FABRICATION OF NIOBIUM-ALUMINUM SUPERCONDUCTING STRANDS USING MECHANICAL ALLOYING

THE DRIVE TOWARD DEVELOPING HIGHER FIELD MAGNETS FOR SUCH APPLICATIONS AS PARTICLE ACCELERATORS AND FUSION ENERGY MACHINES IS LIMITED PRIMARILY BY THE LACK OF CONDUCTOR MATERIAL CAPABLE OF CARRYING HIGH CURRENT DENSITIES AT HIGH FIELD. EXISTING MAGNETS WITH THE HIGHEST PEAK FIELDS USE A15 COMPOUND SUPERCONDUCTING MATERIALS, MOST COMMONLY NIOBIUM-TIN (NB3SN). THE PRIMARY REASON FOR NB3SN SUCCESS IS THAT THIS COMPOUND CAN BE FORMED THROUGH INTERNAL TIN OR BRONZE PROCESSES, WHICH ALLOW A15 PHASE FORMATION AT RELATIVELY LOW TEMPERATURES (<800 DEGREES CENTIGRADE). HOWEVER, NIOBIUM-ALUMINUM (NB3AL) HAS THE ADVANTAGES OF SUBSTANTIALLY HIGHER CRITICAL MAGNETIC FIELD (HC2) AND CONSIDERABLY GREATER STRAIN TOLERANCE THAN NB3SN. PREVIOUS WORK ON NB3AL HAS CLEARLY DEMONSTRATED THE POTENTIAL FOR GOOD CURRENT DENSITY AT HIGH FIELD, BUT LIMITATIONS IN LARGE SCALE PRODUCTION METHODS HAVE PREVENTED THE MATERIAL FROM ACHIEVING THIS POTENTIAL. WORK TO DATE PROVIDES AMPLE EVIDENCE THAT FINER SUBDIVISION OF NB AND AL COMPONENTS IN A COMPOSITE IMPROVES PROPERTIES, WITH CRITICAL TEMPERATURE (TC), HC2, AND HIGH FIELD CRITICAL CURRENT (JC) ALL INCREASING IN INVERSE PROPORTION TO COMPONENT THICKNESS. ONE POSSIBLE METHOD FOR DECREASING COMPONENT THICKNESS WITHOUT VERY HIGH AREA REDUCTIONS--MECHANICAL ALLOYING--WILL BE STUDIED IN THIS PROJECT. THE PHASE I RESEARCH INCLUDES STUDY OF THE MECHANICAL ALLOYING OF NB AND AL TO FORM PRECURSOR RODS THAT HAVE A HIGHLY REFINED DIVISION OF COMPONENTS. THESE RODS ARE ENCASED IN COPPER AND THEN CONVENTIONALLY EXTRUDED AND DRAWN TO WIRE, A TECHNIQUE THAT ALLOWS EVALUATION OF THIS METHOD AS A COMMERCIAL PROCESS FOR FABRICATING LONG LENGTHS OF CONDUCTOR. VARIOUS REACTION HEAT TREATMENTS ARE BEING EXPLORED. TRANSPORT CRITICAL CURRENT DENSITY IS BEING MEASURED AS A FUNCTION OF MAGNETIC FIELD. MICROSTRUCTURAL ANALYSIS IS BEING CONDUCTED IN ORDER TO QUANTIFY THE COMPONENT THICKNESSES AND MORPHOLOGY, BOTH BEFORE AND AFTER HEAT TREATMENT. ANALYSIS IS BEING PERFORMED TO CORRELATE THE MICROSTRUCTURE WITH JC.