Robert E. Barber

Severe plastic deformation of bulk Nb for Nb/sub 3/Sn superconductors

Cast pure Nb with very large grains was processed by multipass equal channel angular extrusion (ECAE) to refine the microstructure. Extrusions were performed on 25 and 50 mm square cross section bars in a right angle die at room temperature following different extrusion routes to strains above nine. The hardness of Nb reaches a saturation level after eight extrusion passes where it is /spl sim/20% above that of fully worked Cu. Recrystallization bands appear to be absent in material processed by a new multipass route (E) which imparts shear on three intersecting planes and gives high processing yields. Hardness and optical microscopy measurements on recrystallized specimens are similar for 25 and 50 mm square bars which indicates a favorable scale-up response. Comparisons to commercially processed Nb and a cost estimate indicate that ECAE may be a viable method for manufacturing fine-grained, homogenous Nb for Nb/sub 3/Sn multifilamentary superconductor applications.

Microstructural refinement of tantalum for Nb/sub 3/Sn superconductor diffusion barriers

Cast pure Ta was deformation processed via equal channel angular extrusion and then recrystallized to produce a uniform, fine-grained bulk material. Extrusions were performed on 25/spl times/25/spl times/150 mm billets at room temperature in 90/spl deg/ tooling to strains of 9.3. The Vickers microhardness reaches near maximum levels after four extrusions. As-worked microstructures are composed of nonuniform submicron grains with apparent banding. A homogeneous and fine grained recrystallized microstructure (average grain size /spl sim/8 /spl mu/m) free of banding is obtained after four extrusions via route E multipass processing and a 1000/spl deg/C heat treatment. The high microstructural homogeneity of such a material may exhibit better co-reduction characteristics with surrounding Cu over conventionally processed Ta for superconductor diffusion barrier applications.

Microstructural Refinement of Niobium for Superconducting RF Cavities

The mechanical properties of commercial polycrystalline pure niobium sheet used for superconducting radiofrequency cavities are known to provide inconsistent yield, springback and surface smoothness characteristics when plastically formed into a radiofrequency cavity. These inconsistent properties lead to significant variations in cavity geometry and thus superconducting cavity performance. One approach to reduce these problems is to refine the microstructure so that its properties are more uniform. Microstructural refinement of Nb sheet for RF cavities using multi-axis severe plastic deformation via equal channel angular extrusion (ECAE) was examined. ECAE was performed on 25 mm square cross-section bars of Reactor Grade Nb in a right angle die at room temperature following different extrusion routes to true strains above nine. This heavily worked material was rolled to 4 mm thick sheet and recrystallized. Measurements of hardness, springback, texture, and microstructural uniformity are reported and compared to those of commercial RRR Grade Nb sheet. Preliminary results show noteworthy promise for bulk Nb processed via severe plastic deformation prior to sheet rolling.

Fine Grained Tantalum for Composite

Poor deformation behavior of commercial polycrystalline Ta sheet used for Sn diffusion barriers in Nb3Sn composite superconductors leads to the use of more Ta than may be necessary in these conductors, and to strand fracture during wire drawing. These problems arise because of nonuniform deformation of the Ta layer when co-drawn with Cu. The origin of the problem resides in the microstructure of the Ta and the code formation mechanics of relatively strong BCC Ta with weaker and more ductile adjacent FCC Cu. In an attempt to remedy this problem, we have processed 25 mm square bars of Ta by multi-axis severe plastic deformation (SPD) via equal channel angular extrusion, then rolled the bars to sheet and annealed the as-worked sheet. The processing is done to refine the microstructure and reduce nonuniformities in grain size and texture. Measurements of hardness, microstructure and mechanical properties are reported for bulk and sheet Ta. The results of SPD processing are encouraging.

{\rm Nb}_{3}{\rm Sn}

Equal channel angular extrusion (ECAE) is used to consolidate amorphous Vitreloy 106a (Zr 58.5 Nb 2.8 Cu 15.6 Ni 12.8 Al 10.3 ) powder into bulk metallic glass. Consolidations are performed on gas atomized powder containing 1280 ppmw oxygen encapsulated in copper cans and 780 ppmw oxygen powder encapsulated in nickel cans in a 90° die-angle tool at temperatures above the glass transition temperature (T g ) but below the rapid crystallization temperature (T x ), in the supercooled liquid region (ΔT). Results show that V106a is successfully consolidated to nominal full density after one extrusion pass. Good particle-to-particle bonding and significant particle deformation are observed in the consolidated alloy. A lower oxygen content is beneficial due to the larger undercooled region and the additional time for processing. The low oxygen content material shows little change in thermal stability after being consolidated when compared to the original powder but fine interparticle cracks are observed in some of the consolidates. ECAE appears to be a viable method for consolidating amorphous metal powders into bulk amorphous metal.

Superconductor Diffusion Barrier Sheet

Poor deformation behavior of commercial Ta sheet used for Sn diffusion barriers in multifilamentary Nb3Sn superconductors leads to excessive Ta-Cu interface roughening and the use of a thick Ta layer to overcome layer fracture. The problem stems from three factors: Ta strain hardens faster and to a much higher flow stress than Cu, pure Cu is adjacent to Ta, and the Ta sheet microstructure is non-uniform and non-optimum. The objective of our work is to fabricate Ta sheet with improved deformation behavior over present day commercial Ta sheet. The hypothesis is that a uniform, fine-grained, and well-textured sheet with a preferred orientation will have better codeformation characteristics. To test this thinking, 25 mm square cross-section bars of Ta were deformed by equal channel angular extrusion (ECAE) to strains of nine, sliced in half, annealed to various levels, and rolled to 0.5 mm thick sheet, and Cu-Ta monofilaments fabricated with the processed sheet. Experimental results and roughness measurements on the Ta layers indicate that severe plastic deformation (SPD) processed sheet shows better characteristics than commercial Ta layers. This results from the uniform microstructure and fine grains in precursor Ta sheet processed by SPD.

