第3元素添加したNb<SUB>3</SUB>Sn超電導線材の弾性定数と内部摩擦―マルテンサイト変態と超電導特性に関連して

Abstract

Effects of ternary additions to the bronze-processed Nb3Sn on its superconducting properties and martensitic transformation have been investigated. The samples were single-core Nb3Sn composite wires prepared using 0-10 at%Ti, Ta, Zr and Hf added Nb cores. Ternary additions increase Hc2 of Nb3Sn composites, which have no bronze matrices, by 0.7-3.5 T. Young’s modulus and internal friction were measured at temperatures from 6 to 300 K using a vibrating-reed technique for the observation of the martensitic transformation in the Nb3Sn compound layers. In the pure Nb3Sn, softening of Young’s modulus occurred with decreasing temperature, and drastic increase in internal friction below the martensitic transformation temperature Tm was observed. The internal friction below Tm is thought to be associated with a stress-induced motion of martensitic domain wall. By the ternary additions, softening of Young’s modulus and the occurrence of the martensitic transformation are strongly affected. Addition of Ti or Ta reduces softening of the Young’s modulus at low temperatures and decreases Tm, and the transformation is suppressed for sufficient amounts of the alloying addition. Addition of Zr or Hf, on the other hand, doesn’t change Tm and is not effective in suppressing the transformation, although it reduces softening of the Young’s modulus at low temperatures. The results mentioned above suggests that the suppression of the martensitic transformation is not a dominant factor of the improvement in Hc2 by the ternary additions. The improvement in Hc2 is thought to be attributed to the increase in ρn.