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)

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

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

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.

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.

Nb-Ti Composite Overview Cu stabilizer (Al can also be used) Multifilamentary Extrusion Billet 47-2000+ filaments Nb sheet or tube prevents reaction between Ti and Cu Monofilament assembly

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

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

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

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.

A.1.1 Strain Space . . . 2 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.

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 =0.69-1.15 HT is 40-80 hrs at 375-420 ºC ef > 4

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 100-300 nm diameter to the pinning scale of 1-5 nm thickness.

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

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

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.

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.

7. Microstructural Refinement Montage by Peter Lee

Jc proportional to % a-Ti

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

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.

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 %

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

Final drawing balances intrinsic and extrinsic effects

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, 0.13-0.17) reduces drawing instability - improving filament uniformity. But there is a trade-off with filament proximity coupling

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.

Progressive Refinement of a-Ti Ppt Morphology

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

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.

Nb-Ti strand production The annual global demand for MRI wire is about 1000 tons (including the dominant copper fraction), or 60,000 - 70,000 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.

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)

APC manufacture

Conventional vs. APC nanostructures

Conventional vs. APC Fp Curves

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!