Stress–strain effects on powder-in-tubeMgB2 tapes and wires

Abstract

The effects of stress–strain on the critical current,Ic, of ex situ powder-in-tube (PIT)-processed Ni-sheathedMgB2 tapes and round wires as well as in situ PIT-processed Cu-sheathed wires at 4.2 K in a magneticfield up to 5 T have been studied. The effect of In powder addition on the Ni-sheathedMgB2 wire was not so clear compared with that in the tape, in which the irreversible strain,εirr, forthe Ic degradation onset increases significantly by the addition. This is attributed to the difference in themicrostructure of the core associated with cold workings. A peak and gradual degradation behaviour ofIc with strainbeyond εirr was found in the wire, whereas no evident peak and a steep degradationbehaviour was found in the tape. As a possible reason, the difference in thetriaxial residual stress state at 4.2 K due to the difference in geometry of thecross-section is suspected. The transverse compression tests revealed thatIc of thewire did not degrade up to 270 MPa. Again, the effect of In addition was minimal. The Young’s modulusof MgB2, 31–41 GPa, at room temperature was estimated by a tensile test of Cu sheath wire using ahigh-accuracy extensometer and the law of mixtures. The tensile strain dependence ofIc in the Cu sheath wire was similar to that in the Ni-sheathed wire,εirr being 0.4%. However, the stress corresponding toεirr, 50 MPa,was about 1/10 of that for the Ni-sheath wire and the irreversible transverse compressivestress, 150 MPa, was also lower. The effect of bending strain on theIc in Cu-sheathed wire was compared with that of the tensile strain.