Abstract
The intrinsic irreversible strain limit εirr,0 of Nb3Sn 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. Nb3Sn 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 Jc is incomplete. Along with RRR and Jc 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.
Highlights
After decades of extensive usage of the ductile Nb-Ti conductor in the fabrication of superconducting magnets for various particle-accelerator facilities, a significant leap will soon take place into a somewhat uncharted territory by introducing the brittle Nb3Sn conductor in the high-luminosity upgrade of the large hadron collider (LHC) at the European organization for nuclear research (CERN)[1,2,3]
All rod-restacked process (RRP) samples investigated in the previous work were given a heat treatment (HT), for reacting Nb3Sn, that followed a three-stage scheme typically used for these wires, with temperature θ at the final stage set at 640 °C for a dwell time of 48 hours
We investigated RRP billet 11976-1, of the design 108/127 [i.e., 108 Nb3Sn sub-elements distributed around 19 Cu sub-elements located at the billet center, making it 127 restacked rods in total], and diameter of 0.82 mm
Summary
Critical-current degradation beyond the irreversible strain limit. Ti-doped billet reacted at 640 and 664 °C to verify that increasing θ slightly to gain higher Jc does not affect εirr,[0]. It is located around the domain of temperatures typically used for heat-treating RRP wires We named this behavior the strain irreversibility cliff (SIC). Steepness of SIC implies that special attention must be paid to the homogeneity and precision of temperature in the furnaces to be used for heat-treating these large, several meters long, and massive magnets This is true if the targeted value of θ is around the SIC tip (which happens to be at 640 °C for the Ti-doped wire studied here). We do not know if Nb3Sn conductors made for the International Thermonuclear Experimental Reactor (ITER), wires made by the internal-tin process, exhibit the SIC behavior[11] We recommend that it be checked, especially because ITER Nb3Sn magnets are even more massive and a tight control of their temperature during heat treatment could be very challenging
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