Towards Correlative Raman Spectroscopy–STEM Investigations Performed on a Magnesium–Silver Alloy FIB Lamella

In this study, a lamella prepared using focused ion beam (FIB) milling from a magnesium-silver alloy wire was investigated. The wire, intended for biomedical applications, was initially degraded in simulated body fluid (SBF) under physiological conditions. Raman spectroscopy was performed across the entire FIB specimen and the results were correlated with findings from scanning transmission electron microscopy (STEM). Our micro-Raman analysis identified chemical compounds at distinct regions within the specimen. Dominant Raman modes at ~1350 cm^-1 and ~1590 cm^-1, likely derived from elemental carbon from the FIB protection layer, were observed. Additionally, modes indicative of the alloy's interaction with SBF, attributable to the constituents of SBF, were detected. Notably, Raman modes at ~3650 cm^-1 corresponding to the OH stretching mode were identified in the targeted areas of the lamella, highlighting the chemical interaction between magnesium (Mg) and the SBF. The micro-Raman mapping images showed localized Mg(OH)₂ distributions, which correlated strongly with the STEM analyses. This study underscores the effectiveness of correlating Raman spectroscopy, revealing chemical changes and STEM, capturing the corresponding microstructural changes. The combined approach is crucial for a deeper understanding of material degradation and reactivity in biocompatible alloys under physiological conditions and advances the characterization of biocompatible materials in physiological environments.