Ultrafine grains and the Hall–Petch relationship in an Al–Mg–Si alloy processed by high-pressure torsion

Experiments were conducted to evaluate the evolution of hardness and microstructure in a commercial Al–0.6% Mg–0.4% Si alloy during processing by high-pressure torsion (HPT). The HPT was conducted under a pressure of 6.0 GPa and disks were torsionally strained to a maximum of 20 turns. It is shown that processing by HPT leads to microstructural refinement with an average grain size of ∼250 nm and to an increase in hardness up to a saturation value at equivalent strains above ∼100. There is a deviation in the Hall–Petch relationship at grain sizes smaller than ∼500 nm and this is consistent with an earlier suggestion that a breakdown may occur if there is an easy movement of the extrinsic dislocations in the non-equilibrium grain boundaries introduced by HPT processing.