The inhomogeneous flow behavior of in iron-based superconducting tape production: Formation mechanism and impact on transport performance

Iron-based superconductors (IBSs) have emerged as promising candidates for high-field applications, with various IBS wires and tapes developed through advanced fabrication techniques. However, the inhomogeneity of superconducting cores in practical conductors remains a persistent challenge, prevalent in nearly all types of powder-in-tube (PIT) superconducting tapes and significantly degrading both transport performance and mechanical properties. Here, we investigate the deformation behavior of Ba_xK_1-xFe₂As₂ (BaK122) tapes and elucidate the formation mechanism of non-uniform superconducting cores. By simulating the actual rolling process of Ag-sheathed BaK122 (BaK122/Ag) tapes, we reveal the in-situ flow behavior of the BaK122 core within the tape. The inhomogeneity of the BaK122 core arises from the plastic flow mismatch with the outer sheath during rolling, attributed to the core's lower plasticity compared to the metal sheath. The periodic characteristics of this inhomogeneous distribution are precisely identified. Furthermore, we demonstrate that rolling speed is a critical factor, with reduced rolling velocity alleviating strain rate mismatch. However, excessively low rolling speeds compromise core densification by enhancing horizontal transverse expansion. An optimal rolling velocity exists to balance uniformity and densification of the BaK122 core. Using the BaK122/Ag tape as a model system, we determine and experimentally validate this optimal rolling velocity through a series of studies, particularly via superconducting transport current measurements. Our findings provide essential guidance for achieving uniform fabrication of heterogeneous composites, such as high-temperature superconducting tapes.