*Corresponding author. Fax: #33-562-172-816.E-mail address: lecoutur@cict.fr (F. Lecouturier).

Physica B 294}295 (2001) 648}652

Ultra high strength nano"lamentary conductors: theway to reach extreme properties

L. Thilly���, F. Lecouturier��*, G. Co!e�, J.P. Peyrade�, S. AskeH nazy�

�Laboratoire National des Champs Magne& tiques Pulse& s, 143, avenue de Rangueil, BP 4245, 31432 Toulouse Cedex 4, France�Laboratoire de Physique de la Matie% re Condense&e, 135, avenue de Rangueil, INSA, 31077 Toulouse Cedex 4, France

Abstract

To enhance the intensity of non-destructive magnetic "elds with long pulse duration, reinforced conductors are neededwith extremely high mechanical strength and good electrical conductivity. The ideal conductors for this applicationshould have an action integral close to that of pure copper. An elaboration process based on cold drawing and restackinghas been developed at LNCMP for this purpose. The best results have been obtained with Cu/Nb nanocomposite wireswith a section of 3�10��mm� composed of a copper matrix embedding 9�10� continuous parallel niobium whiskerswith a diameter of 40 nm. The ultimate tensile strength is 1950 MPa at 77 K. The fundamental properties linked to thee!ect of nanometer size have been investigated. Nevertheless, because of their small section these conductors cannot bepractically used in the winding of our magnets. Therefore, we are elaborating a new generation of optimized Cu/Nbnanostructured wires exhibiting ultra high strength in a section of 2 mm�. The latest developments are presented.Concurrently, we are developing Cu/Ta multi"lamentary conductors. Since the shear modulus of tantalum is greaterthan that of Nb (�

��+2�

��), the Cu/Ta UTS should be enhanced. However, drawing of Cu/Ta billets leads to the

formation of a macroscopic roughness at the Cu/Ta interface and to the fracture of Ta. This phenomenon is interpreted interms of stress-driven rearrangement (Grinfeld instabilities). We have investigated some solutions to prevent itsformation. � 2001 Elsevier Science B.V. All rights reserved.

Keywords: Nano"lamentary wires; Cu/Nb; Cu/Ta

1. Introduction

The Toulouse high "eld facility is concerned withthe production of high-pulsed non-destructivemagnetic "eld with long pulse duration, and it isinvolved in the development of high-strength con-ductors for many years [1].

The duration of the magnetic "eld pulse �t isproportional to the action integral [AI] which de-pends on the physical properties of the conductor:[AI]"�����

��c�/� d¹, where 63 and 250 K, respec-

tively, are the initial and "nal temperatures of thewinding during the shot, c

�is the volume heat

capacity of the conductor and � its resistivity. It isobvious that high heat capacity and high conduct-ivity of the material for the winding will result inlong duration of the "eld pulse.

Among all the possible structures [2], the conven-tional coil, i.e. with uniform current distribution,

0921-4526/01/$ - see front matter � 2001 Elsevier Science B.V. All rights reserved.PII: S 0 9 2 1 - 4 5 2 6 ( 0 0 ) 0 0 7 3 6 - 5

Fig. 1. The four level structure of the nanocomposites. In thelarge magni"cation, the Nb "laments (d

��"524 nm) appear in

white, surrounded by Cu channels in black.

Table 1Properties of the Cu/29.5 vol% Nb wires

d����

(mm)d��

(nm)���

(77 K)(MPa)

RA(%)

[AI]�}��� �

(A� s m� )

2.92 524 1051 99.44 4.96�10��

1.96 352 1206 99.751.00 179 1564 99.930.435 78 1424 810.223 40 1951 95 3.60�10��

gives the longest pulse duration. The stress distri-bution is proportional to the magnetic pressureP��

"B��/2�

�. The maximum value of the Von

Mises stress (������

) which takes into account thetangential and the longitudinal component of stres-ses is 0.6P

��. Thus, the solution to obtain non-

destructive high-pulsed magnetic "elds is to useinsulated conductors with an elastic limit higherthan �

�����(for 100 T, �

�����"2400 MPa). To

improve the characteristics of the magnetic "eldpulse, the conducting materials used for the wind-ing must possess the highest [AI] value (the closestto the copper one) and the best mechanical proper-ties. In this paper, we will only focus on the sophis-ticated continuous nano"lamentary conductorsdeveloped at Toulouse which satisfy these twocriteria.

2. The 5rst generation of Cu/Nb wires

These Cu/Nb conductors were elaborated viaa process of hot-extrusion followed by cold draw-ing and stacking [1]: the initial billet, composed ofa Cu jacket containing a Nb rod, is strongly drawnto a hexagonal shape; 55 hexagonal segments arestacked in a Cu jacket and the process is iteratedagain 4 times. The "nal structure is a Cu matrixcontaining 55 (i.e. 9�10�) continuous parallel Nb"laments with a diameter in the nanometer range.Fig. 1 illustrates the four-level structure of the con-ductors. All the Nb "bers have the same diameter,whereas the Cu matrix is distributed into the inter-"lamentary channels, Cu-0; the super-"lamentarychannels, Cu-1, etc.

