実用超電導線材の極低温における機械的性質と破面観察

抄録

Strain effects and mechanical properties of several practical superconducting wires at cryogenic temperatures were investigated. The results were discussed in the light of morphology of fracture surface.
The NbTi wire showed ductile deformation behavior until fracture in tensile tests even at liquid He temperature. Fatigue cracks were initiated within CuNi sleeve at the surface layer of the wire, especially from inclusions in it. The excellent fatigue behavior is attributed to the hardened microstructure of the CuNi layer.
The critical current I_c of Nb₃Sn wires, especially bronze processed, was sensitive to strain. The reversibility of I_c to the strain was limited by cracking in the Nb₃Sn filaments. The in-situ processed wire, consisted with ultra-fine filaments, showed excellent mechanical properties and I_c vs. strain characteristics as compared with those in the bronze processed. Both the mechanical properties and the I_c vs. strain characteristics changed with wire structure and heat treatment. Fatigue cracks initiated within the brittle Nb₃Sn intermetallic compound layer. They appeared to connect each other gradually and propagate through ductile constituents at the final stage of fracture.