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

Age hardening at 160 degrees C was performed in an Al-6.0 wt.%Zn-2.3 wt.%Mg-I.8 wt.%Cu-0.1 wt.%Zr alloy. It was found that metastable 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 iy 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 eta' precipitates. It was found that the eta' phases in four variants had only three different zone axis: [2 (4) over bar 23](eta'), [10 (1) over bar0] T and [(2) over tilde4 (2) over bar3](eta'), parallel to the [110](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 [110](Al) zone axis was established, which was good in agreement with the experimental result. Simultaneously, Moire fringes analytical technique was also used to quantitatively verify the lattice parameters and orientation variants of eta' precipitates, and demonstrated that Moire fringes with about 0.7 nm spacing were the results of the interaction between the 0004(eta')/4000 (4) over bar (eta'), diffraction eta' of Ti' precipitates in variants 3/4 under the [10 (1) over bar0] y zone axis and the 1 (1) over bar (Al)/1 (1) over bar(1) over bar (Al) of Al matrix under the [110](Al) zone axis. Furthermore, the high-resolution transmission electron microscopy image simulation for Moire fringes of eta' precipitates in the Al matrix presented a good match with the experimental result. (C) 2014 Elsevier B.V. All rights reserved.