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[Y. Hishinuma](https://orcid.org/0000-0002-5112-3065), H. Oguro, H. Noto, H. Yata, [H. Taniguchi](https://orcid.org/0000-0002-2719-6491), [S. Awaji](https://orcid.org/0000-0003-2043-1628), [A. Kikuchi](https://orcid.org/0000-0002-5044-7156)

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[Mechanical Strength Enhancement of the Internal Matrix Reinforced Nb<sub>3</sub>Sn Multifilamentary Wires Using Cu-Sn-In Ternary Bronze Alloy Matrices by HIP Processing](https://mdr.nims.go.jp/datasets/bf90317f-ef3b-49c7-a951-9d4c4efc0fa0)

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Microsoft Word - Transaction article-ASC2024 (2MPo2A-06_ CONF#1173714).doc1 2MPo2A-06  Mechanical Strength Enhancement of the Internal Matrix Reinforced Nb3Sn Multifilamentary Wires Using Cu-Sn-In Ternary Bronze Alloy Matrices by HIP Processing  Y. Hishinuma, H. Oguro, H. Noto, H. Yata, H. Taniguchi, S. Awaji and A. Kikuchi   Abstract—We are approaching to high mechanical strength of bronze processed Nb3Sn wire by internal matrix reinforcement using Cu-Sn-In ternary alloy. The In solute element remained homogeneously in the wire matrix after Nb3Sn synthesis and contributed to transform (Cu, In) solid solutions. (Cu, In) solid solution became as reinforce material of Nb3Sn phase and acted to improve the mechanical strength of the Nb3Sn wire. In this study, we investigated the effect of Hot Isostatic Press (HIP) treatment to the further mechanical strength improvement of the internal matrix reinforced Nb3Sn wire. Tensile stress and strain sensitivities of transport critical current (Ic) property were enhanced by HIP processing because of the drastic reduction of micro-sized vacancies into the transformed (Cu, In) matrix. We found the HIP treatment contributes to mechanical strength enhancement of the internal matrix reinforced Nb3Sn wire.  Index Terms—Nb3Sn, internal matrix reinforcement, ternary bronze matrix, mechanical strength, HIP treatment   I. INTRODUCTION TER project is one of the path-ways to realize nuclear fusion energy generation, and this project is also still in progress. In ITER project, high current capacity Cable in- Conduit (CIC) conductors for the Toroidal magnetic field and Central Solenoid coils were made by the twisted bundles of  This work was financially supported by the National Institute for Fusion Science, the High Field Laboratory for Superconducting Materials, Institute of Materials Research, Tohoku University (GIMRT Project No. 202212-HMKPC-0040), and also the JSPS KAKENHI (Japan Society for the Promotion of Science Grant-in-Aid for Scientific Research (B), 20H01889). Yoshimitsu Hishinuma and Hiroyuki Noto are with National Institute for Fusion Science and Graduate University of Advanced Studies (SOKENDAI), Toki, Gifu, 509-5292 Japan (e-mail: hishinuma.yoshimitsu@nifs.ac.jp and noto.hiroyuki@nifs.ac.jp).  Hidetoshi Oguro is with Tokai University, Hiratsuka, Kanagawa 259-1292 Japan. (e-mail: h-oguro@tokai.ac.jp). Hiroki Yata is with Graduate University of Advanced Studies (SOKENDAI), Toki, Gifu, 509-5292 Japan (e-mail: yata.hiroki@nifs.ac.jp).  Hiroyasu Taniguchi is with Osaka Alloying Works, Co.,Ltd, Fukui, 910-3131 Japan (e-mail: taniguchi@osakagokin.co.jp). Satoshi Awaji is with High Field Laboratory for Superconducting Materials, Institute of Materials Research, Tohoku University, Sendai, Miyagi 980-0812 Japan (e-mail: awaji@imr.tohoku.ac.jp). Akihiro Kikuchi is with National Institute for Materials Science, Tsukuba, Ibaraki, 305-0047 Japan (e-mail: KIKUCHI.Akihiro@nims.go.jp), and then Yoshimitsu Hishinuma is also with as the collaborative guest researcher. Color versions of one or more of the figures in this article are available online at http://ieeexplore.ieee.org many Nb3Sn wires. Through the ITER procurement activities, mechanical strength issues in Nb3Sn wire as the CIC conductor for beyond ITER were pointed out [1]. Especially, transport Ic degradation by mechanical strain applying due to the electromagnetic force is one of the key research issues to be solved for a future high magnetic field superconducting magnet beyond ITER [1]. With regard to mechanical strength improvement of Nb3Sn wire, various reinforcement methods providing Al2O3 dispersion strengthened copper or CuNb alloy to the outside of Nb3Sn wire have been investigated [2, 3].  