Nausite and NbSn2 – Growth and distinction of structural related intermetallic phases in the Cu–Nb–Sn system

Abstract

Processing of superconducting Nb3Sn multifilamentary wires requires several heat-treatment steps forming Nb3Sn at the final stage. During these heat treatments various intermetallic phases from the Cu–Nb–Sn system were intermediately formed, including a ternary phase, sometimes called “Nausite”. This phase derives from the binary NbSn2 phase by partial substitution of Nb by Cu (Martin et al., Intermetallics 80 (2017) 16-21). In this work, several samples containing NbSn2 and Nausite were investigated which show similar growth morphologies for both phases, also with some peculiarities, which are reflected by similarities of their atomic structures. Indeed, both phases belong to closely related crystal structures (CuMg2- and NiMg2-type) with comparable lattice parameters. This leads to similar EBSD Kikuchi patterns which causes difficulties in reliable phase distinction. In order to be able to use EBSD as a useful tool for local phase determination in the sub-μm range, phase distinction will be successful only when distinct minor, less prominent Kikuchi bands can be resolved and were used for indexing. This was revealed in this work using simulated Kikuchi patterns of both phases. Additionally, these bands overcome problems in orientation analysis of NbSn2 which arise from a pseudo-hexagonal symmetry.