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Nb-Ti system is a continuous solid solution alloy with a body-centered cubic structure and a practical composition w (Ti) of about 50%. The alloy has excellent plasticity, mechanical strength and high current carrying capacity under magnetic fields. Since the 1960s, it has been widely used in various fields of superconducting technology and has become the most commonly used practical low-temperature superconducting material in the world at liquid helium temperature (4.2 K).

Niobium-titanium superconducting materials can be made into various composite practical materials through conventional deformation processing and heat treatment processes. When it is used under medium and low magnetic fields of 2~8T, its raw material and manufacturing costs are much lower than other superconductors and other manufacturing technologies. In particular, it is widely used in magnetic resonance imaging (MRI) magnets for modern medical diagnosis. Its annual consumption of NbTi superconducting composite stranded cables has now reached 1,000 tons (reported in 2004). This huge demand has further promoted the development of niobium titanium. The manufacturing cost of superconducting materials has dropped and market advantages have been established.

Another advantage of the niobium-titanium superconducting material is that it undergoes a heat treatment process that creates a flux pinning center prior to the assembly step of twisting cables, wound magnets, or other magnet applications, as opposed to this process of some superconductors. This heat treatment needs to be carried out after the final application assembly step, which greatly facilitates the practical application of the project. People call the former a “reaction first, then wind (R&W)” type superconducting material, and the latter a “first winding, then reaction (W&R)” type superconducting material.

The mechanical tensile strength of niobium-titanium superconducting materials is comparable to that of ordinary steel, and it can well match its supporting structural materials in stress-strain design. This is also an important reason why it is widely used in various fields.

Niobium-titanium superconducting materials are mainly used in: magnetic resonance imaging (MRI) for medical diagnosis, nuclear magnetic resonance spectrometer (NMR), laboratory magnets, high-energy physics particle accelerators, power conditioners, minesweepers, magnetic separation and mineral processing, maglev trains and Superconducting magnet energy storage (SMES), etc.
The alloy composition and superconductivity diagram: the relationship between the upper critical magnetic field Hc2 (T) at 4.2 K and the room temperature resistivity (μΩ·cm) at 293K. It can be seen that the superconducting critical temperature T of Nb-Ti alloy. The peak value of Hc appears in the range of alloys containing 17%~30% (mass) titanium; while the peak value of superconducting upper critical field Hc appears in the range of alloys containing 47%~50% (mass) titanium, 4.2 The highest value of Hc2 at K exceeds 11T. As the titanium content of Nb-Ti alloy increases, its room temperature (293 K) resistivity increases monotonically and rapidly, indicating the completely different nature of normal conductors and superconductors.

Practical superconducting niobium-titanium alloys are generally selected with a titanium content of 45% to 50% (mass fraction), so their Tc value is slightly lower, about 9K. Niobium-rich alloys are single-phase, and their Tc values are relatively certain. As the titanium content increases, Tc becomes related to the composition of the body-centered cubic matrix after α-Ti precipitation and the martensitic transformation.

In order to obtain higher-performance superconducting materials, people have developed ternary alloys or quaternary alloys by adding third or fourth elements based on the niobium-titanium series, such as Nb-Ti-Zr, Nb-Ti- Ta, Nb-Ti-Ta-Zr and Nb-Ti-Ta-Hf. Except for the Nb-Ti-Zr alloy, the Hc2 values of these alloys are higher than those of the best Nb-Ti binary alloy. However, at 4.2 K, this increase in Hc2 is still very limited (about 0.3 T), and the difference between them becomes more significant at lower temperatures. The upper critical field of Nb – 43% (mass) Ti – 25% (mass) Ta alloy reaches Hc2 at 2K and reaches 15.5T. This alloy has been industrially produced and made into Jc of multi-core superconducting wires. The value is approximately 1 800 A/mm2, which is used to manufacture 12T experimental fusion experiment coils.
For the ternary Nb-42% (mass) Zr-6 (mass) Ti alloy, with very little cold working, a high Jc= 2800A/mm2 (5T, 4.2 K), Tc=10.3 K, Hc2= 10.5T. However, this alloy has poor ductility, making it difficult to make practical multi-stranded thin-core superconducting wires.

