| Effet of trace additions of Si,Ag on the microstructure and properties of low Cu:Mg ratio Al-Cu-Mg alloys have been investigated by means of tensile properties measurements,X ray diffraction,Scanning electron microscopy,Transmission electron microscopy observations and Computer Simulation. In order to design advanced Al-Li alloy, the effect of various Li contents on the microstructure and properties of Al-1.5Cu-4.0Mg-0.25Si alloy have been investigated. The main conclusions are as following:(1)The precipitation hardening response of the low Cu:Mg ratio Al-Cu-Mg alloys is enhaced by the additions of Ag and Si. The rate of age-hardening is increased by the addition of Ag, and reduced by the addition of Si.(2)Trace addition of Si does result in a lower density of defect in the as quenched microstructure, and the formation of a fine and uniform dispersion of GPB zones. Addition of Si stimulates the precipitation of S phase, and restrains the formation of T phase. The precipitation of T-phase is completed suppressed when the Si content reach to 0.5wt%.(3)Trace additions of Ag leads to the formation of a large number of Z phase. The precipitation of Z phase is relatively rapid, a large number of Z phase precipitations was observed in the Al-Cu-Mg-Ag quaternary alloy artificially aged for 1h.(4)The simulation results indicate that the additions of Si result in a refinement of Mg clusters and Cu-Mg clusters during early stages of ageing. The addition of Ag lead to the formation of Mg-Ag clusters which act as the nucleation sites of Z phase.(5)When compare to quaternary Al-Cu-Mg-Ag alloy, the combined additions of Ag and Si promote an enhanced age hardening response. However, the precipitation sequence is not altered, the microstructure of Al-Cu-Mg-Ag-Si quinary alloy is also dominated by Z phase.(6)The age hardening response and strength can be promoted by small additions of Li. The precipitation of S phase is stimulated, on the contrary, T phase is restrained in the Li-bearing alloy. |
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