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

Abstract

A review of the new approach to the understanding of the structural relaxation of metallic glasses based on the Interstitialcy theory has been presented. The key hypothesis of this theory proposed by Granato consists of the statement that the thermodynamic properties of crystalline, liquid and glassy states are closely related to the interstitial defects in the dumbbell (split) configuration, called also interstitialcies. It has been argued that structural relaxation of metallic glasses takes place through a change of the concentration of interstitialcy defects frozen-in from the melt upon glass production. Because of a strong interstitialcy-induced shear softening, the defect concentration can be precisely monitored by measurements of the unrelaxed shear modulus. Depending on the relation between the current interstitialcy concentration c and interstitialcy concentration in the metastable equilibrium, different types of structural relaxation (decreasing or increasing c) can be observed. It has been shown that this approach leads to a correct description of the relaxation kinetics at different testing conditions, heat effects occurring upon annealing, shear softening and a number of other structural relaxation-induced phenomena in metallic glasses. An intrinsic relation of these phenomena with the anharmonicity of the interatomic interaction has been outlined. A generalized form of the interstitialcy approach has been reviewed.

Theoretical analysis of the interconnection between the shear elasticity and heat effects in metallic glasses

“…(3) for an analysis of structural relaxation of glass gives a number of successful interpretations of structural relaxation-induced phenomena in metallic glasses (see Ref. [5] for a review and related citations). Structural relaxation within the framework of this approach results in a change of the interstitialcy defect concentration, which through Eq.…”

“…While irreversible structural relaxation below the glass transition temperature T g leads to the heat release, reversible structural relaxation near and above T g results in either heat absorption (upon heating) or heat release (upon cooling) and crystallization is always accompanied by the heat release. Independent of the temperature range, heat release always leads to an increase of the shear modulus while heat absorption always results in its decrease [4,5]. In the literature, heat effects and shear modulus changes taking place within the glassy state are discussed using different approaches [1][2][3].…”

“…Structural relaxation and crystallization are then interpreted as a change of the concentration of interstitialcy defects leading to an increase/decrease of the shear modulus and related exothermal/endothermal heat effects depending on temperature and preliminary heat treatment [4,5]. This conceptual framework offers qualitative and quantitative explanations for quite a few effects associated with structural relaxation and crystallization of metallic glasses (for a review, see Ref.…”

“…This conceptual framework offers qualitative and quantitative explanations for quite a few effects associated with structural relaxation and crystallization of metallic glasses (for a review, see Ref. [5]). In particular, it becomes possible to relate the heat effects occurring upon structural relaxation and crystallization of metallic glass with the relaxation of the shear modulus.…”

Metallic Glasses

“…The atomic structure frozen-in during the ultra-rapid cooling processes needed to generate metallic glasses is usually not stable at room temperature. V. A. Khonik reviews how the presence of interstitialcy defects and the change in their concentration can induce structural relaxation when metallic glasses are heat treated to temperatures below or around the glass transition [15]. Such structural relaxation is important since it can induce changes in many physical properties of the glasses, particularly in the mechanical behavior (elasticity, anelasticity, viscoelasticity, etc.…”