Effect of Cu Cladding on the Mechanical Properties of Iron Sheath Material in the Drawing of Superconducting MgB2 Wires

In the present study, a novel choice of sheath materials for drawing long superconducting MgB2 wire by using the powder-in-tube technique (PIT) is reported. This would eliminate the need for an intermediate strain-relieving annealing process and would reduce the time and cost of fabrication. Our strategy involved the use of a composite sheath instead of a sheath made of a single material. The relatively inert Fe constituted the inner sheath around the MgB2 powder while the Cu-which is capable of efficient heat dissipation-was used as the outer sheath. Important mechanical properties of the wire such as elastic modulus, ultimate tensile strength, yield strength, hardness, and microstructure were carefully studied at different stages of the drawing process using tensile and microhardness tests. To clearly delineate the effect of Cu cladding on the ductile behavior of the iron sheath, another MgB2 wire with only an Fe sheath was prepared; its mechanical properties were measured and compared with those of the composite Cu-Fe-sheathed MgB2 wire. After a few drawing steps, the composite Cu-Fe-sheathed wire showed a lower elastic modulus and tensile strength than those of its Fe sheath counterpart. While both types of wires showed an increase in hardness as the drawing process progressed, the composite-sheath wire consistently showed a lower hardness than that of its counterpart, implying its lower susceptibility to fracture; it can therefore be safely drawn to small diameters without the need for intermediate annealing during the wire drawing process.