Superconducting wire

A Type II superconducting alloy is formed into a wire by extruding a billet of the alloy encased in a copper extrusion can. The extrusion can may include a nose and a tail section having a k factor equal to or greater than that of the body of the core. The nose section may also have a convex inner transverse surface, while the tail section has a concave inner transverse surface. The nose and tail section may be formed of a copper alloy which is of equal or greater stiffness than the alloy of the body of the core. Alternatively, a multifilament wire may be formed by extruding a billet composed of a plurality of separate superconducting wires encased in a copper extrusion can. The nose and tail section of the can may be modified as for the aforementioned monofilament billet.

BACKGROUND OF THE INVENTION 
In the production of high quality fine filament NbTi superconducting wire, 
manufacturers must rely heavily on the integrity of diffusion barriers. 
These barriers, usually Nb, are positioned between the copper cladding and 
the NbTi superconducting alloy that constitutes the bulk of a filament. 
The barrier serves to inhibit the formation of brittle CuTi intermetallics 
at the filament surface. Current processing schemes for the production of 
NbTi superconducting wire containing 4000-7000 filaments, each about 6 
microns in diameter at the final wire size, utilize Nb barriers that 
constitute about 4% of the filament (non-copper) volume. The processing 
will typically subject the barrier to 500.degree.-800.degree. C. 
temperatures over a period of approximately 15 hours, followed by three or 
more 300.degree.-450.degree. C., 40-80 hour heat treatments as the wire is 
brought to final size. Current densities (J.sub.c 's) in excess of 2750 
A/mm.sup.2 at 5 Tesla (T) and 1600 A/mm.sup.2 at 7 T (4.2.degree.) can be 
achieved in fine filament conductors processed in this way (see, for 
example, "Superconducting Wire and Cable for the Superconducting Super 
Collider", T. S. Kreilick, E. Gregory, D. Christopherson, G. P. Swenson, 
and J. Wong, Supercollider 1, Plenum Press, 1989, 235-243). 
While not unacceptable, these current densities are well below the 3800 
A/mm.sup.2 (5 T, 4.2.degree.) that has been achieved in wires with larger 
diameter filaments. The degradation in current density as the filaments 
grow finer is directly attributable to problems with the diffusion 
barrier. 
The long periods at elevated temperatures during processing tend to 
undermine the Nb barrier due to Ti diffusion from the NbTi alloy core. It 
must be remembered that as Ti diffuses into the pure Nb barrier, Ti is 
depleted from the surface of the NbTi alloy filament core, resulting in 
lower overall J.sub.c in the core. The diffusion of Ti through the barrier 
also results in CuTi compound formation at the surface of the filament. 
This brittle CuTi compound fractures during cold reduction, resulting in 
"nodules" that adversely affect both the J.sub.c and the ductility of the 
wire. The surface interfaces between the copper matrix, the Nb barrier, 
and the NbTi core in a filament play a central role in this process. 
In cases where irregular interfaces occur, extensive localized thinning of 
the diffusion barrier is observed. The effect is most pronounced in areas 
where there are projections into the barrier and even into the surrounding 
copper. It is believed that these areas are where CuTi compound will first 
form, and in greater quantity than in areas that do not display such 
thinning. These CuTi regions, the nodules, fracture during wire drawing 
and subsequently cause sausaging of the filaments. As a result, the wire 
displays severely degraded J.sub.c performance and is prone to breakage. 
The obvious solution to the problem of diffusion barrier thinning is to 
simply use a thicker barrier, so that there are no thin spots to cause 
problems. Unfortunately, an increase in the barrier thickness comes only 
at the expense of the NbTi. The barrier supports no J.sub.c at operating 
magnetic fields, so thickening the barrier reduces the overall J.sub.c. In 
addition, increasing the barrier thickness in no way remedies the problem 
of Ti depletion in the NbTi. While not so dramatic as the problem of CuTi 
compound formation, this problem cannot be ignored if current capacity is 
to be improved. So, while increased barrier thickness is an obvious 
solution to the problem of barrier thinning, it is far from ideal. 
