As published in Advances in Crogenic Engineering (Materials), Vol. 38, Edited by F. R. Fickett and R. P. Reed, Plenum Press, New York, pp. 691697, 1992.
IMPROVED RESPONSE TO HEAT TREATMENT OF NB-TI ALLOYS BY THE ADDITION OF ZR
P. J. Lee, J. C. McKinnell*, D. C. Larbalestier+
Applied Superconductivity Center, University of Wisconsin-Madison
Madison, WI 53706-1687
* Now at Superconductor Development Division, TWCA, Albany, OR 97321
+
also Dept. of Materials Science and Engineering
ABSTRACT
In order to increase the rate of microstructural refinement in Nb-Ti alloys we have replaced 1-5 at.% Ti with
Zr, which has a relatively large atomic radius. It was found that both the precipitate size and the final drawing strain
to peak Jc could be significantly reduced by the Zr addition. The magnitude of the peak Jc value was also increased in
many cases (at both 5T and 8T). It was found that a 5T(4.2K) critical transport current density, Jct, of 2500 A/mm2
could be achieved with only one heat treatment and a total strain space of only 10. The microstructural refinement
produced by the Zr addition did not, however, result in a significant reduction in the prestrain, εp, required for
suppression of the deleterious Widmanstätten α-Ti and ω precipitate morphologies.
INTRODUCTION
In order to produce high Jc Nb-Ti wire by conventional heat treatment processing, an extremely high true
strain is required (typically εt≥11). This high level of strain is required in order to provide a sufficient density of the
preferred grain boundary nucleation sites for triple-point α-Titanium precipitation and for the subsequent refinement of
the microstructure to the scale of the fluxoid lattice.1-3 The limited strain space available for commercial wire
manufacture provides a considerable restraint on the processing of Nb-Ti especially for large conductor sizes. If the
rate of microstructural refinement can be improved, the density of grain boundary intersections will be increased and it
may be possible to reduce the total strain space required. To this effect we have added a compatible element with a
relatively large atomic radius, Zr, to two Nb-Ti alloy classes, a standard Nb-47wt.%Ti alloy and a Nb-52wt.%Ti alloy.
EXPERIMENTAL DETAILS
Alloys were prepared at Teledyne Wah Chang Albany to a High Homogeneity grade specification. The target
compositions and final compositions measured by wet chemistry are listed in Table I. The compositions of the final
alloys were extremely close to those specified. In addition the complete wet chemistry results are listed in Table II.
The lower Ti content alloys were chosen to be equivalent to the standard Nb-47Ti class of alloys, replacing Ti with Zr
in order to maintain a constant electron concentration. The higher Ti group of alloys were based on Nb-52wt.%Ti
since this alloy promised excellent properties at low fields4,5. The starting ingot diameter for the ternaries was 145mm
and for the Nb-53wt.%Ti the ingot diameter was 351mm. The final recrystallization anneal was applied at TWCA at a
nominal diameter of 41mm and was received unclad by us for final production into monofilamentary wire at a diameter
of 6.9mm. Three processing schedules were used; a standard aggressive high Jct (3×80hr/420C) heat treatment, a single
aggressive heat treatment, and single aggressive heat treatment preceded by two low temperature nucleation heat
treatments (the schedules are listed in Table III). After final heat treatment, cold drawing strains of up to 5.3 (all
strains are given as true strain) were applied to exceed the Jct optimization strain.
Jct values for the wires were obtained by measuring the critical transport current, Ict, using the standard long
sample method6 at a resistivity criterion of 10-14Ωm and with copper to superconductor ratios measured by the wet
691
Table I
Alloy Specifications and Compositions
I.D.#
Nominal Alloy Specification
weight.%
Nb
Ti
Measured Composition
atomic %
Zr
Nb
weight %
Ti
Zr
Nb
Ti
electron
to
atom
ratio,
e/a
atomic %
Zr
Nb
Ti
Zr
35
52.5
45.6 1.9 36.8 61.9
1.3
52.0
46.1 1.9 36.3 62.4 1.3
4.36
36
52.0
44.2 3.8 36.8 60.5
2.7
52.0
44.4 3.6 36.7 60.7 2.6
4.36
37
48.0
52.0
32.3 67.7
0
47.0
53.0
0
4.31
38
47.6
50.5 1.9 32.3 66.4
1.3
48.0
50.1 1.9 32.6 66.1 1.3
4.33
39
47.1
49.0 3.9 32.3 65.0
2.7
47.3
48.9 3.8 32.4 65.0 2.7
4.32
40
46.3
46.1 7.6 32.3 62.3
5.4
46.7
46.2 7.1 32.5 62.4 5.0
4.33
0
*
31.4 68.6
* = <100ppm
Table II
Complete Wet Chemistry Results for Nb-Ti-Zr Alloys
Alloy I.D.
