The age-hardenable Al–Mg–Si alloy system is strengthened by needle-shaped β” phase precipitates that develop during heat treatment. The phase is fully coherent with the Al matrix, and induces a considerable strain field in the Al matrix, which is key to its hardening property. The lattice parameters of β” have been measured as an average over many precipitates using electron diffraction , but never systematically for specific precipitate shapes. Transmission electron microscopy is ideal for this task, although one must take care to avoid image distortions when accurate distances are to be measured. Multi-exposure scanning transmission electron microscopy (STEM) image series were processed to correct drift and scan distortions . With such datasets, we can investigate the detailed atomic structure and ensure the precipitates to be defect-free, and at the same time measure relative atomic distances accurately. This enables the measurement of the misfit between the β” phase and the Al matrix, for which we employ the geometric phase analysis (GPA) technique . As the misfit in the long direction of the particles is negligible, we concentrate on the a (β’’) and b (β’’) directions, which lie in the cross-sectional plane.
24 precipitates with 15 distinct geometries were investigated . Fig. 1 shows images of representative particles together with the average measured misfits for each geometry. The misfit is found to vary significantly between particles, in the range 1–7%. The misfit in either direction is inversely proportional to the particle width in that direction, as seen in Fig. 2(a). This works as an energy minimization mechanism, as the strain in the Al matrix is distributed evenly in its cross-sectional plane. The elasticity of the β” phase is similar to that of the Al matrix, which is in accordance with related literature . The relative β”–Al misfit area is independent on particle shape, as shown in Fig. 2(b), and has a value of about 7%. A particle will grow during heat treatment until the strain from these extra 7% becomes too much to handle, and the particles loses coherency.
The measured misfits give a good match to reported density functional theory (DFT) simulations for a β” phase with composition Mg5Al2Si4 within a range of precipitate geometries [6,7]. Two other likely compositions were tested, but were found not to fit with the shape-misfit relationship of the experimental data. The conclusions in this study will contribute to improving models for precipitation hardening from atomic to macroscopic scales.
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The authors would like to thank the Research Council of Norway (RCN) for funding of the FRINATEK project “Fundamental investigations of precipitation in the solid state with focus on Al-based alloys”. The TEM work was carried out on the NORTEM instrument JEOL ARM-200F, TEM Gemini Centre, Norwegian University of Science and Technology (NTNU), Norway. Acknowledgements are due to Dr. Lewys Jones for assistance with image processing.
To cite this abstract:Sigurd Wenner, Randi Holmestad; Misfit of coherent precipitate phases in Al alloys measured by scanning transmission electron microscopy. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/misfit-of-coherent-precipitate-phases-in-al-alloys-measured-by-scanning-transmission-electron-microscopy/. Accessed: May 24, 2019
EMC Abstracts - https://emc-proceedings.com/abstract/misfit-of-coherent-precipitate-phases-in-al-alloys-measured-by-scanning-transmission-electron-microscopy/