Relationship between architecture, filament breakage and critical current decay in Nb₃Sn composite wires repeatedly in-plane bent at room temperature

Abstract

Six Nb 3 Sn composite wires with different architectures ('central and near-the-edge reinforcement') were repeatedly in-plane bent at room temperature (in-plane 'pre-bending'). Breakage behaviour was revealed from scanning electron microscopy observations by semi-quantitative analysis of the filament crack formation and evolution. Cracks are formed in the transversal and longitudinal directions. Transversal cracks show some tolerance to the applied bending strain due to the fact that filaments are composite materials; residual Nb core can arrest development of a partial transversal crack initiated in the Nb 3 Sn outer part of the filament. Together with the density of cracks C and the evolution of this parameter with pre-bending strain, ε pb , in different regions of the wire, R-ε pb curves are important to understand breakage behaviour of the wires. R is the ratio (number of full transversal cracks)/(number of full transversal cracks + number of partial transversal cracks). Parameters C and R allow us to reveal and satisfactorily understand the wire architecture-breakage-critical current decay relationship when pre-bending treatment is applied. As a consequence, breakage criteria necessary to minimize I c decay were defined and the positive influence of the reinforcement in preventing breakage was observed. It was also found that, in this regard, more Nb in the CuNb reinforcement, for the investigated wires, is better, if the heat treatment for the wire synthesis is performed at 670 • C for 96 h. A different heat treatment, 650 • C for 240 h, is less efficient in preventing filament breakage. Our results suggest the possibility of control and improvement of the breakage susceptibility of the filaments in the wires and, hence, of the bending I c decay, through the wise design of the wire architecture (i.e. by correlating design with the choice of composing materials and heat treatments).

Performance of as-reacted and multiple bent (‘pre-bent’) practical Nb3Sn bronze route wires with different architectures

“…At present it is unclear how and if such variations are important for the residual strain and thus for critical superconducting characteristics of the as-reacted or pre-bent wires versus their architecture. Our experiments on crack formation, distribution and evolution with pre-bending strain ε pb have shown that the influence of the voids in this respect is relatively low, but it is noticeable for high ε pb 1.2% [6]. Other factors such as morphology of Nb in the CuNb micro-composite may influence mechanical (including residual strain) properties and behaviour of the wires during pre-bending.…”

“…15 and 19 T, respectively). Lower ε pb,max (J c ) than ε pb,max (B c2 ) for some wires is due to irreversible crack formation [6] and their fast development at ε pb 0.8%, leading to an earlier and stronger decay of J c than of B c2 (compare figures 6 and 7(a) and (b)). In other words, J c is strongly dependent on crack occurrence and evolution, while B c2 is weakly dependent.…”

Intra wire resistance and strain affecting the transport properties of Nb3Sn strands in cable-in-conduit conductors

“…Final polishing is conducted with a 0.05 μm silica suspension to permit high-resolution imaging in the SEM. Finally, the sample surface is lightly dip-etched in an HF-HNO 3 solution to remove the polishing deposit and to enhance the contrast around the micro-cracks without enlarging their size [95], [170]. The SEM images allow identifying the locations of the cracks, voids and filaments across the entire width of the filamentary region and along a length of 8 to 9 mm, which corresponds to slightly over two periodic bending wavelengths.…”

“…Several authors have addressed crack dynamics in various types of Nb 3 Sn wires, either mono-or multi-filamentary [168], [169]; ITER-type [93]; or specially reinforced [170]. The crack pattern in isolated filaments induced in TARSIS periodic bending tests at the University of Twente [119] were investigated with a ''postmortem'' microscopic analysis at Florida State University (FSU).…”

“…The distribution of filament cracks in full-size ITER Nb 3 Sn cable-in-conduit conductor samples after cyclic testing is also studied by scanning electron microscopy (SEM) [95], [170], but this method is destructive and time-consuming and therefore less suitable for diagnostics on coil systems. Alternatively, it is proposed that a straight-forward magnetization measurement can also reveal the presence of filament cracks in full-size ITER cable-in-conduit conductors.…”