Aging Behavior of Al 6061 Alloy Processed by High-Pressure Torsion and Subsequent Aging

A process to achieve strengthening in an Al 6061 alloy by grain refinement to ~200 nm using high-pressure torsion (HPT) and fine precipitation using aging treatment is studied. It is shown that although aging of the HPT-processed sample is effective for extra strengthening of the alloy, the imposed shear strain and the aging temperature should be selected carefully. The HPT processing after 5 turns leads high saturation hardness and tensile strength of 163 Hv and 470 MPa, respectively. The hardness at the saturation level remains the same during aging at 373 K (100 °C), while the hardness decreases by aging at 423 K (150 °C). When the disks are processed for 0.75 turns (lower shear strains) and aged at 373 K (100 °C), the hardness increases above the hardness level at the saturation because of the formation of B′ and β′ precipitates. Quantitative analyses indicate that three major hardening mechanisms contribute to the total hardening: grain boundary hardening through the Hall–Petch relationship, dislocation hardening through the Bailey–Hirsch relationship and precipitation hardening through the Orowan relationship. This study shows that the contribution of different strengthening mechanisms can be estimated using a linear additive relationship in ultrafine-grained aluminum alloys.