Rather than compensate for surface irregularities by thickening the 
barrier, we have found that it is better to eliminate those 
irregularities--i.e., to provide as uniform a surface behind the diffusion 
barrier as possible. When it is surrounded by a uniform surface, the 
diffusion barrier becomes more efficient. Nodule formation can be largely 
eliminated and, to the extent that any barrier might have to be thickened, 
the necessary increase can be minimized. The production of uniform NbTi 
surfaces in filaments is the primary objective of the invention. 
Whenever two or more metals are co-processed, the degree of irregularity at 
their interface depends upon a great many factors. Among these are: 
1. The relative hardness of the materials. 
2. The grain size within the materials. 
3. Texture and overall structure within the materials as dictated by prior 
metallurgical and mechanical processing (hot work, cold work, and total 
deformation, e.g.). 
The ideal interface, from the standpoint of uniformity, is that between two 
metals of nearly equal hardness, both having very fine grains aligned 
along the plane of the interface so as to present the smoothest possible 
surfaces. Clearly, rolled sheet materials fit this description well. The 
use of rolled sheet for the purpose of increased filament uniformity forms 
the basis of the present invention. An earlier U.S. Pat. No. 4,646,197, 
issued to Supercon, Inc., related to the fabrication of Ta capacitor wire, 
wherein fine grained Ta sheet was wrapped around a Nb or Ta ingot to 
provide a smooth wire surface with marked resistance to Ta grain growth 
across the wire at elevated temperatures.

In the production of fine filament superconducting wire, the current 
practice is to fabricate a monofilamentary wire from which 
multifilamentary wire is subsequently fabricated through the performance 
of one or more restacking operations. The monofilamentary wire is produced 
from a billet that generally consists of three elements a superconducting 
alloy ingot, a barrier material which is in the form of sheet that is 
wrapped around the ingot, and a copper extrusion can that surrounds both 
the barrier and the ingot. In the present invention, a fourth element is 
added: a series of fine grained (ASTM micro-grain number &gt;8) sheets of 
superconducting material situated between the barrier sheets and the ingot 
(see FIG. 1). These superconducting sheets reduce the degree of surface 
irregularity at the interface between the diffusion barrier and the ingot 
and are also believed to increase J.sub.c by virtue of the increased 
magnetic flux pinning afforded by both the finer grain structure in the 
sheets and the presence of sheet to sheet interfaces. 
The superconducting alloy sheet may constitute all of the superconducting 
cross-sectional area or any portion thereof, the amount to be determined 
by the degree of uniformity required and the particular demands of the 
application. The sheet may be of any composition within the limitation 
that it is the same basic alloy as the central ingot. For example, if the 
ingot is Nb46.5 wt %Ti, the NbTi sheet may have the composition Nb50 wt 
%Ti. Again, this will be determined by the application. 
The smooth sheet masks the minute irregularities of the ingot surface so 
that no localized thinning can occur in the diffusion barrier as a result 
of penetration by grains or by imperfections left over from the ingot 
fabrication. In addition, the ability to vary the sheet composition is a 
major advantage in light of diffusion losses. In the case of NbTi, the use 
of Ti rich sheets around the ingot will serve to compensate for diffusion 
losses so that the J.sub.c of the NbTi region will be maintained. The use 
of higher Ti concentrations (around 50% Ti by weight) in the sheet can 
also provide for improved J.sub.c performance. Ingots composed of these 
.about.50 wt %Ti alloys are difficult to cold-work, but this problem does 
not arise if the superconducting area is properly divided between Ti rich 
sheet and a less rich ingot. 
