Study on the Heat Treatment Microstructure Evolution of High Field NbTi Superconducting Strand

To expand the application of NbTi superconducting strand, this study investigates the microstructure evolution after heat treatment and drawing of samples with higher Critical Current Density 709 A/mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at (4.2 K, 9 T). The experimental results indicate that the combined action of heat treatment and drawing influences the final performance. Beginning the 1st heat treatment, there are a few irregular particle precipitates on the matrix. The precipitates increase with the heat treatment times, and it significantly increases up to 19.7% after heat treatment. After every heat treatment, the original precipitates grow larger, and simultaneously, new precipitates are generated. With the final drawing, the precipitates become strips. The <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i>c (4.2 K, 9 T) is more than 700 A/mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with the proper final strain, compared with the normal NbTi superconducting wire with the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i>c (4.2 K, 9 T) of approximately 580 A/mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>.