Table 1 gives the properties of the Cu/29.5 vol%Nb composites. For further reduction by drawingbelow 1 mm, recrystallization annealing wasneeded. The consequence is the reduction of thecold work rate RA"(S

�!S

�)/S

�for the conduc-

tors with smallest diameters (S�

and S�

are thesection at the heat-treatment stage and the "naldrawn section, respectively). The best result ob-tained so far is an ultimate tensile strength (UTS) of2 GPa at 77 K (for a wire containing Nb "lamentswith a diameter of 40 nm) with an electrical resistiv-ity of 0.6 �� cm at 77 K and an action integral of3.6�10�� A� s m� which is higher than that of the

in situ Cu/Nb wires (3�10�� A� s m� ) [3]. Thestrength is much higher than the strength given bythe rule of mixture (ROM) calculated with fullycold-worked bulk materials. This strong deviationto the ROM has been reported for many highlydeformed nanocomposites but were not clearlyunderstood [3}5].

TEM and HREM studies characterized thenanocomposite structure. The Nb "bers, whichhave developed a strong �1 1 0� texture during thedrawing process are nanowhiskers: their strength isinversely proportional to their diameter and tendsto the theoretical value for perfect crystals, �/2�,where � is the shear modulus, when d

��is smaller

L. Thilly et al. / Physica B 294}295 (2001) 648}652 649

Table 2Properties of the Cu/11 vol% Nb wires (d"2; 1.06; 0.78 mm)with intermediate annealing

d��

(nm)t����

(nm)RA(%)

���

(77 K)(MPa)

���

(77 K)(MPa)

���

(300 K)(MPa)

67 20 80.3 820 3409 241735 11 73.5 850 3681 285626 8 85.7 925 4363 2993

than 200 nm. They present semi-coherent interfaceswith the copper matrix, with the presence of mis"tdislocations, every 8 atomic planes on average,which is the theoretical value for the complete re-laxation of the interface (with a 10% mis"t) [6]. 3Dtomographic analysis and in situ TEM deforma-tion showed that the Orowan mechanism (motionof single-dislocation loops in narrow channels) iscontrolling the dislocation behavior in the coppermatrix, reinforcing its strength [7,8].

3. Optimization: second generation of Cu/Nb wires

From the microstructure characterization andthe in situ tensile tests, the e!ects of the nanometersize on the mechanical properties have been deter-mined:

� the nano"laments are nanowhiskers. Further re-duction will increase drastically their strength.

� a new mechanism of deformation has been dis-covered in the inter- and super-"lamentary Cuchannels. The Orowan stress is inversely propor-tional to the size of the Cu channels [8].

� once the dislocation loops nucleated and ex-panded in Cu, they are blocked at the Cu/Nbinterfaces, where they are assumed to dissociate,to create the semi-coherency observed byHREM [8]. The interface acts as a dislocationbarrier that stops their motion and thus in-creases the strength of the material.

To strengthen the Cu/Nb continuous nanocom-posites, we must increase the number of "lamentswith a smaller diameter and reduce the size of theCu matrix. However, we have to remember that thehigh-pulsed "eld coils must be built with conduc-tors having a cross-section of several mm�.

In a "rst step, we elaborated a Cu/11 vol% Nbconductor containing N"85 "52�10� "la-ments to establish the feasibility of the new elabor-ation process. The "rst tensile tests were performedon conductors containing nano"laments withd��

(100 nm. The properties of the tested samplesare given in Table 2. �

��(300 K) is the strength of

the Nb nano"laments extrapolated from the UTSat 300 K. The UTS of the samples is much lowerthan that of the "rst generation Cu/Nb wires

(925 MPa vs. 2 GPa as best results), but still 35%higher than the value predicted by the ROM(600 MPa at 77 K); this can be explained as follows:

(i) the fraction of reinforcing phase was very muchlowered to increase the electrical conductivity(11.4% vs. 29.5% )

(ii) to reduce the "lament diameter down to 26 nm,many heat treatments were applied to avoidthe fracture of the wire during the drawingprocess.

Thus, the e!ects of the reinforcing phase and ofthe cold working are reduced. However, the ex-trapolated strength of the "laments at 300 K is notnegligible, since it reaches 3 GPa for the "nest wire,which is close to �/30, the lower theoretical boundfor the elastic limit of whiskers (model of MacKen-zie [9]).

The wire with the smallest Nb "bers (26 nm) hasan electrical resistivity of 0.5 �� cm at 77 K: thisvalue should correspond to an action integral of4.3�10�� A� s m� which is higher than the value(3.6�10�� A� s m� ) obtained with our "rst genera-tion of Cu/Nb wires with Nb "bers of 40 nm.

