Cheggour Najib, Stauffer Theodore C, Starch William, Goodrich Loren F, Splett Jolene D
Department of Physics, University of Colorado, Boulder, CO, 80309, USA.
Quantum Electromagnetics Division, National Institute of Standards and Technology, Boulder, CO, 80305, USA.
Sci Rep. 2019 Apr 2;9(1):5466. doi: 10.1038/s41598-019-41817-7.
The strain irreversibility cliff (SIC), marking the abrupt change of the intrinsic irreversible strain limit ε as a function of heat-treatment (HT) temperature θ in NbSn superconducting wires made by the restacked-rod process (RRP), is confirmed in various wire designs. It adds to the complexity of reconciling conflicting requirements on conductors for fabricating magnets. Those intended for the high-luminosity upgrade of the Large Hardon Collider (LHC) at the European Organization for Nuclear Research (CERN) facility require maintaining the residual resistivity ratio RRR of conductors above 150 to ensure stability of magnets against quenching. This benchmark may compromise the conductors' mechanical integrity if their ε is within or at the bottom of SIC. In this coupled investigation of strain and RRR properties to fully assess the implications of SIC, we introduce an electro-mechanical stability criterion that takes into account both aspects. For standard-Sn billets, this requires a strikingly narrow HT temperature window that is impractical. On the other hand, reduced-Sn billets offer a significantly wider choice of θ, not only for ensuring that ε is located at the SIC plateau while RRR ≥ 150, but also for containing the strain-induced irreversible degradation of the conductor's critical-current beyond ε. This study suggests that HT of LHC magnets, made of reduced-Sn wires having a Nb/Sn ratio of 3.6 and 108/127 restacking architecture, be operated at θ in the range of 680 to 695 °C (when the dwell time is 48 hours).
在通过重新堆叠棒材工艺（RRP）制造的NbSn超导线材中，应变不可逆性悬崖（SIC）标志着本征不可逆应变极限ε随热处理（HT）温度θ的突然变化，这在各种线材设计中都得到了证实。这增加了协调制造磁体的导体相互冲突要求的复杂性。那些用于欧洲核子研究组织（CERN）设施的大型强子对撞机（LHC）高亮度升级的磁体，要求导体的剩余电阻率比RRR保持在150以上，以确保磁体抗淬火的稳定性。如果导体的ε处于SIC范围内或底部，这个基准可能会损害导体的机械完整性。在这项对应变和RRR特性进行的联合研究中，为了全面评估SIC的影响，我们引入了一个同时考虑这两个方面的机电稳定性标准。对于标准锡坯料，这需要一个极窄的HT温度窗口，这是不切实际的。另一方面，低锡坯料提供了更宽的θ选择范围，不仅可以确保ε位于SIC平台上，同时RRR≥150，还可以抑制超过ε时应变引起的导体临界电流的不可逆退化。这项研究表明，由Nb/Sn比为3.6且具有108/127重新堆叠结构的低锡线材制成的LHC磁体的HT应在680至695°C的温度范围内运行（当保温时间为48小时时）。
