F Heringhaus

True absolute grain boundary mobility: motion of specific planar boundaries inBi-bicrystals under magnetic driving forces

The migration of planar 90°〈112〉 tilt grain boundaries with the same misorientation but different boundary inclination in Bi-bicrystals was investigated. The driving force for grain boundary motion was created by the action of a magnetic field on bicrystals of bismuth, which is magnetically anisotropic with different susceptibilities parallel and perpendicular to the trigonal axis. The driving force dependency of boundary velocity was measured and the absolute value of grain boundary mobility was determined. The grain boundary mobility was found to strongly depend on grain boundary inclination. The mobility parameters (activation enthalpy and preexponential factor) for a symmetrical 90°〈112〉 tilt grain boundary were much smaller than for an asymmetrical (ψ=45°) boundary. The mobility of the asymmetrical grain boundary depended on the direction of motion. For the given crystallography the boundary moved faster, if the trigonal axis in the growing grain was parallel rather than perpendicular to the direction of motion.

On the correlation of microstructure and electromagnetic properties of heavily cold worked Cu-20 wt% Nb wires

Abstract Fibre or ribbon reinforcedin situ metal matrix composites (MMCs) consisting of Cu and 20 wt% Nb were produced by large strain wire drawing. The microstructure of the composites was investigated by means of optical and electron microscopy. The normal and superconducting properties of the MMC wires in the presence of external magnetic fields were examined and compared to the electromagnetic properties of pure Cu wires. The findings are discussed on the basis of the microstructural changes during deformation. The current results substantiate that the amount of internal boundaries and the filament spacing have considerable influence on the normal and superconducting properties of Cu-20% Nb.

Recent advances in the manufacturing of copper-base composites

Abstract Copper-base fibre-reinforced in situ metal-matrix composites meet the demands concerning strength and electrical conductivity for many applications such as in high field resistive and pulse magnet technology, automobile and aerospace electrics or frames for electronic circuits. In situ formed composites are manufactured by an initial process providing a two- or multi-phase alloy followed by heavy deformation. These initial samples can be supplied by casting procedures or by the sintering of metal powders. Although considerable success was achieved by producing initial specimens by the use of powder metallurgy, some recently developed melting techniques have been established as being competitive or even superior methods. The present study focuses on the recent progress in the metallurgical melting and casting processes of Cu-base two-phase in situ formed composites, namely CuAg and CuNb.

Magnetically forced motion of specific planar boundaries in Bi-bicrystals

For the first time a crystallographically defined planar grain boundary in a specially grown bicrystals was moved under the action of a magnetic driving force. The mobility of 90° tilt grain boundaries with identical misorientation but different boundary inclination in Bi-bicrystals was investigated. The driving force for grain boundary motion was created by the action of a magnetic field on bicrystals of bismuth, which is magnetically anisotropic with different susceptibilities parallel and perpendicular to the trigonal axis. The driving force dependency of boundary velocity was measured and the absolute value of grain boundary mobility was determined. The grain boundary mobility was found to strongly depend on grain boundary inclination. The mobility of the asymmetrical grain boundary was found to depend on the direction of motion.