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Dependence of the critical current in ex situ multi- and mono-filamentary MgB2/Fe wires on axial tension and compression

The transport critical current Ic of single- and four-core MgB2/Fe wires under tensile and compressive axial strain ε was measured with a U-shaped spring set up. In all wires, Ic increases linearly and reversibly with applied tensile strain, up to a sample-dependent reversible strain limit εirr. The same reversible and linear Ic(ε) relation is found when applying compressive strain. This shows how the Ic(ε) increase with tension can be understood as a gradual release of the thermal pre-compression strain induced in the MgB2 filaments of the composite. The value of the reversible strain limit εirr depends mainly on the degree of pre-compression, but also on the shape of the composite wire. It is highest in a square wire and lowest in the flattest tape. Tensile strain levels above εirr cause an irreversible degradation of Ic. This Ic(ε) degradation, due to filament cracking, is significantly steeper in heat-treated wires than in as-deformed ones. The different Ic(ε) regimes are analysed and the mechanical behaviour of the MgB2/Fe composite is compared with that of typical Nb3Sn wires and Bi,Pb(2223) tapes.