Analytical Electron Tomography of Aluminum Alloys with Nano-Precipitates and its Physical Challenges
A. Orthacker
Graz Centre for Electron Microscopy and Institute for Electron Microscopy and Nanoanalysis, Graz University of Technology
11:40 - 12:00 Monday 28 September 2015 Hörsaal I, Alte Technik For a thorough understanding of a material, investigations at the nanoscale are often essential. Analytical techniques like electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDXS) in scanning transmission electron microscopy (STEM) can reveal important chemical information necessary for the development and improvement of high-tech materials. The integrative character of the signal acquired through transmission, however, might hide important structural details of the material, relevant for its properties. Those details can be revealed through electron tomography, where the data is acquired at different tilt angles and, after alignment, reconstructed to form a full 3D model of the material under investigation. The combination of both techniques, analytical STEM and tomography, gives full insight into structure and composition of a material. This novel technique is not yet well established and various experimental challenges remain, especially if a quantitative analysis of the material is the ultimate goal.
In this study an aluminum alloy containing scandium (Sc) and zirconium (Zr) rich nano-precipitates was investigated at different stages of ageing. High resolution STEM and analytical EELS and EDX tomographic investigations were carried out. The resulting 3D elemental reconstructions deliver otherwise inaccessible information on the sample’s chemistry and structure. Additionally EDX-spectra were reconstructed channel by channel, resulting in a data cube, where each pixel contains a whole spectrum. By using an HAADF reconstruction to create a mask the pixels of the core and the shell region of a nano-precipitate were extracted and summed to obtain pure spectra of those regions, thus overcoming the intrinsic limitations arising from the integrative character of analytical STEM.
To approach quantitative analytical tomography also the influence of channeling on the detected signal intensities needs to be understood, to make sure its effect on elemental quantification results is either incorporated in the analysis or the experimental conditions are chosen to minimize such effects. Therefore a comparative study was performed on a rod shaped sample with an off-axis nano - precipitate. Reconstructions from different sets of tilt series performed using different experimental conditions were conducted and compared to investigate the reproducibility of analytical results under different channeling conditions.
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Institute of Solid State Physics
