Atom probe study of the precipitation process in AlCuMgAg alloys

Abstract

The compositions of the G.P. zones, θ″, θ′, and Ω phases in alloys based on the AlCuMgAg system aged under various conditions have been examined by atom probe field ion microscopy. At the peak hardness condition, the composition of θ′ was determined to be Al 2Cu without any partitioning of Ag and Mg. Under the same aging condition, the Ω phase was found to contain a similar amount of Cu as θ′ together with significant levels of Ag and Mg at the α/Ω interfaces. In order to understand the nucleation stage of the Ω phase, the precipitation process during low temperature aging was also studied. By the combination of TEM and FIM observations, it was found that the precipitation sequence at 130°C was supersaturated α → G.P. zones → θ″ →Ω. Uniform nucleation of the Ω phase was observed on the {111} planes independent of the θ″ precipitates on the {001} planes at 130°C at almost the same stage. However, the Ω phase was not observed on aging at 100°C, even after 500 h, although Mg and Ag were detected in some G.P. zones formed at this temperature. The θ″ phase did not appear to contain Mg and Ag, while the Ω phase was found to contain Ag and Mg in this stage. In the as-quenched state, no evidence for co-clustering of Mg and Ag was found.