Thomas Wroblewski

Spatially Resolved Investigation of Solution Cast Nanoparticle Films by X-ray Scattering and Multidimensional Data Set Classification

A combinatorial high-throughput approach is used to investigate a solution cast gradient consisting of colloidal gold nanoparticles on top of a silicon substrate by means of a X-ray nanobeam. Classification algorithms are used to reveal and visualize structural transitions from a frozen colloidal solution to a well-defined nanostructure. Prominent length scales on the order of 100 nm are observed. A periodic change in the nanostructure along the gradient is explained by a simplified stick-slip model.

Residual Stresses in Sintered Diamond-Cobalt Composites

Manufacturing diamond-cobalt composites by sintering results in residual stresses due to the mismatch of thermal expansion coefficients mainly. To understand the influence of manufacturing process parameters on residual stresses of sintered diamond-cobalt composites samples are produced by different process parameters. The investigated diamond composites are pressureless sintered as well as pressureless sintered combined with hot isostatically pressing. Here the influence of powder characteristics and process parameters like compaction pressure and sintering temperature on the residual stresses and microstructure was analysed by X-ray diffraction, microscopy and tomography. The aim of this study is to correlate residual stresses with manufacturing parameters and to give hints for optimising the residual stress state and for improving the lifetime of diamond-cobalt composites.

Energy Dispersive X-Ray Diffraction Imaging

Position resolved structural information from polycrystalline materials is usually obtained via micro beam techniques illuminating only a single spot of the specimen. Multiplexing in reciprocal space is achieved either by the use of an area detector or an energy dispersive device. Alternatively spatial information may be obtained simultaneously from a large part of the sample by using an array of parallel collimators between the sample and a position sensitive detector which suppresses crossfire of radiation scattered at different positions in the sample. With the introduction of an X-ray camera based on an energy resolving area detector (pnCCD) we could combine this with multiplexing in reciprocal space.

Micro Diffraction Imaging of Bulk Polycrystalline Materials

X-ray diffraction imaging applies an array of parallel capillaries in front of a position sensitive detector. Conventional micro channel plates of a few millimetre thickness have successfully been used as collimator arrays but require short sample to detector distances to achieve high spatial resolution. Furthermore, their limited absorption restricts their applications to low energy X-rays of around 10 keV. Progress in the fabrication of long polycapillaries allows an increase in the sample to detector distance without decreasing resolution and the use of high X-ray energies enables bulk investigations in transmission geometry.

Dependence of Oxidation Behavior and Residual Stresses in Oxide Layers on Armco Iron Substrate Surface Condition

Mass gain during oxidation, texture and residual stresses in oxide layers on polycrystalline Armco iron substrates with different surface conditions are investigated using thermogravimetry microscopy and synchrotron X-rays. The mass gain during oxidation in all samples follows a parabolic law. The parabolic oxidation constant increases with increasing roughness of a mechanically ground respectively polished oxide layer. Electrolytic polishing (grain surface etching) reduces while grain boundary etching increases the parabolic oxidation constant compared to the mechanically polished sample. All oxide layers show columnar growth of the magnetite and a moderate fiber texture. The magnetite contains compressive residual stresses. Under the conditions chosen for the oxidation treatment the magnitude of these compressive residual stresses does not depend on the substrate surface condition.

Analysis of Residual Stresses in Micro Powder Injection Moulded Micro Bending Specimens Made of Zirconia

Analysis of the residual stress state, the microstructure and surface topography of ceramic microcomponents made of Y2O3-stabilized ZrO2 produced by micro powder injection moulding were performed. During the production of the microspecimens sintering conditions were varied. The measurement of residual stresses has been carried out using the MAXIM (MAterials XRay IMaging) diffractometer at HASYLAB beamline G3 at DESY in Hamburg (Germany). The microstructure and surface topography of differently moulded specimens were examined using a scanning electron microscope (SEM) and a confocal white light microscope type NANOFOCUS. The findings are used to establish correlations between process parameters and characteristics of the microcomponents. These will allow to improve the production process with respect to the mechanical properties of the microcomponents.

Industrial Research and Development with Synchrotron Radiation

The outstanding properties of synchrotron radiation (SR) allow manifold investigations of materials and processes which are not possible with conventional X-ray sources. Its high brightness allows extremely precise or time/spatial resolved measurements. In combination with its high collimation extremely high angular resolution is achievable. The white spectrum of SR allows tuning of the wavelength for spectroscopic applications or optimization according to the requirements of the experiment.

Recrystallization investigated by X-ray diffraction imaging

X-ray diffraction imaging allows the investigation of a large area of a polycrystalline specimen in a single shot. Dynamic processes like recystallization can, therefore, be studied without prior knowledge of where they occur. Even early stages of nucleation can be traced back using the information from images taken from the fully recrystallized specimen. Experiments performed at HASYLAB beamline G3 on cold rolled Cu and Al showed nucleation and growth behaviour that cannot be explained by classical models.

MAXIM - A Novel Method for the X-Ray Imaging of Strain

A novel method, applying a micro-channel plate as collimator in front of a position sensitive detector, allows the imaging of large areas of polycrystalline materials using the diffracted radiation. A series of images as a function of the diffractometer angles yields up to one million diffraction patterns, one for each pixel of the detector. Sophisticated methods have been developed to reduce this huge amount of data to a few maps displaying the characteristic features of the components in the specimen and/or the diffraction patterns of these components.

Experimental and numerical approach of inter- and transgranular stress- and rotation heterogeneities in the plastic behaviour of a multicrystal

The introduction of micromechanical parameters in the description of the mechanical behaviour of polycrystalline materials is getting more and more important. In the interest of the study of inter- and transgranular stress and strain heterogeneities inside an assembly of grains, the multicrystal is introduced. The complete microstructure is known which makes possible an experimental and numerical approach which takes into account the complete microstructure. The modelling is done by a finite element simulation meshing the grain morphology. The experimental validation is done by x-ray diffraction and optical microscopy. The multicrystal is elaborated on the base of a coarse grained Nickel based 600 alloy. A long heat treatment close to the melting temperature allowed to get into the regime of grain growing. In the finite element calculations the grains are considered as perfect single crystals with only one crystallographic orientation. Naturally, during the elaboration process materials develop sub-grain structures with slight misorientations called small angle grain boundaries or grain mosaicity. The quality of the crystal can be described by the mosaicity. Depending on elaboration parameters, the intergranular misorientations can be distributed more or less randomly. Due to the local distribution of mosaic blocs, the heterogeneity of the measurement depends on the observation scale. The use of optical microscopy allows the determination of plastic activate zones. The observation of heterogeneities inside these zones needs finer measurement techniques such as x-ray diffraction. The measurement of misorientations by rocking the sample around a crystallographic axis allows to quantify the misorientations. This is even more interesting during plastic slip where, due to the anisotropy of the crystal, the mosaicity develops anisotropically, too. Finally, the microscale parameters like the mosaicity are investigated by keeping in mind the influence on the local stress and strain evolution. The comparison to the finite elements simulation is of great interest in relation to the choice of micromechanical parameters which do not contain anything comparable to a mosaic function.