1. Niobium-Titanium Based on
B3.3 Conductor processing of Low Tc materials: The alloy Nb-Ti
Lance Cooley, Peter Lee, and David Larbalestier
Institute of Physics Handbook on Superconductivity (March 2002)

2. Optimized Nb-Ti strands have: ~25% a-Ti
More precipitates than fluxons (full summation)
Very strong flux pinning 5-10% Jd
TEM by Peter Lee

3. Nb-Ti Superconductivity
Standard alloy
Anomalous resisitivity!
Nb-46-48wt%Ti is close to optimum Hc2
but at reduced Tc

4. Nb-Ti Metallurgy Large phase separation produces coring in alloy
High melting point produces low diffusion rates at precipitation temperatures
A hybrid equilibrium phase diagram for Nb-Ti combining the experimentally determined high temperature phase boundaries of Hansen et al with the calculated low temperature phase boundaries of Kaufman and Bernstein modified by Moffat and Kattner. Also shown is the martensite transformation curve (Ms) of Moffat and Larbalestier.

5. Alloy requirements
1. The correct overall alloy composition to optimize Hc2, Tc and precipitation for flux pinning. The acceptable range is Nb-46-weight % Ti to Nb-48 weight % Ti.
2. Uniform composition over the entire billet to ensure optimum physical and mechanical properties over the entire filament.
3. Chemical homogeneity on a microstructural level in order to ensure uniform precipitation of the correct morphology (typically ± 1.5 weight % Ti).
4. Low and controlled levels of impurity elements in order to ensure predictable superconducting and fabrication properties.
5. Elimination of hard particles (typically Nb-rich) as any particle that does not co-reduce with the alloy can result in filament drawing instability and ultimate strand breakage. The exterior of the final Nb-Ti rod must also be free of hard particles and must be smooth enough that it does not easily pick up particles during subsequent handling.
6. A fine (typically ASTM grain size 6 or smaller) and uniform grain size as it controls the distribution of precipitate nucleation sites. A fine grain size also improves diffusion barrier uniformity. Where high critical current is less important a larger grain size has been used in-crease ductility.
7. Low hardness (typically a Vickers hardness <170) to ease co-deformation with Cu stabilizer.

6. Nb-Ti Composite Overview
Cu stabilizer
(Al can also be used)
Multifilamentary Extrusion Billet
filaments
Nb sheet or tube prevents reaction between Ti and Cu
Monofilament assembly

7. To avoid………..
Result of insufficient stirring in melt

8. Overview of Baseline process:
1. Outline of Conductor Fabrication Steps
Strain Space.
Intrinsic Properties
Extrinsic Limitations.

9. Huge size reduction during process
Nb-Ti billets for LHC, courtesy Wah Chang
Strand can have thousands of Nb-Ti strands < 10 µm in diameter
30 cm diameter

10. A.1.1 Strain Space Extremely High Level of Cold Work Strain Required
Strain normally expressed as
True Strain,
where Ao and Do are original transverse cross-sectional area and diameter respectively, and A and D are the final cross-sectional area and diameter respectively after the strain has been applied.

11. A.1.1 Strain Space
The Total Available Strain Space can thus be defined as:
where AR and DR are the Nb-Ti rod/ingot cross-sectional area and diameter respectively, after final recrystallization anneal and Aw and Dw are the final filament cross-sectional area and diameter respectively.

12. A.1.1 Strain Space . . . 3 ep > 4.5 DeHT =0.69-1.15 ef > 4
In general the larger the strain space the higher the Jc.
Available strain is cold work strain after any anneal at ~850C
Extrusion at ~600C is warm work
All wire drawing is cold work
ep > 4.5
DeHT =
HT is hrs at ºC
ef > 4

13. Functions of Cold Work Strain
1. Encourage preferred precipitate phase (a-Ti) and morphology.
2. Improve micro-chemical homogeneity by mechanical mixing.
3. Increase the density of a-Ti nucleation sites.
4. Increase grain boundary density thereby, increasing diffusion rates.
5. Reduce average diffusion distance to the a-Ti precipitate.
6. Increasing the volume of a-Ti by multiple strain/heat treatment cycles.
7. Reduce a-Ti dimensions from the precipitation scale of nm diameter to the pinning scale of 1-5 nm thickness.

