F. Lecouturier

MICROSTRUCTURAL STUDIES OF IN SITU PRODUCED FILAMENTARY Cu/Nb WIRES

E. Snoeck*, F. Lecouturier, L. Thilly, M.J. Casanove, H. Rakoto, G. Coffe, S. Askenazy, J.P. Peyrade, C. Roucau, V. Pantsyrny, A. Shikov, A. Nikulin Centre d’Elaboration de Materiaux et d’Etudes Structurales CEMES-CNRS, BP 4347, 29 Rue Jeanne Marvig, 31055 Toulouse Cedex, France Service National des Champs Magnetiques Pulses SNCMP-CNRS-UPS-INSA, Complexe Universitaire de Rangueil, 118 route de Narbonne, 31062 Toulouse Cedex, France Laboratoire de Physique de la Matiere Condensee LPMC-CNRS-UPS-INSA Complexe Scientifique de Rangueil, 31077 Toulouse Cedex, France Bochvar All Russia Scientific Research Institute of Inorganic Materials, Moscow, Russia

Plasticity of multiscale nanofilamentary Cu/Nb composite wires during in situ neutron diffraction: Codeformation and size effect

In situ neutron diffraction was performed on Cu∕Nb nanocomposite wires composed of a multiscale Cu matrix embedding Nb nanofilaments with a diameter of 267nm and spacing of 45nm. The evolution of elastic strains and peak profiles versus applied stress evidenced the codeformation behavior with different elastic-plastic regimes: the Cu matrix exhibit size effect in the finest channels while the Nb nanowhiskers remain elastic up to the macroscopic failure, with a strong load transfer from the Cu matrix onto the Nb filaments. The measured yield stress in the finest Cu channels is in agreement with calculations based on a single dislocation regime.

Evidence of internal Bauschinger test in nanocomposite wires during in situ macroscopic tensile cycling under synchrotron beam

In situ multiple tensile load-unload cycles under synchrotron radiation are performed on nanocomposite Cu∕Nb wires. The phase specific lattice strains and peak widths demonstrate the dynamics of the load-sharing mechanism where the fine Cu channels and the Nb nanotubes store elastic energy, leading to a continuous buildup of internal stress. The in situ technique reveals the details of the macroscopically observed Bauschinger effect.

FIM AND 3D ATOM PROBE ANALYSIS OF Cu/Nb NANOCOMPOSITE WIRES

Two kinds of Cu/Nb nanocomposite wires were investigated using field ion microscopy (FIM) and 3D atom probe. These two techniques revealed for the first time the nanoscale microstructure of nanocomposite wire cross sections. FIM investigations confirmed the Cu and Nb texture and the disorientation between (111) Cu and (110) Nb planes. Low angle Nb/Nb grain boudaries were also observed. Thanks to 3D atom probe, parts of niobium fibres and copper channels a few nanometer width were mapped out in 3D. Smooth Cu/Nb interfaces were attributed to stress-induced diffusion. Shear bands, observed perpendicular to the wire axis, were attributed to tracks of moving dislocations in a copper channel.

Pulsed magnetic fields in Toulouse: past, present and future

An overview over past and present activities and technical developments in the Toulouse pulsed magnetic field facility is given. Results obtained recently with a 60 T, 150 ms, 1.25 MJ prototype magnet tested with the new 14 MJ generator are presented.

