Structural inhomogeneities in bulk metallic glasses

The aim of this project is to advance a fundamental understanding of both the structural aspects and the mechanical properties of bulk metallic glasses. It is envisioned that understanding the role of structural heterogeneities in the process of heterogeneous flow allows to ultimately tailor ductility in bulk metallic glasses. In this project advanced transmission electron microscopy methods including fluctuation electron microscopy as the main tool to unravel the medium range order, will be combined with X-ray and neutron scattering experiments as well as reverse Monte Carlo simulations. The combination of these state-of-the-art methods including in-situ experiments is expected to lead to a significant leap forward in understanding the intrinsic interrelation between structure and deformation mechanisms of metallic glasses. This challenging research project based on joint preliminary studies will be tackled through concerted efforts and cross-boarder expertise of the participating groups at the University of Vienna (Austria) and the Leibnitz Institute IFW, Dresden (Germany). In order to analyse this complex matter systematically, the following five well-defined goals and their innovative aspects are identified: I. Role of composition on the short-range and medium-range order of different bulk metallic glasses (BMGs) II. Influence of the cooling rate on the structures of different BMGs III. Effect of mechanical pre-treatment on the details of the structures of different BMGs IV. Impact of structural relaxation on various length scales of BMGs in different states V. Modelling of the structure of BMGs by the method of reverse Monte Carlo simulations Our cross-border project is set up to explore cutting-edge aspects in the field of materials science and to deliver a comprehensive picture of the glassy structure. This includes local changes at different length scales caused by thermal or mechanical pre-treatments. Moreover, the expected results are important for establishing principles and routes for the development and design of materials with improved properties and for finally exploiting the full potential of bulk metallic glasses as novel structural material.

In the transnational research project Structural inhomogeneities in bulk metallic glasses, the atomic structure of amorphous metals was investigated and correlated with mechanical properties. Like liquids and glasses, amorphous metals do not have a crystalline (periodic) but an irregular atomic arrangement. This gives the so-called metallic glasses special mechanical properties such as enormous strength combined with high elasticity. Unfortunately, the strong tendency to brittle fracture, which is also inherently associated with the amorphous structure, has so far limited widespread use. The link between structure and property is therefore of fundamental importance for the understanding and the technological application of metallic glasses. Various state-of-the-art methods, including scattering methods with electrons and synchrotron radiation, were used to correlate the amorphous structure and its inhomogeneities with the properties. It has been shown that alloying in CuZr based glasses modifies the local structure and changes its diversity. Depending on the alloying element, crystal-like motifs may occur, which significantly influence the formation and stability of glassy structures. The results also show that very rapid and short-term temperature increases cause a local order change, which can also lead to local crystallization. As an alternative, special mechanical pretreatments were applied to substantially change the structure and the associated properties. Torsional deformation under high pressure increases the volume that an atom can occupy in the structure so that density decreases and mechanical properties such as hardness and elasticity can be adjusted and the homogeneity of the deformation improved. In order to be able to measure structure and properties at the same time, in-situ investigations of metallic glasses were also carried out. Elastic, anelastic and plastic behavior were investigated as well as the effect of nanoscale heterogeneities in the form of crystallites on localized deformation. In this context, it could be shown by means of specially developed analysis techniques that, due to the continuously improvement of the methods and their increased sensitivity, the influence of the method on the experimental results itself is becoming more and more important. The transnational research project was elaborated and carried out in close collaboration with colleagues from the Leibnitz-Institute for Solid State and Materials Science Dresden.