THERMODYNAMIC PROPERTIES OF AMORPHOUS SOLIDS - GLASS-FORMATION AND GLASS-TRANSITION

Glasses are generally produced from the highly undercooled liquid stat e by rapid quenching methods or quasi-statically at slow cooling by th e effective control of potent heterogeneous nucleation sites. For meta llic systems the latter method recently has led to the development of bulk metallic grass with a complex multicomponent chemistry and advanc ed engineering properties. Besides the formation of ''deep eutectics'' due to the strongly varying atomic size of the constituents an enhanc ed oxygen solubility is necessary in order to control heterogeneous nu cleation and produce a bulk metallic glass. As long as crystallization can be avoided the relevant thermodynamic properties of the metastabl e glassy and undercooled liquid phases can be measured below and above the glass transition temperature, respectively. The obtained data giv e new insight into the nature of the glass transition suggesting that it is not a phase transition in the classical sense but kinetic freezi ng triggered by an underlying entropic instability. However, different types of glasses distinguished as ''fragile'' and ''strong'' exhibit different densities of configurational states. Therefore, the thermody namic and transport properties become dependent on the time scales of their exploration. Furthermore, glass formation can be achieved by sol id-state-processing without passing through the liquid state. This cry stal-to-glass transition is observed under a number of different exper imental conditions when a sufficiently high energy level is reached an d 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 ca ses the stability of the undercooled liquid and the non-equilibrium so lid against glass formation is limited by an isentropic condition. Con ceptually, the formation of the glassy state from the liquid and the s olid can then be understood within a thermodynamic framework under app ropriate kinetic constraints resulting in a universal phase diagram wi th a pseudocritical point.