Abstract
The influence of preliminary heat treatment on the formation of a nanocrystalline (NC) structure and hardness of a 7xxx-type aluminum alloy is analyzed. Specimens, 20 mm in diameter and 1 mm thick, cut from a cast and homogenized ingot, after further quenching and artificial aging at 170 °C for 1 to 10 hours were severely deformed by high-pressure torsion (HPT) via 10 revolutions under a pressure of 6 GPa at room temperature. TEM analysis has shown that prior-HPT treatment formed, consequently, the structures with uni- and bimodal distribution of precipitates, differing in size, density and nature. In addition to coherent aluminides of Zr and Sc, ∼20 nm in diameter, which were present in the pre-quenched alloy, aging resulted in precipitation and coarsening of the main strengthening η (MgZn2) type phases. It has been found that the NC structure with a (sub)-grain size of ∼80 nm is developed via continuous dynamic recrystallization in the pre-quenched material only. This alloy behavior is attributed to the formation of plate-shape 11- phases, ∼5 to 15 nm long and one-order higher densities, than that of aluminides. The NC alloy demonstrated much higher hardness than pre-aged ones despite their strong dispersion hardening before HPT.
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