K. Ted Hartwig 1, Robert E. Barber 2, Derek Baars 3 and Thomas R. Bieler 3, 1 Texas A&M University, Dept. of Mechanical Engineering, College Station, TX.

K. Ted Hartwig1, Robert E. Barber2 , Derek Baars3 and Thomas R. Bieler3,

1 Texas A&M University, Dept. of Mechanical Engineering, College Station, TX 77843-3123, USA2Shear Form, Inc, Bryan, TX 77801, USA3 Michigan State University, Dept. of Chemical Engineering and Materials Science, East Lansing, MI 48824-1226, USA

For SRF Materials Workshop

Fermi National Accelerator LaboratoryWilson Hall, Curia IIBatavia, Illinois USA

May 23-24, 2007

* Work Supported by the Department of Energy under contract DE-FG02-05ER84167.

Microstructural Refinement ofMicrostructural Refinement of Niobium for Superconducting RF Niobium for Superconducting RF

CavitiesCavities

MotivationMotivation

Need: RRR Nb sheet for SRF cavity cells.

Problem: Inconsistent and non-uniform “spring back”, and undesirable surface roughness after forming into an SRF cavity shape are common. These problems lead to increased cavity manufacturing cost.

Related work: Improved methods exist for microstructural refinement of bulk material by severe plastic deformation (SPD) processing. Microstructural improvements: grain refinement, microstructural uniformity and texture development in bulk. Property improvements: increased strength, toughness and ductility.

Solution: SPD process bulk Nb to produce a fine and uniform microstructure. Roll SPD processed bulk Nb to sheet, and recrystallize to develop a fine microstructure with preferred texture.

ExperimentalExperimental ProceduresProcedures Materials

1. Commercial RRR grade 4 mm thick Nb sheet (RRR ≥ 250)

2. Reactor grade (RG) bulk (cast) Nb (RRR ~ 300)

Procedures

1. None to as-received commericial RRR sheet

2. SPD preprocess (by ECAE) 25 x 25 x 150 mm bars of bulk RG Nb

3. ECAE process (routes A, B and E) preprocessed RG Nb

4. Roll ECAE processed RG Nb to 4 mm thickness

5. Anneal/recrystallize ECAE/4\rolled RG sheet

Measurements

1. Hardness (Vickers)

2. Tensile Test (commercial sheet)

3. Springback Test (commercial sheet and ECAE processed sheet)

4. Microstructure (grain size, microstructural uniformity and texture) of commercial and ECAE/rolled sheet)

MaterialsMaterials

TABLE I DESCRIPTION OF THE NB MATERIALS EXAMINED

IN THIS RESEARCH

Factor or Property

Low Purity RG Cast Ingot

Commerical RRR Sheet

Supplier CBMM Jefferson Lab (Tokyo Denkai)

Common Name

Reactor Grade RRR Grade

As-Received Form (Annealed)

235 mm dia. casting

405 mm dia. by 4 mm thick disk

Nominal Purity

0.997 0.9995

Substitutional Impurities (ppm)

1500 Ta <1000 Ta

Interstitial Impurities (ppm)

<10 H, <30 C, 67-88 O, 0-10 N

<10 H, <30 C, <40 O, <30 N

RRR (~30-50) >250

Illustration of Route AIllustration of Route A

Illustration of billet orientation and element distortion after one and two ECAE extrusions following route AH. Zapata, “Application of Equal Channel Angular Extrusion to Consolidate Aluminum 6061 Powder, Masters Thesis, pp. 19, 1998

Results of Multiple Results of Multiple ExtrusionsExtrusions Through a 90° DieThrough a 90° Die(1)(1)

ElementTotal Equivalent Equivalent Angle of Element Surface

Number of Strain Reduction Area Element Aspect AreaPasses Intensity Ratio Reduction Inclination Ratio Ratio

(%) (deg.)0 0 0 0 0 1 1.01 1.15 3.2 69 22 5 1.42 2.31 10.2 90 13 17 2.04 4.62 105 99 7 65 3.48 9.24 10100 99.99 3 257 6.0

(1) V.M. Segal, “Materials Processed by Simple Shear”, Mat. Sci. Engr. A, pp. 157-164, 1995.

Material Element Distortion in 50 mm Thick Annealed Copper Bar

Macrostructure of Cast Nb IngotMacrostructure of Cast Nb Ingot

Example of Non-uniform Deformation in ECAE Processed (N=1) Cast Nb

Microstructure of ECAE Route 4E Material at Microstructure of ECAE Route 4E Material at Various Annealing Temperatures (90 min.)Various Annealing Temperatures (90 min.)

