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

Abstract

Several efforts are underway to better understand the current degradation phenomenon in ITER Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn 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 Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands under bending strain. Cracks began to appear at bending strains well below 1%, even in bronze-route Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands, and typically appeared around 0.3%. In internal-tin Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands, the strain causing crack formation decreased further, to about 0.25%. Magnetization measurements detected filament coupling, even in bronze-route Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands. However, the effective filament diameter was so small that its influence on I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> -bending strain characteristics should be limited.