Phase Evolution and Area Fractions of Coarse-Grain and Fine-Grain A15 in APC Nb3Sn Superconductors

Previous studies have shown that APC Nb3Sn strands based on the internal oxidation of Nb-Ta-Zr or Nb-Ta-Hf alloys had higher non-Cu critical current density (Jcs) than the state-of-the-art strands at high fields (e.g., ≥ 12 T) while having lower non-CuJcs at low fields (e.g., ≤ 5 T), which helps to reduce the undesired persistent-current magnetization. Moreover, APC strands reacted at lower temperature tend to have flatterJc(B) curves, possibly allowing the targeting of even betterJcperformance at high fields, while at the same time suppressingJcs at low fields. However, it is required that APC strands attain large fine-grain (FG) area fractions and suppressed coarse-grain (CG) area fractions at low reaction temperature. In this work, the influence of reaction temperature on the evolution of FG and CG Nb3Sn phases in APC Nb3Sn strands was investigated. Our goal is to find more effective methods to increase the FG area fraction as well as the overall performance of APC strands.