Effects of size and geometry on the plasticity of high-strength copper/tantalum nanofilamentary conductors obtained by severe plastic deformation

doi:10.1016/j.actamat.2005.10.031

Acta Materialia

Volume 54, Issue 4, February 2006, Pages 1063-1075

https://doi.org/10.1016/j.actamat.2005.10.031 Get rights and content

Abstract

Copper-based high-strength nanofilamentary wires reinforced by tantalum nanofilaments were prepared by severe plastic deformation (repeated hot extrusion and cold drawing steps) to be used in the windings of high-pulsed magnets. This application requires a complete characterization of the microstructure and the strength and their relationship for further optimization: after heavy strain, the Cu matrix is nanostructured and the Ta nanofilaments develop a strong ribbon-like shape resulting in an early microstructural refinement. The macroscopic strength is greater than rule-of-mixture predictions as confirmed by nanohardness values. The observed size effect is related to the dislocation starvation in the nanostructured materials combined with the barrier role of Cu/Ta interfaces. The strengthening is lower, however, as expected, because of the distorted ribbon morphology of the Ta filaments preventing them from behaving as nanowhiskers, as Nb fibers do in Cu/Nb wires. This shows that size and geometry play key roles in the plasticity of nanomaterials.

Introduction

The design and construction of non-destructive high-field magnets still represent a significant challenge for materials selection and development because the components have both structural (high elastic limit) and functional (high electrical conductivity) requirements that are usually contradictory. As an example, the production of fields above 80 T requires conducting materials possessing an electrical resistivity close to that of annealed copper (0.2 μΩ cm at 77 K) and a yield stress as high as possible (above 1.8 GPa at 77 K) to support the Lorentz forces without damage [1]. Up to now, reinforced copper-based wires have shown the best compromise between these two criteria; in particular, the copper/niobium (Cu/Nb) system has been studied for decades since it has very good mechanical properties that are much better than rule-of-mixture (ROM) predictions when the size of the Nb filaments is reduced to the sub-micrometer range. It should be emphasized that two main fabrication routes have been studied: production of nanofilamentary Cu/Nb wires via severe plastic deformation (SPD) of: (i) arc-melted Cu–Nb ingots, initially consisting of a Cu matrix containing Nb dendrites, so-called “in situ” composites; and (ii) bulk Cu tubes and Nb rods, giving rise to a Cu matrix containing Nb continuous filaments, so-called “continuous” composites [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13].

The Laboratoire National des Champs Magnétiques Pulsés (Toulouse, France) has been involved in the improvement of these continuous composite materials for more than a decade: a fabrication process based on SPD through repeated extrusion, cold drawing and restacking steps has been developed. The best first generation of Cu/Nb “continuous” nanocomposite conductors were composed of a copper matrix containing N = 55⁴ = 9.15 × 10⁶ parallel continuous Nb filaments with a diameter of 40 nm. At 77 K, they possess an ultimate tensile strength (UTS) of 2 GPa and a resistivity of 0.6 μΩ cm for a 0.04 mm² section [14]. However, conductors with a section between 3 and 7 mm² are required for use in pulsed magnets. Therefore, the effect of SPD on the microstructure as well as the influence of nanometer size on the plasticity of nanofilamentary Cu/Nb wires have been studied by in situ deformation using transmission electron microscopy (TEM), tomographic atomic probes, classic tensile tests and nanoindentation tests. The result is the definition of parameters for mechanical optimization (see Ref. [15] for a complete summary of these studies). Thus, a second generation of Cu/Nb wires containing N = 85⁴ = 52.2 × 10⁶ parallel continuous Nb filaments were fabricated: the best nanofilamentary “continuous” conductor, containing Nb filaments with diameters of 142 nm, shows for a section of 5 mm² an UTS of 1.9 GPa and a resistivity of 0.58 μΩ cm at 77 K [16].