Microstructure and superconductivity: The superconducting critical current density Jc value is a sensitive parameter of the crystal structure of the material. In particular, dislocations, desolvated phases and grain boundaries in the microstructure can form magnetic flux pinning centers, significantly affecting the material’s current carrying capacity.
The dislocation structure and desolvated second phase particles in NbTi alloy are the main factors affecting the superconducting critical current density Jc value.
The number and distribution of dislocations are directly related to the cold working and heat treatment of the material. In particular, uneven high-density dislocations (cell structure) have a greater effect than uniform ones. For Cu-coated Nb-44%Ti alloy materials, after 99.999% deep cold working, heat treatment at different temperatures for 1 hour was performed to obtain dislocation cell structures of different scales in the crystal and the critical Lorentz force Fc (i.e., pinning force Fp) mutual relationship. Here, the pinning force Fp (critical Lorentz force Fe) has an almost linear inverse relationship with the dislocation cell size d. That is, the smaller the dislocation cell size, the stronger the pinning force, and the higher the critical current density of the superconducting material. high.
The desolvated second phase is obtained by desolvation heat treatment. It can be seen from the NbTi alloy phase diagram that as the Nb content increases, the phase boundary of the body-centered cubic β phase transforms into a dense hexagonal α phase moves downward, but it is difficult to accurately determine the position of the phase boundary between β and α+β; for Nb- For practical superconducting materials with 50% (mass) Ti, the desolvation heat treatment temperature is generally selected not to exceed 400°C.
Many experiments have proven that the second phase of NbTi alloy desolvated from the β matrix can significantly increase the superconducting critical current density Jc value of the material.
Based on the above, the traditional deep cold working and low-temperature desolvation heat treatment processes can be used for NbTi alloy superconducting materials to obtain an ideal microstructure and improve the superconducting properties of the material.
Multi-core composite wire manufacturing process: The general typical process flow of niobium-titanium superconducting materials is: using electron beam bombardment and arc melting to manufacture Nb-Ti alloy ingots, hot-processing and cold-processing to form rods, and then coating with copper (using high-quality Oxygen-free copper with residual resistance ratio RRR) is made into Nb-Ti/Cu composite single core rod. Then many single core rods are put into copper sleeves, and after extrusion, drawing and heat treatment, practical Nb-Ti multi-core superconducting wires are made.

There will be many differences in the manufacturing process of each manufacturer.
The electrode blank for melting Nb-Ti alloy ingots in an electric arc furnace can be made by cold pressing of titanium sponge and pure memory rods, or it can also be made by stack spot welding of pure titanium sheets and niobium sheets. In order to obtain highly uniform Nb-Ti alloy materials, pure niobium ingots purified by two electron bombardments can be made into fine powder after hydrogenation, and then stirred and mixed with fine titanium powder that meets the requirements, and the piezoelectrode is sintered and degassed. , and then melted and cast into ingots in a secondary electric arc furnace. Such alloy ingots are suitable for manufacturing high Jc superconducting materials due to their uniform composition.
The obtained NbTi alloy ingot can be homogenized; or the NbTi alloy can be made into rods and then subjected to solution treatment.
Nb-Ti/Cu composite process: In some cases, niobium-titanium rods are put into copper tubes to stretch copper cladding, while in others niobium-titanium ingots are put into copper sleeves to extrude copper-cladding.
In most cases, due to the need to stabilize the superconducting system, the diameter of the niobium-titanium single core wire is required to be very thin, so it must be made into a multi-core composite wire. Multi-core composite wires can be made by stretching an outer copper tube to make a multi-core composite wire; but more often, a single core rod is put into an outer copper sleeve and then hot extruded to make a multi-core composite wire.
When superconducting materials are used in changing magnetic fields (including rising and falling fields), alternating magnetic fields, or carrying low-frequency, medium-frequency and power-frequency currents, in order to stabilize the superconducting system and reduce losses caused by various electromagnetic interactions, In addition to the superconducting core wire having to be made very thin (several microns), the multi-core superconducting wires must also be twisted or transposed at a certain pitch. Depending on the application needs, some use high-resistance materials such as Cu-Ni alloy as a barrier layer between the superconducting core wires to make a composite superconducting material with a three-layer structure of NbTi/Cu/CuNi, also known as three-component multi-core Composite superconducting wire.

