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Research on the Rolling Process and Application Characteristics of NbTi Superconducting Sheets and Strips: A Comparative Study with Pure Niobium, Tantalum, Molybdenum, Tungsten, and Ti6Al4V
In addition to wire forms, NbTi superconducting sheets and wide strips are indispensable components in superconducting equipment, frequently utilized in superconducting shielding cylinders, current busbar components, and superconducting laminated assemblies. The production of these sheets relies on repeated rolling and intermediate annealing to control the microstructure, placing stringent demands on rolling plasticity, microstructural uniformity during rolling, surface quality, and resistance to edge cracking. Insufficient material plasticity leads to defects such as edge cracking, delamination, and surface peeling during rolling, rendering the material unsuitable for processing into large-scale superconducting sheets. While many rare refractory metals exhibit excellent low-temperature properties, they are difficult to roll and form, making the production of large, intact superconducting sheets challenging. NbTi alloys possess outstanding plasticity at both cryogenic and room temperatures, allowing them to be rolled into thin, wide superconducting strips; consequently, they offer comprehensive advantages over pure niobium, high-purity tantalum, molybdenum, tungsten, and Ti6Al4V in the field of superconducting sheet and strip fabrication.
Conventional metals exhibit significant limitations when used to produce large-scale superconducting functional sheets. Although the rolling process for 316L stainless steel is mature, its weak magnetism interferes with magnetic fields, and it lacks superconducting properties entirely. Copper and aluminum offer excellent rolling performance but are suitable only as stabilizing layers rather than for superconducting functions. While processing technologies for TA2 and Ti6Al4V titanium alloy sheets are mature and yield stable surface quality, these materials lack the zero-resistance superconducting effect, limiting their use to structural liners rather than functional superconducting components. Conventional metal sheets provide structural support but cannot integrate superconducting functionality, often necessitating multi-layer composite assemblies that increase equipment complexity.
A comparative analysis of rolling characteristics and in-service sheet performance against pure niobium, high-purity tantalum, molybdenum, tungsten, and Ti6Al4V. While pure niobium can be rolled into thin sheets, the process is prone to abnormal grain growth; thick plates suffer from poor internal microstructural uniformity and significant spatial variation in superconducting parameters. High-purity tantalum offers excellent plasticity and allows for the rolling of ultra-thin sheets, but its rapid work-hardening rate necessitates frequent annealing and causes severe die wear; furthermore, the material's high cost makes controlling production expenses for large-scale sheets difficult. Molybdenum and tungsten exhibit pronounced room-temperature brittleness, requiring high-temperature heating during rolling; thin sheets shatter under even slight stress at room temperature, rendering cold rolling impossible and making the fabrication of complex superconducting components extremely challenging. In contrast, Ti6Al4V features a mature rolling process and yields sheets with excellent flatness, yet it lacks superconducting transition capabilities, limiting its use to auxiliary structural applications. NbTi alloys, however, allow for multi-pass cold rolling with high reduction rates and a wide intermediate annealing window; this enables the production of large, intact superconducting strips with minimal edge cracking or delamination, ensuring uniform and stable superconducting performance across the entire sheet.
Within the NbTi alloy system, the standard Nb-47Ti composition offers the most favorable rolling process window. High-niobium NbTi alloys harden rapidly during rolling, limiting the allowable reduction per pass to avoid internal micro-cracking, whereas high-titanium NbTi alloys exhibit excessive plasticity, leading to significant springback and making dimensional precision control difficult. Following optimized rolling and aging heat treatments, fine α-Ti pinning phases precipitate within the NbTi matrix, yielding excellent superconducting performance alongside robust mechanical properties; critical current variations across the sheet are minimized, making the material ideal for manufacturing cylindrical superconducting shields and laminated superconducting assemblies.
Rolled NbTi superconducting sheets are currently employed in superconducting shielding cylinders and busbar assemblies for scientific instruments. By replacing traditional structures made of spliced wires, these sheets simplify component assembly and effectively suppress magnetic field leakage. The drawback of NbTi sheet material is that, while its critical current density in sheet form is slightly superior to that of cast billets, it still lags behind multifilamentary wire, making it unsuitable for fabricating high-current-density coil windings. Future efforts will focus on further optimizing rolling processes and heat treatment regimes to control the size distribution of internal precipitate phases; this aims to enhance the critical current of NbTi superconducting sheets and expand their application scope across various types of precision cryogenic equipment.
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