2005 年 40 巻 11 号 p. 505-509
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 (Ic) 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 Ic prior to the critical strain (εirr) was different depending on the magnetic field; being almost constant at 2 T and increasing with strain at 5 T. The Ic decreased beyond εirr, 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 Ic of the Cu alloy sheath wire did not degrade up to 314 MPa, which is also higher than that of Cu sheath wire.