Nucleation and growth of θ′ precipitation in Sn-modified AlCu alloys: APFIM/TEM observations

We have used atom probe field ion microscopy, together with transmission electron microscopy to follow the precipitation in an Al71.7Cu0.01Sn alloy (at%). We have detected fine clusters of Sn atoms in the as-quenched material. The formation of these clusters clearly precedes the formation of the θ′ phase. Moreover, transmission electron microscopy reveals that further aging leads to the formation of spheroidal particles, which atom probe analysis suggests are composed of Sn. It was observed that θ′ nucleated on these particles. We have also examined the θ′/matrix interface following further precipitate growth to check for Sn segregation as proposed in previous work. Probing of both the broad face and the rim of the plate-like θ′ precipitates has not shown a significant presence of Sn. These results are discussed in terms of previous studies and the factors likely to be involved in the nucleation of θ′ in this and related systems.