We proposed an internal matrix reinforcement method by solid solution strengthening of the wire matrix [4-6]. The principle of the internal matrix reinforcement method is that the wire matrix is changed from the conventional Cu-Sn binary alloy to the Cu-Sn based ternary alloy containing Zn or In elements. The wire matrices of Cu-Sn-Zn or Cu-Sn-In ternary alloys were transformed to homogeneous and harder (Cu, Zn) or (Cu, In) solid solution after Nb3Sn phase synthesis heat treatment, and these solid solutions would act as a protective material for Nb3Sn phase [4, 6]. Furthermore, we confirmed that mechanical strength and intrinsic strain sensitivity in Ic property were improved compared with the conventional bronze processed Nb3Sn wire [7-9]. For the further mechanical strength improvement, we noticed that the Hot Isostatic Pressing (HIP) after Nb3Sn synthesis heat treatment. Previously, T. Fukutake et al. reported that strain dependence of transport Ic property in bronze and Internal Tin processed Nb3Sn wire was improved by HIP treatment [10, 11]. Furthermore, C. Barth also mentioned that the irreversible tensile strain of transport Ic property in Nb3Sn wire was improved by HIP processing [12]. In this study, the internal matrix reinforced Nb3Sn wires after heat treatment were performed to HIP treatment and the comparisons of transport Ic behavior under tensile deformation with and without HIP processing were evaluated.  II. SAMPLE PREPARATION AND EVALUATIONS A. Sample preparation Two kinds of the internal matrix reinforced Nb3Sn multifilamentary precursor wires were prepared. The sample code and specification of wire samples is indicated to Table.1. I2 2MPo2A-06   Fig. 2.  The comparisons of transport Ic behavior at 4.2 K and 15 T as a function of the applied uniaxial tensile stress and strain between the 2.0Inwire samples with and without HIP processing.  The difference between the two kinds of internal matrix reinforced Nb3Sn wires was the composition of the ternary bronze alloy as the wire matrix, which were Cu-10wt%Sn-5wt%In-0.3wt%Ti (Sample code: 5.0In) and Cu-14wt%Sn-2wt%In-0.3wt%Ti (Sample code: 2.0In). These 2.0In and 5.0In precursor wires were carried out the two step Nb3Sn phase formation treatment, which is 550ºC-100 hrs + 650ºC-100hrs in Ar atmosphere.  After that, HIP treatment performed to wire samples at 650ºC-2hrs under 200MPa in Ar atmosphere using NIFS-HIP system. Temperature and pressure profile during HIP treatment is shown in Fig. 1. The pressure in the HIP system was elevated using a two-stage Ar gas compressor and then maintained stably at 200MPa. B. Transport Ic evaluations under tensile deformation with and without HIP processing Transport Ic measurement under uniaxial tensile deformation was carried out using 18 T superconducting magnet with 55 mm bore and the Ic measurement probe with uniaxial tensile deformation mechanism in Institute Material Research, Tohoku University. Before the wire sample mounting to the Ic measurement probe, two strain gages were attached to surface of wire sample. Transport Ic measurement system with tensile deformation mechanism and sample mounting were already reported [13].  The Ic probe after sample mounting was inserted into 18 T superconducting magnet, and transport Ic measurement with tensile deformation was performed at 4.2 K and 15 T of external magnetic field. The tensile strain was defined to the average value of two attached strain gages. A tensile load, which is measured by 1 kN load cell, was applied by the load lever moving. The tensile stress was estimated by the tensile load value divided by the cross-sectional area including the stabilized copper of the wire sample. Transport Ic measurement was carried out using a DC four-probe method, and the Ic criterion was determined by 1 µV/cm electric field generation. III. RESULTS AND DISCUSSIONS