Thermodynamic and kinetic properties of metallic glasses during ultrafast heating

Abstract

When a system is cooled from the high temperature liquid state below its liquidus temperature, the dynamics of the system start to deviate from the ones in the equilibrium high temperature liquid. There are a number of kinetic decoupling phenomena observed, such as the separation of the primary alpha and the secondary beta relaxation and the decoupling of the viscosity from the diffusion. A possible explanation for this behavior is a first order liquid-liquid phase transition which overlaps with a fragility transition. From an experimental point of view the supercooled liquid of metallic glass formers is difficult to access because of their high tendency to crystallize. In this dissertation, the liquid-liquid phase transition of metallic glasses was investigated using an ultrafast heating technique via a capacitor discharge. For this purpose, the capacitor discharge technique was constructed in order to achieve heating rates in the order of 10^6 and 10^7 K/s. Additionally, a novel chopper technique was developed which is capable of enhancing the temporal resolution of any two-dimensional X-ray detector by a factor of thirteen. Thus the chopper method allows structural studies with a high temporal resolution. Using these techniques it could be shown that there is pronounced liquid-liquid phase transition in the metallic glass former Zr65Cu27.5Al7.5 when system transforms from the supercooled liquid into the high temperature liquid. Additionally, by varying the heating rate by eight orders of magnitude in the range from 10^-1 K/s until 10^6 K/s, it could be shown that the shift of the glass transition temperature with heating rate reveals a strong to fragile transition.