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 , USA 2 Shear Form, Inc, Bryan, TX 77801, USA 3 Michigan State University, Dept. of Chemical Engineering and Materials Science, East Lansing, MI , 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-FG02-05ER Microstructural Refinement of Niobium for Superconducting RF Cavities

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.

Experimental Procedures 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)

Materials

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

Material Element Distortion in 50 mm Thick Annealed Copper Bar

Macrostructure of Cast Nb Ingot

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

Microstructure of ECAE Route 4E Material at Various Annealing Temperatures (90 min.) 1100  C Anneal1200  C Anneal 1000  C Anneal900  C Anneal800  C Anneal 900  C Anneal

Recrystallized Grain Size in ECAE Processed Cast Nb

Hardness Measurements

Tensile Test Results

OIM/EBSD of Commercial Sheet

OIM/EBSD of ECAE Sheet

Presentation 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 Production  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

Technical Specifications for Nb Sheet for SRF Cavities Spallation Neutron Tesla Test FacilitySource Project Material Property (TTF) at DESY(Jefferson Labs) RRR > 300 > 250 Grain Size ~ 50 mm ~ASTM #5 (64 m) predominant <ASTM #4 (90 m) locally YS (1) > 50 N/mm 2 > 48 N/mm 2 (7 ksi) TS >100 N/mm 2 96 N/mm 2 (14 ksi) %EL at Frac. (1) 30% > 40% longitudinal > 35 % transverse Vickers Hardness ≤ 50 <50 Impurities Ta ≤ 500 g/g Ta ≤ 1000 g/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