Relationship between architecture, filament breakage and critical current decay in Nb3Sn composite wires repeatedly in-plane bent at room temperature
Relationship between architecture, filament breakage and critical current decay in Nb3Sn composite wires repeatedly in-plane bent at room temperature
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DOI：
10.1088/0953-2048/19/4/014
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发表时间：
2006-04
影响因子：
3.6
通讯作者：
P. Badica;S. Awaji;H. Oguro;G. Nishijima;K. Watanabe
中科院分区：
文献类型：
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作者：
P. Badica;S. Awaji;H. Oguro;G. Nishijima;K. Watanabe
六种具有不同结构（“中心和近边缘增强”）的 Nb3Sn 复合线在室温下反复进行面内弯曲（面内“预弯曲”）。通过扫描电子显微镜观察，通过对细丝裂纹形成和演变的半定量分析，揭示了断裂行为。裂纹在横向和纵向上形成。由于细丝是复合材料，横向裂纹对所施加的弯曲应变表现出一定的耐受性；残留的 Nb 芯可以阻止在灯丝 Nb3Sn 外部引发的部分横向裂纹的发展。连同裂纹密度 C 以及该参数随预弯应变 εpb 在钢丝不同区域的演变，R-εpb 曲线对于了解钢丝的断裂行为非常重要。 R 是比率（完全横向裂纹的数量）/（完全横向裂纹的数量 + 部分横向裂纹的数量）。参数 C 和 R 使我们能够揭示并令人满意地理解应用预弯曲处理时的电线结构-断裂-临界电流衰减关系。因此，定义了最小化 Ic 衰减所需的破损标准，并观察了增强材料在防止破损方面的积极影响。在这方面还发现，如果在 670 °C 下进行 96 小时的线材合成热处理，则对于所研究的线材而言，CuNb 增强材料中的 Nb 越多越好。不同的热处理（650°C 240 小时）在防止灯丝断裂方面效果较差。我们的结果表明，通过线材结构的明智设计（即通过将设计与合成材料和热处理的选择相关联），可以控制和改善线材中细丝的断裂敏感性，从而控制和改善弯曲 Ic 衰减。
Six Nb3Sn 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 Nb3Sn outer part of the filament. Together with the density of cracks C and the evolution of this parameter with pre-bending strain, εpb, in different regions of the wire, R–εpb 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 Ic 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 Ic decay, through the wise design of the wire architecture (i.e. by correlating design with the choice of composing materials and heat treatments).