V. Vidal

Elaboration by Severe Plastic Deformation, Microstructural and Mechanical Study of Cu/X (X =Ta or Nb) Nanofilamentary Wires for Use in High Field Magnet

Nanofilamentary wires consisting of a Cu matrix reinforced by body centred cubic (bcc) nanofilaments were produced by successive hot extrusion and large strain drawing. Effects of this severe plastic deformation on the microstructure and the mechanical properties of two systems, Cu/Ta and Cu/Nb/Cu “co-cylindrical” structure, are presented and compared with the nanofilamentary Cu/Nb wires.

Plasticity Mechanisms in Multi-Scale Copper-Based Nanocomposite Wires

Copper-based high strength nanofilamentary wires reinforced by Nb nanofilaments are prepared by severe plastic deformation (repeated hot extrusion, cold drawing and bundling steps) for the winding of high pulsed magnets. The effects of microstructure refinement on the plasticity mechanisms were studied via nanoindentation, in-situ deformation in TEM and under neutron beam: all results evidence size effects in each nanostructured phase of the nanocomposite wires, i.e. single dislocation regime in the finest regions of the Cu matrix and whisker-like behaviour in the Nb nanofilaments. The macroscopic high yield stress is thus the results of the combination of the different elastic-plastic regimes of each phase that include size effects.

S30 THE BAUSCHINGER EFFECT IN NANOFILAMENTARY Cu/Nb WIRES EVIDENCED BY IN-SITU TENSILE TESTS UNDER SYNCHROTRON RADIATION

Nanocomposite wires are processed by severe plastic deformation (Accumulative Drawing and Bundling) to obtain a multiscale Cu matrix embedding Nb nanotubes. They exhibit high strength that results from size effects in the Cu nanochannels and in the Nb nanotubes (Scripta Mat 57 (2007) 245). The macroscopic stress-strain curve during tensile loading-unloading cycling exhibits an increasing hysteresis, evidencing the presence of internal stresses (Bauschinger effect). By performing such load-unload experiments in-situ under synchrotron radiation at the Swiss Light Source, it is possible to follow continuously the diffraction peak positions and peak widths of the large and the fine Cu channels as well as the Nb nanotubes, revealing the details of the load sharing and deformation mechanisms responsible for the Bauschinger effect. During tensile loading, the large Cu channels provide most of the plastic strain and dislocation storage can be evidenced. The fine Cu channels and Nb nanotubes however mainly store elastic energy. Upon tensile unloading, this energy is partly released via reverse yielding in compression of the large Cu channels: in other words, the large Cu channels are subjected to a built-in true Bauschinger test (inversion of load direction). The observed large plastic strain gradient, added to the sink character of the Cu-Nb interfaces, induce the continuous build-up of internal stresses during co-deformation. These results evidence unambiguously the reverse yielding of the soft phase in composite material with strong yield stress mismatch and bring further understanding to the complex residual stress state of nanocomposite materials obtained by severe plastic deformation processes where repeated loading-unloading cycles are applied (APL 90 (2007) 241907).