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

M.V. Markushev1, Burdastykh Yulia Yulia Leonidovna1, S.V. Krymskiy1, O.S. Sitdikov1
1Institute for Metals Superplasticity Problems RAS, Khalturin St. 39, 450001 Ufa
Abstract
Influence of precipitates, formed upon preliminary heat treatment, on development under severe straining of nanocrystalline (NC) structure and hardness of 7xxx - type aluminum alloy with Zr and Sc additions was investigated. The samples cut from homogenized ingot were processed by high-pressure torsion (HPT) via 10 revolutions under 6 GPa at room temperature. Prior HPT, the alloy was solution treated, water quenched and annealed for 5 hours in the temperature range of 170-250 0C to change its structural heterogeneity - size and density of precipitates of different origin. In addition to coherent disk-type aluminides of transition metals ~25 nm in diameter (so-called dispersoids), that were in pre-quenched alloy, annealing led to precipitation of the main strengthening  (MgZn) - type phases with equivalent diameter from ~10 to 200 nm. Most highly developed NC structure with a (sub)grain size of ~80 nm was processed in the pre-quenched alloy and resulted in its abnormally high hardness. HPT of the pre-annealed at 170 0C alloy with -phase precipitates of less size and one order higher densities than that of dispersoids, on the opposite, produced completely non-recrystallized structure with near 15% reduced hardness owing to the grain refinement suppression. Increasing the temperature of annealing led to coarsening and less densities of - phases, intensifying nanostructuring. However, all pre-annealed NC states demonstrated minimum hardness as their work hardening did not compensate the softening due to -phase coagulation.
Received: 10 February 2017   Revised: 21 March 2017   Accepted: 29 March 2017
Views: 31   Downloads: 15
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