Precipitation behaviour of Al-Zn-Mg-Cu alloy and diffraction analysis from η′ precipitates in four variants

Age hardening at 160 °C was performed in an Al-6.0 wt.%Zn-2.3 wt.%Mg-1.8 wt.%Cu-0.1 wt.%Zr alloy. It was found that metastable η′ phases played a very important role in controlling this behaviour. The alloy reached the 193 HV peak hardness aged for 6 h. Transmission electron microscopy was used to characterise the morphology of η′ phases as main strengthening precipitates, and then the high-resolution transmission electron microscopy, combining with Transition Matrix calculation, was used to analysis the orientations and electron diffraction patterns of η′ precipitates. It was found that the η′ phases in four variants had only three different zone axis: [24̄23]_η′, [101̄0] _η′ and [2̄42̄3]_η′, parallel to the [1 1 0]_Al direction of Al matrix when they were precipitated. By integrating the atomic coordinates, structure factors and double diffraction effect, a new diffraction patterns model under the [1 1 0]_Al zone axis was established, which was good in agreement with the experimental result. Simultaneously, Moiré fringes analytical technique was also used to quantitatively verify the lattice parameters and orientation variants of η′ precipitates, and demonstrated that Moiré fringes with about 0.7 nm spacing were the results of the interaction between the 0004 _η′/0004̄_η′ diffraction of η′ precipitates in variants 3/4 under the [101̄0] _η′ zone axis and the 11̄1_Al/11̄1 _̄Al of Al matrix under the [1 1 0]_Al zone axis. Furthermore, the high-resolution transmission electron microscopy image simulation for Moiré fringes of η′ precipitates in the Al matrix presented a good match with the experimental result.