Surface and subsurface microstructural analysis of cerium under ambient air exposure using in-situ cross sectioning and observation systems

• Microstructure of extremely oxygen-reactive cerium is directly revealed. • FIB and laser milling successfully reveal subsurface microstructures. • Abnormal cerium oxide growth occurs upon brief air exposure. • Growth initiates locally, likely from cerium carbide particles. • Findings provide new insight into early-stage oxidation in reactive metals Cerium, a rare earth element with a strong affinity for oxygen, presents significant challenges for microstructural characterization due to rapid surface oxidation. In this study, cerium was cast in uncoated graphite crucibles under two vacuum levels (∼10 −5 and ∼10 −8 atm) to investigate the influence of the casting environment on surface and subsurface features. Initial cross-sectional analysis using focused ion beam (FIB) milling and scanning electron microscopy (SEM) revealed distinct oxide morphologies, particularly near the surface: discrete particles in high vacuum and a thin continuous layer in low vacuum. Upon brief exposure to ambient air at room temperature, both samples developed accelerated oxidation that occurred preferentially at cerium carbide particles. To investigate these features at greater depth, femtosecond laser beam milling was employed enabling rapid preparation of large area cross sections of about 500 µm width and the depth of more than 1,000 µm. Subsequent observation within the same vacuum chamber and chemical analysis revealed the intrinsic subsurface microstructure of the cast cerium, confirming the presence of metallic cerium, fine cerium oxide particles, and coarse cerium carbide phases. These finding suggest that carbon diffusion from the crucible leads to carbide formation, which may may promote oxidation, though the mechanism remains unclear. Overall, the study demonstrates the utility of in-situ cross sectioning for investigating oxidation behaviour, as well as surface and interface phenomena in highly reactive rare earth metals, and provides a framework for subsurface characterisation.