Scanning transmission electron microscopy (STEM) data with atomic resolution can contain a large amount of information about the structure of a crystalline material. Often, this information is hard to extract, due to the large number of atomic columns and large differences in intensity from sublattices consisting of different elements. In this work, we present a free and open source software tool for analysing both the position and shapes of atomic columns in STEM-images, using 2-D elliptical Gaussian distributions. The software is tested on variants of the perovskite oxide structure. By first fitting the most intense atomic columns and then subtracting them, information on all the projected sublattices can be obtained. From this, we can extract changes in the lattice parameters and shape of A-cation columns from annular dark field images of perovskite oxide heterostructures. Using annular bright field images, shifts in oxygen column positions are also quantified in the same heterostructure. The precision of determining the position of atomic columns is compared between STEM data acquired using standard acquisition, and STEM-images obtained as an image stack averaged after using non-rigid registration.

TEM investigations of two alloys isothermally heat treated at 175°C and 260°C show how Cu additions to the Al–Mg–Si system affect precipitation. Both alloys had a solute content Mg + Si = 1.3 at.%, 0.127 at.% Cu, but with Mg/Si 0.8 and 1.25. Cu-containing Guinier-Preston (GP) zones and three types of Q′ precursors are identified as most common phases at peak-hardness conditions, whereas β″ accounts for maximum 30% of the total number of precipitates. The precursors have needle (L and S precipitates) or plate (C precipitate) morphologies. They consist of different arrangements of Al, Mg and Cu atoms on a grid defined by triangularly arranged Si planes parallel with and having the same period as {100} Al planes. The Si grid is composed of nearly hexagonal sub-cells of a = b = 4.05 Å, c = 4.05 Å. The Cu arrangement on the grid is often disordered in the needle precursors. The plate precursor is ordered, with a monoclinic unit cell of a = 10.32 Å, b = 8.1 Å, c = 4.05 Å, γ = 101°.

The composition of ${\beta }^{″}$ precipitates in an Al–Mg–Si alloy has been investigated by atom probe tomography, ab initio density functional calculations, and quantitative electron diffraction. Atom probe analysis of an Al-0.72% Si-0.58% Mg (at. %) alloy heat treated at $175°\text{C}$ for 36 h shows that the ${\beta }^{″}$ phase contains $\sim 20\text{at}\text{.}\%$ Al and has a Mg/Si-ratio of 1.1, after correcting for a local magnification effect and for the influence of uneven evaporation rates. The composition difference is explained by an exchange of some Si with Al relative to the published ${\beta }^{″}{\text{-Mg}}_5{\text{Si}}_6$ structure. Ab initio calculations show that replacing the ${\text{Si}}_3$ -site by aluminum leads to energetically favorable compositions consistent with the other phases in the precipitation sequence. Quantitative electron nanodiffraction is relatively insensitive to this substitution of Al by Si in the ${\beta }^{″}$ -phase.

This work provides a detailed atom probe tomography study of clustering in the Al–Mg–Si system. Focus is on separating and understanding the influence of natural aging, preaging, and alloy composition on the clustering behavior of solute atoms. Two dilute alloys with the same total solute content have been studied, one Mg-rich and one Si-rich. The detrimental effect of natural aging for these alloys is investigated by comparing directly preaged samples to samples stored at room temperature before the preaging treatment. Clusters were identified in the atom probe datasets by the maximum separation method employing heuristically determined input parameters. It was found that seven days of intermediate natural aging gave a five times lower number density of clusters as compared to direct preaging for both alloy types. The clusters were of comparable size but their compositions depended on heat treatment history. Preaging promoted the formation of clusters with an Mg:Si ratio close to 1 in both alloys, while natural aging produced clusters with Mg:Si ratios more similar to those of the alloys.