Microstructure identification of powder in tube Nb3Al wires during rapid heating process and properties of the transformed superconductors

This paper systematically studied the microstructure and phase formation sequence in the powder in tube (PIT) Nb3Al processed by rapid heating and quenching (RHQ), which is a crucial issue to fabricate reliable high performance practical Nb3Al superconductors. Due to significantly inhomogeneous of the Nb/Al diffusion couple distance, an eutectic phase with liquid characteristic happened during the RHQ process, where the phase formation sequence is: Nb + Al → eutectic phase + NbAl3 → Nb3Al + Nb2Al → grid morphology bcc → band feature bcc with the increase of rapid heating temperature. The phase and microstructure during RHQ are mainly determined by the RHQ condition and local Nb/Al distribution that similar feature was shown in the quenched wires with initial Al content of 22–28 at.% in the raw powders. Grid and band microstructure of the bcc phase was related to the Al content of ~32 at.% and ~ 22 at.%, respectively. After transformation process at 800 °C, quenched wires with coexist of gird and band micro-structure show the best superconducting properties, including Tc, ΔTc, Birr, and Jc. The microstructure identification confirms that uniform and thin Al layer in the precursor wire plays an essential role in obtaining a homogeneous bcc solid phase.