Quantitative Investigation of Precipitates in a High-Zinc Al–9.54Zn–2.10Mg–1.69Cu Alloy with Various Typical Tempers

In order to analyze single stage ageing behavior of a high-zinc Al–9.54Zn–2.10Mg–1.69Cu alloy, the microstructure of the alloy subjected to T6, T76 and T77 states are investigated via transmission electron microscopy (TEM) combined with high-resolution transmission electron microscopy (HRTEM) attached to it. Under the premise in precipitate observations, diameter distributions and average diameter size of precipitates are deduced from Bright-Field TEM (BF TEM) images projected along \( \left\langle {110} \right\rangle_{\text{Al}} \) orientation with the help of an image processing. The results indicate that the main precipitates are GPII zone and η′ phase in the T6 and T77 alloys while η′ and η phase in the T74 alloy. The Bright field TEM observations reveal that the matrix precipitates for the T6 and T77 alloys have small size and dispersive distribution while that for the T74 alloy has big size and sparse distribution. Quantitative precipitate characteristics including diameter distribution and average diameter size have been gained by an image processing relying on BF TEM images projected along \( \left\langle {110} \right\rangle_{\text{Al}} \) orientation. The results reveal that the T6 and T77 alloys have more than a half percentage of precipitates with a size less than 2 nm while the T77 and T74 alloys have broad precipitate distribution range till 14 and 16 nm, respectively. The grain boundary precipitates (GBPs) for the T6 alloy have continuous distribution with small size while that for the T74 and T77 alloys distribute intermittently with big size.