FATIGUE BEHAVIOR AT ROOM-TEMPERATURE AND ITS INFLUENCE ON SUPERCONDUCTING PROPERTIES OF NB3AL COMPOSITE WIRE

The fatigue behavior at room temperature and its influence on the supe rconducting properties of Nb3Al composite wire with a copper ratio of 1.55, containing 241 Nb3Al filaments with an average diameter of 34 mu m, were studied. The main results are summarized as follows. (1) Fati gue cracks nucleated and grew in the matrix (Cu), and thereby caused t he fracture of Nb3Al filaments. The Nb3Al filaments themselves were no t fractured by fatigue. (2) The growth pattern of the fatigue cracks d epends on the maximum-applied-stress (sigma(c,max)) level. (a) At high sigma(c,max), overall fracture of the composite occurs before the gro wth of fatigue cracks. The filaments are merely broken except in the f racture surface as in the case of the static test. (b) At intermediate sigma(c,max), fatigue cracks can grow at several points before the br eakage of the composite. Thus the breakages of the filaments are accum ulated not only in the fracture surface, but also in the whole body. ( c) At low sigma(c,max), one fatigue crack grows in advance of others, causing the breakage of the composite. Thus the filaments are broken o nly in the fracture surface. (3) Corresponding to the growth patterns of the fatigue cracks mentioned in (2), the result of the measurement of critical current (I-c) using the rest part of the broken composite after the fatigue test shows the reduction in I-c at intermediate sigm a(c,max), but not at high and low sigma(c,max).