Free Volume in Bulk Metallic Glasses

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.

Metallic glasses are amorphous metals and as already indicated by the word amorphous they lack any kind of crystalline order. They are more or less dense packed, however, incorporate a high fraction of open space, so-called free-volume. In an amorphous solid this free-volume is equally distributed; it cannot be located at vacancies or grain boundaries as it is the case for crystalline solids. Metallic glasses are by its nature metastable and can only by used at temperatures below their glass transition temperature. Atomic processes necessary to establish distinct properties of these alloys are governed by the kinetics of free-volume. Due to this fact, these processes in metallic glasses occur at low temperatures and cover long time scales. For the study of these processes special experimental methods are necessary. Therefore, the specific and highly sensitive method of LASER dilatometry has been developed. With this technique changes in the free volume can be studied by measuring the simultaneously occurring length change down to a resolution of below 100 nm maintained even for long measurement times. With this technique also the phenomenon of the glass transition was investigated where upon heating the rigid amorphous solid changes to the state of a super-cooled liquid. A glass transition is also observed for other amorphous solids, such as ceramics or polymers and the understanding of the glass transition phenomenon is of utmost importance for sold-state physics. In this project for the first time the lowest possible temperature for such a glass transition has been determined for a specific amorphous metallic alloy. The result is of importance to decide which of the currently controversially discussed theories describing the glass transition phenomenon is the most appropriate one.