THREE DIMENSIONAL ATOM PROBE ANALYSIS OF PRE-PRECIPITATE CLUSTERING IN AN Al-Cu-Mg-Ag ALLOY

Trace element additions in age hardening aluminum alloys often modify precipitation products and kinetics. In the case of Al-Cu alloy, combined addition of trace amount of Mg and Ag leads to precipitation of fine and uniform dispersions of plate-like Ω phase on the {111}Al planes in addition to the θ′ precipitates on the {001}Al planes [1,2]. The Ω phase shows high thermal stability and resistance to coarsening compared with the θ′ phase. Thus Al-Cu based alloy with trace additions of Ag and Mg has improved high temperature strength [3]. The structure of the Ω phase was studied by Knowles and Stobbs [4] and Muddle and Polmear [5], and it was concluded that the Ω phase has a face centered orthorhombic structure. More recently Garg and Howe [6] suggested that the Ω phase formed at 250°C had a tetragonal structure with a = b = 0.607 and c = 0.487 nm using convergent beam electron diffraction, and suggested that Ω is a modified form of the equilibrium phase in Al-Cu binary alloy, θ.