The practice of the present invention has shown an additional advantage to 
the use of NbTi sheet. In brief, the experiment that was performed 
involved the fabrication of two monofilament billets, one containing a 
standard ingot, the other containing NbTi sheets wrapped around a solid 
NbTi core. After processing, it was found that the wire containing the 
sheets displayed a higher current density as compared with the solid NbTi 
material. This was true regardless of wire size and throughout the 1-9 T 
magnetic field range used in the J.sub.c tests. The J.sub.c in the sheet 
area was calculated based upon the known billet dimensions and was found 
to be 8-10% higher than in the solid NbTi. We therefore have reason to 
believe that the use of NbTi sheet can also increase the current density 
of a filament. Most likely this increase is due to an improvement in flux 
pinning caused by the interfaces between the sheets, but as yet that is 
not known with certainty. 
The present invention clearly constitutes a major improvement on current 
barrier technology. Both the existing demand for high quality NbTi 
superconducting wire containing 6 micron filaments and the magnet 
industry's desire for wire having filaments as small as 2.5 micron is 
served by the application of the present invention. In particular, to 
produce the 2.5 micron material required (23,000 filaments) an additional 
restack is necessary. This additional processing step places even more 
pressure on the barriers than is imposed in the processing of 6 micron 
filament material. The present invention helps to maximize the 
effectiveness of barriers so that the most can be accomplished with the 
smallest amount of barrier material. 
A preferred method of practicing the invention is set forth in the 
following non-limiting example. 
EXAMPLE I 
FIG. 1 is a schematic diagram of a NbTi monofilament billet cross section. 
The billet is typical for monofilament NbTi except that Nb50 wt %Ti sheets 
have been included where solid NbTi ingot would ordinarily be. The Nb 
diffusion barrier and the NbTi sheets each occupy about 4% of the 
non-copper volume. The NbTi sheets have the composition Nb50 wt %Ti in 
order to compensate for Ti depletion during the processing and to somewhat 
increase the J.sub.c in the final conductor. The sheet is fabricated from 
a Nb50 wt %Ti ingot by rolling and annealing in such a way that the grain 
size in the 0.015" thick sheet is below a nominal diameter of about 22 
microns (ASTM micro-grain size number &gt;8). 
Three monofilament billets must be constructed in order to have enough 
material for the later multifilament billet. All of the billets are 
constructed and processed in the same way. First, the NbTi ingots and the 
OFHC copper extrusion cans, noses, and tails are machined to the proper 
dimensions, and the Nb and NbTi sheets are cut to size. The standard 
monofilament design calls for a 24" long core, so the NbTi ingots must be 
cut to this length and all of the sheet material must be cut to this 
width. Based on the design in FIG. 1, each billet requires a 69.5" length 
of 0.015" thick Nb sheet and a 68.0" length of 0.015" thick Nb50 wt %Ti 
sheet. Assembly is easier if these are continuous strips, but several 
shorter sections may be used if necessary. 
After all the components have been cleaned, the monofilament billets are 
assembled. Care must be taken to minimize the amount of void space in the 
sheet area. The NbTi ingots are inserted last. The copper nose and tail 
are then attached to the top and bottom of each billet by electron beam 
welding in vacuum. 
The sealed billets are prepared for extrusion by heating them at 
650.degree. C. for 3 hours. The billets are then extruded at 650.degree. 
C. from a 7.125" diameter liner to a diameter of 1.00". The extrusions are 
cropped to remove the excess copper at the noses and tails, and are then 
cold-drawn to restack size: hexagonal wire at a 0.1033" flat-to-flat. 
FIG. 2 is a schematic diagram of the multifilament billet to be constructed 
from the hexed monofilament wire. While the monofilaments are actually 
hexagonal, in cross section they are shown as circles, for convenience. 
After a sufficient quantity of OFHC copper is drawn and hexed, both the 
copper and the NbTi monofilament wire are cut into 31.625" lengths. The 
can, nose, and tail are machined, all components are cleaned, and then the 
billet is assembled. The filaments are arranged symmetrically in the 
proportions indicated in FIG. 2, the intent being to minimize the void 
space that inevitably results when fitting an hexagonal, close packed 
array into a round can. When the filament array is complete, the copper 
can is slipped into position around it. More copper filaments are added to 
the perimeter of the array as necessary to ensure a tight fit. The nose 
and tail are then attached to the can by electron beam welding in vacuum. 
