Microstructural refinement of niobium for superconducting rf cavities
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Microstructural Refinement of Niobium for Superconducting RF Cavities. 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 77843-3123, USA 2 Shear Form, Inc, Bryan, TX 77801, USA

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Microstructural refinement of niobium for superconducting rf cavities

Microstructural Refinement ofNiobium for Superconducting RF Cavities

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, USA

2Shear Form, Inc, Bryan, TX 77801, USA

3 Michigan State University, Dept. of Chemical Engineering and Materials Science, East Lansing, MI 48824-1226, USA

For

SRF Materials Workshop

Fermi National Accelerator Laboratory

Wilson Hall, Curia II

Batavia, Illinois USA

May 23-24, 2007

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


Motivation

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.


ExperimentalProcedures

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)



Illustration of route a
Illustration of Route A

Illustration of billet orientation and element distortion after one and two ECAE extrusions following route A

H. Zapata, “Application of Equal Channel Angular Extrusion to Consolidate Aluminum 6061 Powder, Masters Thesis, pp. 19, 1998


Results of multiple extrusions through a 90 die 1
Results of Multiple Extrusions Through a 90° Die(1)

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





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

800C Anneal

900C Anneal

1000C Anneal

1100C Anneal

1200C Anneal

900C Anneal



Hardness Measurements Annealing Temperatures (90 min.


Tensile Test Results Annealing Temperatures (90 min.


OIM/EBSD of Commercial Sheet Annealing Temperatures (90 min.


OIM/EBSD of ECAE Sheet Annealing Temperatures (90 min.


Presentation Conclusions Annealing Temperatures (90 min.

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

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

  • 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 production
Challenge Questions for Nb Sheet Production Annealing Temperatures (90 min.

  • To manufacture adequate and reproducible product:

    • What knowledge is needed?

      • Microstructure-processing-property relationships

    • What material characteristics give the most favorable behavior?

      • Experiments should be used to set limits on chemistry and microstructural characteristics.

    • How can it be made?

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

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

      • What microstructure is preferred?

      • How can this microstructure be developed most economically?

      • How will cavity manufacture and use affect performance, and what can be done to minimize negative factors?


OIM/EBSD Results Annealing Temperatures (90 min.


Technical Specifications for Nb Sheet for SRF Cavities Annealing Temperatures (90 min.

Spallation 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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