Significance of grain refinement on microstructure and mechanical properties of an Al-3% Mg alloy processed by high-pressure torsion

Significant grain refinement is attractive for improving mechanical properties at both ambient and elevated temperatures and improvements in the properties of light-weight metals has become indispensable for the practical and societal needs of materials selection. Experiments were conducted to examine the influence of grain refinement on the mechanical properties of an Al-3% Mg alloy processed though high-pressure torsion (HPT) at room temperature under a compressive pressure of 6.0 GPa for up to 10 turns. The hardness values from Vickers microhardness measurements demonstrated a strain hardening behavior with increasing torsional straining and a high level of homogeneous microstructure was achieved along the disk diameter after HPT for 10 turns. An X-ray diffraction analysis at the peripheral regions of the disks outside of the central areas showed an evolution towards a reasonably random texture together with increases in the dislocation density and lattice parameter of Al with increasing numbers of HPT revolutions. A maximum elongation of ∼430% was recorded in the alloy after HPT for 5 turns when testing at 673 K at 1.0 × 10⁻⁴ s⁻¹but there was evidence for grain growth during testing and the flow behavior was controlled by viscous glide. Analysis shows that the improved hardness through HPT is well expressed by the Hall-Petch relationship with only limited contributions from solid solution strengthening and precipitation hardening.