Dog-bone copper specimens prepared by one-step spark plasma sintering

Copper dog-bone specimens are prepared by one-step spark plasma sintering (SPS). For the same SPS cycle, the influence of the nature of the die (graphite or WC–Co) on the microstructure, microhardness, and tensile strength is investigated. All samples exhibit a high Vickers microhardness and high ultimate tensile strength. A numerical electro-thermal model is developed, based on experimental data inputs such as simultaneous temperature and electrical measurements at several key locations in the SPS stack, to evaluate the temperature and current distributions for both dies. Microstructural characterizations show that samples prepared using the WC–Co die exhibit a larger grain size, pointing out that it reached a higher temperature during the SPS cycle. This is confirmed by numerical simulations demonstrating that with the WC–Co die, the experimental sample temperature at the beginning of the dwell is higher than the experimental control temperature measured at the outer surface of the die. This difference is mostly ascribed to a high vertical thermal contact resistance and a higher current density flowing through the WC–Co punch/die interface. Indeed, simulations show that current density is maximal just outside the copper sample when using the WC–Co die, whereas by contrast, with the graphite die, current density tends to flow through the copper sample. These results are guidelines for the direct, one-step, preparation of complex-shaped samples by SPS which avoids waste and minimizes machining.

New Developments at the National Pulsed Field Laboratory in Toulouse

Pulsed-magnet users are requesting higher fields with longer pulse-times in larger bores. This request led logically to user facilities similar to facilities existing for continuous fields. The Laboratoire National des Champs Magnetiques Pulses (LNCMP) in Toulouse (FRANCE) has a user community from all over Europe. We report on current efforts in Toulouse to fulfill the request of users. We developed and tested coils up to 80 T and reduced the cool-down time. Moreover, we created special design installations: a transportable pulsed-field installation to be used at other large instruments and coils with optimized field profiles for optical experiments. We are currently investigating the sources of noise on our fields and have successfully tested a preliminary method to reduce this noise.

Ultra high strength nanocomposite conductors for pulsed magnet windings

A process based on cold drawing was developed to elaborate high strength nanocomposite wires for pulsed magnet windings. The best results have been obtained for Cu/Nb nanocomposites composed of a copper matrix embedding 9.10/sup 6/ continuous parallel niobium fibers, with a diameter of 40 nm: their ultimate tensile stress is 1950 MPa and /spl rho/=0.6 /spl mu//spl Omega/.cm at 77 K. TEM and HREM studies characterized the nanocomposite structure: the Nb fibers are nanowhiskers embedded in a copper matrix with semi-coherent interfaces. 3D tomographic analysis and in-situ deformations showed that the Orowan mechanism is controlling the dislocations behaviour in this structure. Optimized conductors are developed: the new Cu/Nb nanocomposites will contain 4.10/sup 9/ Nb fibers with smaller diameter d/sub Nb/ (down to 10 nm) to increase the whisker effect, proportional to 1/d/sub Nb/. Concurrently, Cu/Ta conductors are fabricated. The theoretical strength of a whisker is /spl mu//2/spl pi/ (/spl mu/ is the shear modulus). Since /spl mu//sub Ta//spl ap/2/spl mu//sub Nb/, the Cu/Ta UTS should be enhanced. However, the drawing of Cu/Ta billets lead to the formation of a macroscopic roughness at the Cu/Ta interface and the fracture of Ta. This phenomenon is interpreted in terms of stress driven rearrangement (Grinfeld instabilities) and solutions are given to prevent its formation.

Axial and radial interface instabilities of copper/tantalum cylindrical conductors

The shape evolution of conductors is investigated when axial and radial sinusoidal fluctuations appear simultaneously on the cylindrical surface of a stressed surface. An energy variation calculation is performed to determine the theoretical wavelengths of these oscillations, while the profile of the surface is predicted. A study of the rod surface kinetics is also performed to characterize the evolution of the roughness vs time. The results are compared with the experimental observations of the interface of copper/tantalum conductors. The good agreement between the theory and the experiment confirms the interpretation of the phenomenon in terms of Grinfeld instabilities.

Copper/Stainless Steel Polyhelix Magnets

The LNCMI has been involved since many years in the research and development of copper/stainless steel (Cu/SS) macrocomposite conductors for wire wound pulsed field magnets, generating magnetic fields up to 80 Tesla. The mechanical and electrical properties are adjusted to the magnet requirements by selecting the area fraction of the stainless steel reinforcement and the work-hardening state at the end of the drawing procedure.