In order to make the multi-strand wires in the cross-section of the composite multi-core superconducting wire evenly arranged, when assembling single core rods with large copper sleeves, they should be packed as tightly as possible to reduce the porosity after packing. A copper tube with an outer hexagonal shape and an inner circle can be used to insert an NbTi alloy round rod to form a hexagonal Nb-Ti/Cu composite single core rod. This hexagonal composite single core rod can then be assembled into a large copper sleeve to form a tight assembly. ingot.

Studies have also found that during the processing of multi-core composite wires, uneven deformation between NbTi and Cu will occur along the length of the wire, resulting in a sausage-like structure, which reduces the current carrying capacity of the superconducting wire. This is related to the different plastic deformation resistance of the NbTi alloy and the base metal highly conductive copper, as well as to the process quality of the cluster assembly. Strictly controlling the deformation process and using NbTi alloy raw materials with uniform chemical composition can overcome or reduce the uneven sausage-like structure produced by the NbTi core wire in the length direction.
As discussed earlier, NbTi alloy superconducting materials can improve the microstructure of the alloy and enhance the superconducting properties of the material through deep cold working and low-temperature heat treatment processes. The design of the processing rate (section reduction rate) of superconducting materials is very important. Generally, when the total processing rate of the material reaches 99.99~99.999%, the best superconducting critical current density Jc value can be obtained. To achieve a high processing rate, it is necessary to reduce the diameter of the niobium-titanium alloy core wire to a very small size (about 10 ~ 25 μm). However, in order for the superconducting material to carry a large enough superconducting current, the superconducting material must be processed into a multi-core composite wire structure.
The desolvation heat treatment of NbTi alloy wire is very critical, which directly affects the number, distribution, size and morphology of dislocations and desolvated second phases in the alloy. The final heat treatment is mostly set at a certain stage before the final finished product size (retaining a certain amount of additional processing of the final product), the temperature is 350 ~ 400°C, and the heat treatment time can be as long as about 100 hours.
According to different process designs, a certain amount of additional cold deformation can be performed after the final heat treatment to further improve superconductivity. In addition, process procedures such as twisting of multi-core wires are also carried out after the final thermal treatment.

“Artificial Pinning Center” (APC) process technology Almost all commercial Nb-Ti superconducting wires obtain α-Ti desolvation precipitation through heat treatment, thereby generating magnetic flux pinning points in the superconductor and obtaining high criticality of the material. Current density Jc, this process is called “conventional process method”. However, there is a certain limit to improving superconducting performance through α-Ti desolvation and precipitation. The critical current density Jc of Nb-Ti superconducting wire using conventional technology is approximately 3 800 A/mm2 under a magnetic field of 4.2K and 5T.

A new process technology was developed in the 1980s, called the “Artificial Pinning Center” (APC) process technology. It is different from the “conventional process method” in that it eliminates any desolvation and precipitation heat treatment and replaces the conventional process with an artificial combination of superconducting and non-superconducting phases.

“Artificial Pinning Center”: (APC) process flow diagram. The upper left end represents the manual assembly of APC composite materials. It is suitable for smaller quantities of manufacturing. The size of the assembled units (rods, tubes, hollow wires, etc.) must be small, only millimeters in diameter, which is more difficult. Then, in order to make the small unit size Reducing it to 10 nm requires a processing rate of 96% to 97% and 3 to 4 assembly extrusions. The upper right end shows the nanostructure of the cross-section of the superconducting wire. The size of the Nb nails and Ti nails is about 100 nm, and the shape and arrangement of the nails are relatively uniform. The lower left end represents a larger amount of APC composite material technology, which uses various micron-scale units such as powder foils, coils, or multi-layer sheets. Because its starting size is small, 92% of the final nanostructure needs to be achieved. ~ 95% processing rate and 1 to 2 assembly extrusions are enough, which is simpler than manual assembly of APC; the disadvantage is that the arrangement and distribution of artificial nails cannot be controlled.
The APC combination process has poor processability and high cost, and the commercial APC process is still under research and development. But APC superconducting wire has shown its excellent low-field performance. For a superconducting wire containing 25% Nb pinning the center in an Nb-47.5% (mass) Ti alloy, the critical current density Jc reaches 7500 A/mm2 under a magnetic field of 4.2K and 3T: while under a magnetic field of 4.2K and ST, the critical current density Jc also reaches 4600 A/mm2.