US3476615A - Niobium-titanium superconductor - Google Patents

Description

NOV 4. 1969 D. F. FAIRBANKS r-:TAL 3,476,615

NIOBIUNFTITNIUM SUPERCONDUCTOR Filed Sept. 6. 1966 Ov Om ON Nmsxm/ United States Patent U.S. Cl. 14S- 11.5 4 Claims ABSTRACT F THE DISCLOSURE Niobium-titanium superconductor wire is fabricated from ingot to wire through a series of cold working reductions with two intermediate anneals, the total cold working being less than 1500 times and less than 90 times before the second anneal, resulting in substantially increased critical current density.

The present invention relates to the production of a niobium-titanium superconductor, and a principal object of the invention is to provide such a superconductor of relatively large cross-sectional area having a high-field critical current density considerably greater than has been obtained by prior techniques for such large crosssectional areas.

PRIOR ART The basic utility of niobium-titanium alloys for superconducting uses is described in the patent to Matthias 3,167,692 which shows the importance of cold work in increasing superconductive properties. The patent to Kneip et al., 3,215,569, Nov. 2, 1965, discusses in some detail the cold work and heat treatment of alloys of niobium with other Group IVA metals. Specic reference to cold working and heat treatment of niobium-titanium alloys for increasing current density is found in the Journal of Applied Physics, vol. 6, No. 3, p. 1179, March 1965, wherein the cold work and heat treatment of a niobiumtitanium alloy containing about niobium is discussed by Vetrano and Boom. Similarly, the doctoral thesis of Ralls, entered in the Massachusetts Institute of Technology Library in June 1964, describes the general development of niobium-titanium alloys and the elect of heat treatment. Parallel Work of Westinghouse is described in British Patent 1,019,888, published Feb. 9, 1966. The most recent literature known to applicants regarding the cold work and heat treatment of niobiumtitanium alloys is the Work published by Atomics International in the Quarterly Reports on Contract No. NAS 8-5356. In this work a titanium ingot having 22 atomic percent niobium was swaged to 0.025 inch, stress relieved for 4 hours at 550 C., then drawn to 0.010 inch and subsequently annealed for various times at various temperatures.

The present invention is particularly concerned with the production of superconductors having high critical current density at high magnetic elds by multiple cold work and multiple heat treatment steps during the formation of the superconductor wire or ribbon. In particular, the' invention is directed to the use of at least 2 heat treatment steps before the alloy has been cold reduced by more than about 90 times.

In the present invention the multiple heat treatment operations are accomplished sequentially so as to give a product having a critical current density at a given size substantially in excess of the critical current density which can be obtained by prior art techniques. Specifically, it enables a critical current density in a 20 mil wire of about l 105 amp/cm.2 at 60 kilogauss which is equal 3,476,615 Patented Nov. 4, 1969 ice to the critical current density only previously obtained obtained by reducing the wire to 10 mils.

While the exact mechanism involved in the present invention is not completely understood, the dramatic effects of the invention are clearly demonstrable. It is believed that in providing numerous alternate cold work and heat treatment steps various preferred metallographic structuresare obtained. This is believed to be due to the fact that after `a given amount of cold work, for example a reduction of 6 times, a number of defect centers are created in the crystal lattice. Precipitates are preferentially formed at these centers when the alloy is subjected to suitable heat treatment to precipitate a second phase. With the initially formed defect centers llled by the precipitated second phase, additional cold working will provide additional defect centers at which additional precipitation can occur on subsequent heat treatment. Without giving specific quantitive data regarding the defect structures and their exact description, it is believed that the multiplicity of alternate cold work and precipitation steps provides extremely uniform distribution of defect structures throughout the alloy.

The present invention also contemplates, as a part of the sequential heat treatment, that the heat treatment be less than is necessary to bring the second phase to equilbrium so that all of the potential second phase will not be precipitated in the earlier heat treatment operations. Conversely, subsequent heat treatments must be at a sufciently low temperature so that the previously precipitated second phase will not be redissolved. Accordingly, the temperature of the subsequent heat treatments is preferably the same as, or lower than, the temperature of the preceding heat treatments.

With this general discussion of the invention in mind, reference should be had to the following detailed discussions taken in connection with the drawing wherein:

FIG. l is a graph showing the Icritical current as a function of wire diameter for several different Wires having the same composition and drawing techniques, but with different heat treatment steps.

Referring now to a specific embodiment of the invention, there is set forth below in Example l one preferred method of practicing the invention.

