J. Freudenberger

Supersaturated solid solution of niobium in copper by mechanical alloying

Abstract Alloys with both high strength and high conductivity have been produced by mechanical alloying. In the present study, copper was mechanically alloyed with 5, 10 and 20 at.% Nb using a planetary ball mill. The Cu–Nb phase diagram shows a negligibly low mutual solubility in the solid state, but high energy ball milling can largely extend the region of solid state solution. Previously, it was observed that niobium partly dissolves in the copper lattice during milling. The present investigation demonstrates that this limit can be extended to a strongly supersaturated Cu solid solution of up to 10 at.% Nb provided the appropriate mechanical alloying method is applied. The change in the powder microstructure was followed by scanning and transmission electron microscopy (TEM) as well as by X-ray diffraction (XRD) analysis. In the case of Cu–5%Nb and Cu–10%Nb a homogeneous single-phase microstructure was obtained after 30 h of milling. Elemental Nb could no more be detected, indicating the formation of a metastable supersaturated Cu–Nb solid solution.

Critical current scaling and anisotropy in oxypnictide superconductors.

Having succeeded in the fabrication of epitaxial superconducting LaFeAsO(1-x)F(x) thin films we performed an extensive study of electrical transport properties. In the face of multiband superconductivity we can demonstrate that an anisotropic Ginzburg-Landau scaling of the angular dependent critical current densities can be adopted, although being originally developed for single band superconductors. In contrast with single band superconductors the mass anisotropy of LaFeAsO(1-x)F(x) is temperature dependent. A very steep increase of the upper critical field and the irreversibility field can be observed at temperatures below 6 K, indicating that the band with the smaller gap is in the dirty limit. This temperature dependence can be theoretically described by two dominating bands responsible for superconductivity. A pinning force scaling provides insight into the prevalent pinning mechanism and can be specified in terms of the Kramer model.

Superconductivity and disorder in YxLu1−xNi2B2C

Abstract The superconducting transition temperature T c , the temperature dependence of the upper critical field H c2 ( T ), and the lattice structure of polycrystalline Y x Lu 1− x Ni 2 B 2 C samples have been studied by susceptibility, resistivity, and X-ray measurements, respectively. All samples exhibit the same LuNi 2 B 2 C-type structure and the lattice parameters depend linearly on the yttrium content. The residual resistivity ratio (RRR) decreases steeply first for slight deviations from both pure stoichiometric limiting contents and then saturates at a low level in a broad plateau in between 0.1 x T c ( x ) and H c2 ( x ) at fixed T change nearly parabolically. The H c2 ( T )-curves show a positive curvature for all samples. This dependence can be described within a broad temperature region 0.3 T c T ≤0.95 T c by a simple and convenient empirical expression H c2 ( T )∝(1− T / T c ) α . The parameter α >1 describing the positive curvature of H c2 ( T ) characterizes the sample quality and decreases with increasing disorder in the Y x Lu 1− x Ni 2 B 2 C samples. However, the case of complete disorder (dirty limit) where α =1 is not reached yet in the investigated samples. The nonmonotonous dependence of T c and H c2 upon x is attributed to partial disorder within the Y–Lu basis layers. Together with the RRR-saturation they reflect the complex multiband character of the materials under consideration.

Microstructural evolution and its effect on the mechanical properties of Cu–Ag microcomposites

Abstract The microstructure and the mechanical properties of Cu–Ag alloys with 7 and 24 wt.% Ag are investigated. The microstructure of the alloys is mostly determined by the silver content. That of Cu-24 wt.% Ag alloys consists of a Cu-rich solid solution and the eutectic. Otherwise, the microstructure of Cu-7 wt.% Ag alloys consists of primarily solidified dendrites of a Cu-rich solid solution and small Ag-rich particles. The composition strongly influences the work-hardening rate. In order to achieve an ultimate tensile strength of 1 GPa, a logarithmic cold-deformation strain, η, of about 3.7 is required (η = 1n A0/A) for the 7 wt.% Ag alloy, whereas for Cu-24 wt.% Ag alloys η = 3.1 is sufficient. In as-cast alloys with 7 wt.% Ag a strong segregation is observed, which, consequently, leads to a strong decrease of the age-hardening effect. Therefore, the Cu-7 wt.% Ag alloy has to be homogenised before aging. The application of Cu–Ag alloys with a Ag-content below 8 wt.%, i. e. an the maximum solubility ...

Mechanical properties of Cu‐based Micro‐ and Macrocomposites

There is a need for high-strength and highly-conducting materials for applications such as pulsed high magnetic field coils. Two different approaches were studied in order to strengthen copper-based conductor materials. On the one hand, microcomposite Cu-Ag alloys yield high strength as a consequence of their nanoscale microstructure and, on the other hand, a Cu-based macrocomposite can be strengthened by the use of a steel jacket. In both cases the increase of strength coincides with a decrease of conductivity. Thus, the ideal material balances between these two competing properties.