35
36
37
38
39
40
Nominal
45.6Ti,1.9Zr
44.2Ti,3.8Zr
52Ti
50.5Ti,1.9Zr
49Ti,3.9Zr
46.1Ti,7.6Zr
Heat #
594416
59400
594036
594404
594408
594412
Pacs #
70194
70242
67512
70191
70192
70193
S.O.#
5634.1
5519-1
5519-5
5519-2
5519-3
5519-4
Element
Content in ppm, except * = wt.%
Ti
46.1*
44.4*
53.0*
50.1*
48.9*
46.2*
Zr
1.9*
3.6*
<100
1.9*
3.8*
7.1*
Al
<25
<25
<25
<25
<25
<25
C
60
60
40
60
60
40
Cr
<50
<50
<50
<50
<50
<50
Cu
<10
<10
<10
11
<10
<10
H
28
26
16
27
26
28
Mg
<25
<25
<25
<25
<25
<25
N
44
58
42
62
44
58
Ni
<25
<25
<25
<25
<25
<25
O
570
590
520
570
610
590
Si
<100
<100
<100
<100
<100
<100
Sn
<40
<40
<40
<40
<40
<40
Ta
710
710
1040
720
710
720
Table III
Processing Schedules
Composite I.D.
UWxx18*
UWxx34*
UWxx38*
HT
Strain,
εt
Time,
hrs
Temp.,
C
Strain,
εt
Time,
hrs
Temp.,
C
Strain,
εt
Time, hrs Temp.,
C
1st
7.12
80
420
4.65
3
300
4.65
80
420
2nd
-
-
-
5.96
3
300
5.96
80
420
3rd
-
-
-
7.12
80
420
7.12
80
420
* = “xx” refers to alloy ID. given in Tables I and II.
692
Figure 1
Jct(5T) versus final cold drawing strain for monofilament series xx18, 1×80hrs/420C.
chemical technique. Selected samples were also prepared for Transmission Electron Microscopy, TEM by jet
electropolishing transverse wire cross-sections2. Quantitative microstructural information was obtained from multiple
tilt series TEM images using a Megavision XM1024 high resolution image processor7.
RESULTS
The Jct versus strain curves for schedules for the 1×80hr/420C, 3×80hr/420C and 2×3hr/300C + 80hr/420C
schedules at 5T (4.2K) are shown in figures 1,2 and 3 respectively. In all cases the addition of Zr produced a
significant (∆εf ≥ 1) shift of the strain of peak Jc (εpk) to lower strain. The addition of Zr also tended to reduce the
magnitude of the peak Jc although this was not always the case. For some of the wires the Jc versus strain slope
indicates a peak in the slope at lower strains than we were able to measure for this experiment (εf < 3). The single
80hr/420C heat treatment produced Jcs of less than 2500A/mm2 (5T, 4.2K) in all but the Nb-52wt.% case (which
surpassed 2600A/mm2) but the Jc versus strain slope indicates that with the addition of 3.9wt.% Zr that level may be
surpassed at final strains below 3. The single heat treatment was not sufficient to produce Jct values (5T, 4.2K) greater
than 1800A/mm2 for the Nb-47wt.Ti equivalent alloys. Increasing the Zr content from 1.9wt.Zr to 3.8wt.%Zr in the
47wt.%Ti equivalent alloys changed the Jc(5T) slope from a remarkably flat curve from 3 < εf < 5 in the case of the
1.9wt%Zr ( the only alloy of this heat treatment schedule for which the addition of Zr did not push the peak Jc below
an εf of 3) to a relatively steep linear slope for the 3.8wt.% Zr. The addition of the two low temperature heat
treatments had very different effects on the different alloys (Fig. 2). In the case of the 52wt.%Ti alloy, the Jc (5T) was
increased from just over 2600 A/mm2 for the single 80hr/420C heat treatment to almost 3500A/mm2 when preceded by
the two 3hr/300C heat treatments. The addition of Zr above 1.8wt.%Ti, to this alloy, however, reversed this trend with
Figure 2
Jct versus final cold work strain for xx38 series monofilaments, 3 × 80hrs/420C, and a Nb-46.4wt.%Ti
monofilament given similar processing4.
