Microstructural changes produced in a multifilamentary Nb-Ti composite by cold work and heat treatment

The microstructural changes produced by cold work and heat treatment of a full-scale filamentary Nb-Ti superconducting composite have been investigated using transmission electron microscopy. Full details of the processing conditions used are given, and both longitudinal and transverse sections of the microstructure have been examined. Heat treatments are found to give a complex precipitate morphology; the precipitates produced by heat treatments at 648 K (375 °C) were found to range from a thin boundary film 3 to 6 nm thick, through small approximately ellipsoidal precipitates (~5 x 10 nm), to larger (~50 to 500 nm) equiaxed precipitates. The larger precipitates could be identified as α-Ti. Cold drawing was found to refine the microstructure, but an even greater degree of refinement was observed when intermediate heat treatments were applied. The observations of this investigation show a strong interdependence between the cold-worked substructure and the precipitate morphology in these heat-treated and heavily cold-worked (ε ~ 5 to 9) composites.