A. Transport Ic behavior under tensile deformation of the 2.0In wire sample with and without HIP processing The comparisons of transport Ic behavior at 4.2 K and 15 T as a function of the uniaxial tensile stress and strain between the 2.0In wire samples with and without HIP processing is shown in Fig. 2. Here, each normalized Ic values shown in Fig. 2 corresponded to the Ic values loaded to some tensile stress (σ) or strain (ε) divided by the maximum Ic value (Icmax), and these normalized Icσ/Icmax or Icε/Icmax values indicate the Ic degradation rate by the uniaxial tensile stress and strain. In this study, the uniaxial tensile stress and strain shown in Icmax value are also defined as the peak stress and strain, respectively. Icmax value of the 2.0In wire sample was roughly increased from 93 A to 113 A by the HIP processing. Normalized Ic values loaded to zero tensile stress and strain (Icσ0/Icmax and Icε0/Icmax) were decreased by the HIP processing. These suggested that the high pressure of 200 MPa by HIP process effectively applied to the inside of the the 2.0In wire sample TABLE I THE SAMPLE CODE AND WIRE SPECIFICATION OF THE INTERNAL MATRIX REINFORCED NB3SN MULTIFILAMENTARY WIRES IN THIS STUDY  Sample code 2.0In 5.0In Wire matrix Cu-14Sn-2In-0.3Ti Cu-10Sn-5In-0.3Ti Matrix composition (Sn, In and Ti: wt%) Sn In Ti Sn In Ti 14.05 1.99 0.35 10.17 5.06 0.31 Number of Nb fila. 7771 7771 Diameter of Nb fila. 3.4 μm 3.4 μm Barrier material Nb Nb Final wire diameter 0.9 mm 0.9 mm Cu ratio 1.30 1.30  Fig. 1. Temperature and the pressure profiles during HIP processing in this study. 3 2MPo2A-06   Fig. 4.  The comparisons of X-ray Diffraction (XRD) patterns in wire matrix region of the 5.0In and 2.0In wire samples between before and after Nb3Sn synthesis heat treatment using micro-focused XRD system. and increased the residual stress and strain. Icσ/Icmax and Icε/Icmax values improved by the applying tensile deformation. These would be caused by the release of the residual stress and strain into the wire. The peak stress and strain of the 2.0In wire sample with HIP processing were obtained to be approximately 162 MPa and 0.299 %, and these values were much higher than those of the sample without HIP. The fracture stress and strain were estimated to be approximately 474 MPa and 0.963 %, and then they were improved roughly 15 % by the HIP processing. The summary of major mechanical properties in the 2.0In samples with and without HIP processing is indicated to Table. 2. We found that mechanical strength of the 2.0In wire sample was enhanced by the HIP processing.  B. Transport Ic behavior under tensile deformation of the 5.0In wire sample with and without HIP processing Similar to Fig. 2, the comparisons of transport Ic behavior at 4.2 K and 15 T as a function of the uniaxial tensile stress and strain between the 5.0In wire samples with and without HIP processing is shown in Fig. 3. Similar to the 2.0In wire sample, normalized Icσ0/Icmax and Icε0/Icmax were decreased by the HIP processing. This suggested that the high pressure of 200 MPa by HIP process effectively also applied to the inside of the 5.0In wire sample and increased the residual stress and strain. The peak stress and strain of the 5.0In wire sample with HIP processing were obtained to be approximately 308 MPa and 0.3600 %, and these values were much higher than those of the sample without HIP. The fracture stress and strain were estimated to be approximately 578 MPa and 1.1668 %. The summary of major mechanical properties in the 5.0In wire sample with and without HIP processing is indicated to Table. 3. We found that mechanical strength of the 5.0In wire sample was also enhanced by the HIP processing.  C. The comparisons of the HIP effect on transport Ic behavior under tensile deformation between the 2.0In and 5.0In samples As indicated in Tables. 