An Intermetallic Precipitate Strengthened Alloy for Cryogenic Applications
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An Intermetallic Precipitate Strengthened Alloy for Cryogenic Applications Ke Han, Yin Xin and Robert.P.Walsh National High Magnetic Field Laboratory, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, Florida, 32310, USA ABSTRACT In high field magnets, most of the structural materials operate at very low temperatures (e.g. 1.8 K) and high magnetic fields (e.g. >20 T). Such an extreme environments ask for demanding properties from the materials. Austenite stainless steels have been used for such an application for decades and provide good combination of mechanical strength and toughness at cryogenic temperatures. In the National High Magnetic Field Laboratory, we have studied intermetallic precipitate strengthened alloys for high field magnet and cryogenic applications. An examination of the data from an intermetallic precipitate strengthened alloys shows that intermetallic precipitate strengthened alloys can be used at cryogenic temperatures and have some superior properties in extreme environments. Based on our experimental data, we developed a new alloy that has even better properties than the current intermetallic precipitate strengthened alloys. INTRODUCTION Austenitic stainless steels have been used for cryogenic applications (mainly at 4 K or below) for decades because of their good combination of the mechanical strength and toughness at cryogenic temperatures. The structural materials in high field magnets, such as the materials in fusion magnets and high field research Series-Connected Hybrid (SCH) magnets [1], also operate in high fields. Significant numbers of stainless steels will have phase transformation at a high filed, therefore are not suitable for such applications. In addition, the structural materials, such as the conduit alloy for Cu-Nb-Sn wires in high field magnets, are exposed to a heat treatment at temperatures between 600 -700 ºC for more than 24 hours in order to transform the precursor to Cu/CuSn/Nb3Sn composites. At such a heat treatment condition, most of the stainless steels are sensitized and become brittle at cryogenic temperatures. Low carbon 316 LN (denoted as 316LLN in this paper) is one of the austenitic stainless steels that can work in such an extreme environment: cryogenic temperatures, high magnetic fields and experience in sensitization temperature range for a long time. The low carbon content retards the formation of the grain boundary carbides and makes the materials less vulnerable to sensitizations than commercial 316LN. However, their yield strength in annealed status of 316 LLN is limited to a range of 1000 MPa at 4 K partially due to their low carbon content. In addition, the intermetallic compounds may form at grain boundaries at certain temperature ranges and make the materials less ductile than the one heat treated at optimized heat treatment temperatures. The researchers in the national high magnetic field laboratory (NHMFL) studied other materials for cryogenic applications and focused the efforts on Haynes 242 alloy as a candidate allo
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