Internal co-precipitation in aged Al-1.7Cu-0.3Mg-0.1Ge (at.%) alloy

A trace (0.1 at.%) addition of Ge to a base Al-1.7Cu-0.3Mg alloy resulted in considerable refinement of the θ′ (Al2Cu) precipitate dispersion as well as stimulating the formation of 〈0 0 1〉α lath-shaped precipitates in a fine and uniform distribution. These precipitates contain mainly Ge and Mg. The formation of the S(Al2CuMg) and σ(Al5Cu6Mg2) phases was suppressed, presumably due to the stronger interaction between Ge and Mg over Cu and Mg. Significantly, the trace addition of Ge produced an internal structure within the θ′ precipitate phase with its habit plane parallel to the {1 0 1}θ′ planes. Our investigations reveal that this unusual internal structure is secondary precipitation within θ′ precipitation of a Mg-Ge rich phase thought to be an allomorph of Mg2Ge, and this influences both the nucleation and growth of θ′ precipitates. This internal structure is designated as K phase, possessing a close packed hexagonal structure (a = 0.405 nm, c = 0.607 nm) and oriented such that, (0 0 0 1)K//(1 0 1)θ′ and 211¯0]K//[1 0 0]θ′. Prolonged ageing at 200 °C led to replacement of the 〈0 0 1〉α lath-shaped precipitates through their dissolution and reprecipitation of a more stable form of Mg2Ge (face-centred cubic, Fm3¯m, a = 0.638 nm).