Visualization of the grain structure in high-performance Ba1−x K x Fe2As2 superconducting tapes

Abstract

122 type iron-based superconductors (IBS) are potential for high-field applications, and the fabrication of high-performance IBS wires and tapes is essential. By using the powder-in-tube method, transport critical current density (J_c) of hot-pressed silver-sheathed Ba_1-xK_xFe₂As₂ (Ba-122) tapes has reached 1.5 × 10⁵ A/cm² at 4.2 K and 10 T. However, the J_c of flat-rolled tapes is 6.2 × 10⁴ A/cm² (4.2 K, 10 T), less than half of the former. The grain orientation, grain size and grain shape are important parameters for the understanding of superconducting properties of IBS tapes. Such grain structure data can be provided by electron backscatter diffraction (EBSD) technique. In this work, we extensively adopted EBSD to visualize the microstructure of state-of-the-art hot-pressed and flat-rolled Ba-122 tapes. The grain orientation, grain connectivity, grain size, and grain shape aspect ratio of these two types of tapes are quantitatively analyzed. The c-axis texture is commonly found in both hot-pressed and flat-rolled tapes, but no in-plane texture is discovered. The texture and grain connectivity in hot-pressed tapes are better than that in flat-rolled tapes. The grain size of flat-rolled tapes is smaller than that of hot-pressed tapes, and the hot-pressing processes can promote the growth of grains along the ab plane. We recommend that strong texture will promote the growth of grains, while the small-sized grains will return to limit the formation of texture. Therefore, finding a balance point that plays a synergistic promote effect of grain orientation and grain size is a strategy to improve the transport properties of IBS tapes further. The present results demonstrate that there is still margined to enhance the properties of IBS tapes.