Metallographic analysis of 11 T dipole coils for High Luminosity-Large Hadron Collider (HL-LHC)

Abstract

For next-generation accelerator magnets for fields beyond those achievable using Nb-Ti, Nb 3 Sn is the most viable superconductor. The high luminosity upgrade for the Large Hadron Collider (HL-LHC) marks an important milestone as it will be the first project where Nb 3 Sn magnets will be installed in an accelerator. Nb 3 Sn is a brittle intermetallic, so magnet coils are typically wound from composite strands containing ductile precursors before heat treating the wire components to form Nb 3 Sn. However, some mechanical assembly is still required after the coils have been heat-treated. In this paper, we present direct evidence of cracking of the brittle Nb 3 Sn filaments in a prototype dipole that resulted in degraded magnet performance. The cracking of the Nb 3 Sn, in this case, can be attributed to an issue with the collaring process that is required in the assembly of dipole accelerator magnets. Metallographic procedures were developed to visualize cracks present in the cables, along with quantitative image analysis for location-based crack analysis. We show that the stresses experienced in the damaged coil are above the critical damage stress of Nb 3 Sn conductor, as evidenced by a measured Cu stabilizer hardness of 85 HV 0.1 , which is higher than the Cu stabilizer hardness in a reference Nb 3 Sn cable ten-stack that was subjected to a 210 MPa transverse compression. We also show that once the collaring procedure issue was rectified in a subsequent dipole, the Nb 3 Sn filaments were found to be undamaged, and the Cu stabilizer hardness values were reduced to the expected levels. This paper provides a post-mortem verification pathway to analyze the damage, provides strand level mechanical properties, which could be beneficial for improving model prediction capabilities. This method could be applied beyond Nb 3 Sn magnets to composite designs involving high work hardening materials.

Numerical simulation of mechanical behaviors and intergranular fracture of polycrystalline Nb3Sn and superconducting filaments

“…In order to evaluate the adverse effects of mechanical and brittle fracture behavior, researchers report a large number of experimental results. Including direct or indirect evaluation of the mechanical properties of Nb 3 Sn filaments, Nb 3 Sn wires, Rutherford cables and coils [30][31][32][33][34], as well as their critical performance experiments under coupling fields. West and Rawlings [35] observed that the polycrystalline Nb 3 Sn filament failed in an intergranular fracture manner without any previous dislocations by scanning electron microscopy.…”

Effect of transverse compressive stress applied at room temperature on Nb3Sn Rutherford cables

“…In the longitudinal cross-section, few cracks were observed in the strands located at the edge of the cable-called kink, see figure 1. They are mainly oriented in the longitudinal direction of the sub-element, named longitudinal cracks in figure 17(c) and in [1,2], or straight cracks in [21]. Other cracks oriented with wave shape around the porosity in the core of the sub-elements are also visible, named wavy cracks in figure 17(c) and in [21].…”

“…To study the electromechanical degradation of Nb 3 Sn cable-in-conduct-conductors (CICC) used in fusion reactors, metallographic observations were performed after electromechanical loadings in [9][10][11][12][13][14]. For accelerator magnets, Balachandran et al [21] performed post-mortem metallographic analysis of an 11 T dipole short model coil and of Nb 3 Sn stack samples, composed of ten cables, subjected to transverse pressure. Cracks were observed in the dipole coil resulting in degraded magnet performance.…”

Study on Internal Strain Distribution of the High-Field Nb₃Sn Superconducting Accelerator Magnets With Homogenization Theory

“…(3) becomes a limit form of Eqs. (2). The stress and strain in the equation depend on macro-variables and satisfy the general equilibrium equation.…”

“…In addition, in a quench analysis of the magnet, it was found that the degradation of some coils was due to the fracture of filaments in the strand caused by excessive prestress [2]. Therefore, it is necessary to evaluate the stress of coils with internal microstructures.…”