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The mechanical properties of Cu and Cu-alloy (Cu–Zr, Cu–Be and Cu–Cr) sheath in situ PIT-processed MgB 2 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. 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 I c prior to the critical strain ( e irr ) was different depending on the magnetic field; being almost constant at 2 T and increased with strain at 5 T. The I c decreased beyond e irr , which was much larger for Cu-alloy sheath wires as compared with Cu sheath wire. The magnitude of e irr is due to the difference in the thermal compressive strain in the MgB 2 core, which was relaxed 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 about 95 MPa, which is also higher than that of Cu sheath wire. |
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