Deformation twins and stacking faults were observed in nanostructure Al-Mg alloys subjected to high pressure torsion.These observations are surprising because deformation twinnings have never been observed in their coarse-grained counterparts under normal conditions.Experimental evidences are introduced on non-equilibrium grain boundaries,deformation twinnings and partial dislocation emissions from grain boundaries.Some of these features can be explained by the results reported from molecular-dynamics simulations of pure FCC metals.Special emphasis is laid on the recent observations of high density hexagonal and rhombic shaped nanostructures with an average size of 3 nm in the Al-Mg alloys processed by high pressure torsion.A possible formation process of these nanostructures is proposed based on molecular-dynamics simulations. Deformation twins and stacking faults were observed in nanostructure Al-Mg alloys subjected to high pressure torsion. Deformations have been never observed because deformation twinnings have never been observed in their coarse-grained counterparts under normal conditions. Experimental evidences are introduced on non-equilibrium grain boundaries , deformation twinnings and partial dislocation emissions from grain boundaries. Home of these features can be explained by the results reported from molecular-dynamics simulations of pure FCC metals. Specific emphasis is laid on the recent observations of high density hexagonal and rhombic shaped nanostructures with an average size of 3 nm in the Al-Mg alloys processed by high pressure torsion. A possible formation process of these nanostructures is proposed based on molecular-dynamics simulations.