The crystal structure of the β'-phase including Ag in Al-Mg-Si-Ag alloy

In the present work, β' phase in alloys Al -1.0 mass% Mg ₂Si -0.5 mass% Ag (Ag-addition) and Al -1.0 mass% Mg ₂Si (base) was investigated by high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) to understand the effect of Ag addition. The β' phase is rod-shape with the longest direction parallel to 〈001〉 _Al. HRTEM images and SAED patterns obtained along the direction were similar for the β' phase in both alloys. The unit cell of β'-phase in Ag-addition alloy is hexagonal with the same c-axis dimension as the Ag-free β', but shorter a-axis. Ag was found in the composition of the rod-shaped precipitates in Ag-addition alloy by energy dispersive X-ray spectroscopy (EDS). In addition, the distribution of Ag was investigated by energy filtered mapping and high annular angular dark field scanning transmission electron microscopy (HAADF-STEM). The Ag-containing atomic column was observed in every β' unit cell, and the unit cell symmetry is slightly changed as compared with the Ag-free β'. The Ag-containing β' rods have complicated domain structures. The interfaces of these particles are enriched with Ag atoms that occupy the lattice positions in the Al matrix. The occupancy of the Ag-containing atomic columns seem to vary both inside particles, as well as at the interfaces.