Filamentary MgB2 wires manufactured by different processes subjected to tensile loading and unloading

A reversible strain effect on the transport critical current (I-c) of filamentary MgB2 wires manufactured by three different processes has been examined at 4.2 K and under an external field of 5 T. MgB2 wires with a Nb barrier and a Monel (R) outer sheath made by powder-in-tube ex situ, in situ and by diffusion of magnesium into the boron process, have been examined. The wire samples were loaded and partially unloaded at progressively higher strain levels to determine the irreversible strain limit (epsilon(irr)), which is defined as the ultimate strain where the critical current (I-c) is still reversible. It was found that the strain tolerances of the tested MgB2 wires are affected by the production process. The highest annealing temperature (> 900 degrees C), applied in the ex situ process, causes an apparent softening of the Monel (R) and, together with the poor grain connectivity of MgB2 filaments, leads to the lowest strain tolerance (epsilon(irr). =. 0.20%). The best grain connectivity, in internal Mg diffusion IMD)-made MgB2, combined with a stronger Monel (R) sheath heat treated at a lower temperature similar to 640 degrees C) results in the best strain tolerance epsilon(irr). =. 0.55%).