Improved critical current density of Cu-doped Nb3Al wire through optimized Rapid-Heating Quenching and Transformation method

We have prepared 2.5% Cu-doped Nb3Al wire by the Rapid-Heating Quenching and Transformation (RHQT) method with optimized annealing. XRD results show that A15 and supersaturated bcc phase are formed after high current RHQ process, and the supersaturated bcc phase is transformed into A15 phase after conventional annealing (800 °C\10 h). We observed the microstructures at different annealing time by field-emission scanning electronic microscope (FESEM) and found that σ phase is generated from the decomposition of the metastable A15 phase after prolonged annealing due to its lower free energy at 800 °C, and gradually accumulates Cu to form the Cu-rich σ phase after 20 h. The low-field superconducting properties were also given through magnetic measurement. Both Tc and Bc2 decreased slightly along with the increase of σ phase volume when the annealing is prolonged, while Jc reached its peak after 20 h\800 °C annealing, which were 1836 A/mm2 at 4.2 K and 5 T and 822 A/mm2 at 8 K and 5 T for 800 °C\20 h annealing, about 34% and 36% higher than that of the conventional transformed sample, respectively. The calculated Fpmax of the samples with optimized annealing condition also increased accordingly. The fitting analysis of fp using the Dew-Hughes model showed that the pinning mechanism was surface pinning. The additional diffusion channels for Cu provided by newly generated interfaces of σ and Nb(Al) particles enhance the Cu segregation on the A15 boundaries and may be the source of the pinning force improvement.