Effect of an Aging Heat Treatment on the 4 K Fracture and Fatigue Properties of 316LN and Haynes 242

Since the introduction of the cable‐in‐conduit conductor (CICC) concept, a variety of alloys have been proposed for fabricating the jacket. The jacket provides primary containment of the liquid helium coolant and is typically also the primary structural component for the magnet coils. These functions create requirements for strength, toughness, fatigue crack resistance, and fabricability. When the CICC uses Nb3Sn superconductor, the conduit alloy must retain good mechanical properties after exposure to the superconductor’s reaction heat treatment. Here we present data from cryogenic fracture toughness and fatigue crack growth rate tests on 316LN and a Cr‐Mo‐Ni base super‐alloy (Haynes 242) at 4 K before and after the exposure to the heat treatment. These alloys are presently being considered as candidates for use in the next‐generation series connected hybrid magnet for the NHMFL. Both of the alloys are found to have adequate fatigue and fracture properties for the CICC application while the superalloy has distinctly better elastic properties of modulus and thermal expansion.