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

Sigurd Wenner; Lewys Jones; Calin D Marioara; Randi Holmestad

doi:10.1016/j.micron.2017.02.007

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

Micron. 2017 May:96:103-111. doi: 10.1016/j.micron.2017.02.007. Epub 2017 Feb 28.

Authors

Sigurd Wenner¹, Lewys Jones², Calin D Marioara³, Randi Holmestad⁴

Affiliations

¹ Department of Physics, NTNU - Trondheim, NO-7491 Trondheim, Norway. Electronic address: sigurd.wenner@ntnu.no.
² Department of Materials, University of Oxford, Parks Road, OX1 3PH Oxford, UK.
³ Materials and Chemistry, SINTEF, NO-7491 Trondheim, Norway.
⁴ Department of Physics, NTNU - Trondheim, NO-7491 Trondheim, Norway.

PMID: 28282549
DOI: 10.1016/j.micron.2017.02.007

Abstract

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 Al₆Mg₆Si_7.2Cu₂.

Keywords: Aluminium alloys; Energy-dispersive X-ray spectroscopy; Precipitation; Scanning transmission electron microscopy.