Master and PhD projects

On this page you can find information about master and PhD projects that are available or ongoing in the research group.

Supervised by Herwig Peterlik

Ongoing: Temperature behavior of the elastic properties of carbon fibers

The goal of this PhD thesis is the determination of the elastic properties of carbon fibers for temperatures up to 2000°C.  A small diameter (approx. 10µm) poses a rather challenging situation for the determination of their temperature. The difficulty in measuring the lateral strain lies in the tiny changes in diameter, which are expected to be in the range of a few nm. Therefore, it is necessary to develop and build new measuring devices for temperature and lateral strain.

Ongoing: Relaxation and recovery in biological fibers with focus on hair

In this master thesis, we are investigating the relaxation behaviour of biological fibers (such as hair), a possible recovery at different strain levels, and the correlation to structural features, which are measured using small-angle X-ray scattering (SAXS).

Hair consists of keratin filaments with a typical filament distance of about 8 nm, which is an optimum range for SAXS. Also, a number of other parameters such as the bilayer distance of lipids is available from this measurement technique. If the material is strained to a high value, on the the molecular scale a transition of the structure from alpha-helices to beta-sheets was proposed /e.g. 1/. A change in the X-ray signal, which is observable in SAXS, led to the hypothesis of this transition, but its precise nature is still a matter of scientific discussions. Whereas data on bending recovery and relaxation of hair in dependence on humidity are available /2/, the relation of the mechanical parameters to the structure of hair is still missing.

In this thesis, we are loading fibers of natural hair up to different strain levels and follow the macroscopic recovery (by recording stress-strain curves) and the nanoscopic recovery (a possible backshift of the X-ray signal together with the time constant). If possible, we perform these experiments in-situ. However, this could be limited by the available X-ray intensity in the laboratory. One way to overcome this limit is the use of a higher number of single hair strands. The time-dependent loading experiment should allow to obtain information not only on the deformation on the respective scale, but also to gain insight into possible self-healing mechanisms, which could be able to recover the original molecular structure by refolding.

/1/ W.T. Astbury and H.J. Woods, Nature 126 (1930) 913-914.
/2/ F.J.Wortmann, M. Stapels, and L. Chandra. J. Appl. Polym. Sci. 113 (2009) 3336-3344.

Ongoing: Measurement of lateral strain of technical and biological fibers

Technical fibers, such as those used in composite materials or textiles, are produced synthetically and therefore differ from natural fibers in their chemical composition, structure and in many mechanical properties. Natural fibers include, on the one hand, plant fibers such as cotton, flax, jute or hemp, and, on the other hand, animal fibers such as hair or wool and silk. Natural fibers usually have a greater degree of variability in their mechanical properties than artificial fibers. These include the diameter, the cross-section, the surface properties and the strength. Flax fibers, for example, have a higher tensile strength than technical fibers.

The mechanical properties of fibers are determined, among other things, by the strain, the longitudinal strain being measured along the load direction and the transverse strain perpendicular to it. The measurement of the transverse strain of fibers proves to be difficult because of the small diameter of the fibers and the small strains associated therewith or because of inhomogeneous properties; While there are already generally used standardized methods for determining the longitudinal expansion, the transverse expansion is still relatively untapped. Therefore, the transverse strain of various biological and technical fibers is to be examined in this master's thesis. If changes in the transverse strain occur, accompanying investigations such as small-angle X-ray scattering or scanning electron microscopy should be carried out in order to determine the structural causes for this. This could be the case in particular in the high load region (stress-strain curves up to breakage). Since many fibers are not round, but rather have an elliptical cross-section, the experiments are carried out on a newly developed tensile test machine in which the clamping jaws rotate symmetrically, with which the cross-section and its change during the experiment can be measured. The method is to be further investigated in this master's thesis by applying it to different fibers.

These investigations are intended to provide a comprehensive insight into the structure of the fibers under mechanical stress. In addition, the stretching and structural properties of natural and artificial fibers are to be compared in order to investigate possibilities of characterizing fibers or application-relevant questions, since alternatives are being sought in many industrial areas to replace artificial fibers with natural ones, as these have a lower density than, for example technical fibers and are also biodegradable.

