Effects of Ternary Additions on Young’s Modulus and the Martensitic Transformation of Nb₃Sn

Abstract

Recent measurements on bronze-processed Nb₃Sn using a vibrating reed technique have shown that Young’s modulus at low temperatures decreases to 0.4 of its room temperature value and that internal friction increases dramatically below 50 K, the martensitic transformation temperature.¹ In the present paper, this technique was used to study softening and the occurrence of the martensitic transformation in bronze-processed Nb₃Sn containing ternary additions. The additions studied were Ta and Ti, which are known to cause substantial improvements of critical current density at high fields,^2–4 and Zr, which improves the growth kinetics of bronze-processed Nb₃Sn.⁵ The additions were added to the Nb core and are believed to occupy Nb sites in the A15 lattice. Results show that ternary additions to Nb₃Sn can affect both its structure by suppressing the martensitic transformation and the state of stress, when included in a matrix, by causing large changes in Young’s modulus at low temperatures. Both of these effects are of technological interest since the upper critical field, H_c2, and critical temperature depend on whether Nb₃Sn is cubic or tetragonal^{6, 7} and, in the case of composite conductors, on the combined strain resulting from the compression due to the matrix and externally applied stress.^{8, 9}

Work supported in part by the U.S. DoE.