High Strength Materials

Our efforts in material research and development programs related to the high field magnets have continuously concentrated on:

• Development of various fabrication routes for different conductors and reinforcement materials with nanostructures in collaboration with industrial partners
• Investigation of the physical properties, atomic structure and microstructure of both kinds of materials
• Exploration of new materials that have the potential for fabrication of next generation magnets

The program emphasizes the requirement to develop fabrication routes capable of producing the conductors and reinforcement materials with desired and homogeneous properties and with appropriate sizes.

The study of fabrication routes and properties of the conductors took an approach to relate the properties both to design requirements and to the service life of the magnets. Fabrication of Cu, Cu-Ag, Cu-Nb, and Cu+Al₂O₃ aims to make high strength conductors with the appropriate cross-section required for the magnets. Consideration has been given to the role of atomic structure distortion on both the elastic-plastic transition and the mechanical instability of heavily deformed conductors. Assessments were also made on Young’s modulus, yield stress and conductivity of the materials at room temperature and 77 K. In both Cu-Ag and Cu-Nb conductors, the strain hardening introduces nanostructure and crystallographic lattice distortion and consequently no sharp elastic-plastic transition can be observed. Cyclic deformations, such as mechanical fatigue, reduce the effect of the lattice distortion on the mechanical properties of the materials. More severe lattice distortions were found in Cu-Nb (fcc-bcc composite) than Cu-Ag (fcc-fcc composite) conductors and resulted in more rounded stress-strain curves. The deformed Cu+Al₂O₃ materials have the most abrupt elastic-plastic transition among all the conductors investigated, because the reinforcement nano-particles essentially are not shear-susceptible at the stresses applied and little lattice distortion occurs. Because the lattice distortion is related to the fatigue behavior of the conductors, it affects the service life of the pulsed magnets.

The exploration of new conductors is concentrated on cryogenic deformation of Cu and Cu+Al₂O₃. The assessment of the microstructure and properties of cryogenic deformed pure Cu indicates that the structure and strength formed by 77 K deformation need to be stabilized. Therefore, low temperature rolling and drawing have been undertaken on Cu+Al₂O₃ conductors. Preliminary results indicate that the low temperature deformation indeed introduces more strain hardening in Cu+Al₂O₃ conductors at low strain levels.

The reinforcement materials investigated are cobalt-nickel alloys and high purity maraging steels. The cobalt-nickel alloys are reinforced mainly by dislocations and coherent defects or precipitates which are only a few atomic layers thick. The high purity maraging steels have a low interstitial level and thus a higher fracture toughness value than conventional maraging steels. Both materials have a higher Young's modulus than other currently available reinforcement materials, e.g. stainless steels. However, the lack of available data on the heat treatment and cold work conditions resulted in inconsistent and lower than optimized mechanical properties of cobalt-nickel alloys. A systematic investigation has been undertaken on the thermo-mechanical processing variables and their effect on the strength, ductility and fatigue life of cobalt-nickel alloys. The properties are related to the structure of the materials from the microscopic scale to atomistic scale. Currently, the optimized fabrication routes supply an MP35N alloys in a repeatable manner with an ultimate tensile strength of 2500 MPa at 77 K. The materials can survive more than 2000 cycles at a maximum stress level of 2300 MPa at 77 K. These properties meet the aggressive requirements of the current pulse magnet designs.

In addition to nanostructure materials, we also studied various macro-composite. Currently, macro-composites made of stainless steel jacked Cu, Cu+0.3wt%Al₂O₃ and Cu+1.1wt%Al₂O₃ have been fabricated. The preliminary results indicate that the conductors have the potential to achieve high strength with large cross-sections. We also studied Cu+SiC composite and the materials reached strength level of 1300 MPa at room temperature with conductivity of 70% IACS.

For more information, please contact Dr. Ke Han at han@magnet.fsu.edu or (850) 644-6746.