Revisiting the precipitation sequence in Al–Zn–Mg-based alloys by high-resolution transmission electron microscopy

The precipitation process in Al–Zn–Mg-based alloys has been revisited using high-resolution transmission electron microscopy. It was found that there is an intermediate phase between the η′ phase and the η phase. This intermediate phase with a hexagonal structure can be termed the η precursor, since its a lattice parameter is the same as that of the η′ phase and its c lattice parameter is approximately the same as that of the η phase. Transformation from the η′ phase to the η phase occurs smoothly with this η precursor existing between them.