2 and 3, the major mechanical property factors were improved by the HIP treatment, and it contributed to the mechanical strength enhancement of the internal matrix reinforced Nb3Sn wire. However, difference in the HIP effect was observed between the different compositions of the wire matrix. This was caused by the difference in the microstructure of the wire matrix before HIP treatment. X-ray Diffraction (XRD) patterns in wire matrix region of the 5.0In and 2.0In wire samples between before and after Nb3Sn synthesis heat treatment using micro-focused XRD system is shown in Fig. 4.  Fig. 3.  The comparisons of transport Ic behavior at 4.2 K and 15 T as a function of the applied uniaxial tensile stress and strain between the 5.0In wire samples with and without HIP processing.  TABLE III THE SUMMARY OF MAJOR MECHANICAL PROPERTIES IN THE 5.0IN WIRE  SAMPLE WITH AND WITHOUT HIP PROCESSING IN THIS STUDY  5.0In sample Without HIP With HIP Increment (%) Maximum Ic (A) 64.60 63.19 - 2.17 Fracture stress (MPa) 564.84 578.83 + 2.48 Fracture strain (%) 0.9320 1.1668 + 25.19 Peak stress (MPa) 265.89 308.29 + 15.95 Peak strain (%) 0.3052 0.3600 + 17.96  TABLE II THE SUMMARY OF MAJOR MECHANICAL PROPERTIES IN THE 2.0IN WIRE  SAMPLE WITH AND WITHOUT HIP PROCESSING IN THIS STUDY  2.0In sample Without HIP With HIP Increment (%) Maximum Ic (A) 93.33 113.33 + 21.42 Fracture stress (MPa) 412.30 474.24 + 15.02 Fracture strain (%) 0.8385 0.9626 + 14.80 Peak stress (MPa) 104.30 162.79 + 56.08 Peak strain (%) 0.1760 0.2986 + 69.66  4 2MPo2A-06   Fig. 5.  The volume change of the wire matrix in the 2.0In and 5.0In wire samples between before and after Nb3Sn synthesis heat treatment The X-rays were focused to a diameter of 100 µm by the collimator and irradiated only to the wire matrix region. In previous studies, we already confirmed that the wire matrix after Nb3Sn synthesis heat treatment has transformed to a homogeneous (Cu, In) solid solution with an initial In element composition [6-9].  As shown in Fig. 5, both the Cu-Sn-In ternary bronze alloy and the (Cu, In) solid solution have the Face-Centered Cubic structure, the volume change of the wire matrix before and after the Nb3Sn formation could be calculated based on the change of the lattice parameter in the wire matrix estimated by XRD. In the 2.0In wire sample, where the HIP effect was significant, the lattice constant changed from 0.3695 nm to 0.3632 nm and the volume decreased by approximately 5 %. In the 5.0In wire sample, the lattice constant changed from 0.3670 nm to 0.3666 nm, resulting in a volume decrease by approximately 2 %. The decrease in volume in the wire matrix was one of the factors of the void formation in the wire matrix, and these voids would become the origin point of mechanical fracture. The 2.0In wire sample have formed more voids in the wire matrix after Nb3Sn formation compared with the 5.0In wire sample. The HIP treatment increased the mechanical strength by removing many voids. In references [10-12], the HIP treatment improved the mechanical strength properties in critical current properties and concluded that the void reduction was the main reason for this improvement. In other words, HIP treatment of the internal matrix reinforced Nb3Sn wire using a ternary bronze alloy acted more effectively to the larger volume change of the wire matrix during Nb3Sn synthesis heat treatment.   IV. CONCLUSIONS The peak stress and strain after HIP processing were clearly improved. HIP treatment was one of the effective processes to improve mechanical strength of the internal matrix reinforced Nb3Sn wire. Because the voids into the wire matrix after Nb3Sn formation formed by the volume change of the wire matrix were reduced by the HIP process. And the HIP effect had appeared more effectively to the wire with larger volume change of the wire matrix due to the Nb3Sn phase formation.  ACKNOWLEDGMENT We would like to thank Ms. Kawashima of the Collaborative Research Center in Meisei University for her assistance in the use of the micro-focused XRD system for phase transformation analysis on the wire matrix before and after Nb3Sn synthesis. REFERENCES [1] A. Devred, I. Backbier, D. Bessette, G. Bevillard, M. Gardner, C. Jong, F. Lillaz, N. Mitchell, G. Romano and A. Vostner, “Challenges and status of ITER conductor production”, Supercond. 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