Stress/strain characteristics of Cu-alloy sheath MgB2 superconducting wires

The mechanical properties of Cu and Cu-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 (Ic) of the wires have also been studied at 4.2 K and in magnetic fields up to 5 T. Alloying the Cu sheath significantly increased the yield stress of the wires. The 0.5% flow stresses of the Cu-alloy sheath wires were 147–237 MPa, whereas that of Cu was 55 MPa. At RT, the serration in the stress–strain curves corresponding to the multiple cracking was observed around a strain of 0.4% and the curve almost saturated beyond that point. The strain dependence of Ic prior to the critical strain (εirr) was different depending on the magnetic field; being almost constant at 2 T and increased with strain at 5 T. The Ic decreased beyond εirr, which was much larger for Cu-alloy sheath wires as compared with Cu sheath wire. The magnitude of εirr is due to the difference in the thermal compressive strain in the MgB2 core, which was relaxed by yielding in the sheath materials. The transverse compression tests revealed that the Ic of the Cu-alloy sheath wire did not degrade up to about 95 MPa, which is also higher than that of Cu sheath wire.