Interstitial clustering in metallic systems as a source for the formation of the icosahedral matrix and defects in the glassy state

Abstract

The paper presents molecular dynamics and -statics simulations of a prototypical mono-atomic metallic system (aluminum) and its defects in the crystalline and glassy states. It is shown that there is a thermodynamic driving force for the association of dumbbell interstitials in the crystalline lattice into clusters consisting of different amounts of defects. Clusters containing seven interstitials constitute perfect icosahedra. Within the general framework of the interstitialcy theory, melting of simple metallic crystals is intrinsically related to a rapid increase of the concentration of dumbbell interstitials, which remain identifiable structural units in the liquid state. Then, the glass produced by rapid melt quenching contains interstitial-type defects. The idea of the present work is to argue that the major structural feature of many metallic glasses—icosahedral ordering—originates from the clustering of interstitial-type defects frozen-in upon melt quenching. Separate defects and their small clusters represent the defect part of the glassy structure.

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

“…The IT was originally derived as an approach providing a physical connection between the liquid (both equilibrium and metastable), glassy and crystalline states. The theory argues that melting of simple crystals is related to the rapid multiplication of interstitial defects in the dumbbell (split) configuration, in line with recent experimental observations [52,53]. Computer simulation shows that these defects remain identifiable structural units in the liquid state [54].…”

“…These properties lead to the conclusion that the excess internal energy (enthalpy) of glass with respect to the counterpart crystal is determined mostly by the elastic energy of the defect system [58,76] (see also equation ( 22) below). Besides that, the defects under consideration should also reveal large formation entropy, which is intrinsically related to the shear softening, as well as to the low-frequency modes in the vibrational spectra of the atoms belonging to the defects [50,53]. Provided that these properties are present, the defects can be termed as interstitial-type defects.…”

A simple kinetic parameter indicating the origin of the relaxations induced by point(-like) defects in metallic crystals and glasses

“…This result suggests that the relaxation induced by interstitialtype defects should be considered as more preferable in glassy Al and FeNiCrCoCu. A decrease of |K| upon amorphization of Al can be understood in the view of the work reference [28], which argues the presence of icosahedral clustering of interstitial-type defects frozen-in upon melt quenching of Al. This clustering results in a decrease of their shear susceptibility and, as a result, |K| should decrease as well (see equation ( 5)).…”

“…The physical origin of this inequality is quite clear. While interstitials and vacancies provide a similar increase of the volume [28], the diaelastic effect produced by interstitials is much larger. The physical origin of this effect was sketched above.…”

“…To do this, one needs the information on the shear susceptibilities β and relaxation volumes r for interstitials and vacancies in both materials under investigation. For aluminum, the relaxation volumes were derived earlier, r i = 1.76 and r v = −0.49 [28], the shear susceptibilities in the single-crystalline state were calculated in reference [17], β i = 22.9 and β v = 2.2. For polycrystalline Al, we performed Reuss-Voigt averaging of the shear moduli as described above and calculated the shear susceptibilities in the polycrystalline state, β i (poly) = 19.3 and β v (poly) = 2.8.…”