Effect of the Powder Strain State on the Mechanical Properties of MgB 2 Tapes

Starting from commercial MgB 2 powder of 325 mesh, we further reduced the agglomerates by attrition milling in nitrogen atmosphere to minimize oxidation. Using different milling media (tungsten carbide, yttria stabilized zirconium oxide and stainless steel balls), we obtained different milling energies and, consequently, powders with different strain states. Crystallite size and micro-strain within the grains were estimated by x-ray diffraction. We investigated the changes in the superconducting properties with the use of different milling media by measuring the magnetic moment as a function of temperature in a SQUID magnetometer, after zero field cooling. Single-filament stainless steel-sheathed wires and tapes were prepared with the obtained powders. The flexural properties of the tapes were determined by 4-point bending micro-tests. The use of a precursor powder with a higher strain resulted in more flexible wires and tapes, with lower flexural strength. Three different regions were determined for the maximum normal stress-strain curves. Some in-situ monel-sheathed hollow wires and ex-situ titanium-sheathed tapes and monel-sheathed 7-filament tapes were measured for comparative purposes. We discuss the correlation between sheath yielding and superconducting core cracking with the possible critical current degradation due to mechanical failure of the wires and tapes.