1051-8223 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TASC.2016.2517412, IEEE Transactions on Applied Superconductivity > Manuscript Tracking ID: MT24-4OrBB-03 1 Bending Behavior of High-Strength Conductor Ke Han, Robert Goddard, Rongmei Niu, Tienlei Li, Doan Nguyen, James R. Michel, Jun Lu and Victor Pantsyrny Abstract Conductors in pulsed magnets in the US National High Magnetic Field Laboratory (MagLab) are rectangular cross- section wires of relatively large thickness. During the manufacture of the magnets, some of these conductors are wound to small-diameter coils (less than 15 mm). Because of the large thickness and the small bending radii for winding, the wires undergo large bending strain, sometimes causing breakage. We studied the bending behavior of high-strength conductor wires. In most materials, maximum bending strain can usually be calculated from elongation values obtained in tensile tests. In our study, however, the maximum bending strain exceeded the elongation of most of our high-strength wires so we could not estimate bending strain from elongation. The large bending strain that occurs during the manufacture of coils in pulsed magnets causes an increase in residual strain and a decrease in packing factor. Due to the anticlastic effect, the cross-section of the wire changes from rectangular to keystone shape, with the keystone angle up to 10 degrees. This introduces gaps into the coils, thus reducing the magnetic field by an amount that must be taken into an account. In both tensile and compressive strained regions, we observed significant microstructure changes. Certain properties, such as tensile mechanical strength and electrical conductivity, depend directly on microstructure. This paper summarizes our work on both geometry and microstructure evolution in conductors exposed to different bending strain values. Index Terms—high strength conductor, bending strain. Coil winding, Cu-Nb, plastic deformation. I. INTRODUCTION URING the first decade of this century, researchers in the National High Magnetic Field Laboratory (MagLab) with their partners developed conductors of such high strength that they were able to build a nondestructive pulsed magnet capable of generating a field of 100T, still the strongest of this type in the world. This magnet consists of an outer coil and an insert with 8 individual helix coils, all from rectangular cross- section wires of relatively large thickness (up to 9 mm[1]). Each of the inner coils is wound separately to a different diameter and connected to the others with extended leads. Each diameter corresponds to a fixed bending strain. The magnet has a bore diameter of only 10 mm. To The work was undertaken in the National High Magnetic Field Laboratory, which is supported by NSF DMR-1157490, the State of Florida. and DOE. Ke Han, Robert E. Goddard, Rongmei Niu, and Jun Lu are with the National High Magnetic Field Laboratory, Tallahassee, FL32309 USA (e- mail: han@magnet.fsu.edu). Tienlei Li is now with Danfoss Turbocor Compressors Inc.. Doan N, Nguyen and James R. Michel are with the National High Magnetic Field Laboratory, Los Alamos National Laboratory, Los Alamos, NM USA (e-mail: doan@lanl.gov). Victor Pantsyrny is with Nanoelectro Co., Russia (e-mail: pantsyrny@gmail.com). achieve such a core size, the wire used to build the coil must be subjected to considerable bending strain. If the bending strain exceeds the elastic deformation strain of the conductors, significant plastic deformation may occur, which have at least two significant effects on magnet construction, and magnet performance. First, large plastic deformation strain causes changes in the cross-section area and the shape of the wire, decreasing the packing factor of the conductors and consequently reducing the power of the magnet. Second, if the bending strain nears that of the homogeneous deformation strain in tensile tests, premature failure of the conductor may occur. To increase the magnetic field of future magnet, the bore size may have to be smaller than 10 mm, in which case plastic deformation may be even higher than we have previously encountered. This paper reports our investigation of the deformation of conductor wire that occurs at different bore diameters between 6.4 mm and 12.7 mm. II. EXPERIMENTAL METHODS A. Materials and Fabrication 3 mm x 5.8 mm CuNb conductor wires were used in our experiment. The measured elongation is between 1.7 and 2.9%. The measured true strain to failure from reduction-in- area of fracture is between 14-30%. The coils from these wires are manufactured by taking into account of spring-back to ensure coils to be tightly packed in the magnet assembly. For example, the innermost layer is wound on a 9.3 mm former. The spring-back assists this coil to be installed on a 10-mm mandrel. B. Macrostrain Measurements To track the macro deformation strain on wire surface due to the winding, we used laser micro machining to mark a grid of tiny grooving lines on the wide surfaces of the wire, (Fig. 1). The grid consists of 21 lines along the wire length (wire axis, the horizontal lines in the figure) and 4 lines perpendicular to the wire axis (along the width, the vertical lines in the figure). The deformation strain can be determined from the new distances between the vertical lines (for example, lines labelled as 2, and 3 in Fig. 1). (a) (b) (c) Fig. 1. Grid lines engraved by laser: (a) sample without any bending strain; (b) sample in compressive bending strain; and (c) sample in tensile bending strain. The vertical lines allow us to track the deformation along the width of the conductor. The horizontal lines were D