Transmission electron microscopy analysis of grain boundary precipitate-free-zones (PFZs) in an AlCuSiGe alloy

We have characterized the elevated temperature (190 °C) precipitation sequence near the grain boundaries of an AlCuSiGe alloy, comparing these results to the binary AlCu and the ternary AlSiGe. In the quaternary alloy, there is a graded microstructure that evolves with increasing distance from the boundaries, which is generally a superposition of the precipitate-free-zones (PFZs) in the binary AlCu and in the ternary AlSiGe. After aging for 3 h, this graded area consists of an approximately 140 nm wide region that is entirely precipitate free, followed by a 400 nm wide region that is denuded of Si–Ge and θ′ precipitates. Rather than containing the (Si–Ge)–θ′ pairs observed in the bulk, this 400 nm wide region contains only homogeneously nucleated θ′′. Only in the overaged condition (144 h) are the near grain boundary θ′′ replaced by a coarse distribution of large plate-like θ′. In the alloys, the solute depleted zones are much narrower than the total length of the PFZ. For example, in both AlCu and AlCuSiGe, the Cu depleted zone is only 30 nm wide. This underscores the need for vacancies during precipitation of not only θ′ and Si–Ge, but of θ′′ as well.