Severe plastic deformation (SPD) techniques are known to promote exceptional mechanical properties due to their ability to induce significant grain and cell size refinement. Cell and grain refinement are driven by continuous newly introduced dislocations and their evolution can be followed at the earliest stages of plastic deformation. Pure metals are the most appropriate to study the early deformation processes as they can only strengthen by dislocation rearrangement and cell-to-grain evolution. However, pure metals harden also depend on texture evolution and on the metal stacking fault energy (SFE). Low SFE metals (i.e., copper) strengthen by plastic deformation not only by dislocation rearrangements but also by twinning formation within the grains. While, high SFE metals, (i.e., aluminium) strengthen predominantly by dislocation accumulation and rearrangement with plastic strain. Thence, in the present study, the early stages of plastic deformation were characterized by transmission electron microscopy on pure low SFE Oxygen-Free High Conductivity (OFHC) 99.99% pure Cu and on a high SFE 6N-Al. To induce an almost continuous rise from very-low to low plastic deformation, the two pure metals were subjected to high-pressure torsion (HPT). The resulting strengthening mechanisms were modelled by microstructure quantitative analyses carried out on TEM and then validated through nanoindentation measurements.