material science

University of Wisconsin-MadisonUniversity of Wisconsin-MadisonApplied Superconductivity CenterApplied Superconductivity Center

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90Composition, Weight Percent Niobium

Liquid

+

Liquid +

Nb-47 weight % Ti

Ms

Composition, AtomicPercent NiobiumTe

mpe

ratu

re,

C

0 20 30 40 50 60 70 80 10010

0 20 30 40 50 60 70 80 10010

A hybrid equilibrium phasediagram for Nb-Ti combining theexperimentally determined hightemperature phase boundaries ofHansen et al ( ) with thecalculated low temperature phaseboundaries of Kaufman andBernstein ( ) modified by Moffatand Kattner ( ). Also shown is themartensite transformation curve(Ms) of Moffat and Larbalestier().

IIIaIIIa. Nb-. Nb-Ti Ti Phase DiagramPhase Diagram

With thanks to Peter Lee,Abridged metallurgy of DuctileAlloy Superconductors, IEEEEncyclopeadia of ElectricalEngineering


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14

0 10 20 30 40 50 60 70 80

Weight percent Ti

T c, K

and

Hc2

, T

20

30

40

50

60

70

80

90

Res

istiv

ity,

. c

m

Hc2 at 4.2 KTcResistivity

Variation in Hc2 (4.2K data of Muller, 1987) and Tc (data of Moffat,1985) for single phase Nb-Ti. Hc2 is defined as the linear extrapolation ofthe high field pinning force (Fp) to zero (today would be defined as H*).

IIIbIIIb. Primary Basic Parameters. Primary Basic Parameters

Notice the anomalous resistivity due tosoft phonons


TEM image of very high Jc Nb-47 weight % Ti at final sizein transverse cross-section reveals a densely folded arrayof second phase, non-superconducting -Ti pins that closelymatches the scale of the equilibrium fluxoid spacing at 5T(inset). Coherence length is 5nm, Hc2(4.2K) 12T.

IIIcIIIc. Optimum nanostructure. Optimum nanostructure

-Ti formsribbons 1-2 nmthick and severalm long.

Matrix is~Nb35wt.%Ti


Micro-chemical Ti-Nb segregationinhomogeneity revealed by etchingdue to wide separation of liquidusand solidus. The control of thissegregation is vital as this controlsthe uniformity of -Ti precipitatesize and flux pinning center. Sampleshown is a high homogeneity Fedoped Nb 46 weight % Ti alloymade by Teledyne Wah Chang,Albany OR.

Ingot is shown at large size.Desired final size of precipitates is1-2 nm thick.

IIIdIIId. Final size nanostructure. Final size nanostructurerequires macro homogeneityrequires macro homogeneity


High resolution back-scattered electron scanningelectron microscope image ofthe Nb-Ti adjacent to a Nbdiffusion barrier (white) afterthe third and final -Tiprecipitation heat treatment.

At this stage the -Ti is ~100nm in diameter and severaltimes longer. A final reductionin wire diameter of 8-10x willturn the precipitates intoribbons 1-2 nm thick.

IIIeIIIe. Uniform . Uniform --TiTi pinning array at pinning array atlast heat treatment last heat treatment sizesize


Final Recrystallization Anneal

1st Heat Treatment

2nd Heat Treatment

3rd Heat Treatment

Final Wire Size

Monofilament Composite Assembly

Multifilament Composite Assembly

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RodDrawing

RodandWireDrawing

Extrusion

Extrusion

Col

d W

ork

Stra

in in

Nb-

Ti

Schematic of the process forNb-Ti superconductors in termsof cold work strain.

Strain e = 2 ln Dstarting/Dfinal

300 mm diameter to 0.5-1 mmdia.

The 3 HT produce the 20vol.% of-Ti flux pins.

The final draw thins the pins tooptimum flux pinning size.

This is one of the most severecold work manufacturingprocesses employed anywhere.

50,000 lengths of wire notunusual

IIIfIIIf. The Fabrication Process. The Fabrication Process


Alloy Composition, wt.% Ti

Col

d W

ork

Pres

train

,

p

0

5

10

15

6540 45 50 55 60

m

m mm

m

m

3

-Ti at grain boundaryintersections

mix

ed p

reci

pita

te ty

pes

3

The morphology and locationof precipitation in coldworked Nb-Ti alloys isdetermined by the amountof cold work in themicrostructure. As the Ticontent is increased the coldwork required to produceoptimum -Ti (only at grainboundary intersections)increases. In thiscompilation of data (39, 40,44) optimum precipitation isdenoted by the symbol ,and mixed precipitatemorphology is denoted bythe letter m for heattreatment at 420 C. Thenumber 3 identifies wheremixed mode precipitationhas also been observed for375 C heat treatments.

