> 2PoCK-04< 1 The effects of Ti addition and high Cu/Sn ratio on tube type (Nb,Ta) 3 Sn strands, and a new type of strand designed to reduce unreacted Nb ratio Xingchen Xu, Edward Collings, Michael Sumption, Chris Kovacs and Xuan Peng Abstract—In this work we report the properties of two tube type Ta doped Nb 3 Sn strands: one strand was additionally Ti doped by way of a Sn-Ti alloy core, and the other had high Cu/Sn ratio within the filaments. Higher irreversibility field (B irr ) was obtained on the quaternary strand with respect to the (Nb- 7.5wt.%Ta) 3 Sn strand. High Cu/Sn ratio decreased the amount of coarse grain (CG) formation, but also degraded the layer J c of the tube type strand by depressing the Sn content in the fine grain (FG) layer. A new type of strand, the subelement of which is composed of 7 bare Cu-Sn cored Nb tube filaments, was designed with the aim to reduce the unreacted Nb area fractions. The test results of the first experimental strand are reported. The unreacted Nb ratio is reduced relative to normal tube type strands and the FG area fraction is improved. The unique structure of this strand makes it also possible to improve the stoichiometry of FG and reduce the effective diameter (d eff ). Index Terms— irreversibility field; layer J c ; Nb 3 Sn; tube type. I. INTRODUCTION LTHOUGH the rod-in-tube (RIT) or restack-rod-process (RRP) Nb 3 Sn strands generally have large effective subelement diameters (d eff ) and thus poor low field stability [1]-[3], they are the principle Nb 3 Sn strands presently being considered for the high energy physics (HEP) applications thanks to their high critical current density J c [4], [5]. On the other hand, although the non-Cu J c s of the best powder-in-tube (PIT) and tube type strands are somewhat lower than the best RIT strands, their fine grain (FG) layer J c s are comparable [6], [7]. In work [6] we show that both the stoichiometry and the grain sizes of the best RIT and tube type strands are similar. This offers the possibility of improving the non-Cu J c of tubular strands by suitable adjustment of the architecture of the subelements. There are two reasons for the lower FG area fractions in tubular strands: the relatively high unreacted Nb and coarse grain (CG) area fractions. In work [8] a model predicting the amounts and radial extents of CG and FG areas as functions of the starting Cu and Manuscript received July 16th, 2013. This work was supported in part by the U.S. Dept. of Energy, Office of High Energy Physics, under Grants No. DE-FG02-95ER40900 (OSU) and a DOE SBIR. X. Xu, E. Collings, M. Sumption and C. Kovacs are with the Ohio State University, Columbus, Ohio 43202 USA (X. Xu phone: 515-441-3429; e- mail: xu.452@osu.edu). X. Peng is with Hyper Tech Research Inc., Columbus, Ohio, USA. Sn amounts in the filament is presented, and it contends that elevated Cu/Sn ratios diminish the formation of CG and increase the FG area fractions. In this work a strand with high Cu/Sn ratio was tested to find out whether the high layer J c can be maintained as the Cu/Sn ratio is increased. We also analyze the reason why tubular strands have higher unreacted Nb area fractions than RIT strands, and then offer a new type of strand that not only reduces the unreacted Nb ratio with respect to normal tube type strands but also decreases the persistent-current magnetization with respect to RIT strands. The effects of Ti doping on tube type strands are investigated. In a (Nb-7.5 wt.%Ta) 3 Sn RIT strand improved irreversibility field (B irr ) was reported due to Ti addition from the inclusion of Nb-Ti filaments in the starting billet [9]. In this work Ti was introduced into the tube type strands by way of a Sn-Ti alloy core. The effects of Ti doping on B irr and J c of (Nb,Ta) 3 Sn tube type strands are reported. II. STRAND SPECIFICATIONS AND EXPERIMENT DETAILS A. Strands prior to heat treatment Four strands were studied in this work, with their specifications presented in Table I. All of them are Ta doped by using Nb-7.5 wt.% Ta filaments. T1505 is a standard tube type strand with the highest 12 T non-Cu J c and serves as the benchmark in this work. T2631 is additionally Ti doped via using Sn-1.5 at.% Ti alloy in the core. The strand T2637 has a very high Cu/Sn ratio within the filaments. In addition, a new type T3203 strand, the subelement of which is composed of seven very high Sn/Nb ratio tube type filaments (without Cu matrix), was also studied. The SEM images of subelements (or filaments) of these strands prior to the heat treatments are shown in Fig. 1. TABLE I HERE FIG. 1 HERE B. Heat treatments and measurements All these strands were wound and heat treated on ITER barrels [10]. Except T3203, which was reacted at 650 °C for 120 h, other strands were all reacted at 625 °C. The reaction times for T1505, T2631, and T2637 were 500 h, 150 h, and 150 h, respectively. Detailed descriptions of preparations, transport measurements and SEM observations can be found A This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication. The final version of record is available at http://dx.doi.org/10.1109/TASC.2013.2291159 Copyright (c) 2013 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.