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2 Décembre 2011 - C. Mariora - Structural investigations of the main hardening phase b” in Al-Mg-Si alloys by Transmission Electron Microscopy (TEM), Atom Probe Tomography (APT) and first principles calculations

par LABGPM - publié le

2 Décembre 2011 - 10h30

Title : Structural investigations of the main hardening phase b” in Al-Mg-Si alloys by Transmission Electron Microscopy (TEM), Atom Probe Tomography (APT) and first principles calculations

Abstract : b” phase is always associated with peak hardness conditions of Al-Mg-Si alloys. Therefore considerable effort has been invested in solving its crystal structure and to produce industrial alloys that are optimised for this type of precipitate. The first breakthrough took place when using a combination of High Resolution TEM (HRTEM) and quantitative electron diffraction a model was proposed describing b” as monoclinic with C2/m space group and Mg5Si6 composition. However, APT studies indicated this phase was likely to contain 20% to 30% Al. This result was supported by first principles calculations showing that two structures with compositions Mg5Al2Si4 and Mg4Al3Si4 have lower formation enthalpies as compared to the initial Mg5Si6 model. The new structures are structurally identical with the initial model, but having one Si site replaced by Al, or the same Si site and in addition one Mg site replaced by Al. Quantitative electron diffraction proved insensitive to such substitutions due to the similarities in the scattering factors for Mg, Al and Si. With the development of more powerful TEM techniques such as aberration-corrected High Angle Annular Dark Field Scanning TEM (HAADF-STEM) images can be recorded that contain Z-contrast in addition to spatial resolutions towards 0.1 nm. Such images from b” precipitates have been recorded in Rouen and are in a process of analysis in order to determine if the above-mentioned compositional changes can be detected. Increased computing power has led to the possibility of relaxing b” units of different compositions in an Al block. Work is under progress that indicates composition variations inside the same precipitate as function of distance from the core to the interface, due to small size distortions caused by strain. Therefore, despite being under study for more than 14 years the debate about b” is far from over. Improved models will hopefully lead to the possibility of simulating strain fields and the calculation of macroscopic hardness for a given precipitate microstructure.