Influence Of Composition Distribution On Phase Formation And Superconductivity Of Rapid Heating Quenching Nb₃Al Wires

Magnetic confinement fusion technology has received widespread attention because of its advantages of sufficient raw materials,safety and reliability,and no environmental pollution.Fully superconducting magnetically confined tokamak devices are the most effective way to realize fusion energy utilization.Large superconducting magnets are the key to tokamak devices,in which high-performance superconducting wires are the necessary materials for magnet winding.The primary practical superconducting materials currently applied to high-field magnets above 10 T are Nb₃Sn and Nb₃Al.Nb₃Al has a higher superconducting transition temperature and critical current density than Nb₃Sn.Significantly,the excellent strain tolerance makes Nb₃Al an ideal candidate for the next generation of high field magnets.At present,kilometer-scale Nb₃Al wires with high critical current density（J_c）have been successfully prepared by the rapid heating,quenching and transformation（RHQT）process,which is considered as the most promising technology for large-scale practical application.The phase formation sequence and microstructure evolution during the heating process play a crucial role in the stable preparation of high-performance wires.This thesis is focused on the phase formation process and superconductivity of Nb₃Al wire during the RHQT process.The phase evolution during the RHQ treatment of PIT and JR wires is systematically explored,and the relationship between the phase formation process and the superconductivity of the wires is summarized.The influence of Nb-Al local composition distribution on the phase formation pattern of Nb₃Al wire is comprehensively analyzed to provide a theoretical basis for optimizing the wire properties and preparing practical superconducting materials.Precursors with initial Al content of 22 at%-28 at%are prepared by the powder in tube method and rapidly heated at different energy densities.The phase and microstructure of wires are mainly determined by the RHQ conditions and the local Nb-Al distribution.The phase formation process of PIT wires with different initial Al contents has a universal pattern,and the change of the initial components does not significantly affect the phase formation process of the wires.The phase formation sequence in PIT wire can be summarized as follows:Nb+Al→eutectic phase+Nb Al₃→Nb₃Al+Nb₂Al→grid morphology bcc→band feature bcc.Due to the inconsistency of the initial Nb-Al components,differences in the composition of the different cores arise.The result leads to differences in the lattice parameters and the actual Al content of the A15 phase of the transformed wires,which affects the superconductivity of the wires.In the PIT wires,the bcc region can be subdivided into three steps according to the differences in microstructure and local components,with Al content of 24～32 at%,20～25 at%,22 at%.After 800°C transformation heat treatment,wires quenched at step 2 of bcc region show the best superconducting properties in the 26 at%Al precursor wire.The transformed superconductors show range of T_c from 16.6 K to 17.1 K,ΔT_c of 0.5 K-0.8 K,and J_c at 8 K,5 T from 4.7×10⁴ A/cm² to 2.0×10⁵ A/cm²,B_irr of 14.5 T-15.7 T at 8 K in precursor wires with initial Al content between 22 at%to 28 at%.The filamentary precursors are prepared by jelly-roll method.Different RHQ currents are applied to the precursors to reach different temperature intervals,and the phases and microstructures of the as-quenched wires are analyzed.For JR mono-filamentary wires,A15,σ,and eutectic phases appear at lower energy density,and bcc phase forms with increasing temperature.The microstructure of the bcc phase formation region shows a similar change pattern as that of PIT wires.Nb（Al）_ss has a fiber weave in the ND（110）direction.The bcc grain boundary angle is significantly dependent on the RHQ conditions,and the higher the energy density,the more concentrated in the small-angle grain boundary.With the increase of RHQ temperature,the Nb（Al）_ss grains increase significantly.The high temperature leads to excessive grain growth and is not conducive to grain refinement.Unlike the phase formation process of mono-filamentary wires,the solid phase Nb（Al）_ss with sufficient reaction exists in JR multi-filamentary wires.The uniformity of the core is further enhanced and the component differences are eliminated.The results of the microstructural evolution indicate that controlling the thickness of the Nb layer well below the Al diffusion distance plays a crucial role in obtaining a homogeneous bcc solid phase.The differences in the phase formation process also lead to different intervals where the wires have the best properties.When the thickness of Nb-Al layer in the precursor is wide,the wire is heated to the liquid phase temperature region to melt the core completely.It makes the Nb-Al elements in the wire fully react and form a uniform phase,and the superconductivity is excellent.When the thickness of the Nb-Al layer is small,the precursor is quenched at solid bcc phase region to obtain the stoichiometric ratio Nb（Al）_ss.The superconductivity of the wire can be controlled stably.