Examination of factors affecting strain tolerance of multifilament MgB2 wires with Fe barrier

Abstract There are two fabrication processes, the wind and react method and the react and wind method, to manufacture magnesium diboride (MgB2) magnets. The react and wind method is desirable to simplify the coil fabrication process, but the understanding of strain tolerance of the sintered MgB2 wire is insufficient. Hence, this study focused on determining the main factors that contribute to the strain tolerance of the MgB2 wire with Fe barrier. It is generally thought that irreversible strain of sintered MgB2 wire is mainly determined from the residual strain of MgB2 filament, which is caused by the difference in the coefficient of thermal expansion among MgB2 and metal sheaths. To estimate the residual strain of MgB2 wire, the effect of yielded material must be considered. We calculated the residual strain of each constituent material used in the ten filaments MgB2 wire in which copper was yielded during the cooling process from sintering temperature to room temperature. The calculated residual strains of metal sheaths except copper were compared with yield strains, and whether they yield was confirmed. The estimated residual strain of the MgB2 filament was also compared with experimentally obtained irreversible strain, and the difference in these strains suggests the strain tolerance of the MgB2 filament. By measuring irreversible strains of the MgB2 wires having different sintering times but the same residual strain, it was confirmed that the effect of the mechanical strength of the MgB2 filament on the strain tolerance of the MgB2 wire is slight but certain.