Due to progress in materials physics and materials science in recent years more and more metallic alloys can now be produced with amorphous atomic structures. The formation of the so-called bulk metallic glasses upon solidification of a supercooled melt is characterized by a strong increase in the viscosity within a very narrow temperature range in the vicinity of the glass transition temperature, which characterizes this transition. Good progress has been made in characterizing the atomic structure of the amorphous solids and also the glass transition is well understood from a thermodynamic point of view. However, a profound understanding of the atomistic processes governing this glass transition, the ductility and related the role of the free volume is still lacking. The present research project will investigate the kinetics of free volume below and in the vicinity of the glass transition of bulk metallic glasses with focus on the formation of free-volume as structural defect. For this study in-situ, time-dependent length change measurements during equilibration after fast temperature changes close below the glass temperature will be employed. The experimental method which is specific and sensitive for the measurement of free volume in solids will mainly be applied to the family of Zr/Cu based bulk metallic glasses.
The specific points addressed in this project are:
Fraction of the (reversibly) formed free volume and its temperature dependence.
Comprehensive characterization of the kinetics of the formation/disappearance of free volume, i.e. determination of time constants and migration enthalpies and entropies.
Dependence of the kinetic parameters on the number of components, the composition, the glass forming ability, the viscosity, the fragility of the investigated bulk metallic glasses and its relation to the mechanical properties, i.e., the ductility.
Comprehensive elucidation of the atomic kinetics of the free volume leading to the glass transition and its influence on the formation of shear bands which accompany the ductile behavior. What is the nature of the free volume (localized or delocalized)?
From basic science point of view the results will contribute to a profound understanding of the fundamental atomic processes and mechanisms governing the glass transition in bulk metallic glasses. The results will also be most helpful with respect to the development of new materials such as amorphous Cu-alloys or amorphous steels. As far as covalently bound network glasses or amorphous polymers are concerned the result may also provide clues to a deeper understanding of this kind of materials.