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
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
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.
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.
1. Commercial RRR grade 4 mm thick Nb sheet (RRR ≥ 250)
2. Reactor grade (RG) bulk (cast) Nb (RRR ~ 300)
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
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 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
(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.)
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.
OIM/EBSD Results Annealing Temperatures (90 min.
Technical Specifications for Nb Sheet for SRF Cavities Annealing Temperatures (90 min.
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