Thermal strain exerted on superconductive filaments in practical Nb3Sn and Nb3Al strands

Practical superconductive (SC) wires such as the Nb3Sn and Nb3Al strands used in fusion reactors are typical composite materials consisting of brittle superconducting intermetallic compounds. Thermally induced strain is inevitably generated in the composite due to the different coefficients of thermal expansion and different moduli of elasticity among the constituent components. In order to evaluate quantitatively the thermal strain, local strain measurements during heating by means of quantum beams, and room temperature tensile tests were carried out. The stress versus strain curves of both Nb3Sn and Nb3Al strands showed a typical elasto-plastic behavior, which could be numerically evaluated on the basis of the rule of mixtures. The local strain exerted on SC filaments along the axial direction was compressive at room temperature and tensile at high temperatures, which is common for Nb3Sn and Nb3Al strands. Their temperature dependence was numerically evaluated by means of the iteration method. As a whole, it has been established that the temperature dependence of thermal strain can be reproduced well by the numerical calculation proposed here. It is pointed out that the thermal strain in SC filaments is affected by the creep phenomenon at high temperatures above a threshold temperature.