Behaviour of filamentary MgB2 wires subjected to tensile stress at 4.2 K

Abstract

Different filamentary MgB2 wires have been subjected to tensile stress at 4.2 K. Stress–strain and critical current versus stress and strain characteristics of wires differing by filament architecture, sheath materials, deformation and heat treatment were measured and compared. It was found that the linear increase of critical current due to the pre-compression effect (ranging from 5% up to ≈20%) is affected by thermal expansion and the strength of used metallic sheaths. The values of irreversible strain εirr and stress σirr depend dominantly on the applied outer sheath and its final heat treatment conditions. Consequently, the strain-tolerance of MgB2 wires is influenced by several parameters and it is difficult to see a clear relation between Ic(ε) and σ(ε) characteristics. The lowest εirr was measured for Monel sheathed wires (0.3–0.6%), medium for GlidCop® sheath (0.48–0.6%), and the highest εirr = 0.6–0.9% were obtained for MgB2 wires reinforced by the stainless steel 316L annealed at temperature between 600 and 800 ° C. The highest εirr = 0.9% and σirr = 900 MPa were measured for the work-hardened steel, which is not considerably softened by the heat treatment at 600 ° C/2.5 h.