relaxed state, D is zero up to T ≈ 530 K again, indicating the absence of structural relaxation. At higher temperatures, D becomes positive and rapidly increases with temperature, suggesting rapid defect multiplication near T g , which is manifested by strong shear softening and heat absorption [77]. Similar results were obtained on glassy P d 40 Cu 30 N i 10 P 20 [73].…”

“…One can point out a good agreement between calculated and experimental heat release below T g and heat absorption above T g . Similar agreement was found for other Pd-and Zr-based metallic glasses [77][78][79]. Below T g , a decrease of the interstitialcy defect concentration upon heating leads to the heat release and related increase of the shear modulus.…”

“…Since the density is increasing upon structural relaxation of initial MGs below T g , it is widely believed that elementary structural relaxation events take place in the regions of smaller local density, i.e., in the regions containing some excess "free volume" [4,9,12,13]. In spite of the fact that the "free volume" has no clear theoretical definition, as repeatedly mentioned in the literature (e.g., [17,85]), and the application of this concept to the interpretation of experimental data sometimes leads to evident inconsistencies [16,77], the free volume-based notions still remain quite popular [4,11,15,86]. In this context, it is important to estimate what kind of volume effects could be associated with interstitialcy defects in glass and to compare these with volume effects attributed to the free volume.…”

“…This approach was developed in [77,91]. It was shown that: (i) these defects lead to the elastic softening of glass with respect to the reference crystal; and (ii) their stored elastic energy, which is released as heat upon structural relaxation below and above T g , as well as upon crystallization of glass, closely corresponds to the heat effects observed experimentally.…”

“…These predictions were tested in [73,76]. (10)) and the related increase of the shear modulus and heat release [77]. Finally, at T > 670 K, D becomes positive and rapidly increases with temperature due to the fast defect multiplication ( dc dT > 0), which is manifested in strong shear softening and heat absorption upon approaching the glass transition [77].…”