Structural impact on the Hall-Petch relationship in an Al-5Mg alloy processed by high-pressure torsion

The evolution of microstructure and microhardness was studied in a commercial 5483 Al-5Mg alloy processed by high-pressure torsion (HPT) under a pressure of 6.0 GPa up to 10 turns. Significant grain size refinement was observed even after 1/4 turn, and additional processing led to a further grain size reduction and a shift in the distribution of grain boundary misorientation angles towards higher values. An essentially fully homogeneous microstructure was reached after 10 turns with a final grain size of ~70 nm, a saturation Vickers microhardness of Hv≈240 which was attained at and above equivalent strains of ~150, a relatively narrow grain size distribution and a fraction of ~80% of high-angle grain boundaries. Analysis shows the Hall-Petch plot deviates from the conventional linear relationship for samples processed through small numbers of turns, but after 3 or more turns there is a direct correlation between the results obtained in HPT processing and coarse-grained samples.