Experimental and theoretical study of the microstructures in rapidly solidified Cu–75 at% Sn alloy

The microstructure of Cu alloys has attracted increasing interest for the development of fluoride-ion batteries (FIBs), owing to their potential as cathode materials. However, the mechanisms governing non-equilibrium microstructural evolution in Cu–Sn alloys, particularly those affecting F-ion transport pathways, remain insufficiently understood. In this study, we investigate the microstructural evolution of a Cu–75 at% Sn alloy prepared by rapid solidification followed by thermal aging, with a particular focus on the role of Sn as a potential diffusion pathway for F ions. Transmission electron microscopy (TEM) and scanning TEM (STEM) analyses reveal nanoscale compositional modulations that form alternating Cu-rich and Cu-poor domains, reminiscent of spinodal decomposition, in the rapidly solidified state. Rapid solidification also promotes interstitial incorporation of Cu into the β-Sn phase, increasing the Cu solubility up to 1.65 at% and inducing lattice contraction. During subsequent aging, Cu atoms redistribute, resulting in the recovery of the β-Sn lattice parameter, as supported by first-principles calculations and STEM–energy-dispersive X-ray spectroscopy. These findings provide fundamental insights into the non-equilibrium microstructural modulation of Cu–Sn alloys and offer useful guidelines for tailoring cathode materials for next-generation FIBs. • Rapid solidification suppressed Cu₃Sn formation, yielding a microstructure of β-Sn and Cu₆Sn₅ with refined grains and lattice disorder. • Thermal aging drove structural recovery and Cu redistribution, sharpening diffraction peaks, restoring lattice parameters, and forming secondary Cu₃Sn. • Hardness rose in rapidly solidified samples due to nanoscale modulation from spinodal decomposition, but fell after aging from domain coarsening and homogenization. • DFT and STEM-EDS showed Cu solubility in β-Sn rises under rapid solidification via interstitial Cu and lattice contraction.