A comprehensive TEM study on the structure and interfacial characteristics of S’ and S phases in aluminum alloys

The structural and interfacial characteristics of S′ and S phases in an α-Al matrix aged at 523 K were investigated using transmission electron microscopy, diffraction pattern simulation, and lattice misfit analysis. Both phases adopt the orientation relationship [100] S’/S // [100] Al, [010] S’/S // [02-1] Al, [001] S’/S // [012] Al. A key distinction was identified in the interfacial alignment: the (02-3) S plane is parallel to the (001) Al plane, whereas the (02-3) S′ plane exhibits a systematic angular deviation of ~2.5 ± 0.5ᵒ about (001) Al. Lattice misfit values were calculated to be 2.2% for S′ and 1.9% for S phases. These findings demonstrate that subtle deviations in the (02-3) S′ and (02-3) S interfacial orientations directly influence phase stability and morphology, offering new insights into the crystallographic relationship and transformation behavior of S′ and S precipitates, which can help guide the control of precipitation during heat treatment and alloy composition design for improving the mechanical performance of Al-Cu-Mg-(Si) alloys. • -TEM reveals identical orientation relationships for S’ and S precipitates in α-Al alloys • HRTEM shows a ~2.5ᵒ interfacial deviation of (02-3) S’ from (001) Al, unlike S phase • -TEM-based lattice misfit analysis links S’ higher strengthening than S phase