Microstructural investigation of internal-tin Nb3Sn strands

Various types of internal-Sn multifilamentary strand were studied in an attempt to correlate fracture, subelement hardness, and interfacial conditions. Wire fracture (one of the main factors limiting the piece-length of multifilamentary precursor wires) is empirically known to increase with strand complexity. In an attempt to quantify this, a number of internal-tin multifilamentary precursor wires exhibiting various levels of drawability were investigated by microhardness as well as optical and electron microscopy. The progression of cross sectional stability was observed via optical microscopy as a function of wire drawing. Microanalysis both in the Scanning Electron Microscope (SEM) and the Scanning Transmission Electron Microscope (STEM) were used to study the interdiffusion of various elements through the interface between the restacked subelement and between the outer Cu sleeve, as well as inside the subelement. Comparing the hardness distribution along different regions of the interfaces between sleeve and subelement as well as between the subelements suggests possible causes for the occurrence of fracture.