M. Kerber

Coincidences of hypercubic lattices in 4 dimensions

Summary We consider the CSLs of 4-dimensional hypercubic lattices. In particular, we derive the coincidence index Σ and calculate the number of different CSLs as well as the number of inequivalent CSLs for a given Σ. The hypercubic face centered case is dealt with in detail and it is sketched how to derive the corresponding results for the primitive hypercubic lattice.

Impact of Ball Milling and High-Pressure Torsion on the Microstructure and Thermoelectric Properties of p- and n-Type Sb-Based Skutterudites

For thermoelectrics it is important to produce thermodynamically stable bulk nanostructured materials. Ball milling/hot pressing was shown to reduce the crystallite size by a factor of 100 and to reach about 100 nm with dislocation densities of 1012 – 1013m-2. Thereby thermoelectric properties of single, double and multifilled Sb-based skutterudites were improved significantly leading to figures of merit ZT, which in some cases are twice as high as those of their microstructured counterparts. With HPT treatment the crystallite size can be decreased to even 50 nm with dislocation densities as high as 1015m-2. The small grains as well as the high dislocation density result in a further lowering of thermal conductivity holding a high potential for future enhancement of ZT.

Plasticity and X-ray Line Profile Analysis of the semicrystalline polymer poly(3-hydroxybutyrate)

The evolution of the microstructure during compressive deformation of the biodegradable polymer poly(3-hydroxybutyrate) (P3HB) was investigated in-situ via X-ray diffraction using synchrotron radiation. Flow curves were measured in-situ together with X-ray profiles for several degrees of deformation. The profiles were analysed using Multi-Reflection X-ray Line Profile Analysis (MXPA) adapted by the authors for semicrystalline polymers providing lamella thickness, crystallinity, and the presence and density of dislocations as a function of the deformation. In contrast to previous investigations in α crystallised isotactic polypropylene (α-iPP), P3HB does not exhibit a deformation induced increase of the dislocation density which suggests mechanisms other than dislocations to be involved in plastic deformation of P3HB.

Changes of Thermoelectric Properties and Hardness After HPT Processing of Micro- and Nanostructured Skutterudites

In this paper the influence of the starting material on the physical properties after severe plastic deformation (SPD) will be discussed. A bulk p-type skutterudite DD0.44Fe2.1Co1.9Sb12(DD stands for didymium which consists of 4.76% Pr and 95.24% Nd) was (1) hand milled and hot pressed, resulting in crystallite sizes in the μm range and (2) ball milled and hot-pressed, reducing the crystallite size to about 100nm, and afterwards deformed using high pressure torsion (HPT). It could be shown that in both cases the lattice parameters were slightly higher after HPT processing, the difference of the electrical resistivity values between heating and cooling was much larger for the skutterudite, which stems from the microstructured alloy. The thermopower data for both alloys are slightly higher, resulting in power factors at 800K almost like (originally nanosample) or even higher (originally microsample) than before HPT. As the thermal conductivity is always lower after SPD, a much higher ZT can be expected. After deformation hardness measurements showed a much higher increase for the nanosample compared to that of the microstructured one.

Head-on collision of ultrarelativistic charges

We consider the head-on collision of two oppositely charged point particles moving at the speed of light. Starting from the field of a single charge we derive in a first step the field generated by a uniformly accelerated charge in the limit of infinite acceleration. From this we then calculate explicitly the burst of radiation emitted from the head-on collision of two charges and discuss its distributional structure. The motivation for our investigation comes from the corresponding gravitational situation where the head-on collision of two ultrarelativistic particles (black holes) has recently aroused renewed interest.

Nanocrystallization and Dissolution of Immiscible Powder Alloys Using High Pressure Torsion

Precompacts out of immiscible systems CuCr (75/25 wt%) and WCu (80/20 wt%), respectively, were made by pressing mixed powders and sintering. By applying different strains and hydrostatic pressures of HPT at room temperature, disc-shaped samples with a diameter of 8 mm were produced. They were investigated by Light Microscopy, Scanning-Electron Microscopy using Back-Scattered Electrons, and X-ray Line Profile Analysis. In addition, Vickers microhardness data were collected. Both systems showed the highest microhardness at a shear strain of about γ = 170. The density (for the case of Cu25Cr) of the consolidated material could be increased to the theoretical value. Microhardness and grain sizes were studied individually for each of the phases, too.

In Situ X-Ray Synchrotron Profile Analysis During High Pressure Torsion of Ti

X-Ray Line Profile Analysis is a powerful method to characterize the microstructure of deformed materials, especially when high energy and brilliant Synchrotron radiation enables investigations with high time and spatial resolution. Parameters like dislocation density, dislocation arrangement as well as scattering domain size and it’s distribution are parameters of a physical model of peak broadening, which can be applied to high quality diffraction measurements. A small high-pressure-torsion-machine was designed in order to perform in situ diffraction experiments during the deformation process at hydrostatic pressures up to 8 GPa in order to follow the strain as well as pressure induced microstructural characteristics of any material deformed. This was possible with the ideal design and equipment at the High-Energy-Materials-Science-beamline at PETRA III in Hamburg. Recent and First results of experiments on HPT-deformed Ti show that at 6 GPa the high pressure \( \omega \)-phase is initiated only with additional shear deformation.

Phase transformations of severely plastically deformed Ti–Ni–Pd high-temperature shape memory alloys

Severe plastic deformation of a Ti50Ni25Pd25 high-temperature shape memory alloy yields a nanoscale mixture of an amorphous phase and retained lamellae of the parent B19 martensite. The thermal stability of the deformed martensite is significantly enhanced. Upon heating, B2 austenite occurs within the martensite and by crystallization of the amorphous phase, concomitant with recovery and the formation of an ultrafine grained structure. In the small grains, the B2 austenite to B19 martensitic transformation is shifted to lower temperatures and incomplete.