693
Figure 3
Jct versus final cold work strain for monofilament schedule xx34, 2×3hrs/300C + 80hrs/420C and a Nb46.5wt.%Ti binary alloy given the same processing8.
the Jc dropping by almost 1000A/mm2. The remaining alloys performed very similarly to their single heat treatment
counterparts. With 3 × 80hr/420C (Fig. 2) the addition of Zr to the Nb-47wt.%Ti alloys resulted in a depression of the
peak Jc
(∆Jc > 400 A/mm2). The shapes of the two ternary curves were very similar, with the 3.8wt.%Zr curve being
~250A/mm2 lower than for the 1.8wt.%Zr. The shift in the peak to lower εf, however, results in higher Jc for the
1.8wt%Zr ternary than for the binary alloy for final strains below 4.5. The addition of up to 3.8wt.% Zr results in an
improvement in the peak Jc for the Nb-52wt.%Ti equivalent alloys, in particular the Nb-50.5wt.%Ti-1.9wt%Ti alloy
surpassed 3300A/mm2 at a final strain below 4.
At 8T (Figs. 4,5 and 6) the trend to lower εpk for the ternary wires continued. In the case of the single heat
treatment (Fig.4) the peak Jc for the Nb-52wt.%Ti equivalent alloys was increased by ~10% with Zr additions of
1.9wt.% and 3.8wt.%. while the εpk was reduced by ~0.8. Unlike at 5T, however, of the alloys given the single heat
treatment, only the Nb-44.2Ti-3.8Zr and Nb-46.1Ti-7.6Zr appeared to peak below a final strain of 3. With three
80hr/420C heat treatments, all three ternary Nb-52wt.%Ti equivalent alloys performed significantly better (∆Jc max
>200A/mm2) and at lower final strains (∆εf ≥ 0.4) than the binary alloy. In the case of the binary Nb-47wt.%Ti
equivalent ternaries, the Jc was superior to the binary for final strains below 4.9. In both cases increasing the Zr
content decreased the εpk. In general, increasing the Zr content decreased the final drawing strain to peak Jc but also
tended to reduce the magnitude of the Jc.
The amounts of precipitate produced in two of the ternary alloys by three 80hr/420C heat treatments are listed
in Table IV, along with comparison data for the Nb-53wt.%Ti binary and similarly processed Nb-46.5wt.%Ti
material.1,9 The replacement of Ti with Zr considerably reduced both the cross-sectional areas of the α-Ti precipitates
as well as the quantity. In the case of the Nb-46.5wt.Ti alloy, the addition of 1.9wt.%Zr reduced the precipitate crosssectional area to 12% of the average value in equivalent binary alloys but also reduced the volume of precipitate to
66% of the original. For a 3.9wt.%Zr addition to Nb-53wt%Ti, the precipitate C.S.A. dropped to 39% of the original
binary and the quantity to 57% of the binary. The addition of Zr also drops the standard deviation in the precipitate
cross-sectional areas to less than 25% of the binary equivalents. The nominal precipitate diameters given in Table IV
have been calculated from the cross-sectional areas by assuming a circular precipitate cross-section, although this is
very much an approximation, these values are useful in interpreting the microstructural significance with respect to
fluxoid dimensions. The reductions in the precipitate diameters and distributions are clearly illustrated in figure 7
where the distributions of precipitate diameters with respect to volume are illustrated. In all of the 3 × 80hr/420C heat
treatment monofilaments, the dominant form of precipitation was α-Ti at grain boundary triple points (thus the fine
scale distribution of triple-point α-Ti also reflects the fine scale of the β-Nb-Ti microstructure). In the high Ti series the
Widmanstätten intra-granular form of α-Ti could be observed in isolated regions after heat treatment.
DISCUSSION
The relationship between final strain, microstructural dimensions and Jc in Nb-48wt.%Ti has been thoroughly
characterized2,3. As the wire is drawn after final heat treatment, the precipitate dimensions are reduced until the
precipitate thickness and separation are similar (and in most cases less) than the dimensions of the fluxoid lattice. Thus
it is no surprise that by reducing the precipitate dimensions by alloy addition, the final drawing strain to peak Jc can be
usefully reduced. These results compare very favorably with our previous attempts to reduce εpk in binary Nb-Ti by
694
Figure 4
Jc (8T, 4.2K) versus final cold work drawing strain for monofilaments with 1 × 80hrs/420C heat
treatments.
Figure 5
Jc (8T, 4.2K) versus final cold work drawing strain for monofilaments with 3 × 80hrs/420C heat
treatments.
Figure 6
Jc (8T, 4.2K) versus final cold work drawing strain for monofilaments with 2 × 3hrs/300C plus 1 ×
80hrs/420C heat treatments.
695
Table IV
Microstructural quantification after final heat treatment.