The multifilament billet is HIP'd to consolidate the numerous subelements. 
As a result of the monofilament design, the HIP'd multifilament billet has 
a filament spacing-to-diameter ratio (s/d) of 0.168. Here, the diameter 
refers to the average NbTi filament diameter, including its barrier, and 
the spacing refers to the distance between adjacent filaments. A low s/d 
is important in preventing filament sausaging during extrusion; if the 
filaments are closely packed relative to their diameter, they provide 
mechanical support to one another. An s/d of 0.168 is adequate for a 
multifilament of this kind. 
After the HIP, the nose and tail of the multifilament billet are machined 
to match the post HIP can diameter. Next, the billet is heated and then 
extruded from a 12.36" diameter liner to 3.25" in diameter. The extrusion 
is cropped and then drawn to the diameter at which the heat treatment 
schedule begins. The application of heat treatments separated by cold work 
serves to optimize the J.sub.c of the final conductor. The heat treatment 
schedule is open to informed modification, but the following is typical 
for a fine filament NbTi conductor: 375.degree. C. for 40 hours at each 
wire diameter of 1.00", 0.640", 0.460", and 0.325". 
After the last heat treatment, the multifilament is drawn to a final 
diameter of 0.0318", where the filament diameter is .about.6 microns. Such 
a conductor is well suited to use in advanced machines like the 
Superconducting Super Collider. The application of the invention should 
significantly increase J.sub.c over the 2750 A/mm.sup.2 (5 T, 4.2.degree. 
K) typically obtained in comparable fine filament NbTi conductor. Piece 
length (the average length of the wire segments that reach final size) 
should also increase substantially due to the greater ductility of the 
nodule-free wire. 
In accordance with the invention as described in Example I, the titanium 
losses can be offset by wrapping the NbTi ingot in the monofilament with 
NbTi sheets having a higher percentage of titanium than the ingot. 
However, this method requires that two alloys be melted, which is 
expensive. Furthermore, the higher titanium alloys are more difficult to 
cold-work than standard alloys, so the fabrication of high titanium sheet 
becomes a problem. 
An alternative to the use of high titanium alloy sheets is to use sheets 
having the same composition as the ingot and to alternate these sheets 
with relatively thin layers of pure titanium. The thin titanium layers 
serve as a reservoir of titanium, diffusing into the alloy sheets even as 
those same sheets are losing titanium to the niobium barrier. 
In general, the titanium sheets should be much thinner than the NbTi sheets 
so as to promote complete diffusion of the titanium. A NbTi to titanium 
sheet thickness ratio of 7:1 or greater is preferred. The overall volume 
of NbTi and titanium sheet should not exceed .about.15% of the filament 
volume, since the titanium depletion will not be significant farther into 
the filament. Both the NbTi and titanium sheets should have a nominal 
grain diameter less than 22 .mu.m (ASTM micro-grain number &gt;8) in order to 
ensure maximum barrier efficiency. The outermost layer of titanium should 
be inside the outermost layer of NbTi. That is, the Nb barrier should not 
be in direct contact with titanium, since this situation only promotes 
titanium diffusion into the barrier. 
Although the above discussion focuses on NbTi, it is obvious that these 
principles can be applied to any filamentary type II superconductor 
subject to analogous diffusion losses (NbZr losing Zr, e.g.). The same is 
true for the following non-limiting example, which sets forth a preferred 
method of practicing the invention: 
EXAMPLE II 
FIG. 3 is a schematic diagram of a NbTi monofilament billet cross section. 
The billet is typical for monofilament NbTi except that a series of Nb46.5 
wt %Ti and pure titanium sheets have been included where solid NbTi ingot 
would ordinarily be. The Nb diffusion barrier occupies about 4% of the 
non-copper volume, while the NbTi/titanium sheet volume is about 5%. Both 
the NbTi and the titanium sheets are fabricated from ingots by rolling and 
annealing in such a way that the nominal diameter of their grains is below 
about 22 .mu.m (ASTM micro-grain number &gt;8). The titanium sheet should be 
of Grade 2 purity or better for optimum workability. 