IIIgIIIg. Pinning centers are alpha-. Pinning centers are alpha-TiTi


TEM image in bright field, (a) and(b), and dark field using an -phasereflection, (c), of a Nb-54 weight% Ti alloy after precipitation heattreatment. The heat treatment wasapplied at a cold work prestrain of5 which is insufficient to avoid thedeleterious phase and or intra-granular Widmanstaetten -Tiforms of precipitation. In (a) themicrostructure is shown inlongitudinal cross-section with thedrawing axis running down the page.Images (b) and (c) are of the sametransverse cross-sectional areawith the dark field image bringingout contrast from the phase.

IIIhIIIh. The precipitates must have. The precipitates must havethe right shapethe right shape


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Volume of -Ti precipitate (%)

Crit

ical

cur

rent

den

sity

(A/m

m)

Nb-47wt.%Ti, 5 T, 4.2 K (41)SSC Nb-47wt.%Ti, 5 T, 4.2 K (43)Nb-50wt.%Ti (42)Nb-47wt.%Ti, 8 T, 4.2 K (41)

5 T

8 T

y = 120x + 675

y = 41x + 470

Maximum Jcincreases linearlywith % a-Tiprecipitate from3-25 vol. % atboth 5 T and 8 T.

IIIiIIIi. Full summation of pinning forces. Full summation of pinning forces


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46 47 48 49 50 51 52 53

Initial alloy composition (wt.% Ti)

Volu

me

of a

-Ti

and

Res

idua

l Ti c

onte

nt b

y w

eigh

t of m

atrix

(%)

3 HT matrix composition2 HT matrix composition1 HT matrix composition

3 HT volume of precipitate2 HT volume of precipitate1 HT volume of precipitate

63 64 65 66 67 68

Initial alloy composition (atomic % Ti)

-Ti Precipitation rateincreases strongly with %TiAdditional heat treatmentsfurther increase the amountof precipitate.As additional heat treatmentand strain cycles are applied,and more precipitate isproduced, the residual Ticontent of the -Nb-Ti matrixdrops until insufficient Ti isleft to drive furtherprecipitation.The average residual matrixcomposition is calculatedassuming an -Ti compositionof Nb-3.75 atomic % Ti.

IIIjIIIj: Composition determines both: Composition determines bothpin density (and Hpin density (and Hc2c2))


1 m

f = 0

f = 4.0

f = 2.5

f = 1.1

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0 5 7 9 10 11B (T)

F p (G

N/m

)3

f= 5.3

= 4.4 f

= 3.4 f

= 2.5 f

= 1.1 f

precipitatesFiner andmore denselypacked

864321

Bulk pinning force increases with drawing strain after the last heat treatment, asprecipitate size and spacing are reduced to less than a coherence length in thickness.The refinement of the microstructure with increasing strain for the same strand isshown schematically in transverse cross-sections with the a-Ti precipitates in black.

IIIkIIIk. Pinning Force Curve Shape. Pinning Force Curve ShapeChangeChange


High quality strand at final sizemanufactured by IGC-AS for theSuperconducting Supercollider.

An overview of the strand cross-section is superimposed on a highermagnification image showing thefilament cross-sections in moredetail.

Both the filament size (diameter of6 m) and spacing is very uniform.A Nb diffusion barrier (light-appearing ring) which preventsreaction of Cu and Ti surroundseach filament.

III l. The Whole CompositeIII l. The Whole Composite


IIImIIIm. Summary for Nb-. Summary for Nb-TiTi Mature product (~3 Ton per day for MRI, ~60,000 km per

year) 1/3 ton billets without breaks

Cost ~$1/kA.m (Cu ~$20/kA.m) Large use for MR Imaging magnets Seems to be optimized now

Full summation of vortices against pins (15GN/m3), ~20% ofproximity-coupled pin in wire

But artificial pin arrays have yielded >40GN/m3 so there is someresidual uncertainty about the pinning mechanism

a particular point of uncertainty concerns vortex shape changeas it sits on a proximity-coupled -Ti precipitate

Anomalous phonons due to incipient BCCHCP transition arevital to developing the high Hc2 of Nb-Ti! Hc2 nTc, means 12T, not 6T!

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