Alloy
Composition
wt.%
Volume
% α-Ti
46.5Ti
α-Ti Precipitate Diameter+, nm
Cross-Sectional Area, nm2
Mean
Median
σn-1
Mean
Median
σn-1
Vol. Wtd.
d*
21
41882
25676
49724
201
181
112
256
231
46.1Ti-1.9Zr
14
5095
1633
9350
62
46
52
121
81
53Ti
28
12327
1049
26658
82
37
94
272
125
48.9Ti-3.8Zr
16
4846
2520
5931
65
57
44
117
79
+
Calculated from area assuming a circular cross-section, * from mean C.S.A.
restricting the temperature and duration of heat treatments.10 The limited number of heat treatments attempted in this
study indicate that further manipulation of heat treatments to utilize fully these alloys should be fruitful. However, the
replacement of Ti by Zr in these alloys results in a reduction in the quantity of α-Ti precipitate. The magnitude of the
Jc obtainable in Nb-Ti increases markedly with the percentage of precipitate10,11; thus the performance of the ternary
monofilaments is probably limited by the reduced level of precipitation. This could be overcome by more aggressive
heat treatment but this is likely to increase the risk of superconductor-copper reaction and will reduce the size
advantage by precipitate coarsening10. Another possibility is to replace the Nb with Zr rather than the Ti, this approach
has the disadvantage of a likely deterioration in the Hc2 and thus the high field properties. The deterioration in high
field properties can be seen in comparing the Nb-46.5Ti equivalent alloys with the Nb-53Ti equivalent alloys in figure
5: within the alloy groups the replacement of Ti with Zr does not result in a large suppression in the magnitude of Jc
(in fact the Zr improved the performance of the high Ti alloy) however the higher Nb ternaries always did better than
their lower Nb equivalents. At 5T the same high Ti monofilament ternaries outperformed the lower Ti content
material, presumably through the increase in precipitate quantity. The Jc values produced by the ternary alloys were
higher than would be expected from the Jc versus %α-Ti relationship described previously10, suggesting a higher
pinning efficiency from the α-Ti precipitates in the ternaries, this may be related to the segregation of Zr to the α-Ti
precipitates noted in an earlier STEM-EDS analysis on a Nb-54wt.%Ti-4wt.%Zr alloy12.
The results of this study have been very promising and future performance improvements are likely to be
obtained by a combination of alloy and heat treatment modifications tailored to the appropriate field range of
application.
Figure 7
696
α-Ti precipitate size distributions after 3 × 80hr/420C heat treatments expressed in terms of % of total
alloy cross-section.
SUMMARY
1.
2.
3.
The addition of Zr to binary Nb-Ti alloys significantly refines the precipitate size at heat treatment.
Replacement of Ti by Zr reduces the precipitate quantity and thus the magnitude of Jc.
The replacement of Ti with Zr does not significantly impair the high (8T) field properties and in some cases
improves them.
ACKNOWLEDGEMENTS
We are grateful to P. O’Larey and Teledyne Wah Chang Albany for the preparation and initial processing of the
alloys. W. Starch supervised wire drawing and A. Squitieri performed additional Jc characterization. The work was
funded by the U.S. Department of Energy, Division of High Energy Physics under grant number DE-AC02-82ER4007.
REFERENCES
1
2
3
4
5
6
7
8
9
10
11
12
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Mag., 25, pp. 1918-1921 (1989).
P. J. Lee and D. C. Larbalestier, Development of nanometer scale structures in composites of Nb-Ti and their
effect on the superconducting critical current density, Acta. Met., 35, pp. 2526-2536, 1987.
C. Meingast, P. J. Lee and D. C. Larbalestier, Quantitative description of a high Jc Nb-Ti superconductor
during its final optimization strain: I. Microstructure, Tc, Hc2 and resistivity, J. Appl. Phys., 66, pp. 59625970 (1989).
J. C. McKinnell, P. J. Lee, R. Remsbottom, D. C. Larbalestier, P. M. O’Larey, and W. K. McDonald, High
Titanium Nb-Ti Alloys-Initial High Critical Current Density Properties, Adv. Cryo. Eng., 34, pp. 1001-1007
(1988).
J. C. McKinnell, P. J. Lee, D. C. Larbalestier, The effect of Titanium content on the pinning force in
Nb44wt.%Ti to Nb62wt.%Ti, IEEE Trans. Mag., 25, pp. 1930-1933 (1989).
W. H. Warnes and D. C. Larbalestier, Critical current distributions in superconducting composites,
Cryogenics, 26, pp. 643-653 (1986).
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EMSA, 45, pp. 358-359 (1987).
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produced by heat treatments of varying temperature duration and frequency, J. Mat. Sci., 23, 3951-3957
(1988).
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Nb-46.5wt%Ti, Adv. Cryo. Eng., 36, 287-294 (1990).
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B, 60 (1989).
P. J. Lee, unpublished data, Applied Superconductivity Center, University of WI-Madison, Madison, WI.
697