k. ted hartwig 1, robert e. barber 2, derek baars 3 and thomas r. bieler 3, 1 texas a&m...
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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
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