EXAMPLE 1 An arc melted ingot was formed under argon to give an ingot 3.5 inches in diameter by w20 inches long. This ingot had the following composition:

The ingot was forged at 480 C. to 2% inches in diameter. It was then put in a copper can having 3.5 inches O D. and 2% inches LD. and then the NbTi and Cu were extruded at 650 C. to 3%: inch I D. (.6 inch diameter NbTi) and cold swaged to a NbTi diameter of .2 inch. The NbTi ingot had thus been cold worked from 3.5 to 2%, a reduction of 1.6 times as calculated by dividing the cross-sectional area of the ingot before working by the cross-sectional area of the bar after working. It was hot worked 22 times and cold worked a further 9 times by swaging. In this connection, the 480 C. forging is metallurgically cold Working while the 650 C. is metallurgically hot. The .2 inch rod was then heated at 450 C. for 4 hours. Thereafter the rod was cold drawn to .08 inch wire (a further reduction of 6 times) and heat treated at 450 C. for 4 hours. The wire was then nally reduced to .020 inch (a further reduction of 16 times) 3 and tested with the results shown in FIG. 1. The total cold reduction was about 1400 times.

EXAMPLE 2 In this case the procedure was essentially the same as that used in Example l except that a third heat treatment of 2 hours at 250 C. was employed after the wire was reduced to the nal diameter.

EXAMPLE 3 This was identical to Example l except the 450 C. heat treatment at .2 inch was eliminated so that the wire was only heat treated after cold work at .08 inch.

EXAMPLE 4 This was identical to Example l except that the 450 C.

heat treatment at .08 inch was eliminated so that the wire was only heat treated after cold work at .2 inch.

In FIGURE 1 the test results of the various wires prepared in Examples 1 through 4 are plotted to show current density at 60 kilogauss applied magnetic iield as a function of wire diameter. In running these tests, samples of the wire were taken at the various indicated diameters and the current density measured for each sample. In all cases the actual current measured was normalized to a l0 mil wire diameter to give current density. Some of the curves do not cover the whole wire size range, but the trendsv-`v can be seen. From the results plotted, it is apparent that the three heat treatments of Example 2 give the highest current density particularly at the higher wire diameters. The next best is the wire of Example 1. Examples 3 and 4 which had only one heat treatment had significantly lower current densities at the larger diameters.

While several specific embodiments of the invention have Ibeen described above, all of these being based on two specilic temperatures of heat treatment, numerous modifications thereof may be made without departing from the spirit of the invention. For example, the ternperature range of the intermediate heat treatments can be preferably between about 300 C. and about 500 C. The time of heat treatment can range on the order of minutes to 20 hours. Naturally, from an economic standpoint, the shorter time is desired. With respect to the final heat treatment (astypiiied by Example 2), when the product is subject t0 no further cold working, the temperature should be rather low, between about 200 C. and 350 C., and the time should be between about M1. hour to 5 hours. Quite satisfactory results are obtained when the procedure of Example 2 is utilized, e.g. 250 C. for 2 hours.

In connection with the inal heat treatment of Example 2, it should be pointed out that this has the important Vadditional eect of annealing the copper so as to increase .the conductivity ratio to a number as high as about 200.

This conductivity ratio is the ratio of the conductivity of the copper and 4.2 K. and the conductivity of the copper at room temperature, 'both being measured under zero field. The high conductivity of the copper at the very low temperature provides excellent shunt paths for the superconducting current in the event that the niobium-titanium superconductor goes normal due to iux jumping in a magnet embodying this superconductor.

While one specific alloy has been given in the examples, numerous other modifications of the titanium-niobium system may be employed. In general; the titanium content runs between about 10% and about 70%.

Since certain changes Caftfbe.,madein` the above process rvvitlflout departing from the 'scope' of the invention herein involved, it is intended that all matter contained in the above description shall be )interpreted as illustrative and not in a limiting, sense.

What is claimed is:

1. A method of increasing the high-field critical current density of a niobiurn-titanium,` superconductor which comprises cold reducing a niobium-titanium body by N1 times, heat treating said cold worked body to atemperature between about 300 C. to 500 C. for a time on the order of -30minutes.to;20 hours,furt,her cold. working ,v the heat treated. body with` the-reduction-of at. least 1 N2 times," heatr treatingvsaidY further .reduced producty to a temperature between 300 C. and 500. C. `for a-time ranging from 30 minutes to 20 hours and further cold reducing said body by UN2; lfiriiestlie product of N1 times N2 times N3 being less than 1500.

2. The method of claim 1 wherein the product of N1 times N2 is between 30 and 150.

3. The method of claim 1 wherein N3 is between l0 and 150.

4. The method of claim 1 wherein the finally cold reduced superconductor is given a heat treatment between about .200 C. ,and 35.0 C. for a time ofy between 11thour and 5 hours. ,p

. i References Cited vUNITED-STATES 'PATENTS 8/1966- Reynolds 14S-11.5

OTHER REFERENCES Baranov et al., Determining the Optimum Conditions of Heat Treatment for the Superconductive Alloy Nbpercent Zr, Mar. 22, 1966, pp. -150.

`HYLAND BIZOT, Primary Examinerv

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