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Study on the Prestress-Bearing Performance of NbTi Superconducting Magnets: A Comparison with Nb₃Sn, Tungsten, Molybdenum, C103, and Ti6Al4V
Superconducting magnets generate immense electromagnetic forces upon excitation, subjecting the coils to intense outward expansion and compressive stress. To prevent coil displacement, deformation, or insulation failure, a preload (prestress) is typically applied to the superconducting coils during assembly to counteract the electromagnetic stresses encountered during operation. This necessitates that the superconducting conductor itself be capable of withstanding specific levels of pre-compression or pre-tension without significant degradation of superconducting properties, plastic deformation, or cracking. Some refractory metals, while possessing high strength, are brittle and prone to cracking under prestress; other superconducting materials exhibit a rapid drop in critical current when subjected to stress. NbTi alloy combines high yield strength with excellent ductility, enabling it to withstand assembly-induced prestress loads while maintaining stable superconducting performance under such conditions; this offers unique engineering value compared to Nb₃Sn, tungsten, molybdenum, C103, and Ti6Al4V.
Conventional metals used as prestress-bearing components each have limitations. High-strength stainless steel can withstand significant prestress but is weakly magnetic—interfering with the magnetic field—and lacks superconducting properties. Copper alloys offer good ductility but have relatively low yield strength, making them prone to plastic creep after pre-tensioning, which causes the prestress to gradually dissipate over time. Ti6Al4V titanium alloy is strong and non-magnetic, making it suitable for external prestress bands or casings, but it cannot serve as the primary superconducting conductor; the coil body itself still requires superconducting material. Conventional metals can only function as external load-bearing structures, whereas the coil body must rely on superconducting material to both carry current and withstand stress.
A comparative analysis of the performance of NbTi against Nb₃Sn, tungsten, molybdenum, C103 niobium alloy, and Ti6Al4V under prestress conditions. Nb₃Sn is a brittle intermetallic compound with a very low strain tolerance; if the assembly preload slightly exceeds the threshold, micro-cracks form within the matrix, leading to a drastic reduction in critical current—damage that is irreversible once cracking occurs. Tungsten and molybdenum exhibit significant brittleness at both room and cryogenic temperatures, making them suitable only for external rigid constraint structures rather than as primary materials for superconducting coils. C103 niobium alloy offers excellent strength and toughness at low temperatures and can withstand preload, but it is a structural alloy lacking superconducting capabilities. Ti6Al4V is widely used for external preload hoops on magnets due to its reliable mechanical properties, yet it lacks the zero-resistance conductivity characteristic of superconductors. In contrast, NbTi can withstand a certain degree of tensile and compressive preload; within a reasonable stress range, the degradation of critical current is minimal, allowing the coil itself to both carry high superconducting currents and bear a portion of the assembly preload.
Within the NbTi superconducting system, the Nb-47Ti alloy offers the best balance between mechanical properties and superconducting performance. High-niobium NbTi alloys possess higher strength but reduced ductility, resulting in a lower strain tolerance; high-titanium NbTi alloys offer better ductility but lower yield strength, making them prone to plastic deformation under preload, which leads to a loss of pretension. NbTi subjected to cold working and aging treatments achieves an optimal balance of strength and ductility; within the typical preload ranges used for magnets, it does not undergo irreversible plastic deformation, and the flux-pinning structure remains intact, ensuring the magnet's superconducting performance after energization.
Currently, a vast number of commercial MRI magnets and scientific cryogenic magnets utilize NbTi conductors; despite the application of preload during coil assembly, no stress-induced performance degradation has been observed over years of operation. The limitations of NbTi lie in the fact that performance degrades when stress exceeds the material's threshold, and its high-field performance is inferior to that of Nb₃Sn. Future efforts will focus on fine-tuning alloy composition and optimizing processing techniques to further enhance the stress tolerance of NbTi, thereby simplifying magnet preload structure design and reducing both manufacturing complexity and costs.
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