Low temperature phase transformation in Nb<inf>3</inf>Sn multifilamentary wires and the strain dependence of their critical current density

Abstract

Low temperatre X-ray diffraction analysis of Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn multifilamentary wires produced by the bronze route has been performed at different orientations. An important volume fraction of the Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn filaments is found to undergo a stress-induced cubic-tetragonal transformation with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1 - c/a \cong 0.008</tex> at 10 K. Strong texturing of the tetragonal phase has been detected: the shorter c axis is preferentially oriented along the wire axis. The present data furnish a direct indication for the uniaxial stress distribution in the wire. Careful H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</inf> (T) measurements on a wire at the vicinity of T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> reveal a sharp change of the slope dH <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</inf> /dT|T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> . This is interpreted as a physical proof for the coexistence of two phases at low temperature, with T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> (cub.) < T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> (tetr.), but H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</inf> (cub.) > H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</inf> (tetr.). Based on these data, the variation of the ratio J <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cm</inf> /J <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">co</inf> of Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn multifilamentary wires under uniaxial stress can be correlated with the presence of the tetragonal phase. The applied strain is believed to affect as well the the tetragonal volume fraction as the ratio c/a of the tetragonal phase.