| As heat treatable aluminum alloys,Al-Mg-Si alloys are widely used in the automotive and aerospace industries because of their moderate strength,good formability and low cost.The alloys precipitate nano-scale precipitates during aging,which strengthen the alloys by impeding dislocation movement.It has been found that the addition of rare earth elements(RE)to Al-Mg-Si alloys can bring grain refinement and improve the mechanical properties of the alloys.The effects of RE elements Sc and Er on the aging precipitation of Al-Mg-Si alloys have also received increasing attention in recent years.In this thesis,the effects of Sc and Er on the microstructure and mechanical properties of Al-Mg-Si alloys during aging were systematically investigated using microhardness test,tensile test,transmission electron microscopy,high-angle annular dark-field scanning transmission electron microscopy,and first-principles calculations.The results show that:(1)When artificially aged at 180 or 250 ℃,the additions of Sc and Er have no significant effect on the peak hardness of Al-Mg-Si alloys,but can improve the strength and ductility of the alloys.(2)Both Sc and Er additions significantly change the aging precipitation sequence of Al-Mg-Si alloys.In the alloys containing Sc or Er,in addition to the precipitation sequence of the early-stage β " phase to the late-stage β’ phase that commonly observed in Al-Mg-Si alloys,there exists another precipitation sequence,i.e.,the disordered β" phase(early-aging stage)to the composite precipitates(overaging stage).Some composite precipitates without β " phase or its substructure observed in the Er-containing alloy,may be nucleated directly from the Al-matrix.Atomic-resolution transmission electron microscopy and first-principles calculations show that Sc atoms enter the β’ and B’ phases and occupy some of the Mg sites in them,while Er atoms enter the β" phase in small amounts and occupy some of their Mg sites.(3)The additions of Sc and Er induce the formation of a large number of(Al,Si,RE)-rich secondary phases in Al-Mg-Si alloys,which consume a certain amount of solute Si atoms and affect the contribution of precipitation strengthening during aging.In the early stage of aging,the Si concentration in the matrix of the RE-containing alloys is lower than that of the RE-free alloy,and thus the nucleation of the β " phase is suppressed.Therefore,the precipitation strengthening contribution in the early stage of aging for the RE-containing alloys is not as good as that for the RE-free alloy.In the peak aging state,the precipitation strengthening contributions of the alloys with and without RE are comparable due to the small reduction in solute Si atoms that does not have a significant effect on the growth of the β" phase.The strength enhancement of the RE-containing alloys in the peak aging state is mainly attributed to grain refinement and secondary-phase strengthening,although it is negatively affected by the lower solid solution strengthening.The ductility enhancement of the RE-containing alloys is mainly due to the formation of the secondary phases and grain refinement. |
