Effect of Ta Additions upon in situ Prepared Nb 3 Sn-Cu Superconducting Wire
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INTRODUCTION
AT the present time the most developed process for producing multifilamentary superconducting wire based on Nb3Sn is the bronze process, where the fabrication relies upon mechanical reduction of bronze billets containing Nb rods followed b y a tin diffusion step. ~ An alternate process which may prove to be more economical is the socalled in situ process, 2 where fabrication involves mechanical reduction of Cu-Nb castings followed by tin plating and then the diffusion step. Since the advent of the in situ process 3 it has been consistently found that wires prepared by this process have lower He2 values than bronze processed wire. Recent work4 has shown that optimum Jc values for in situ wire require close control of Nb filament diameter and amount of reduction in the wire drawing step. The values of Jc are improved by increasing the area reduction ratio, R, upon drawing, but a decrease in J~ begins to occur when the reduction becomes sufficient to produce Nb filament diameters so small that they coarsen during the tin diffusion step. For Cu-Nb castings containing Nb dendrites of around 8 /zm diameter, optimum J~ values are obtained for an R of around 2500 which produces Nb ribbon filaments of 500 to 700 ,~ thickness, followed by a tin diffusion anneal at 550 ~ for four to six days. Even in these optimized wires, 4 H* remains less than that of bronze processed wires 5 by 3 to 5T. Hence, it was found that the J,. values of optimized in situ wire are inferior to bronze processed wire above around 12T. In recent research Suenaga and others have shown significant improvements in He2 of bronze processed wire through the addition of Ta 6'7'8 and Ti 9 to the Nb cores. The addition of Ta is particularly attractive to the in situ process because one expects the Ta to segregate almost completely to the primary Nb dendrites during solidification of Cu-Nb-Ta alloys. Two investigations of (NbTa)aSn-Cu in situ wires have been reported, ~~ but in both cases the alloys were prepared J.J. SUE, formerly a Graduate Assistant at Ames Laboratory, is currently a Senior Project Engineer at Union Carbide Corporation, Coating Services Division, Indianapolis, IN 46224; J. D. VERHOEVEN is a Professor, Department of Materials Sciences and Engineering, Iowa State University, and a Senior Metallurgist, Ames Laboratory; E. D. GIBSON is an Associate Metallurgist at Ames Laboratory; J.E. OSTENSON is an Associate Physicist at Ames Laboratory; and D.K. FINNEMORE is a Professor, Department of Physics, Iowa State University, and a Senior Scientist, Ames Laboratory, Ames, IA 50011. Manuscript submitted January 4, 1983. METALLURGICAL TRANSACTIONS A
without care to optimize the filament size. In addition, both investigations utilized graphite crucible materials, and recent work 4bhas shown that the carbon pickup is sufficient to produce a significant drop in Jc. Therefore, in the present study we report on the critical superconducting properties of (NbTa)3Sn-Cu in situ wires prepared in Y203 crucibles utilizing near optimum control of t
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