Unravel the Spinodal Decomposition Kinetics in (FeCoCrNi)85(AlCu)15 Alloy through Small-Angle Neutron Scattering

High entropy alloys (HEAs) constituted of single solid solution phase, but remains chemical inhomogeneity in nature due to its multi-principal composition. Currently, existence of nanoscale spinodal decomposition (SD) phase in matrix was found to have significant impact on the properties of HEAs. Nevertheless, the morphology evolution and the kinetics of SD is not clear, which hinders in-depth understanding of the structure–property relationship. In this study, we examine the spinodal structures in (FeCoCrNi)85(AlCu)15 HEAs at different states using in-situ small-angle neutron scattering (SANS), in conjunction with transmission electron microscopy technique. The result demonstrates that SD occurred when aging the HEA samples at temperatures ranging from 500 to 800 °C, which leads to the phase constitution of NiAlCu-rich and FeCoCr-rich spinodal phases, L1₂ ordered phases, and FCC matrix. The characteristic wavelength of SD (\({\lambda }_{\text{SD}}\)) grows from 5.31 to 51.26 nm when aging temperature rises from 500 to 800 °C, which explains the enhancement of the alloy’s microhardness. The SD kinetics was unraveled by fitting the time-dependent \({\lambda }_{\text{SD}}\) through in-situ SANS measurement at 700 °C. During isothermal treatment at 700 °C, the \({\lambda }_{\text{SD}}\) increases from 10.42 to 17.43 nm with prolonged time, and SD is in the late stage from the exponential trend of the \({\lambda }_{\text{SD}}\) over time. Moreover, comparing with aging temperature, the aging time has a relatively minor impact on the coarsening of SD.