Interaction between θ'-Al<sub>2</sub>Cu and Al<sub>3</sub>Sc in the Heterogeneous Precipitation in an Al-Cu-Sc Alloy

Abstract

Interaction between the plate-like θ'-Al 2 Cu and the spherical Al 3 Sc phase in mutual heterogeneous precipitation at 623K in an Al-2.5mass%Cu-0.23mass%Sc alloy was investigated on the basis of TEM observation. Growth rate of the θ'-Al 2 Cu phase is much larger than that of the Al 3 Sc phase at 623K. As the θ'-Al 2 Cu precipitates grow in the early stage of the aging, the θ'-Al 2 Cu/matrix interface changes from of coherent to semi-coherent nature, until the interface dislocations are induced. With further increasing aging time, the Al 3 Sc-PFZ is formed in the vicinity of the θ'-Al 2 Cu phase, probably due to the migration of Sc atoms in the matrix to the interface dislocations. After a prolonged aging, the θ'-Al 2 Cu precipitates become emaciated while the equillibrium θ-Al 2 Cu phase begins to form and the θ'-Al 2 Cu/matrix interfaces become coherent with the matrix again. Then, the Al 3 Sc phase begins to form at the θ'-Al 2 Cu/matrix interface. These observations imply that the strain fields of the interface dislocations as well as the coherency strain fields at the θ'-Al 2 Cu/matrix interface plays an important role in the process of heterogeneous precipitation of the Al 3 Sc phase on the θ'-Al 2 Cu phase. The Al 3 Sc precipitates with different size were obtained by preaging at 733K for different aging time, and the precipitation behavior of the θ'-Al 2 Cu on the Al 3 Sc was studied by aging at 623K. The heterogeneous precipitation of the θ'-Al 2 Cu phase on the Al 3 Sc phase was identified only where the Al 3 Sc particles grew to the critical diameter(∼40nm) with the formation of the interface dislocation at the Al 3 Sc/matrix interface. These observations suggest that the strain fields of the interface dislocations at the Al 3 Sc/matrix interface.