Obtaining highly homogeneous Nb₃Al superconductor with improved critical current density by understanding the phase evolution sequence of mono-core and multifilamentary wires

Abstract Mono-core and multifilamentary (MF) Nb–Al precursor wires were obtained by a rapid heating, quenching and transformation (RHQT) process to investigate the phase evolution sequence, in order to find a way to fabricate highly homogeneous Nb 3 Al superconductor. The microstructure and composition distribution of the Nb 3 Al superconductor is the same as that of quenched wires. Phase evolution (1) in mono-core wire is as follows: Nb + Al→ eutectic phase +Nb 2 Al + Nb 3 Al→ grid-shape Nb(Al) ss → band-like Nb(Al) ss , where nonuniform microstructure is formed in the transformed Nb 3 Al because the Nb layer is too thick; (2) in MF wire is as follows: Nb + Al→ Nb 2 Al + Nb 3 Al→solid Nb(Al) ss → two-phase-segregation→ liquid Nb(Al) ss , where much thinner Nb layers completely react to Nb 3 Al and Nb 2 Al, but the Al content of Nb 3 Al phase is still inhomogeneous. Restricted by the precursor wire fabrication ability, further decreasing the Nb layer thickness fails to improve the homogeneity of Nb 3 Al phase. Therefore, the author’s route of afresh heating of the quenched Nb 2 Al + Nb 3 Al wire was carried out to increase the atomic diffusion coefficient due to the non-equilibrium crystal defects present in the quenched wires. As a result, a highly composition homogeneous Nb 3 Al phase was realized in the whole superconducting filament with a scale of about ∼100 μ m. The magnetic critical current density ( J c ) of the afresh heated Nb 3 Al superconductor reached 2066 A mm −2 @12 T, 4.2 K, showing an improvement of about 60% compared to the best J c result for the MF RHQT Nb 3 Al wire.