Characterization of super conducting property from the interdisciplinary viewpoint of superconducting materials technology and solid mechanics

超导线材在制造和使用过程中，受到机械力和电磁力的作用，其超导特性如临界温度、电流和上限磁场等都会发生变化。本文结合超导材料和力学的知识，阐明了Nb_3Al和Nb-Ti复合线材在静应力和循环应力作用下损伤的形核、生长和累积及其对超导性能的影响。主要结果如下。(1)用中子衍射法研究了丝的择优取向和稳定铜的取向。(2)Nb_3Al丝以脆性方式断裂，而具有延展性的Nb-Ti丝由于与周围的铜的机械相互作用而出现多次颈缩，从而在高应变下保持高的临界电流密度。(3)尽管Nb_3Al丝的强度分散性较大，但复合丝的强度分散性较小。这一特性可在计算机上用MonteCarlo模拟方法再现。本研究结果首次证明了复合丝的高可靠性。(4)在循环应力下，裂纹首先在铜中形成（阶段I））并扩展到芯部（阶段II）。在此过程中，细丝被破坏，导致低临界电流和低残余强度。

The superconducting properties such as critical temperature, current and upper magnetic field change when mecahnical and electro-magnetic forces are exterd on the superconducting wires during fabrication and/or service. In this work, by combining the knowledges in the fileds of superconducting materials and mechanics, the nucleation, growth and accumulation of damages under staic and cyclic stresses and their influences on superconduing properties of Nb_3Al and Nb-Ti composite wire were clarified. Main results are summaized as follows. (1) The preferred orientation of the filaments and stabilizing copper were clarified by the neutron diffraction method. (2) Nb_3Al filament is broken in a brittle manner, while Nb-Ti one, having ductility, shows mumtiple necking due to the mechanical in teraction with surrounding copper, which acts to keep high critical current density up to high applide strain. (3) In spite of large scatter of strength of Nb_3Al filaments, the scatter of strength of composite wire is small. This feature could be reproduced by a Monte Carlo simulation method in the compuetr. The present results first demonstrate the high reliability of composite wire. (4) Under cyclic stress, crack forms first in the copper (stage I)) and propagate into the core portion(Stage II). In this process, filamenst are broken, resulting in low critical current and low residual strength.