Mechanical and metallurgical assessment of austenitic stainless steel as structural materials for superconducting high field magnets at cryogenic temperature

Characterization of materials at low temperatures is one of the fundamental goals of the Materials Metrology and Non-Destructive Testing (MM) section at the European Organization for Nuclear Research (CERN). The MM section is part of the Mechanical and Materials Engineering (MME) group at CERN. With years of expertise in such domain the group is recognized as one of the world's leading laboratories in the field for materials testing and analysis. A big part of the current generation of particle accelerators such as the Large Hadron Collider (LHC) and High Luminosity LHC (HL-LHC) at CERN and the future fusion devices such as ITER rely in superconductivity to achieve the required magnetic fields. Superconducting magnets must operate below 4.2 K to maintain their strong magnetic fields which necessitates materials with exceptional strength and stability to withstand high Lorentz forces at cryogenic temperatures. Austenitic stainless steels with their advantageous properties such as high strength low magnetic permeability and resistance to fracture are ideal candidates for these applications. The aim of this thesis is to provide a broader and deeper understanding of the mechanical properties of austenitic stainless steels at 4.2 K focusing on the mechanical characterization using tensile testing and fracture toughness. It introduces innovative crack characterization methods and proposes new interpretations of the data to discern between stable and unstable crack growth at very low temperatures (4.2 K) advancing knowledge in support of high-field magnet applications for fusion devices and particle accelerator systems.