Atomic-resolution chemical mapping of ordered precipitates in Al alloys using energy-dispersive X-ray spectroscopy

• DOI: 10.1016/j.micron.2017.02.007
• Publication Date: 2017-03-01T01:53:05Z
• Authors: Sigurd Wenner; Lewys Jones; Calin D. Marioara; Randi Holmestad
• Keywords: Energy-dispersive; X-ray spectroscopy; Aluminium alloys; Precipitation; Scanning transmission electron microscopy; 02 engineering and technology; 0210 nano-technology

Scanning transmission electron microscopy (STEM) coupled with energy-dispersive X-ray spectroscopy (EDS) is a common technique for chemical mapping in thin samples. Obtaining high-resolution elemental maps in the STEM is jointly dependent on stepping the sharply focused electron probe in a precise raster, on collecting a significant number of characteristic X-rays over time, and on avoiding damage to the sample. In this work, 80kV aberration-corrected STEM-EDS mapping was performed on ordered precipitates in aluminium alloys. Probe and sample instability problems are handled by acquiring series of annular dark-field (ADF) images and simultaneous EDS volumes, which are aligned and non-rigidly registered after acquisition. The summed EDS volumes yield elemental maps of Al, Mg, Si, and Cu, with sufficient resolution and signal-to-noise ratio to determine the elemental species of each atomic column in a periodic structure, and in some cases the species of single atomic columns. Within the uncertainty of the technique, S and β" phases were found to have pure elemental atomic columns with compositions Al2CuMg and Al2Mg5Si4, respectively. The Q' phase showed some variation in chemistry across a single precipitate, although the majority of unit cells had a composition Al6Mg6Si7.2Cu2.