Glass transition criterion and plastic deformation of glass

Abstract

We develop the notion that amorphous substances undergo reversible configurational structural changes accompanied by local expansion and compression (atom delocalization) near the glass transition temperature. They are similar in nature to configurational changes in the structure of glasses in the case of reversible frozen (plastic) deformation and its thermally stimulated relaxation. We assume that the glass-liquid transition is associated with the process of atom delocalization caused by bond breaking and formation of elementary excitations e.g. configurons. We discuss the possibility of detection of configuron formation and atom delocalization near glass transition based on temperature dependence of X-rays or neutron first sharp diffraction (pair distribution function) minimum.

Undercooled phase behind the glass phase with superheated medium-range order above glass transition temperature

“…This gap is considered as due to the bonds which are broken or built at the temperature T in the formation model of moving entities. The change of Arrhenius law appears in the derivative of this relation as already shown with other phenomenological laws for the viscosity [34,35,42]:…”

“…The theoretical Gibbs free energy of configurons is also used and successfully introduced in that of Phase 3 [36]. It is proportional to the temperature as predicted by Sanditov et al and determine the fraction of configurons involved in kinetic units having a coherence length ratio of /0 [42]. The threshold degree   ±  of configuron percolation involved in kinetic units associated with medium-range order in phase 3 is confirmed.…”

“…One method is based on previous studies of Rammal and Benoit and Henley devoted to spin glasses predicting that the relaxation time   exp(W/T) increases with W  ln where  is the coherence length of percolating clusters [40,41]. The energy W also contains the delocalization energy ee of atoms calculated by Sanditov, Ojovan and Darmaev which is related to the ultimate displacement of a bridging atom at Tg and above Tg [42]. This occurs when the energy of thermal vibrations of the lattice per atom becomes equal or lower than ee.…”

On Viscous Flow in Glass-Forming Organic Liquids

“…Mazurin noted that the widespread idea that the glass transition temperatures of all glasses corresponds to the temperature at which the glass viscosity is 10 12 Pa·s is not justified, although most of the glasses do obey this rule [ 44 ]. These data, along with data on the viscosities of other materials at different T g values [ 45 ], question the concept of the universality of defining T g values from the equation Log[η(T g )] = 12.…”

“…The transition of glass is most often considered to be a gradual change from ergodic to non-ergodic states without structural changes, where it is assumed that the glassy state of the amorphous materials is inherently non-ergodic [ 46 ]. The observed glass transition viscosity levels below 10 12 Pa·s for some materials, along with evident changes in the heat capacity and thermal expansion coefficient at T g , indicate that glass transition is a phase transformation [ 28 , 45 , 47 , 48 ]. This transformation is similar to a second-order phase transition in the Ehrenfest sense, with continuity of the volume and entropy and discontinuity of their derivatives, which are used in practice to identify T g .…”