Superconducting properties of Nb3Al wire fabricated by theclad-chip extrusion method and the rapid-heating,

quenching and transformation treatment

Sakae Saito a,*, Hiroharu Sugawara b, Akihiro Kikuchi c, Yasuo Iijima c,Kiyoshi Inoue c

a Ashikaga Institute of Technology, Ashikaga 326-8558, Japanb Graduate Student of Ashikaga Institute of Technology, Ashikaga 326-8558, Japan

c National Research Institute for Metals, Tsukuba 305-0003, Japan

Abstract

The study of a fabrication process for Nb3Al intermetallic compound wire and measurements of its superconducting

properties are presented. The adopted process of fabrication for the superconducting wire consists of the clad-chip

extrusion (CCE) method and the rapid-heating, quenching and transformation (RHQT) treatment. The former (CCE)

is the processing of Nb/Al composite wire characterized by the extrusion of thin chips of Nb/Al clad-sheet in which the

thickness ratio of the Nb-layer to the Al-layer corresponds to Nb3Al. The latter (RHQT) is the heat-treatment method

to transform the CCE-processed Nb/Al composite wire into the Nb3Al intermetallic compound, which gives a nearly

stoichiometric composition and a fine grain of Nb3Al for high performance of superconductivity. The experimental

results show that the wire is successfully fabricated by the CCE method and the RHQT treatment, and its supercon-

ducting properties are improved to 18.02 K of Tc and 318 A/mm3 of Jc at 20 T and 4.2 K. It is also shown that 26.3 T ofHc2 is estimated by a Kramer plot.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Nb3Al; CCE method; RHQT treatment; Critical current density

1. Introduction

The intermetallic compound Nb3Al is apromising superconductor for high magnetic fieldapplications because of its excellent inherent su-perconducting properties. However, the brittlenessof this compound at room temperature makes itdifficult to establish a commercially available fab-

rication process of the wire. Up until now, variousmethods have been explored to produce Nb3Alwire, such as the jelly-roll (JR) process [1], thepowder metallurgy method [2], the Nb-tube (rod intube, RIT) process [3], and the clad-chip extrusion(CCE) method [4]. They are two-step operations.The first step of each is the working of Nb/Alcomposite wire with a fine (<0.1 lm) dual layeredstructure, and the second is the subsequent diffu-sion treatment to form Nb3Al at temperaturesbelow 1273 K to suppress grain coarsening. Thosemethods are effective to some extent, but the

Physica C 372–376 (2002) 1373–1377

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*Corresponding author. Fax: +81-284-62-9802.

E-mail address: ssaito@ashitech.ac.jp (S. Saito).

0921-4534/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

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intrinsic excellent superconducting properties can-not be obtained because of their heat treatmentprocess to form Nb3Al. That is, such a relativelylow temperature does not promote the diffusionreaction resulting in a non-stoichiometric compo-sition of the Nb3Al phase. A nearly stoichiometriccomposition and fine grains are both necessary forthe wire to exhibit superior performance in a highmagnetic field.Recently the rapid-heating, quenching and

transformation (RHQT) method [5] was developedfor Nb3Al wire, with not only a nearly stoichio-metric composition but also fine grains. The pro-cess is as follows. Rapid heating to a very hightemperature, around 2200 K, is applied to Nb/Alcomposite wires for a short time, followed by rapidquenching to room temperature, to form a super-saturated solid solution of Nb(Al)ss, and the sub-sequent transformation annealing at relatively low

temperatures below 1073 K. The RHQT-methodhas been successfully applied to both the JR-pro-cessed [6,7] and the RIT-processed [5] Nb/Alcomposites wire, but it had not been appliedto CCE-processed Nb/Al composites wires. It isimportant to study the wide applicability of theRHQT-method to Nb/Al composite wire pro-cessed with other fabrication processes.The present study is focused on the fabrication

of Nb3Al wire by the combination of the CCEmethod and the RHQT treatment, and on theevaluation of its superconducting properties.

2. Experimental

The CCE process is schematically shown in Fig.1. It consists of clad-rolling to obtain a Nb/Al-layered composite sheet, cutting the clad-sheet

Fig. 1. Schematically drawn fabrication process of Nb3Al wire by the CCE method.