These results are strongly dependent on the Nbcontent. Therefore, we are now elaboratingnano"lamentary Cu/Nb wires with the same ge-ometry but with higher Nb content (30 vol%) inorder to reach a strength greater than 2 GPa ina section of 2 mm�, which could be used as thewinding of the `Coilina in the Coilin/Coilex system[10] in order to reach at "rst 80 and 100 T in thefuture.

The next step deals with the study of the funda-mental properties of these nanostructured wires atthe atomic scale: we have planed the elaboration of

650 L. Thilly et al. / Physica B 294}295 (2001) 648}652

the 5th stage of bundling in order to reduce the85� Nb whiskers (i.e. 4�10�) to the atomic scale.

4. First studies on the Cu/Ta multi5lamentary wires

The maximum strength for whiskers is �/2�n.The Cu/Ta system should enable signi"cant pro-gress, since the shear modulus of Ta is twice as largeas that of Nb: one can expect to obtain conductorswith a strength close to 4 GPa.

Our "rst experiments have been performed ona Cu billet with a Ta core, which has been drawn atroom temperature. After a reduction in area of60%, the Ta surface presents a macroscopic rough-ness characterized by two sets of perpendicularundulations. This phenomenon, observed for the"rst time in bulk materials, is interpreted in termsof Grinfeld instabilities [11}13], and can be linkedto the non-hydrostatic stress "eld (applied at theCu/Ta interface during the drawing stage) and tothe di!erence of shear modulus. Let us analyze thestresses applied to the wire during the drawingprocess:

� the "rst step is the section reduction occurring inthe die: the stress "eld is mainly radial and isassumed to create the "rst set of undulationsalong the Ta diameter.

� in the second step, the wire undergoes tensilestress along the drawing axis, which can be re-sponsible for the second set of oscillations alongthe Ta axis.

If we apply higher deformation to this system, weobserve fractures of the Ta rod which forbid theelaboration of nano"lamentary wires. The fracturesare assumed to originate from the second set ofoscillations.

We investigated some solutions to prevent theformation of such oscillations: we have successfullydrawn a Cu/Ta billet, "rstly deformed by hot-extru-sion, to hexagonal shape. The reduction in area ishigher than 99.5% without fracture. Thus, we havestacked 85 hexagonal segments in order to followthe process with the con"guration N"85�. Thehot-extrusion seems to be a very promising way todeform such composite materials. An explanationcan be given: during the extrusion, only a radial

stress is applied in the die that leads to the forma-tion of the "rst set of oscillations only [14]. Theevolution of these radial oscillations does not leadto the fracture of the Ta. Further observations willbe performed in order to clearly understand thee!ect of the extrusion.

5. Conclusion

The strengthening mechanism of the Cu/Nbnano"lamentary wires containing 9�10� nanow-hiskers of Nb has been studied to optimize theproperties of conductors with cross-section of sev-eral mm�. The results obtained with the wire con-taining 11 vol% of Nb spread over 52�10� Nbwhiskers show that we are on the right way for theoptimization: we are now elaborating conductorswith the same geometry but containing 30 vol% ofNb, in order to reach a strength higher than 2 GPain a section of 2 mm�.

The Cu/Ta system, investigated for its highervalue of shear modulus, has been deformed by colddrawing: the oscillations observed on the Ta sur-face lead to fracture. To prevent this phenomenon,a Cu/Ta billet has been hot-extruded and success-fully drawn: 85 hexagonal segments have beenstacked in order to continue the process with thecon"guration N"85�.

References

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H. Rakoto, G. Co!e, S. AskeH nazy, J.P. Peyrade,C. Roucau, V. Pantsyrny, A. Shikov, A. Nikulin, ScriptaMat. 38 (11) (1998) 1643.

[4] W.A Spitzig, A.R. Pelton, F.C. Laabs, Acta Metall. 35 (10)(1987) 2427.

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[7] X. Sauvage, L. Thilly, F. Lecouturier, A. Guillet,D. Blavette, Nanostruct. Mater. 11 (8) (1999) 1031.

[8] L. Thilly, O. Ludwig, M. VeH ron, F. Lecouturier, J.P.Peyrade, S. AskeH nazy, Philos. Mag. A. (2000), submittedfor publication.

L. Thilly et al. / Physica B 294}295 (2001) 648}652 651

[9] J.K. MacKenzie (1949), Bristol, UK.[10] S. AskeH nazy, Physica B 216 (1996) 221.[11] L. Thilly, J. Colin, F. Lecouturier, J.P. Peyrade, J. GrilheH ,

S. AskeH nazy, Acta Metall. 47 (3) (1999) 873.

[12] M.A. Grinfeld, Soviet Phys. Dokl. 31 (1986) 831.[13] J. Colin, L. Thilly, F. Lecouturier, J.P. Peyrade, J. GrilheH ,

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