Advancing small-angle X-ray scattering for complex metallic systems: Ti ₂ Cu precipitation in a martensitic near-α Ti alloy

Small-angle X-ray scattering (SAXS) experiments performed at synchrotron radiation sources enable the <i>in situ</i> study of precipitation behaviour, providing crucial information for alloy design. Numerous studies have demonstrated the successful use of SAXS to investigate precipitation phenomena across a wide range of complex metallic systems. Nevertheless, despite its advantages, the application of SAXS remains often overlooked owing to the challenges associated with data analysis, especially under non-isothermal conditions in which a metal matrix is continuously evolving due to simultaneous phase transformations and microstructural coarsening. This work presents a novel SAXS modelling approach developed for the quantitative evaluation of precipitate formation in metallic systems exhibiting scattering signals superimposed on directionally streaked signals originating from the embedding matrix, as commonly found in martensitic microstructures. Two-dimensional synchrotron data recorded during a continuous <i>in situ</i> heating experiment on a Cu- and Si-containing near-α Ti alloy are used to demonstrate how the evolving SAXS signal can be separated into matrix- and precipitation-related contributions. A Guinier-Porod function linked to a grain coarsening model was used to describe the background signal from the matrix, while Ti₂Cu precipitates were modelled using an ellipsoidal model function combined with a lognormal size distribution. This combined approach enabled the evaluation of precipitate volume fraction and size distribution throughout the heat treatment. The SAXS results were validated through complementary transmission electron microscopy and atom probe tomography experiments, showing excellent agreement in both size and phase fraction. The presented methodology allows the successful capture of early-stage precipitation and provides an adaptable solution to background modelling challenges in non-isothermal SAXS experiments. This approach expands the applicability of SAXS for precipitation studies in structurally complex alloy systems.