Effect of microstructure on the mechanical properties of as-cast Ti–Nb–Al–Cu–Ni alloys for biomedical application

The correlation between the microstructure and mechanical behavior during tensile loading of Ti68.8Nb13.6Al6.5Cu6Ni5.1 and Ti71.8Nb14.1Al6.7Cu4Ni3.4 alloys was investigated. The present alloys were prepared by the non-equilibrium processing applying relatively high cooling rates. The microstructure consists of a dendritic bcc β-Ti solid solution and fine intermetallic precipitates in the interdendritic region. The volume fraction of the intermetallic phases decreases significantly with slightly decreasing the Cu and Ni content. Consequently, the fracture mechanism in tension changes from cleavage to shear. This in turn strongly enhances the ductility of the alloy and as a result Ti71.8Nb14.1Al6.7Cu4Ni3.4 demonstrates a significant tensile ductility of about 14% combined with the high yield strength of above 820 MPa already in the as-cast state. The results demonstrate that the control of precipitates can significantly enhance the ductility and yet maintaining the high strength and the low Youngs modulus of these alloys. The achieved high bio performance (ratio of strength to Youngs modulus) is comparable (or even superior) with that of the recently developed Ti-based biomedical alloys.

The High Field Project at Dresden/Rossendorf: A Pulsed 100 T/10 ms Laboratory at an Infrared Free-Electron-Laser Facility

This article describes the project to build a pulsed magnetic field user laboratory at the Forschungszentrum Rossendorf near Dresden. Using a 50 MJ/24 kV capacitor bank, pulsed fields and rise times of 100 T/10 ms, 70 T/100 ms, and 60 T/1 s should be achieved. The laboratory will be built next to a free-electron-laser-facility for the middle and far infrared (5 to 150 µm, 2 ps, cw). We describe the work which has been performed until now to start the construction of the laboratory in 2003: coil concepts and computer simulations, materials development for the high field coils, and design of the capacitor bank modules. In addition, a pilot laboratory has been set up where fields up to 62 T/15 ms have been obtained with a 1 MJ/10 kV capacitor module. It is used to gain experience in the operation of such a facility and to test various parts of it. In this test laboratory special devices have been developed for measurements of magnetization and magnetoresistance, and have been successfully used to investigate various materials including semiconductors and Heavy Fermion compounds. In particular, metamagnetic transitions in intermetallic compounds and the irreversibility field of a high-Tc superconductor have been determined. Shubnikov–de Haas oscillations have been observed in the semimetallic compound CeBiPt. Resistance relaxation has been observed to start less than 1second after the field pulse. It could be shown for the first time that nuclear magnetic resonance (NMR) is detectable in pulsed fields.

Highly alloyed Ni–W substrates for low AC loss applications

Cube texture formation has been studied in Ni?W alloys with a W content of 9?at.% and above. These alloys show a low magnetization at 77?K and below, and are therefore excellent candidates for use as substrates of coated conductors in AC applications. The application of a modified deformation and annealing sequence leads to a highly textured surface of Ni9W and Ni9.5W tapes with cube texture fractions above 96%. YBCO (YBa2Cu3O7??) layers obtained on these substrates using a standard buffer architecture showed a critical current density exceeding 1.5?MA?cm?2 at 77?K, similar to those for films on commercial Ni5W tapes. In contrast, only a weak cube texture was achieved in Ni10W tapes. The rolling texture of this alloy showed a significantly increased Goss component, which could not be reduced by applying intermediate annealing treatments. The influence of this texture on the cube texture formation will be discussed in detail.

Evidence of tetragonal to orthorhombic distortion of HoNi2B2C in the magnetically ordered state

Using high resolution neutron scattering on a powder sample of HoNi2 11B2C magnetoelastic effects are observed in this compound for the first time. At low temperatures the tetragonal lattice is distorted along the [110] direction, in which the long-range ordered Ho moments are aligned. The length of the unit cell in [110] direction is shortened by about 0.19% compared to its length in [110] direction. This lattice distortion is considered to be related to the existence of the commensurate c-axis modulated antiferromagnetic structure.

Specific heat and disorder in the mixed state of non-magnetic borocarbides

The temperature and magnetic-field dependence of the specific heat cp(T, H) in the superconducting (sc) mixed state as well as the upper critical field Hc2(T ) have been measured for polycrystalline YxLu1−xNi2B2C and Y(Ni1−yPty)2B2C samples. The linear-in- T electronic specific-heat contribution γ(H) · T exhibits significant deviations from the usual linear-in-H law resulting in a disorder-dependent negative curvature of γ(H). The Hc2(T )d ata point to the quasi-clean limit for (Y, Lu)-substitutions and to a transition to the quasi-dirty limit for (Ni, Pt)-substitutions. The γ(H)-dependence is discussed in the unitary d-wave as well as in the quasi-clean s-wave limits. From a consideration of γ(H )d ata only,d-wave pairing cannot be ruled out.