The evolution of precipitates in an Al–Zn–Mg alloy

The precipitation sequence in Al–Zn–Mg alloys has been subject to many revisions over the years as more structural details of the early-stage Guinier-Preston (GP) zones and precipitates have been uncovered. To further investigate this, a 7003 aluminium alloy naturally aged for one year was subjected to artificial ageing at 140 °C to investigate the evolution of early-stage precipitates. Scanning transmission electron microscopy was coupled with atom probe tomography and hardness measurements to characterise the precipitate structure and chemistry of the alloy at different stages in the heat treatment. The naturally aged condition contained a dense population of GPI zones, co-existing with a smaller distribution of η′ precipitates. After artificial ageing for 10 min, the hardness decreased from 121 HV to 88 HV. The number densities of clusters were similar in the two conditions, while the average size of the clusters had increased during the first 10 min of artificial ageing. The average Zn/Mg ratio of the clusters decreased from 2.0 to 1.7 and the clusters no longer exhibited the GPI zone atomic structure, indicating that the GPI zones had dissolved. Concurrently, the fraction of precipitates having η′ structure increased from 2% to 18%. During further ageing, the hardness increased again, reaching a peak after 5 h. In this condition, η′ was the dominant phase, co-existing with η1, η2 and T′ phases. Atomically resolved scanning transmission electron microscopy images in addition to precession electron diffraction patterns of the disputed T′ phase are presented and compared to literature.