Effect of precipitates on nanostructuring and strengthening of high-strength aluminum alloys under high pressure torsion

Effects of secondary phases on the development of a nanocrystalline structure under severe plastic deformation (SPD) of two high-strength Al-Zn-Mg-Cu alloys with different additions of transition metals (TM) and on parameters of their hardness and tensile strength were investigated. The variety of heterogeneity of the alloy initial structure was conditioned by changes of sizes, densities and volume fractions of precipitates of TM aluminides and main strengthening phases, owing to alloying and heat treatment. Prior SPD the homogenized ingots were solution treated, water quenched and annealed in the temperature range of 170-250 degrees C to different times. Therewith in addition to coherent disk-shape 10-40 nm in diameter TM aluminides in the pre-quenched condition, further annealing led to formation of zones and precipitates of the main strengthening phases with equivalent diameter from several to about hundred nanometers. The disk-shape samples were further SPD processed by high-pressure torsion (HPT) via 10 revolutions under pressure of 6 GPa at room temperature. It was found that HPT of both alloys in the pre-quenched state, having nanosized precipitates of TM aluminides only, resulted in the most developed nanostructure with a (sub) grain size of about 80 +/- 20 nm, and their maximum hardening. Straining of the pre-aged alloys, containing precipitates of the both types of fewer sizes and an order higher total density, on the contrary, completely suppressed grain refinement. Development of a work hardened, non-recrystallized structure was accompanied by the hardness and yield strength loss up to 25%. Preliminary overaging, leading to a strong coarsening and reduction in densities of main strengthening phases, activates nanostructuring of the alloys matrix. However, it was accompanied with formation of a bit coarser (sub) grains and demonstration of less hardness and strength than in the pre-quenched condition. Nature of the alloys structure and strength evolutions, depending on processing regimes and heterogeneity of the initial structure, have been discussed in details. (c) 2018 Elsevier B.V. All rights reserved.