Effect of pre-annealing on microstructure, mechanical properties and current-carrying properties of Bi-2212 wires

• Published in: Fusion engineering and design 2020-07, Vol.156, p.111606, Article 111606
• Main Authors: Hao, Q.B., Li, C.S., Xu, X.Y., Liu, G.Q., Jiao, G.F., Zheng, H.L., Zhang, S.N., Li, G.S., Zhang, C.P., Yu, Z.M., Bai, L.F., Feng, J.Q., Zhang, P.X.
• Format: Article
• Language: English
• Subjects: Annealing; Bi-2212 CICC; Bi-2212 wire; Bismuth strontium calcium copper oxide; Carrying capacity; Conductors; Core loss; Critical current density; Crystal defects; Filling ratio; Heat treating; Heat treatment; Mechanical properties; Melting; Overpressure; Phase composition; Porosity; Pre-annealing; Surface defects; Tensile strain; Theoretical density; Twisting; Yield strength; Yield stress
• ISSN: 0920-3796; 1873-7196
• DOI: 10.1016/j.fusengdes.2020.111606

[Display omitted] •The pre-annealing of the wire can achieve the purpose of not reducing the wire diameter during overpressure heat-treatment of Bi2212 magnet.•The pre-annealed wires had excellent mechanical properties, so that the conductors with smaller pitch and low hysteresis loss could be prepared.•The crystallinity of Bi-2212 powder of pre-annealed wires is greatly improved, which is conducive to the simultaneous melting the powder. In order to solve the problem of excessive porosity of Bi-2212 Cable in Conduit Conductor (CICC) after the overpressure heat treatment, which leads to a significant decrease of mechanical stability of the conductor, a pre-annealing of Bi-2212 wires with a total pressure of 50 bar has been carried out in this paper. After the pre-annealing, the density of oxide cores of Bi-2212 wires was close to the theoretical density, which avoided the problem of diameter reduction of the wires in Bi-2212 CICC during overpressure heat treatment. At the same time, the mechanical properties, microstructures and current carrying properties of pre-annealed wires have been also analyzed. It showed that after pre-annealing, the Bi-2212 wires had not only high yield strength σ0.2, which reached 167 Mpa, but also great tensile strainsε, which reached 7.4 %. High yield strength of the wires could effectively avoid serious scratches on the surface of wires during the preparing process of Bi-2212 CICC. Large tensile strain of the wires could prevent the crack of the wires caused by large compressive deformation or twisting deformation of the wires during preparation of Bi-2212 CICC, so that the conductors with smaller pitch and low hysteresis loss could be prepared. In addition, the phase composition of oxide cores in Bi-2212 wires did not change obviously during the pre-annealing process, but the crystallinity of Bi-2212 powder of the cores increased significantly, which was beneficial to the simultaneous melting of Bi-2212 powder of the cores during the overpressure heat treatment and the improvement of the content of Bi-2212 phase of the final wires. As a result, the current carrying capacity of the wires was slightly improved, and its engineer critical current density (Je) reached 825 A/mm2 (4.2K, 12T), which could meet the requirements of future fusion systems. In summary, it is expected that Bi-2212 CICC with less surface defects of the wires, low hysteresis loss, high filling ratio and excellent current-carrying performance can be obtained