Three monofilament billets must be constructed in order to have enough 
material for the later multifilament billet. All of the billets are 
constructed and processed in the same way. First, the NbTi ingots and the 
OFHC copper extrusion cans, noses, and tails are machined to the proper 
dimensions, and the niobium, NbTi, and titanium sheets are cut to size. 
The standard monofilament design calls for a 24" long core, so the NbTi 
ingots must be cut to this length and all of the sheet material must be 
cut to this width. Based on the design in FIG. 3, each billet requires a 
72.0" length of 0.015" thick Nb sheet, a 68.0" length of 0.015" thick 
Nb46.5 wt %Ti sheet, and a 68.0" length of 0.002" thick titanium sheet. 
Assembly is easier if these are continuous strips, but several shorter 
sections may be used if necessary. 
After all the components have been cleaned, the monofilament billets are 
assembled. Care must be taken to minimize the amount of void space in the 
sheet area. The NbTi ingots are inserted after the sheet. The copper nose 
and tail are then attached to the top and bottom of each billet by 
electron beam welding in vacuum. The sealed billets are prepared for 
extrusion by heating them at 650.degree. C. for 3 hours. The billets are 
then extruded at 650.degree. C. from a 7.125" diameter liner to a diameter 
of 1.00". The extrusions are cropped to remove the excess copper at the 
noses and tails, and are then cold-drawn to restack size: hexagonal wire 
at a 0.1033" flat-to-flat. 
The product is then treated as previously discussed in connection with FIG. 
2. After a sufficient quantity of OFHC copper is drawn and hexed, both the 
copper and the NbTi monofilament wire are cut to 31.625" lengths. The can, 
nose, and tail are machined, all components are cleaned, and then the 
billet is assembled. The filaments are arranged symmetrically in the 
proportions indicated in FIG. 2, the intent being to minimize the void 
space that inevitably results when fitting an hexagonal, close packed 
array into a round can. When the filament array is complete, the copper 
can is slipped into position around it. More copper filaments are added to 
the perimeter of the array as necessary to ensure a tight fit. The nose 
and tail are then attached to the can by electron beam welding in vacuum. 
The multifilament billet is HIP'd to consolidate the numerous subelements. 
As a result of the monofilament design, the HIP'd multifilament billet has 
a filament spacing-to-diameter ratio (s/d) of 0.168. Here, the diameter 
refers to the average NbTi filament diameter, including its barrier, and 
the spacing refers to the distance between adjacent filaments. A low s/d 
is important in preventing filament sausaging during extrusion; if the 
filaments are closely packed relative to their diameter, they provide 
mechanical support to one another. An s/d of 0.168 is adequate for a 
multifilament of this kind. 
After the HIP, the nose and tail of the multifilament billet are machined 
to match the post-HIP can diameter. Next, the billet is heated and then 
extruded from a 12.36" diameter liner to 3.25" in diameter. The extrusion 
is cropped and then drawn to the diameter at which the heat treatment 
schedule begins. The application of heat treatments separated by cold work 
serves to optimize the J.sub.c of the final conductor. The heat treatment 
schedule is open to informed modification, but the following is typical 
for a fine filament NbTi conductor: 375.degree. C. for 40 hours at wire 
diameters of 1.00", 0.640", 0.460", and 0.325". 
After the last heat treatment, the multifilament wire is drawn to a final 
diameter of 0.0318", where the filament diameter is .sup. 6 .mu.m. Such a 
conductor is well suited to use in advanced machines like the 
Superconducting Super Collider. The application of the Example II 
embodiment should significantly increase J.sub.c over the 2750 A/mm.sup.2 
(5 T, 4.2.degree.) typically obtained in comparable fine filament NbTi 
conductor. Piece length--the average length of the wire segments that 
reach final size--should also increase substantially due to the greater 
ductility of the nodule-free wire. 
