Thermodynamic Properties of Amorphous Solids —Glass Formation and Glass Transition— (<I>Overview</I>)

Glasses are generally produced from the highly undercooled liquid state by rapid quenching methods or quasi-statically at slow cooling by the effective control of potent heterogeneous nucleation sites. For metallic systems the latter method recently has led to the development of bulk metallic glass with a complex multicomponent chemistry and advanced engineering properties. Besides the formation of "deep eutectics" due to the strongly varying atomic size of the constituents an enhanced oxygen solubility is necessary in order to control heterogeneous nucleation and produce a bulk metallic glass.As long as crystallization can be avoided the relevant thermodynamic properties of the metastable glassy and undercooled liquid phases can be measured below and above the glass transition temperature, respectively. The obtained data give new insight into the nature of the glass transition suggesting that it is not a phase transition in the classical sense but kinetic freezing triggered by an underlying entropic instability. However, different types of glasses distinguished as "fragile" and "strong" exhibit different densities of configurational states. Therefore, the thermodynamic and transport properties become dependent on the time scales of their exploration.Furthermore, glass formation can be achieved by solid-state-processing without passing through the liquid state. This crystal-to-glass transition is observed under a number of different experimental conditions when a sufficiently high energy level is reached and kinetic conditions prevent the establishment of equilibrium. In some instances it can be shown that basically the same glassy state can be reached approaching it from the liquid or the solid state. In both cases the stability of the undercooled liquid and the non-equilibrium solid against glass formation is limited by an isentropic condition. Conceptually, the formation of the glassy state from the liquid and the solid can then be understood within a thermodynamic framework under appropriate kinetic constraints resulting in a universal phase diagram with a pseudo-critical point.