Composition fluctuations at interfaces in Mg-Al-Ca alloys revealed by quantitative three-dimensional X-ray energy dispersive spectroscopy

Nanoscale microstructure has a significant impact on the properties of materials, defining the high-temperature mechanical properties of metal alloys for aerospace and automotive applications. Quantifying the three-dimensional composition of a material across interfaces within such microstructure is therefore essential. Here, we report three-dimensional (3D) nanoscale composition quantification across interfaces in an Mg-Al-Ca alloy using scanning transmission electron microscopy-based X-ray energy dispersive spectroscopy. We use this demonstration to evaluate two 3D quantification approaches: (1) absolute quantification employing experimentally calibrated ionization cross-sections and compressed sensing tomography based on second order total variation regularisation and (2) relative quantification based on physical parameters extracted from the MC X-ray programme and tomography using the simultaneous iterative reconstruction technique (SIRT). X-ray absorption and shadowing corrections were integrated with both reconstruction methods. The results offer a methodological demonstration of absorption correction with absolute quantification as well as insight into the differences in absorption correction for CS-TV² and SIRT. In turn, these findings reveal composition changes immediately at the interfaces between the α-Mg matrix and the intermetallic skeleton microstructure characteristic of Mg-Al-Ca alloys. These advances in microscopy methodology to probe 3D compositional fluctuations at buried interfaces establish a route for quantitative analysis of alloys and materials with surface oxides as well as samples containing several elements with overlapping X-ray absorption edges.