In addition to the problems of wire ductility and current density addressed 
in the foregoing examples, the latest processing schemes for NbTi wire 
suffer from significant losses during the extrusion step. These losses are 
caused by the non-uniform flow patterns in the metal at both front and 
rear as the copper and NbTi are co-extruded. At the start of extrusion, 
the center of the billet moves through the extrusion die first, followed 
later by the outer regions. A similar effect results in a hollowing at the 
rear of the extrusion. Distinctive nose and tail defects are produced in 
the extruded rod, as shown in FIG. 4. Such regions must be discarded, 
resulting in a significant loss of valuable NbTi superconductor. These 
losses increase the cost of the remaining conductor. Improving the yield 
from extrusions would thus have important economic benefits in the 
fabrication of fine filament NbTi superconducting wire. 
Referring again to FIG. 4, the degree of non-uniformity in an extrusion is 
quantified in the "flowshape factor", defined as the ratio of the length 
of the distorted region, L, to the core diameter, D. The higher the 
flowshape factor, the higher are the losses. In an ideal world, one would 
be able to predict the flowshape factor for any extrusion and thereby 
anticipate the losses. Unfortunately, it is not possible to do this 
because there are so many variables involved in the formation of the 
flowshape. Among them are the following: 
1. Areal reduction and billet length. 
2. Billet composition. 
3. Billet and liner temperatures. 
4. Lubrication. 
5. Die geometry. 
It is beyond the scope of the present discussion to detail the effects of 
each of these factors. Suffice it to say that changing any one of the 
factors can alter, to one degree or another, the flow of material during 
an extrusion. 
The flowshape factor comes into play at two stages in the fabrication of 
fine filament NbTi wire: extrusion of the monofilament billet and 
extrusion of the multifilament billet. A typical flowshape factor for a 
monofilament (areal reduction, R=51) is L/D=115, and for a multifilament 
(R=15, L/D=12). The difference is mostly due to the fact that L/D is, in 
general, proportional to R, although in comparing the monofilament to the 
multifilament one must also consider the much different core compositions 
in the two types of billets. In real terms, the overall losses from 
typical NbTi monofilament and multifilament extrusions are about 15% of 
the initial useable volume. 
There are two methods for increasing extrusion yields that are both simple 
and effective. The first is to adjust the billet composition so that the 
stiffness of the nose, tail, and core are as close as possible. As common 
sense indicates, like materials tend to flow more uniformly than unlike 
materials. In an extrusion, the determinant of similarity is the "k 
factor". The mathematical definition of k is: 
EQU k=F/(A.sub.o InR) 
where F is the force applied during extrusion, A.sub.o is the initial 
billet cross-sectional area, and R is the areal reduction employed. With F 
in tons and A.sub.o in square inches, k has the units tons-per-square-inch 
(tsi). All other things being equal, the greater the force required to 
extrude a material, the greater the value of k. In this sense, k is a 
measure of stiffness in a material. Its value is, of course, strongly 
dependent upon the temperature of the material. At 650.degree. C., the k 
factor for Nb46.5 wt %Ti is about 24 tsi, while the value for copper is 
around 10 tsi. This is a substantial difference, and it causes a larger 
flowshape factor in conventional copper-clad NbTi billets than could be 
obtained if something with a higher k factor than copper were used at the 
billet nose and tail. 
Investigation reveals that the cupro-nickel alloy Cu10 wt %Ni, with a k 
factor of 20-25 tsi at 650.degree. C., is a close match to Nb46.5 wt %Ti. 
