Stress/Strain Characteristics of Cu Alloy Sheath in situ Processed MgB₂ Superconducting Wires

Abstract

The mechanical properties of copper and copper alloy (Cu-Zr, Cu-Be and Cu-Cr) sheath in situ PIT-processed MgB₂ superconducting wires were studied at room temperature (RT) and 4.2 K. The effects of stress/strain on the critical current (I_c) of the wires have also been studied at 4.2 K and in magnetic fields up to 5 T. It has been clarified that alloying the Cu sheath significantly increases the yield and flow stresses of the wires at both RT and 4.2 K. The 0.5% flow stresses of the Cu alloy sheath wire were 147-237 MPa, whereas that of Cu was 55 MPa. At RT, serration corresponding to multiple cracking was observed around a strain of 0.4% and the stress-strain curves saturated beyond that point. The strain dependence of I_c prior to the critical strain (ε_irr) was different depending on the magnetic field; being almost constant at 2 T and increasing with strain at 5 T. The I_c decreased beyond ε_irr, which is much larger for Cu alloy sheath wires as compared to Cu sheath wire. This is due to the difference in the residual compressive strain in the MgB₂ core during cooling from the heat-treatment temperature to 4.2 K, which is determined through relaxation by yielding in the sheath materials. The transverse compression tests revealed that the I_c of the Cu alloy sheath wire did not degrade up to 314 MPa, which is also higher than that of Cu sheath wire.