Deformation-processed wire prepared from gas-atomized Cu-Nb alloy powders
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I.
INTRODUCTION
DEFORMATION-processed Cu-refractory metal materials have been under development for some time, originally as a precursor material for the fabrication of superconducting wire by the so-called in situ bronze process. Currently, these materials are of interest for their combination of high electrical conductivity and high mechanical strength, tl,21 They are produced by large mechanical deformation of Cu-X alloy billets, where X is a refractory metal, e . g . , niobium or tantalum. The precursor billets are usually produced by consumable arc melting and chill casting. It was demonstrated t3,4~ that a reduction in scale of the original cast microstructure would produce increased strength at a given level of deformation strain. A new route for the production of Cu-Nb alloy powder with a very fine microstructure has become available to the authors with the installation of highpressure gas atomization (HPGA) facilities at the Ames Laboratory (Ames, IA). Powders of Cu-Nb have been prepared by HPGA, consolidated by conventional powder processing techniques to prepare precursor billets, and reduced to wire by deformation processing. In this work, the electrical and mechanical properties of deformation-processed wires made from gas-atomized copper-niobium alloy powders have been measured and are related to microstructural characteristics.
II.
EXPERIMENTAL PROCEDURE
A Cu-Nb alloy powder was produced by HPGA from a melt of Cu-20 vol pct Nb using argon for the atomization gas. The starting elemental charge was C10100
Cu and 99.9 pet Nb prealloyed by consumable arc melting followed by chill casting. The alloy charge was melted with an induction furnace at 3000 Hz in a fully dense cer-met crucible composed of 40 vol pct ZrO2 in a Mo matrix. The melt temperature at the start of the gas atomization pour was 2000 ~ The 1-kg powder yield that resulted was collected, stored, and sieved under an argon atmosphere. A 38 to 63 /xm sample was obtained and consolidated by cold isostatically pressing at 310 MPa to produce a material approximately 85 pct dense with open porosity. The cylindrical compact was placed in a copper can and hot vacuum outgassed at 800 ~ for 4 hours, sealed by electron beam welding, and then hot isostatically pressed ("hipped") at 310 MPa for 4 hours at 800 ~ After consolidation, the material was cold swaged from 25.4 to 2.9 mm, then drawn to 1.3 mm, where fracture by "center bursting" became a problem. At this point, the as-drawn material was swaged to 0.79 mm and drawn to a final diameter of 0.36 mm. This procedure was done using only material deformed to one die before center bursting was noted. Further mechanical reduction was discontinued due to the reoccurrence of center bursting of the wire. Mechanical testing was done at a strain rate of 0.25 m m / m i n after a gage section was produced by abrasive polishing. All testing was at ambient temperature. Electrical resistivity measurements were done by the standard four-probe method, as previously described, t2] Heating rates during the resisti
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