The effect of trace additions of sn on precipitation in Al-Cu alloys: An atom probe field ion microscopy study

We have studied the early stages of θ′ precipitation in an Al-Cu-Sn alloy by atom probe field ion microscopy (APFIM) and transmission electron microscopy (TEM). Clusters of Sn atoms were observed in as-quenched samples, and their formation clearly precedes the formation of the θ′ phase. Transmission electron microscopy also revealed that elevated temperature aging leads to the formation of spherical particles. Atom probe analysis and microbeam electron diffraction suggested that these particles were β-Sn (I4₁/amd, α = 0.583 nm, and c = 0.318 nm). The TEM observations showed that θ′ nucleated heterogeneously on these particles. We have also examined the θ′/α interface following further precipitate growth to check for possible Sn segregation. Atom probe analysis of both the broad face and the narrow rim of the platelike θ′ precipitates has shown no evidence of Sn segregation at θ′/α interfaces. It was also found that cold work prior to aging inhibits the formation of Sn particles resulting in a lower number of these types of nucleation sites for the θ′ phase. Atom probe analysis has also revealed solute depletion at grain boundaries during the early stages of aging in the Al-Cu-Sn alloy, and this, in conjunction with vacancy depletion, explains the formation of narrow precipitate-free zones observed following further aging.