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The degradation mechanisms of state-of-the-art Nb-Ti/Cu superconductors are described, based on in-situ synchrotron X-ray diffraction measurements during heat treatment. A quantitative description of the Nb-Ti/Cu degradation in terms of critical current density, Cu stabiliser resistivity and mechanical composite strength is presented. In an applied magnetic field a significant critical current degradation is already observed after a 5-minute 400 °C heat treatment, due to variations of –Ti precipitate size and distribution within the Nb-Ti alloy filaments. A strong degradation of the strand mechanical properties is observed after several minutes heating above 550 °C, which is also the temperature at which the formation of Cu-Ti intermetallic phases is detected. Several minutes heating at 250 °C are sufficient to increase the RRR of the strongly cold work strands inside a Rutherford type cable from about 80 to about 240. Heating for several minutes at 400 °C does not cause a significant conductor degradation in self-field and, thus, leaves enough temperature margin for the electrical interconnection of Nb-Ti/Cu conductors with common low temperature solders.
The load-independent Vickers hardness (HV) of the individual phases in a non-reacted Nb3Sn
wire has been measured and the yield strength (YS) of the individual components estimated
from the HV values. The YS and tensile strength (Rm) of the composite wire are calculated
according the rule of mixtures (ROM). Calculated tensile properties are compared with tensile
test results that were obtained for the entire composite wire, for the wire after removal of the
Cu stabiliser and for single Nb-7.5wt.%Ta fibres.