Nucleation of Omega phase in an Al-Cu-Mg alloy containing small additions of Ag

The role of small additions of Ag in increasing the nucleation frequency of a plate shaped precipitate having {111}(Al) habit plane, designated Omega, in an Al-Cu-Mg alloy has been investigated using transmission electron microscopy (TEM), electron diffraction and energy dispersive X-ray spectroscopy. The results provide for the first time direct evidence showing association of Omega plates with small Ag-rich precipitates of the constituent binary Al-Ag system during the early stages of artificial aging. Given the strong attraction between Ag and Mg owing to the large difference in electronegativity between them, Mg simultaneously combines with the Ag-rich precipitates formed on the {111}(Al) matrix planes, and makes such sites suitable for the heterogeneous nucleation of Omega phase.