Thermophysical and structural properties of the equilibrium and undercooled melt of bulk metallic glasses investigated by electrostatic levitation

摘要

Upon quenching a liquid into a glass, its viscosity increases drastically. A "strong" liquid is characterised by an Arrhenius-like temperature dependency of the viscosity with a temperature independent activation energy. For a "fragile" liquid, the activation energy increases monotonically. In many bulk metallic glasses (BMGs) both strong and fragile behaviour can be observed at different temperatures. However, most of these strong-to-fragile crossovers take place in the undercooled state, where experimental data is rare because this region is masked by crystallisation. This fact and the chemical reactivity of metallic melts make investigations of an underlying transition and of accurate melt viscosities experimentally challenging.In this work, electrostatic levitation was used for investigations of Zr-based BMGs, providing precise high temperature data over a broad temperature range and enabling access to the deeply undercooled melt. With this new data, we confirmed that the investigated BMGs exhibit different fragilities.Studies of the undercooled melt revealed indications for a transition in specific heat capacity and in local structure changes at the same temperature.These indications are associated with a structural transition and are attributed to be the origin of the difference in fragility. The underlying mechanism may be due to a liquid-liquid transition. Whether there are chemical composition changes in the different liquid phases is subject to further studies.