Microstructural Observation of ITER $\hbox {Nb}_{3}\hbox {Sn}$ Strands Under Bending Strain

Several efforts are underway to better understand the current degradation phenomenon in ITER Nb₃Sn cable-in-conduit (CIC) conductors, using an electric circuit model of the strand interior. To support these efforts, this paper explores crack formation and filament coupling through microstructural observation of ITER-type Nb₃Sn strands under bending strain. Cracks began to appear at bending strains well below 1%, even in bronze-route Nb₃Sn strands, and typically appeared around 0.3%. In internal-tin Nb₃Sn strands, the strain causing crack formation decreased further, to about 0.25%. Magnetization measurements detected filament coupling, even in bronze-route Nb₃Sn strands. However, the effective filament diameter was so small that its influence on I_c-bending strain characteristics should be limited.