Progress in Consolidation of Amorphous Zr-based Powder into Bulk Metallic Glass

Fabrication of niobium (Nb) superconducting radio frequency cavities by hydroforming is desirable because of production economy and mitigation of defects. Favorable material characteristics for hydroforming include: (a) good ductility, (b) sufficient strain hardening, and (c) microstructural homogeneity. Seamless Nb cavities are attractive because they do not contain welds. Welds can act as performance inhibitors due to defects, local chemistry changes, pits, etc. The objective of the work reported is to provide a cost-effective processing strategy involving severe plastic deformation and thermo-mechanical processing to produce uniform fine grain Nb microstructures in seamless RRR Nb tubes. An example of a successful implementation of the process is presented, which provides a fine grain size and possible texture control involving different strengths of 〈111 〉/〈100〉 component in the hoop direction. A fine grain size and suitable texture should lead to less deformation heterogeneity, better surface properties, and a lower tube failure rate from hydroforming.

Fabrication of Tantalum Sheet for Superconductor Diffusion Barriers

A strategy to obtain higher critical current density (Jc) in Nb3Sn conductors is to increase the Nb fraction. This can be achieved by improving the deformation behavior of the Nb filaments so less Cu is needed between filaments enabling smaller diameter filaments and more filaments in the subelement region. The objective of this project is to better understand the influence of different pre-processing strain path (texture) on the deformation of a Nb filament in a Cu-matrix. Initial severe plastic deformation by equal channel angular extrusion (ECAE) was used to obtain a uniform fine grain size with slightly different textures in starting Nb rods. Cu-Nb composite bars were warm extruded and wire drawn to evaluate the performance of different starting Nb fine grained microstructures on Nb filament deformation behavior. The initial textures in the various Nb samples were different but quite weak; little difference is seen in the final Nb filament circularities. However, this study indicates the Cu-Nb interface roughness is influenced somewhat by non-uniformity in the Nb microstructure. The evidence suggests that to obtain better deformation characteristics in Nb filaments, one should start with fine-grain weak or untextured Nb with a globally uniform microstructure. This should enable finer as-drawn Nb filaments with better concentricity, composite wires with a higher Nb loading fraction, and conductors with higher Jc.

Nb Tubes for Seamless SRF Cavities

Large grained pure niobium was processed by multi‐pass equal channel angular extrusion and then annealed with the objective of producing a fine‐grained uniform recrystallized microstructure. Extrusions were performed on 25 mm square cross‐section bars in a right angle die at room temperature. Material was processed through 1, 2, 4 and 8 consecutive extrusion passes corresponding to plastic strains of 1.15, 2.3, 4.6 and 9.2. The Vicker’s hardness increased from 66, for the cast material, to 155 after eight extrusions. The temperature for which full recrystallization is achieved for two pass material is near 1100°C and decreases for higher strains. The recrystallized grain size after two passes is near 50 μm but spans the range 10μm to 100μm with distinct regions or “bands” showing different average grain size. The average recrystallized grain size decreases with increased strain and becomes more uniform but larger for higher annealing temperatures. Work on improved thermo‐mechanical processing strategies to eliminate “banding” and produce further reductions in the recrystallized grain size is underway.Large grained pure niobium was processed by multi‐pass equal channel angular extrusion and then annealed with the objective of producing a fine‐grained uniform recrystallized microstructure. Extrusions were performed on 25 mm square cross‐section bars in a right angle die at room temperature. Material was processed through 1, 2, 4 and 8 consecutive extrusion passes corresponding to plastic strains of 1.15, 2.3, 4.6 and 9.2. The Vicker’s hardness increased from 66, for the cast material, to 155 after eight extrusions. The temperature for which full recrystallization is achieved for two pass material is near 1100°C and decreases for higher strains. The recrystallized grain size after two passes is near 50 μm but spans the range 10μm to 100μm with distinct regions or “bands” showing different average grain size. The average recrystallized grain size decreases with increased strain and becomes more uniform but larger for higher annealing temperatures. Work on improved thermo‐mechanical processing strategies t...

Influences of Different ECAE Routes on Filament Deformation in Cu Clad Nb Composite Wires

Tantalum (Ta) diffusion barriers used in Nb3Sn superconductors deform nonuniformly during wire drawing. A strategy sometimes used to mitigate this problem is to place a sacrificial layer of niobium (Nb) next to the Ta to act as a mechanical transition zone between the softer Cu-Nb composite and the harder Ta. This approach can promote more uniform co-deformation of the Ta layer enabling thinner Ta layers and a reduced chance of layer rupture, and has the added benefit of providing additional Nb for conversion to Nb3Sn. A thinner Ta layer translates to less Ta volume and less cost. In the current work the authors find that a layer of Nb adjacent to a Ta layer embedded in Cu significantly increases the composite wire strain to fracture, and appears to decrease the Ta layer interface roughness.