Interaction between theta'-Al_2Cu and Al_3Sc in the Heterogeneous Precipitation in an Al-Cu-Sc Alloy

摘要

Interaction between the plate-like theta' -Al_2Cu and the spherical Al_3Sc phase in mutual heterogeneous precipitation at 623K in an Al-2.5mass percent Cu-0.23mass percent Sc alloy was investigated on the basis of TEM observation. Growth rate of the theta'-Al_2Cu phase is much larger than that of the Al_3Sc phase at 623K. As the theta'-Al_2Cu precipitates grow in the early stage of the aging, the theta'-Al_2Cu/matrix interface changes from of coherent to semi-coherent nature, until the interface dislocations are induced. With further Increasing aging time, the Al_3Sc-PFZ is formed in the vicinity of the theta' -Al_2Cu phase, probably due to the migration of Sc atoms in the matrix to the interface dislocations. After a prolonged aging, the theta'-Al_2Cu precipitates become emaciated while the equillibrium theta-Al_2Cu phase begins to form and the theta'-Al_2Cu/matrix interfaces become coherent with the matrix again. Then, the Al_3Sc phase begins to form at the theta'-Al_2Cu/matrix interface. These observations imply that the strain fields of the interface dislocations as well as the coherency strain fields at the theta'-Al_2 Cu/matrix interface plays an important role in the process of heterogeneous precipitation of the Al_3Sc phase on the theta'-Al_2Cu phase. The Al_3Sc precipitates with different size were obtained by preaging at 733K for different aging time, and the precipitation behavior of the theta' -Al_2Cu on the Al_3Sc was studied by aging at 623K. The heterogeneous precipitation of the theta' -Al_2Cu phase on the Al_3Sc phase was identified only where the Al_3Sc particles grew to the critical diameter(approx 40nm) with the formation of the interface dislocation at the Al_3Sc/matrix interface. These observations suggest that the strain fields of the interface dislocations at the Al_3Sc/matrix interface.