Investigating site occupancy via atom probe microscopy

This video shows an Atom Probe Tomography reconstruction generated from the analysis of an Al-Mg-Li alloy heat treated for 8h at 150°C after solution treatment. Every dot in this tomogram corresponds to an individual atom that has been detected and reconstructed. The Al atoms are shown in blue, the Li in pink, the Mg in green and the Zr in purple.

The latter forms large Al₃Zr dispersoids, a few of which are imaged in this volume, while the Li forms δ’ precipitates of composition Al₃Li. These precipitates are ordered: within the precipitates the Al atoms occupy the faces of the face-centered cubic lattice, while the Li sits at the edges of the cube. Precipitates are clearly visible when an 8 at.% Li isoconcentration surface is superimposed to the three-dimensional map. As the thin slice is moved through the data, a large number of precipitates are seen and can be individually interrogated.

The study aimed to reveal how the Mg partitions between the precipitates and the matrix, and within the precipitates, whether the Mg occupies the sites of the Li or those of the Al. We found that the composition is very similar in the matrix and the precipitates, however, in the matrix, the Mg randomly occupies any site, within the precipitates, it selectively replaces the Li.