Structural features, microhardness, and mechanical properties of three binary Al-Mg alloys and a commercial AA5182 alloy subjected to high pressure torsion at room temperature were comparatively investigated using transmission electron microscopy, high-resolution transmission electron microscopy, and quantitative X-ray diffraction measurements. Average grain sizes measured by dark-field images are in the range 71-265 nm while the sizes of coherent domains decreased tremendously from 86 to 46 nm as the Mg content increased from 0.5 to 4.1 wt%. The average dislocation density in the deformed alloys is in the range 0.37 x 10(14)-4.97 x 10(14) m(-2). Both the microhardness and tensile strength of all the deformed alloys increased dramatically as compared to the undeformed counterparts. The yield strength with values ranging from 390 to 690 MPa in the deformed alloys is typically five to seven times higher than that of the same undeformed alloys. Calculations based on the Hall-Petch and Taylor equations suggest that the strengthening mechanisms contributing to the very high strength may depend not only on the conventional mechanisms of grain size strengthening and dislocation strengthening, but also on the additional mechanisms related to the contributions from stacking faults and nanotwins, and nonequilibrium GBs observed in the deformed alloys.