| No matter what kinds of energy on the earth, such as mineral fuel, water, wind or storage in the animal and plant body, they all come from the sun. The energy of the sun is produced by nuclear fusion, so if the human can control the nuclear fusion, it means that the human master the energy source of the sun. At present, the project of International Thermonuclear Experimental Reactor(ITER) will make this ideal be achieved by taking use of nuclear fusion power generation technology with superconducting controllable magnetic confinement(Tokamak). Nb3Sn and NbTi superconducting material have been applied in Tokamak magnet, but in the future fusion demonstration reactor or commercial reactor, the requests for the magnetic field generated by the magnets and the stress-strain tolerance of the superconducting coil are higher. As the next generation superconducor to substitute for Nb3Sn and NbTi, there are a lot of works to be done for the Nb3Al superconducor. It is different from the intermetallic compound Nb3Sn, Nb3Al can not be fabricated with the Nb-Sn-Cu diffusion heat treatment method at a low temperature. The Nb3Al superconducting wires must be heat treated at 2000℃ in order to obtain better properties, the heating time need to be as short as possible to prevent the Nb3Al grain growing coarse. So, the two aspects listed following are the key points which confine the Nb3Al superconductor applied in practical:in one side, it is the fabrication technology of the Nb3Al precursor wires, due to the heat treatment are carried out at 2000℃ only for a short time, the wires and filaments diameters need to be small, even for the diffusion heat treatment method at a temperature low than 2000℃ for a long time, the distance of between Nb-Al need to be shorter than 1μm, so, the precursor wires are diffcult to fabricate; on the other side, if the heat treatment need to quench at 2000℃, the RHQT apparatus and the process are too complicated to control exactly, the wires may fracture because of the heat stress, besides, the contamination caused by the quenching liquid Ga also make the process of covering the Cu stable layer diffculty, all factors mentioned above restrict the fabrication of long Nb3Al superconducting wires. In this thesis, the thermokinetics transformation process of supersaturated solid solution Nb(Al)ss fabricatied with the mechanical alloying method to Nb3Al was first systematically studied. The Nb3Al superconductor fabrication by mechanical alloying method was optimized, and the possibility of the in-situ PIT method to prepare Nb3Al superconducting wires were also explored. In addition, the Nb-Al diffusion behavior was first investigated by Nb-Al diffusion couple. The effects of raw material, coductor structure and the processing route on the mechanical properties of wires prepared by rod in tube method were studied, the improvements were also proposed. After heat treatent, Nb3Al superconducting wires were obtained.The thermokinetics transformation process of supersaturated solid solution Nb(Al)ss fabricatied with the mechanical alloying method to Nb3Al was studied with the solid sinter and differential sanning calorimetry method. The thermodynamics transformation condition was satisfied only if the heat treatment temperature reached 800℃,when the temperature rised to 1000℃, Nb3Al phase separated to poor Al Nb3Al phase and Nb2Al phase accompany with diffusion. The smaller and decreased with the reaction proceeding characteristics of the activation energy in transformation process indicated that the potential barrier of the reaction was small and the reaction was easy to generate, the nucleation and the growing up of the Nb3Al crystal was easy too. By optimizing the milling time, elements composition, the ratio of ball:powder and annealing temperature, pure Nb3Al phase with porous structure was obtained after heat treated at 800℃ and the highest onset superconducting transition temperature (Tc-onset) reached 15.8 K. Base on the above results, the in-situ powder in tube method was used to fabricate Nb3Al superconducting wires. Two kinds of powder were used to compare the differences in properties and structures, one was directly blended and the other was mechanical alloyed. The results suggested that:Purer Nb3Al phase can be obtained when the heat treating temperature was up to 1400℃, but there were so many pores that it was difficult to rise the current carrying ability, which confined this method to be industrialized; The critical current density can reach 103A/cm2 at 4.2 K, in the field of 6 T in the wires fabricated with the mechanical alloying powder close to the wires heat treatment with RHQT method. If the filaments structure could be denser, then the current carrying ability was improved through the method that the filaments were composite further to increase their deformation degree, it was very promising to fabricate the in-situ PIT Nb3Al superconducting wires in combination with the mechanical alloying method in practical.Moreover, this thesis also explored the rod-in-tube (RIT) method to make the Nb3Al precursor long wires. First, we got the Nb-Al diffusion phase formation behavior and the factors that affected the phase formation. The content of Mg and the thickness of Cu sheath had obvious effects on the mechanical properties of Nb3Al precursor wires. The wires would provide excellent mechanical properties and get uniform Al filaments if the suitable Al alloy and sheath were chosen. Combing with drawing with roller dies, rolling with grooved rools, composite assembly, hot-extrution, drawing and rolling process, and consequently 156 filaments Nb3Al precusor superconducting wires with bundle structure had been made. The wires had excellent mechnical properites and the filaments deformation were uniform. After heat treatment with the RHQT method, the Nb3Al superconducting layers in the wires were compact and with well grains-connectivity. Though there were still big pores in the filaments, the onset superconducting transition temperature reach about 17.3 K, close to the wires reported abroad. If the technology of double composite hot-extrution could be conquered, the size of the Nb3Al filaments could reduce to 1-2μm or lower, using the RIT Nb/Al precursor wires and RHQ heat-treatment was promising to develop the next-generation high-performance Nb3Al superconducting wires for high-field magnet application. |
