Li, Ag And Sc On The Microstructure And Properties Of Al-cu-mg Alloy,

The present work deals with the effects of individual and combined additions of Li. Ag . Sc alloying elements on the microstructures and mechanical properties of the low Cu/Mg ratio and high Cu/Mg ratio Al-Cu-Mg alloys. Various testing methods were used to examine the effects of these elements, including Vickers hardness and ambient tensile properties measurements, Transmission electron microscopy (TEM) and Scanning electron microscopy (SEM) observations. The main conclusions are as folio wings:1.In the Al alloy, the Ag addition stimulated the formation of Ag-Mg co-clusters, which further promoted the homogeneous and dispersed precipitation of Z phases. However, the combined addition of Ag and Li produced minor coarse Z phases, due to the strong interaction between Li and Ag atoms. In the A5 alloy with high Li content, numerous 5'phases were precipitated at the initial stage of aging; numerous Z phases were precipitated at the prolonged aging. Due to the combined strengthening effect of 8'phases and Z phases, the A5 alloy had the higher strength.2.Two hardness peaks occurred on the ageing curves of the A5 alloy aged at 170 and 200 . The former peak was contributed to the precipitation of 5'phase, and the later to the formation of Z phase.3.Compared with the A6 alloy with only Ag content, the combined addition of Ag and Li increased the strength and ductility of the A3 alloy and retarded over-ageing. This difference was contributed to the addition of Ag, which changed the species of the strengthening phases. The strengthening phases were S' phases and small account of 5' phases in the A6 alloy. The main strengthening phases in the A3 alloy consisted of Z phase and 8' phase.4.In comparison with the A6 alloy, the combined addition of Li and Sc elements apparently improved mechanical properties of the A7 alloy. The addition of Sc promoted the homogeneous precipitation of 8' phase at the initial stage of ageing, retarded coarsening of 8' phase, stimulated the homogeneous precipitation of S' phase and inhibited over-ageing of the A7 alloy.5.Despite of the homogeneous precipitation of Z phase in the Al alloy containing Ag, the A6 and A7 alloys without Ag addition also consisted of Z phase at the over-ageing stage; therefore, Ag is not the necessary condition to the formation of Z phase.6.In the D6 alloy containing Sc, the finely-dispersed A^Sc particles provided the preferential nucleation sites for the formation of 9'phase, and also stimulated the precipitation of 0' phase in the D7 alloy containing Ag and Sc. Due to the addition of Ag, the interaction between Ag and Mg atoms promoted the precipitation of Q phase and increased the peak hardness of the D7 alloy.