Abstract
The morphological evolution of the precipitates in Al–7.4Zn–1.7Mg–2.0Cu (wt%) alloy was studied by high-resolution transmission electron microscopy (HRTEM). Statistics reveal that the hardness of the alloy changes accordingly with the change of the average thickness–diameter ratio of precipitates. The GPII zones are mainly responsible for the first and also the highest hardness peak. They grow in diameter and keep 7-atomic-layer in thickness. Once the thickness changes, the phase transformation from GPII zone to η′ or η-precursor would occur. The resultant metastable η′ and η-precursor precipitates grow in both diameter and thickness, but much faster in the former. After the first hardness peak, the metastable η′ precipitates and η-precursor, coexisting with part of GPII zones, are counted as the main hardening precipitates.
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