Studies of strengthening mechanisms have led to different ways of optimization. One is based on the following result: the strength of Nb nanofibers was observed to be inversely proportional to their diameter and approached, for the smallest diameters, the theoretical strength for perfect crystals, μ/2π, where μ is the shear modulus; the Nb filaments can therefore be considered as nanowhiskers [13], [17]. Since the shear modulus of tantalum (Ta) is higher than that of Nb (μ_Ta = 69 GPa > μ_Nb = 40 GPa), the Cu/Ta system is assumed to induce better mechanical properties. The same trend is expected using Fe as the reinforcing phase, since μ_Fe = 80 GPa (in this case, however, poorer electrical properties are expected due to the miscibility between Cu and Fe at high temperatures). In the literature, numerous studies of heavily deformed composites which consist of a face-centered cubic (fcc) matrix and a body-centered cubic (bcc) reinforcing phase are reported: “in situ” Cu/Nb [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], Cu/Ta [18], [19] and Cu/Fe [20], [21] systems. They all illustrate the considerable interest in Cu-based fcc/bcc composites because of the good combination of conductivity and strength. In particular, the improved strength observed in “in situ” Cu/Ta nanocomposites compared to “in situ” Cu/Nb ones [18], [19] seems to confirm that Cu/Ta wires exhibit better strengthening mechanisms. The crucial point is to understand the origin of such increased strength: does it derive from intrinsic properties of the Ta component (in the bulk state, Ta is harder than Nb) or from microstructural features such as geometry of the reinforcement or size effect on the composite plasticity (whisker effect in the Ta fibers or other dimensional dependence)?

In order to attempt to answer this question, we report here some recent developments of “continuous” Cu/Ta ultrahigh-strength conductors, and compare their properties to “continuous” Cu/Nb nanofilamentary wires.

Therefore, this paper reports on the complete characterization of different Cu/Ta samples taken throughout the fabrication process. The electrical resistivity was measured using the four-probe technique, while the microstructure was characterized using scanning electron microscopy (SEM), TEM and X-ray diffraction (XRD); the mechanical properties were investigated using macroscopic tensile tests, as well as nanoindentation. Particular care was taken to link the characterization results obtained at different length scales (micro-/macroscale).

Section snippets

Fabrication process of Cu/Ta nanofilamentary wires by SPD

As mentioned earlier, the Cu/Ta system was chosen for the higher value of shear modulus of Ta with respect to Nb. Its co-deformation behavior was initially investigated by cold drawing: radial and longitudinal oscillations were observed at the Cu/Ta interface leading to untimely fracture [22]. To prevent this phenomenon, a hot extrusion stage was added prior to the cold drawing process. The stress field is indeed simplified during extrusion since only section reduction occurs in the die;

Texture analysis

The development of the macroscopic texture during the repeated hot extrusion and cold drawing stages was first studied using XRD measurements. The local texture of Cu and Ta was also investigated using TEM selected area electron diffraction (TEM SAED) as discussed in Section 3.2.

XRD patterns were obtained using a Seifert diffractometer. The texture developed in the drawing axis direction was characterized by standard θ–2θ scans using Cu Kα radiation. θ–2θ scans were obtained from cross-sections

Electrical characterization

The electrical resistivity of Cu/Ta conductors was measured using the classic four-probe technique with a maximum current of 100 mA to avoid heating of the wires. The resistivity values for samples a9 (d = 2.41 mm), a10 (d = 1.83 mm) and a11 (d = 1.398 mm) at 77 K are, respectively, 0.375, 0.382 and 0.401 μΩ cm. The decrease of the electrical conductivity with a reduction in diameter results from the increase of dislocation density, the dislocations being scattering centers for electrons, and also from the

Experimental procedure

This section describes the mechanical testing of the Cu/Ta wires via macroscopic tensile tests as well as a nanoindentation study of the local mechanical properties with the indentation axis parallel to the filament axis. Segments of the nanocomposite conductors were cut using a metallographic saw and embedded in epoxy resin so that the cross-sectional surface of the wire can be perfectly perpendicular to the indenter axis. The samples were then mechanically polished. Nanoindentation

Conclusion

Cu/Ta nanofilamentary wires obtained by SPD applied via successive hot extrusion and cold drawing steps were characterized and compared to Cu/Nb conductors. During the fabrication process, a very fine microstructure is achieved in the Cu/Ta wires:

•
The Cu microstructure appears similar to that observed in Cu/Nb wires: (i) the largest Cu channels correspond to heavily deformed Cu composed of grains in the micrometer and nanometer range with a high dislocation density; (ii) the finest Cu channels

Acknowledgements

The authors thank the technical team of LNCMP, L. Bendichou, G. Coffe, N. Ferreira, J.M. Lagarrigue and J.P. Laurent, who participated in the fabrication of the conductors.

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Effects of size and geometry on the plasticity of high-strength copper/tantalum nanofilamentary conductors obtained by severe plastic deformation

Abstract

Introduction

Section snippets

Fabrication process of Cu/Ta nanofilamentary wires by SPD

Texture analysis

Electrical characterization

Experimental procedure

Conclusion

Acknowledgements

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J Cryst Growth

Adv Eng Mater