Effect of secondary phases on nanostructuring and hardness of severely deformed high-strength aluminum alloy

Abstract

Influence of precipitates, formed upon preliminary heat treatment, on the development of a nanocrystalline (NC) structure and hardness of high-strength aluminum alloy with Zr and Sc additions under severe plastic deformation was studied. The samples cut from homogenized ingot were processed by high-pressure torsion (HPT) via 10 revolutions under pressure 6 GPa at room temperature. Prior to HPT, the alloy was solution treated, water quenched and annealed for 5 hours in the temperature range of 170-250°C to vary structural heterogeneity via changing the sizes and densities of precipitates. In addition to coherent disk-shape aluminides of transition metals about 25 nm in diameter (so-called dispersoids) in pre-quenched alloy, further annealing led to precipitation of the main strengthening η-type (MgZn) phases with equivalent diameter from 10 to 200 nm. The most developed NC structure with a (sub)grain size near 80 nm was processed in the pre-quenched alloy and resulted in its abnormally high hardness. HPT of the alloy pre-annealed at 170°C which contained η-phase precipitates of lesser size and an order of magnitude higher densities than that of dispersoids, on the contrary, produced a completely non-recrystallized structure with an up to 15 % reduction in hardness owing to the suppression of grain refinement. Increasing the temperature of annealing led to coarsening and reduction in density of η-phases, intensifying the recrystallization. However, all pre-annealed NC states demonstrated even less hardness as their work hardening could only partially compensate the alloy softening due to η-phase coagulation.