On the S-phase precipitates in 2024 aluminum alloy: An atomic-scale investigation using high-angle annular dark-field scanning transmission electron microscopy

The present article reports on a comprehensive atomic-scale investigation on S-series precipitates in 2024 aluminum alloy. Cs-corrected high-angle annular dark-field scanning transmission electron microscopy is applied to reveal the fine atomic-scale structure of precipitates at early ageing state. Geometrics phase analysis is used for elucidating the induced strain field from precipitates. The precipitate sequence of S-series precipitates in 2024 Al alloy is identified as follows: super saturated solid solutions (s.s.s.s.) → clusters (GPB zone) → S′ phase → S phase. The interfaces between precipitates acting as precursor of S′ phase are well characterized. One typical characteristic of S-series phase precipitates is the coexistence of clusters and subsequent metastable phases. Transformation of metastable phases is characterized. Corresponding hardness structure relationship is revealed, and S′ phase is considered as the key strengthening structure in S-series precipitates in 2024 Al alloys.