Enhanced characterization of non-metallic inclusions in ultra-high-strength steels using advanced synchrotron-based X-ray absorption spectroscopy and chemometric analysis

This study leverages synchrotron radiation-based X-ray absorption photoemission electron microscopy (X-PEEM) to investigate non-metallic inclusions (NMIs) in ultra-high-strength steels. By acquiring Ca-L_2,3-edge X-ray absorption spectra from six NMIs at both room temperature and 400 °C, we analyze their structural changes, chemical compositions, and phase transformations. To extract meaningful insights from complex X-PEEM images, advanced chemometric data analysis is employed. Multivariate Curve Resolution (MCR-ALS) is utilized to provide an unbiased analysis, revealing pure spectra and concentration profiles of NMIs before and after annealing. This approach maintains the integrity of the results while uncovering NMI behaviors, including the identification of new calcium phases. The challenge of assigning calcium phase identities to the spectra extracted by MCR-ALS is addressed using a comprehensive reference database and the Hit Quality Index (HQI), which effectively matches most NMIs before annealing to the calcium phosphate phase. Principal Component Analysis (PCA) and Hierarchical Clustering Analysis (HCA) further elucidate the relationships between calcium phases and NMIs during annealing. For classifying NMIs based on annealing history and calcium phase changes, Partial Least Squares Discriminant Analysis (PLS-DA) and Soft Independent Modeling of Class Analogy (SIMCA) are applied, with PLS-DA demonstrating superior classification accuracy. By analyzing complex X-PEEM data, we uncover valuable information about NMIs' responses to annealing processes and their classification based on calcium phase changes.