Cheggour Najib, Stauffer Theodore C, Starch William, Lee Peter J, Splett Jolene D, Goodrich Loren F, Ghosh Arup K
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. 2018 Aug 29;8(1):13048. doi: 10.1038/s41598-018-30911-x.
The intrinsic irreversible strain limit ε of NbSn superconducting wires, made by the restacked-rod process and doped with either Ti or Ta, undergoes a precipitous change as a function of temperature θ of the final heat-treatment for forming the A15 phase. NbSn transitions from a highly brittle state where it cracks as soon as it is subjected to an axial tensile strain of any measurable amount, to a state more resilient to tensile strain as high as 0.4%. The remarkable abruptness of this transition (as most of it occurs over a range of only 10 °C) could pose real challenges for the heat-treatment of large magnets, such as those fabricated for the high-luminosity upgrade of the Large Hadron Collider (LHC). We named this behavior the strain irreversibility cliff (SIC) to caution magnet developers. The approach to fulfilling application requirements just in terms of the conductor's residual resistivity ratio RRR and critical-current density J is incomplete. Along with RRR and J wire specifications, and sub-element size requirements that reduce wire magnetization and instabilities effects, SIC imposes additional constraints on the choice of heat-treatment conditions to ensure mechanical integrity of the conductor.
通过重新堆叠棒材工艺制造并掺杂钛或钽的铌锡超导线材的本征不可逆应变极限ε，会随着形成A15相的最终热处理温度θ的变化而急剧改变。铌锡从一种高度脆性的状态转变而来，在这种状态下，一旦受到任何可测量量的轴向拉伸应变就会开裂，转变为一种对高达0.4%的拉伸应变更具弹性的状态。这种转变的显著突然性（因为大部分转变发生在仅10°C的温度范围内）可能给大型磁体的热处理带来实际挑战，比如为大型强子对撞机（LHC）的高亮度升级而制造的磁体。我们将这种行为命名为应变不可逆悬崖（SIC），以提醒磁体开发者。仅从导体的剩余电阻率比RRR和临界电流密度J的角度来满足应用要求的方法是不完整的。除了RRR和J导线规格以及降低导线磁化和不稳定性影响的子元件尺寸要求外，SIC对热处理条件的选择施加了额外的限制，以确保导体的机械完整性。