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into pieces, filling up the pieces into a container,and extruding a billet, followed by wire drawing.The practical experimental conditions are as fol-lows.Nb sheets of 1 mm thickness and the Al foils of

0.14 mm thickness were used as starting materials.Three layered Al/Nb/Al clad-sheets were obtainedby cold rolling at 70% reduction per pass to thefinal thickness of 0.2 mm followed by cutting theminto pieces of roughly 10 mm2. They were put intoa Cu-container and then extruded at room tem-perature. After removing the Cu-surface layer, theresulting Nb/Al composite bar was put into an Nb-tube followed by re-extrusion to get the Nb/Alcomposite with Nb-surface layer. This processis necessary for the RHQT treatment. The Nb-sheathed composite bar was t drawn to wire of0.9 mm diameter.The Nb/Al composite wire obtained was then

subjected to rapid-heating and quenching treat-ment using the apparatus shown in Fig. 2, where itwas set to wind off of the supply reel and onto thewinding-up reel in a vacuum chamber. The Nb/Alwires were ohmic-heated between the electrodepulley and Ga-bath to a very high temperature(nearly 2200 K) and instantaneously quenched ina molten gallium bath at about 320 K. Moltengallium acts simultaneously as an electrode forohmic heating and a coolant for quenching. The

winding-up speed was 1 m/s. The applied voltagewas changed from 15 to 17 V to control the tem-perature of the specimens. The wire obtained bythis process was a supersaturated solid solutionof Nb(Al)ss. It was then transformed to the inter-metallic compound Nb3Al by annealing at tem-peratures from 973 to 1023 K for holding timesfrom 4 to 160 h.The conventional four-terminals method was

used to evaluate the superconducting propertiesof the critical temperature Tc, and the criticalcurrent Ic under the applied magnetic field B up to28 T. The critical current density Jc was obtainedby dividing Ic by the cross-sectional area of Nb3Alwithout the Nb-surface layer.

3. Results and discussion

Fig. 3 shows the longitudinal cross-section ofthe extruded composite, where white and blacklayers correspond to Nb and Al, respectively. Thefollowing metalworking was successfully carriedout to fabricate Nb/Al composite wire that had adiameter of 0.9 mm and a length of over 40 m forthe RHQT treatment. It had a fine Nb/Al dualstructure in the cross-sections as shown in Fig. 4.The thickness of the Nb and Al layers was esti-mated to be about 140 and 40 nm, respectively, by

Fig. 2. Schematically drawn ohmic-heating and rapid-quench-

ing apparatus.

Fig. 3. Longitudinal cross-section of the extruded Nb/Al

composite.

S. Saito et al. / Physica C 372–376 (2002) 1373–1377 1375

a microscopic examination and a calculation of thereduction ratio in this metal working process. Sucha fine structure was suitable for forming a super-saturated solid solution of Nb(Al)ss by the rapid-heating and quenching treatment. The chemicalcomposition was estimated to be 76.5 of Nb and23.5 at.% of Al by calculating the thickness ratioof Nb and Al layers in the clad-sheet used as astarting material.The measurements of the critical temperature

Tc were carried out for various conditions of theRHQT treatment. Most values were above 17 K,and the highest one was 18.02 K, which was neverbefore obtained by conventional heat-treatment[4,8]. This result reveals that the RHQT-treatment

has been successfully applied to the CCE-pro-cessed Nb/Al composite wires in order to obtain anearly stoichiometric Nb3Al, although the precisemicrostructure has not been investigated in thisstudy.Fig. 5 shows the comparison of Jc–B curves

between the best result obtained in this study andthe data from Ref. [8]. It reveals that the RHQTtreatment enhances the Jc-property compared withthe conventional heat-treatment [4,8]. Fig. 6 showsa Kramer plot from the data shown in Fig. 5, bywhich the critical magnetic field (Hc2) is estimatedto be 26.3 T at 4.2 K. This value corresponds tothe improvement of Hc2 by 3 T, compared with thedata in Ref. [8]. Those enhancements are causedby the microstructure introduced by the RHQTtreatment. That is, the fine grain structure witha nearly stoichiometric Nb3Al in the wires mightincrease the pinning force in the high magneticfields. Further enhancement of superconductingproperties may be possible after investigating thechemical composition and reducing the layerthickness of Nb and Al in the CCE-processed Nb/Al composite wires and optimizing the RHQTtreatment conditions.

Fig. 5. Critical current density (Jc) versus applied magnetic field(B) curves.

Fig. 4. Cross-section of the Nb/Al composite wire: (a) trans-

verse, and (b) longitudinal.

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4. Summary

The Nb3Al wire was successfully fabricatedby the combination of the CCE method and theRHQT treatment. The superconducting properties

have been improved to 18.02 K of Tc and 318 A/mm3 of Jc at 20 T and 4.2 K. It is also shown that26.3 T of Hc2 is estimated by a Kramer plot. Thesevalues should be compared with 17 K of Tc, 200A/mm3 of Jc and 23.2 T of Hc2 obtained from theconventional CCE-processed wire. There remainmore opportunities for enhancement of supercon-ducting properties by optimizing process parame-ters in both methods.

Acknowledgements

The superconducting properties were measuredin the High Magnetic Field Research Station ofthe National Research Institute for Metals, Tsu-kuba, Japan. We are indebted to their staff forhelping us to carry out the experiments.

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Fig. 6. Kramer plot of J 0:5B0:25 versus applied magnetic field(B).

S. Saito et al. / Physica C 372–376 (2002) 1373–1377 1377