Phase transformations of structural related phases in the Nb–Cu–Sn system and insights in the corresponding phase diagram
Processing of Nb3Sn superconductor in multifilamentary wires for application in high-field magnets requires several heat-treatment steps. Starting mainly from Nb, Cu and Sn as precursor materials, different intermetallic phases were formed as intermediates which will affect the microstructure and thus the superconducting properties of the finally formed Nb3Sn. Due to that, phase information of the ternary Nb–Cu–Sn phase diagram is a crucial knowledge for optimizing the heat-treatment parameters of these wires. However, the currently available ternary phase diagrams are not complete regarding the existing phases. In the last years, in contradiction to long-term belief, a ternary phase with the estimated composition Nb0.75Cu0.25Sn2 was found and called “nausite”, in honour of its discoverer. This phase is not included in available Nb–Cu–Sn phase diagrams, also due to lack of data.
It has to be emphasized that the ternary nausite phase derives from the binary NbSn2 by partial substitution of Nb by Cu, causing a change in crystal structure from CuMg2-type to NiMg2-type. Both crystal structure types are closely related to each other which is reflected in their atomic structures as well as their related lattice parameters. Furthermore, a transition from one phase to the other can be realized by a change in stacking sequence of parallel Nb(-Cu) chains along the c-axis which was observed in samples containing NbSn2 and nausite simultaneously. These structural similarities lead to similarities in growth morphology and electron backscatter diffraction (EBSD) Kikuchi patterns causing problems in distinction between both phases. This can be overcome by resolving minor, less prominent bands in experimental patterns with sufficient image quality.
The present work now fills the mentioned gap by investigation of a variety of nausite containing samples which were used to elaborate its homogeneity and stability range. In detail, coated substrates and ternary powder mixtures were heat-treated at different temperatures in Ar atmosphere and quenched in ice water. Energy dispersive X-ray spectroscopy and EBSD were used to determine the chemical composition and phase equilibria of nausite, as well as differential scanning calorimetry and in-situ X-ray diffraction to investigate its decomposition. Information gained was basis for re-evaluating the ternary phase diagram using the CALPHAD approach. It is shown how the generated data fit in the already available phase diagram.