HITEMAL-an outer sheath material for MgB2 superconductor wires: The effect of annealing at 595–655 °C on the microstructure and properties

Highlights: • Al wire fabricated from a fine Al powder was studied as outer sheath material for lightweight MgB₂ superconductors. • The effect of annealing realized at 595-655 °C on microstructure, mechanical and electrical properties was investigated. • The as-rolled wire showed ultrafine-grained Al structure with 1.6% of nanometric Al₂O₃ dispersoids dispersed in Al matrix. • The Al₂O₃ dispersoids effectively stabilized Al grain structure during annealing, and only minor Al grain growth occurred. • Wires annealed up to 642 °C showed attractive strength and electrical resistivity at room and cryogenic temperatures. An aluminum material stabilized with a 1.6 vol% of nanometric Al₂O₃ dispersoids, named HITEMAL, was fabricated by hot extrusion of a fine atomized Al powder. The Al₂O₃ dispersoids stemmed from a native Al₂O₃ films on an aluminum powder surface. An extruded rod of the material was then cold-deformed out into a thin wire by rotatory swaging, hydro extrusion and rolling. Such severely deformed HITEMAL was studied as a prospective outer sheath stabilizer material for MgB₂ core superconductor wires. An emphasis was put on the effect of annealing realized at various temperatures close to the melting point of Al, which is typically applied to form an in-situ MgB₂ superconductive core, on the mechanical and electrical properties of the HITEMAL wire. A systematic investigation was performed of changes to a severely deformed Al grain structure and Al₂O₃ dispersoids upon annealing the as-deformed wire from 595 to 655 °C for 30 min. Mechanical and electrical properties of the wire were measured at room and cryogenic temperatures. The results showed that the ultrafine-grained Al + 1.6 vol% Al₂O₃ wire exhibited advantageous properties, and surface integrity after annealing up to 642 °C. It is thus a lightweight material of choice for outer sheaths of MgB₂ superconductors.