Stress–strain effects on powder-in-tube MgB2 tapes and wires

The effects of stress-strain on the critical current, Ic, of ex situ powder-in-tube (PIT)-processed Ni-sheathed MgB2 tapes and round wires as well as in situ PIT-processed Cu-sheathed wires at 4.2 K in a magnetic field up to 5 T have been studied. The effect of In powder addition on the Ni-sheathed MgB2 wire was not so clear compared with that in the tape, in which the irreversible strain, epsilonirr, for the Ic degradation onset increases significantly by the addition. This is attributed to the difference in the microstructure of the core associated with cold workings. A peak and gradual degradation behaviour of Ic with strain beyond epsilonirr was found in the wire, whereas no evident peak and a steep degradation behaviour was found in the tape. As a possible reason, the difference in the triaxial residual stress state at 4.2 K due to the difference in geometry of the cross-section is suspected. The transverse compression tests revealed that Ic of the wire did not degrade up to 270 MPa. Again, the effect of In addition was minimal. The Young's modulus of MgB2, 31-41 GPa, at room temperature was estimated by a tensile test of Cu sheath wire using a high-accuracy extensometer and the law of mixtures. The tensile strain dependence of Ic in the Cu sheath wire was similar to that in the Ni-sheathed wire, epsilonirr being 0.4%. However, the stress corresponding to epsilonirr, 50 MPa, was about 1/10 of that for the Ni-sheath wire and the irreversible transverse compressive stress, 150 MPa, was also lower. The effect of bending strain on the Ic in Cu-sheathed wire was compared with that of the tensile strain.