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 similar to 250 nm and to an increase in hardness up to a saturation value at equivalent strains above similar to 100. There is a deviation in the Hall-Petch relationship at grain sizes smaller than similar to 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. (C) 2011 Elsevier B.V. All rights reserved.