Available: Simulation of the vibrational modes of solid bodies

The vibrational modes of tetrahedrons and spheres shall be visualized in order to be more easily able to map the measured to the calculated eigenfrequencies.  A meaningful mapping is necessary to acquire the elastic constants of the material.

Available: Measurements of SiC fibers with small angle X-ray scattering at high temperatures

The kinetic of structural change of silicon-carbide fibers shall be measured with small angle X-ray scattering. Scanning electron microscopy measurements complement the investigation into the structural change.

Supervised by Erhard Schafler

Ongoing: Strain-rate sensitivity in high pressure deformation of steel, iron and nickel

Severe plastic deformation and specifically high pressure torsion (HPT) are used to achieve an ultrafine-grained or even nanocrystalline structure of materials. This leads to improvement of mechanical, but also functional properties of the material. Deformation parameters like pressure, strainrate and temperature influence the resulting microstructure.

In this thesis, the influence of strainrate during HPT of nickel and iron will be investigated, which serve as examples for facecentered and bodycentered cubic metals, respectively. In addition to room temperature experiments, low temperature deformations as well as investiagtions at elevated temperatures should complete understanding of the strain rate sensitive behaviour of the investiagted materials with respect to this specific deformation treatment (high hydrostatic pressure, ultra-high strain), that has not been investigated so far at all.

Ongoing: Tuning nanostructured Nickel by high pressure annealing

The aim of this work is to study the difference between annealing at atmospheric pressure and high pressure annealing at the same temperature, and changes in the recrystallisation process.

Our material of choice is Ni discs of 5 mm radius deformed by high pressure torsion (HPT). For the HPT-processing, the samples are subjected to 5 revolutions. We differ between two initial conditions after the HPT deformation. “Unloaded” for samples were the hydrostatic pressure was released and then loaded again before the heat treatment, and “loaded” for samples where the pressure was not released but only reduced.

The HPT-processed samples will subsequently be annealed under different temperatures and different pressures. Each measurement series was performed at one temperature and one pressure. For reference, atmospheric pressure annealing is included in every measurement series, executed in a differential scanning calorimeter (DSC).

For investigation of mechanical properties, microhardness measurement will be performed and the grain size will be analysed by SEM measurements.

Ongoing: Strengthening of Zn-rich alloys by high pressure torsion and thermal treatment

The aim of this master thesis is to investigate the strength increase of two different zinc-magnesium alloys by serve plastic deformation (SPD) and subsequent heat treatment. The focus is not only on the formation of dislocations but also on the generation of vacancy agglomerates.

The sample material consists of extruded zinc rods with 0.5 and 1 wt% magnesium. After sample preparation, which includes cutting and grinding, the samples are annealed under argon protective gas atmosphere to a defined initial state (homogenization). Subsequently, the samples are deformed by high pressure torsion (HPT) at room temperature and heat treated at different temperatures in a silicone oil bath.

The dislocation and vacancy densities are determined by x-ray diffraction (XRD) line profile analysis and differential scanning calorimetry (DSC). The mechanical properties are determined by micro hardness measurements.

Supervised by Bodgan Sepiol

Available: Short-range order in crystal structures

The diffuse scattering on intermetallic crystals (Ni-Al, Fe-Al) is measured with an X-ray detector with a good energy resolution of 200 eV. The scattering function is compared with a simulated crystal model providing information about the short-range order of atoms in the investigated structures.

Available: Simulation of intermetallic phases and ionic glasses

We are familiar with measuring elementary jumps in ordered intermetallic phases and in amorphous glasses using X-ray correlation spectroscopy (XPCS). The central problem here is the adaptation of particular short-range order models. The question is to simulate short-range order with interatomic potentials known from the available literature and to compare them with diffuse scattering measurements. This problem should be clarified in the context of a master's thesis. The basic knowledge of programming is required for this work in order to write the corresponding software.

Available: Dynamic Light Scattering measurements of diffusion under the action of driving forces

The basic idea behind Photon Correlation Spectroscopy (PCS) is based on measuring the fluctuations in the scattered intensity over time. These provides information about the diffusion or particle sizes in the sample. Usually this effect, called also Brownian diffusion, is studied in thermodynamic equilibrium. It’s much more complicated to extract this information in systems under the influence of driving forces such as gravity or electric fields. The development of measurement technique and of PCS theory is the main task of this thesis.