1100C Anneal 1200C Anneal

1000C Anneal900C Anneal800C Anneal

900C Anneal

Recrystallized Grain Size in ECAE Recrystallized Grain Size in ECAE Processed Cast NbProcessed Cast Nb

0

50

100

150

200

250

300

350

400

0 2 4 6 8 10Number of Extrusions (N)

Gra

in S

ize

(mm

)

1000°C1100°C1200°C

TABLE III HARDNESS MEASUREMENTS ON ECAE PROCESSED AND

COMMERCIAL NB SHEET

Specimen Description

Mechanical Processing1

Vickers Microhardness (HV300)

Disk 1 As-received 79 ± 3 Disk 2 As-received 63 ± 4

RG1 ECAE1 + R + A 81 ± 2

RG3 ECAE3 + R + A 80 ± 2

RG32 ECAE3 + R + A 76 ± 1

RG5 ECAE5 + R + A 77 ± 0.5 1 R stands for rolled and A stands for annealed. 2 High temperature annealing.

Hardness MeasurementsHardness Measurements

Tensile Test ResultsTensile Test Results

TABLE IV TENSILE TEST RESULTS ON COMMERICIAL RRR GRADE NB SHEET

Specimen ID Sample Orientation 1 YS

(N/mm2) TS (N/mm2)

EL (%)

Disc 1 A 55 128 43 Disc 1 90° from A 77 152 61

Disc 2 B 54 127 62

Disc 2 90° from B 87 163 58 1 Because of disc shape of the source material, the orientation of Aand B with respect to the rolling direction is unknown.

2Dis1fine03, Middle, 0.1, 7.7, max 10.6

2Dis1fine04, Lower-middle, 6.2, 1.3,max 6.7

2Dis1fine05, Upper-middle, 2.7, 2.3,max 3.8

2Dis1fine06, Upper-edge, 1.7, 4.0,max 5.1

2Dis1fine07, Middle, 0.3, 6.2, max 7.9

2Dis1fine08, Middle, 0.0, 8.0, max 11.0

ND

TD

ND

TD2Dis1coarse09

As-received RRR Nb 2Dis

Consistent in middle, Variable in surface regions

OIM/EBSD of Commercial SheetOIM/EBSD of Commercial Sheet

ND

Center

Surface RD (b)

Average Grain Sizes, Diameter

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

Average Grain Size Weighted by Area

Av

era

ge

Dia

me

ter

(mic

ron

s)

F1 F3 F5 L1 L3 L5

1Dis1 2Dis1 20D2 26D2

"As-received Com RRR"

OIM/EBSD of ECAE SheetOIM/EBSD of ECAE Sheet

Presentation ConclusionsPresentation Conclusions

1. The microstructure and mechanical properties of commercial Nb sheet for SFR cavities are non-uniform and inconsistent between sheet batches.

2. Sheet made from ECAE processing bulk Nb can be finer grained than SFR Grade commercial Nb sheet.

3. Preliminary experiments indicate that it may be possible to fabricate fine grained sheet with a favorable texture (for deep drawing) from ECAE processed bulk Nb.

Challenge Questions for Nb Sheet Challenge Questions for Nb Sheet ProductionProduction

To manufacture adequate and reproducible product:To manufacture adequate and reproducible product:– What knowledge is needed?What knowledge is needed?

Microstructure-processing-property relationships Microstructure-processing-property relationships – What material characteristics give the most favorable What material characteristics give the most favorable

behavior?behavior? Experiments should be used to set limits on chemistry and Experiments should be used to set limits on chemistry and

microstructural characteristics.microstructural characteristics.– How can it be made?How can it be made?

Consistent thermo-mechanical processing with verification of Consistent thermo-mechanical processing with verification of chemistry and microstructure specifications. chemistry and microstructure specifications.

– What questions remain regarding material behavior, What questions remain regarding material behavior, manufacturing methods and operational performance? manufacturing methods and operational performance?

What microstructure is preferred?What microstructure is preferred? How can this microstructure be developed most economically?How can this microstructure be developed most economically? How will cavity manufacture and use affect performance, and what How will cavity manufacture and use affect performance, and what

can be done to minimize negative factors?can be done to minimize negative factors?

OIM/EBSD ResultsOIM/EBSD Results

Technical Specifications for Nb Sheet for SRF CavitiesTechnical Specifications for Nb Sheet for SRF CavitiesSpallation Neutron

Tesla Test Facility Source Project

Material Property (TTF) at DESY (Jefferson Labs)

RRR > 300 > 250

Grain Size ~ 50 mm ~ASTM #5 (64 mm) predominant <ASTM #4 (90 mm) locally

YS (1) > 50 N/mm2 > 48 N/mm2 (7 ksi)

TS >100 N/mm2 96 N/mm2 (14 ksi)

%EL at Frac.(1) 30% > 40% longitudinal > 35 % transverse

Vickers Hardness ≤ 50 <50

Impurities Ta ≤ 500 mg/g Ta ≤ 1000 mg/g, W ≤ 100

O ≤ 10 O ≤ 40, Ti ≤ 40

N ≤ 10 N ≤ 30, Si ≤ 50

C ≤ 10 C ≤ 30, H ≤ 10

H ≤ 2 Other metallic ≤ 50 each(1) The YS and Elongation at Fracture in the longitudinal and tranversed directions should

not differ by more than 5%, for the SNS Project specification