The use of this alloy at the nose and tail of Nb46.5 wt %Ti billets will 
thus result in more uniform flow during extrusion, which in turn gives 
higher yields. In monofilament extrusions, a stiffer nose will help 
prevent the bulbous "dog bone" seen at the right in the diagram of FIG. 4, 
since this particular defect is primarily the result of extruding a high k 
material behind a low k material. Dog boning is not so severe in 
multifilament extrusions because the copper matrix in the core brings the 
k factor down to about 17 tsi, more in line with copper. Still, the use of 
CuNi is of value in multifilament extrusions, particularly at the tail, 
where a stiffer material--in this case, stiffer even than the core--is 
less susceptible to the collapse that results in hollowing at the rear of 
the extrusion. 
The drawback to the use of CuNi noses and tails is that the higher k factor 
of the CuNi substantially increases the "upset force", the force required 
to initiate extrusion of the billet. For this reason, the areal reductions 
or temperatures normally employed during the extrusion of conventional 
billets may have to be adjusted for the modified billets, but this will 
depend on the billet sizes and the extrusion force available. 
The second method by which to efficiently increase yield is to simply 
compensate for the losses by incorporating an offset in the initial 
billet. This is accomplished by shaping the billet nose and tail, making 
the inner surface of the former convex, and of the latter concave. FIG. 5 
shows a diagram of a billet with this type of nose and tail compensation. 
Also shown is a diagram of a conventional billet. The degree of shaping at 
the nose and tail of a billet is determined empirically from standard 
billets, those constructed with flat surfaces at the nose and tail. Direct 
measurement of the extruded rods from such billets will, by extrapolation, 
yield the correction required at the nose and tail for those materials at 
the given temperature and reduction. 
Although effective, this method of correction is not suitable for 
application to monofilament NbTi because it entails shaping the NbTi ingot 
as well as the nose and tail. The NbTi lost during the shaping would 
largely offset any gains in yield. This technique is, however, perfect for 
multifilamentary billets, since the stack of small rods that makes up the 
core naturally conforms to the shape of the tail upon which it sits. 
The two methods described here for improving extrusion yields are not new 
(see, for example, The Extrusion of Metals, Claude E. Pearson, pg. 110, 
John Wiley & Sons Inc., N.Y., 1953; and Nuclear Reactor Fuel Elements, 
Edited by A. R. Kaufmann, Chpt. 12, "Cladding and bonding Techniques":, F. 
M. Yans, P. Loewenstein, and J. Greenspan, 455-457, Inner Science 
Publisher's Division, John Wiley and Sons Inc., N.Y., 1962), but they have 
not generally been applied to NbTi superconducting wire fabrication. 
Considering the expense of NbTi alloys, it is important that such proven 
techniques be utilized in this industry. The 15% losses currently 
tolerated in each extrusion can be reduced substantially if the present 
invention is properly applied. 
In accordance with the invention, fine filament NbTi superconducting wire 
can be fabricated so as to maximize extrusion yields as described in the 
following non-limiting example: 
EXAMPLE III 
Three monofilaments like that diagramed in FIG. 1 are fabricated as 
described in Example I, except that Cu10 wt %Ni is used for the billet 
noses and tails in place of pure copper. The k factor of the CuNi is close 
to that of Nb46.5 wt %Ti at the 650.degree. C. extrusion temperature, so 
the flowshape factor is relatively low, resulting in extrusion yields 
higher than the approximately 85% achieved with pure copper can 
components. After extrusion, the monofilament is processed as described in 
Example I. A multifilament like that of FIG. 2 is then assembled as 
described in Example I, except that Cu10 wt %Ni is used for the nose and 
the tail, both of which components are shaped to offset losses in the 
extrusion. Longitudinal cross sections of these components are shown 
schematically in FIG. 6 (tail) and FIG. 7 (nose). The degree of curvature 
on the nose and tail is chosen so as to offset an approximately 7.5% loss 
at each end of the extrusion (15% overall). 
Once assembled and welded, the multifilament billet is extruded (at 
approximately 650.degree. C.) and then processed as described in Example 
I. Between the shaping and the increased stiffness of the nose and tail, 
losses in the multifilament extrusion are greatly reduced. In addition, 
the material benefits from the barrier technology detailed previously and 
applied in Example I, above.