Fig. 1 Typical EBSD map showing the microstructure of the as-homogenized Al-1.0?wt% Mg alloy.

Fig. 2 Grain size reduction with increasing HPT revolutions.

Fig. 3 (a) TKD map showing the microstructure of Al-1.0?wt% Mg alloy after 1 HPT revolution, and (b) GND distribution map of the same zone.

Fig. 4 APT characterization of the Al-1.0?wt% Mg alloy after 1 revolution. (a) A reconstructed atom map of Al atoms (in blue) and Mg atoms (in red) in an analyzed volume. Select one box marked by the white dashed lines (20?nm?×?20?nm?×?50?nm) for a detailed view of the grain boundary and the Mg-depletion zone. A LAGB is distinguished by a small change through the pole of Al, which is marked by the white arrow. (b) 2D density plots of Al atoms in the slices marked in (a) by black rectangles. Different poles are visible in these plots. (c) and (d) 1D concentration profiles across Interface 1 and Interface 2, respectively, using a measurement bar with cross-section 20?nm?×?20?nm and a bin size 0.5?nm.

Fig. 5 APT characterization of Al-1.0?wt % Mg alloy processed by 10-turn HPT at room temperature. (a) A reconstructed atom map of Mg atoms (in red), (b) and (c) 1D concentration profiles across GB1 and GB3 in the analyzed volume, as marked by green arrows in (a) using a measurement bar with cross-section 10?nm?×?10?nm and bin size 0.5?nm.

Fig. 6 Schematic illustration of three mechanisms for formation of the Mg-depletion zone. (a) Vacancy assisted diffusion (b) GB migration (c) Solute drag by dislocations.