The effect of Zn/Mg ratio on the precipitation of T’ in Al Zn Mg Cu alloys

Recently, the T’ phase, the precursor to the equilibrium phase T- , has gained interest in the community. This is partly due to its significant contribution to strength in the novel cross-over alloys. However, the relation between the alloy composition and its occurrence is not fully understood. In this work, the effect of varying Zn/Mg ratio on precipitation of T’ in three alloys at peak-aged hardness was investigated by scanning precession electron diffraction and high-angle annular dark-field scanning transmission electron microscopy. We report on a novel application of 4D-STEM phase mapping on the system to estimate phase fractions of T’ and various η -type phases in two samples with different Zn concentrations. A correlated diffraction–imaging study was conducted in order to verify results from the phase maps. An increase in both Vickers hardness and precipitate density with increasing Zn content was observed. The three samples all exhibited a dense distribution of highly disordered, metastable η -type precipitates, and the disorder was characterized in terms of disruptions in the stacking sequences of the rhombohedral structural units of the η phase. Through imaging of particles within regions from which phase maps were acquired, and inspection of corresponding diffraction patterns, the designation of T’ particles in the phase maps was verified. The occurrence of T’ was observed to be highest for the sample with the highest Zn content, and a decrease in the number of highly disordered η precipitates with increasing Zn content was observed. • Scanning electron diffraction based phase mapping gives novel insight into precipitation characteristics. • Diffraction combined with high-resolution imaging allows for more detailed precipitate statistics, and novel insight into T’ precipitation. • T’ forms preferentially in the alloy sample with the highest Zn content. • A large number of η precipitates are highly disordered in the low-Zn alloy.