Nanocrystals in bulk metallic glasses studied by TEM

Bulk metallic glasses are a hot topic of materials science due to their unique properties. Their high strength usually has the drawback of low ductility caused by the formation of a shear band leading to catastrophic failure. By introducing nanocrystals into the amorphous materials it is possible to achieve some ductility. Simulations indicate that the interface between the amorphous and crystalline phase could be important for the mechanical properties. Still, there is little experimental evidence on their structure. Therefore in this project, state of the art transmission electron microscopy methods will be used to gain a clear understanding of these interfaces at the atomic level. Up till now investigations of nanocrystals in an amorphous matrix were frequently hindered by images affected by delocalization and Fresnel-fringes, therefore in this study it is proposed to use aberration corrected transmission electron microscopy methods at the cutting edge. Amorphous Co3 Ti containing homogenously distributed nanocrystals will be produced in the bulk by severe plastic deformation. In the first part of the project atomic resolution microscopy including exit wave reconstruction will be used to image the atomic structure of the amorphous/crystalline interface. It can be expected that the transition from the crystalline structure to the amorphous one does not occur in an abrupt manner. It is planned to study the interface of the nanocrystals in order to detect surface relaxations as reported for free-standing nanocrystals. It is envisioned that the local atomic arrangement will be linked to both the atomic structure of the nanocrystal and the structure of the surrounding amorphous matrix. In the second part smaller nanocrystals will be studied to analyse the transition from well-developed nanocrystals (> 5 nm) to those on the verge to medium-range order (sizes around 3 nm) and finally to the medium-range ordered clusters (< 2 nm). A systematic study will be carried out to understand the relationship between the structure of nanocrystals and medium-range ordered clusters. In addition to atomic resolution images, nanodiffraction with different beam diameters will be used. By sub-nanometer beam diameters single and interconnected clusters can be revealed. In the case of larger beam diameters it is planned to detect medium-range order by nanodiffraction fluctuation microscopy. It is expected that close to the nanocrystals the concentration of medium-range ordered clusters could be higher. The results will be of great importance for a wide range of materials. The interface-structure in amorphous/crystalline nanocomposites influences the properties not only in metallic glasses but also in e.g. semiconductors and ceramics. A clear understanding of the structures at the atomic level will allow the comparison with theoretical models and thus facilitate the intelligent design of materials.

The aim of the present project Nanocrystals in bulk metallic glasses studied by TEM was to use cutting-edge electron microscopy to reveal the structure of nanocrystals in metallic glasses and to study their influence on the mechanical properties. Metallic glasses are amorphous materials; in contrast to their crystalline counterparts they lack a long-range periodic crystal structure. Due to their exceptional hardness, they attract wide scientific attention, but they lack ductility at room temperature as deformation localization in the form of shear bands leads to catastrophic failure. Introducing nanocrystals into the amorphous material allows to increase the ductility. In the frame of the present project, in situ deformation Experiments could be carried out in the electron microscope to directly observe the influence of the nanocrystals on the deformation behaviour of the material on a local scale. The experiments revealed that the nanocrystals have a large influence on the deformation as they can interact with the shear bands. In addition to the nanocrystals the structure of the metallic glass was studied in the project. In addition to the short-range order that is also known for liquids, metallic glasses have a medium-range order on the lengthscale of several atomic distances. The medium-range order can be studied with statistical analysis of electron diffraction experiments from areas that are as small as a nanometer. To achieve the goals of the project analysis routines were developed allowing to analyse the large amounts of data collected during the diffraction experiments. Systematic experiments could show a large effect of the chemical composition of the metallic glass on the medium-range order. The scientific results were presented at international conferences and generated a large interest, as can be seen from the large number of invited talks. A large part of the project was carried out at the University of California, Berkeley and at the National Center for Electron Microscopy (NCEM) that is also in Berkeley and has led to a large number of international cooperations.