Powder metallurgical alternative for the processing of (Nb - Ta)<inf>3</inf>Sn multifilamentary wires, starting from elementary Nb and Ta powder

Abstract

Ordinarily the Nb-Ta cores of alloyed Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn wires are prepared by electron beam melting followed by remelting for better homogenity. The present work describes the first attempt to replace the melting process by a more economical powder metallurgical technique, starting with Nb and Ta elementary powders. After the processing to fine wires, the ductile Ta particles are deformed to ribbon-like filaments, the average distance between two Ta "Walls'' being in the range of the A15 grain size. Thus an improvement of the pinning behavior of Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn multilfilamentary wires was expected. Nb/Ta powde mixtures (up tp 20 wt Z Ta) were extruded and deformed to rods. wich were inserted in a bronze matrix, the superconductor wires being produced by the conventional bronze route. It was found that during the reaction heat treatment Ta is partly dissolved in the A15 layer, the dissolved amount depending strongly on the reaction temperature. As a main result of the present investigation, the field dependence of J <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> for our wires after a reaction of 120 h at 675°C is similar to that of binary Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn wires, the absolute value of J <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> below 13 T being substantially higher than that of the latter. After 64 h at 700°c and 750°c, J <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> has the characteristics of alloyed Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn wires the absolute values of J <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> in the A15 phase being comparable to those of the best conventionally processed (Nb-Ta) <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn wires. These high critical currents after the reaction at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T \leq 675\deg</tex> C are discussed as a consequence of "artifical pinning" by the remaining Ta inclusions.