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Bulk metallic glasses

Bulk metallic glasses are a relatively new kind of materials with very interesting properties (e.g. exceptional strength and an impressive elastic limit). The properties are associated with the strong tendency to vitrification upon cooling leading to an amorphous structure without long-range atomic order. Still, some ordering on short-range (up to 0.5 nm) and medium-range (up to 2 nm) scales is present.
In a newly funded transnational research project (FWF I1309) state-of-the-art electron microscopy including fluctuation electron microscopy is used to study the fundamental role of the structural aspects on the mechanical properties and the deformation processes in bulk metallic glasses.

Electron diffraction pattern of a bulk metallic glass
Tilted dark-field image of Cu-Zr alloy

Amorphous thin films

Due to the lack of long-range order in amorphous structures the interrelation of the mechanical behaviour and the atomic structure is not easily amendable even for the case of elastic deformation. However, we demonstrated the applicability of atomic-level elastic strain measurements using electron scattering of micro/nanoscaled thin films in the transmission electron microscope by analyzing distorted selected area electron diffraction (SAD) images that can be acquired down to submicron scale [C. Ebner at al., Ultramicroscopy 2016; doi:10.1016/j.ultramic.2016.04.004]. In order to obtain the 2D atomic level strain tensor from the geometric changes of the first diffuse ring with sub-pixel accuracy an automatic evaluation procedure was developed.

We are using this method to study in-situ in the TEM the structural and mechanical response of amorphous thin films deformed within the elastic regime. Hitherto, we focused (i) on the elastic stress-strain response to calculate Young’s modulus and Poisson’s ratio, (ii) on the local atomic-level elastic strain as a function of sample position (strain mapping), (iii) on uncoupling anelastic and elastic strain and (iv) on the time dependent viscoelastic strain response. 

(a) Color coded image overlay of a distorted SAD pattern and its by 90° rotated and inverted counterpart. (b) Atomic level elastic strain (as a function of azimuthal angle χ) measured at two different stress states of the same sample position.

Nanostructured Materials

Nanostructured materials attract considerable scientific and technological interest since they exhibit new and enhanced physical and mechanical properties. In our group nanostructured materials are mainly processed by methods of severe plastic deformation (SPD) such as high pressure torsion and accumulative roll bonding applying deformations up to 100 000%. The focus of our research comprises fundamental aspects of grain refinement, defect evolution and phase stability of the SPD processed nanostructured materials. Bulk nanostructured metals, alloys and intermetallic compounds based on different long-range ordered structures are investigated. The materials are analysed by novel transmission electron microscopy (TEM) methods (like fluctuation microscopy and quantitative profile analysis of selected area electron diffraction PASAD) and on an atomic scale using high-resolution TEM. In addition, TEM in-situ methods (straining, heating and cooling) are applied. DSC is used to analyse various phase transformations. The TEM and DSC results are complemented by data obtained by X-ray and synchrotron scattering experiments.

Nanostructured shape memory alloys

Reversible shape changes of ferroelastic materials showing martensitic phase transformations can be controlled by temperature, stress or magnetic fields. Grain size at a nanoscale can strongly impact the martensitic phase transformation and therefore the shape memory effect and superelasticity. Nanocrystalline and ultrafine grained shape memory materials including NiTi alloys, low-hysteresis NiTiPd alloys, and ferromagnetic high-temperature NiMnGa alloys are processed by methods SPD. The lattice structures of the martensitic phases, self-accommodated martensitic domains, transformation temperatures, as well as the enthalpy and entropy changes upon transformation are studied by systematic experiments. These results are analysed using thermodynamic models considering the total Gibbs free energy of ferroelastic domains confined to small grains.

TEM lattice fringe image of two twin related variants in an ultrafine grain of NiMnGa

Materials driven far from thermodynamic equilibrium

In our group various ordered intermetallic alloys (based on B2, L12, Heusler and L21 lattice structures) are subjected to severe plastic deformation including HPT and ARB. As a result of the systematic studies, different metastable structures driven far from thermodynamic equilibrium were obtained. Concomitant to the strong grain refinement the chemical long range order of intermetallics can be strongly reduced by severe plastic deformation. The local destruction by fragmenting chemically ordered domains are monitored by TEM methods. Disordered phases such as a fcc lattice structure in NiMnGa alloys can arise not observed in the undeformed, coarse grained material. Even the topological order of some intermetallics can be almost completely destroyed; strain induced amorphization occurs e.g. in NiTi-based alloys and Co3Ti. Applying appropriate heat treatments, the kinetics and structural pathways towards thermal equilibrium such as reordering and nanocrystallization from an amorphous phase are studied.

Color coded TEM image of chemically ordered nanodomains in FeAl using superlattice reflection [100]

Nanostructured thin films

The deformation behavior of nanocrystalline metallic thin films is influenced by the dimensional constraints given by the grain size and the film thickness. In-situ TEM concomittant with stress-strain measurements is used to study the deformation processes of freestanding nanocrystalline thin films at different stages of deformation.

Movie: Dislocation mediated deformation of nanocrystalline gold during in-situ TEM. The central grain is about 200 nm in size. 

Dislocation mediated deformation of nanocrystalline gold during in-situ TEM

Research Group Physics of Nanostructured Materials
Faculty of Physics

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