14. 1. Encourage formation of the preferred precipitate phase and morphology.
Good
Bad
Peter Lee (UW)

15. 2. Improve micro-chemical homogeneity by mechanical mixing
Micro-chemical inhomogeneity in a Nb-Ti alloy can be revealed a composition sensitive etch as in this example of an high homogeneity Fe doped Nb-46 weight % Ti alloy produced by Teledyne Wah Chang.
Local chemistry affects precipitation rate and precipitate morphology.
Image by Peter Lee

16. 3. Increase the density of precipitate nucleation sites.
Ideally precipitate nucleation sites are at grain boundary triple points
Image by Peter Lee
4. Increase grain boundary density thereby, increasing diffusion rates (grain boundary diffusion being considerable faster than bulk interdiffusion). 5. Reduce the average diffusion distance to the precipitate nucleation site.

17. 6. Increasing the volume of precipitate by multiple strain/heat treatment cycles.
Precipitation rate increases strongly with Ti content.
Additional heat treatments further increase the amount of precipitate.
As heat treatments are applied, residual Ti content of the b -Nb-Ti matrix drops until insufficient Ti is left to drive further precipitation.
The average residual matrix composition is calculated assuming an a -Ti composition of Nb-3.75 atomic % Ti.
Matrix Ti%
a-Ti content
Increasing Ti % increases precipitation rate < 46 wt % too low.

18. 7. Microstructural Refinement
Montage by Peter Lee

20. Jc proportional to % a-Ti

21. Final Strain increases Jc vs. Vol.% Precipitate Gradient
Nb-47wt% Ti, 5 T, 4.2 K
More precipitate and more final strain means more Jc

22. Heat Treatment Temperature
Typically 375 ºC to 420 ºC
Lower temperatures produce finer precipitates at HT size allowing smaller ef to optimization
Higher temperatures produce higher precipitate volumes (and thus great potential Jc) but larger size increases ef required for optimization.

23. Heat Treatment Time HT times typically 40-80 hrs
Increased HT time increases precipitate volume with reduced impact on precipitate size and barrier breakdown
Increased HT time increases processing costs
Precipitation rate is slow after initial 20 vol. % of precipitate. Extremely long HTs (500 hrs or more) can increase Jc and volume %

24. Bulk Pinning Force and Nanostructure
For conventionally processed Nb-Ti, Fp increases with drawing strain after the last heat treatment ef. The increase occurs at all fields as the precipitate size and spacing are reduced to less than a coherence length (x) in thickness. The refinement of the microstructure with increasing strain for the same strand is shown schematically in transverse cross-sections with the a-Ti precipitates in black.
Meingast, Lee and Larbalestier JAP 1989

25. Final drawing balances intrinsic and extrinsic effects

26. Extrinsic Limitations
Any hard particle of similar size to the final filament size will cause degradation of wire breakage
Nb and Cu matrix react to for hard Cu-Ti intermetallics
This is overcome by a Nb diffusion barrier
Drawing and Spacing/Filament Diameter Ratio
Designing the composite with a close-packed array of filaments (High S/D ratio, ) reduces drawing instability - improving filament uniformity. But there is a trade-off with filament proximity coupling

27. Diffusion Barrier
High resolution back-scattered electron scanning electron microscope image of the Nb-Ti adjacent to a Nb diffusion barrier (white) after final precipitation heat treatment. Non-uniformity of the barrier is shown along with a zone of reduced precipitate (black) next to the barrier.

28. Progressive Refinement of a-Ti Ppt Morphology

29. Extrinsic Limitation 2
Bonding - The bonding between composite elements must be strong
Composite surfaces are etched clean and assembled in a clean room or glove box.
Hotter extrusions promote bonding but reduce effective cold work
HIP (hot isostatic pressing) prior to extrusion promotes bonding and allows lower extrusion temperatures

30. Extrinsic Limitation 3 Cu location
With S/D optimized there is usually excess Cu. This can be place in the center to reduce center burst in extrusion or outside to reduce cabling degradation.

31. Nb-Ti strand production
The annual global demand for MRI wire is about 1000 tons (including the dominant copper fraction), or 60, ,000 km, worth about $50M.
Accelerator Nb-Ti demands can be high - this year another 1000 tons for LHC.
Nb-Ti is the lowest cost superconductor, by as much as an order of magnitude over it’s nearest competitor, Nb3Sn.

32. APC Nb-Ti Artificial Pinning Center:
mechanically assembling the final microstructural components at large size and then mechanically reducing their size.
Complete control of components and compositions
ef = es Hard to maintain microstructural homogeneity over increased ef
Works well for low field
Relatively expensive (labor + extrusions)

33. APC manufacture

34. Conventional vs. APC nanostructures

35. Conventional vs. APC Fp Curves

36. Nb-Ti Summary Well optimized standard process
Optimization came after scientific understanding
Very strong flux pinning – full summation
APC shows that other pins might be better
Presently ~10%Jd at 0T and ~5% at 5T, 4.2K
Anomalous resistivity vital to high field use!