| As typical high-performance lightweight aluminum alloys,6××× series Al-Mg-Si alloys are extensively used in automotive and high-speed train because they exhibit an attractive combination of high strength-to-weight ratio,good formability and corrosion resistance.Understanding the evolution of precipitate microstructure in the heat treatment process is essential for improving the properties of the alloys.Using advanced electron microscopy in association with phase transformation theories and first-principles calculations,this thesis has clarified several important questions left in the past about the precipitation behaviors in Al-Mg-Si alloys.The mostly believed precipitation sequence of Al-Mg-Si alloys is described as: supersaturated solid solution → clusters → GP zones of β"→ pre-β"(/β")→ β’(/B’/U1/U2)→ β,where the β’(/B’/U1/U2)-phases form after the β"-phase.Our results show that when the Si-supersaturation in the Al matrix is low,since the nucleation energy barrier of the β’(/B’/U1/U2)-phases is lower than that of the β"-phase,the β’(/B’/U1/U2)-phases can form directly upon aging,i.e.,a reversal of the precipitation sequence can occur.Such that the precipitation sequence of the alloys is: supersaturated solid solution → clusters → β’(/B’/U1/U2)→ β.This implies that nucleation of the β’(/B’/U1/U2)-phases and nucleation of the β"-phase can be independent of each other.Hence,by their major hardening phase to be formed upon artificial aging rather than by their composition,Al-Mg-Si alloys can be more specifically classified into “normally-β"-hardened” alloys and “normally-β’-hardened” alloys.In the slightly pre-deformed Al-Mg-Si alloys,dislocation-induced precipitates usually line up,forming sophisticated precipitate microstructures.Using atomic-resolution electron microscopy in association with hardness measurements,we systematically investigated these precipitates in relation to the age-hardening responses of the alloys.Our study reveals that the majority of dislocation-induced complex precipitates are actually short-range ordered while long-range disordered polycrystalline precipitates and multiphase composite precipitates,including U2 polycrystalline precipitates,B’/U2,B’-2/U2,B’/B’-2/U2 and β’/U2 composite precipitates.It is suggested that the formation of these complex precipitates is mainly owing to a high nucleation rate and rapid growth of different precipitate phases parallel to the associated dislocation lines.Since dislocation-induced precipitates consume more Mg than Si from the matrix and have a high formation kinetics,they will have different impacts on the matrix precipitation in different types of Al-Mg-Si alloys.Our results further demonstrate that for the “normally-β"-hardened” alloys,their formation leads to a coarser precipitate microstructure in the matrix,whereas for the “normally-β’-hardened” alloys,their formation reverses the precipitation pathway in the matrix,resulting in a reduced age-hardening potential of the former alloys and an improved age-hardening potential of the later alloys.Natural aging has a significant effect on the age-hardening response during subsequent artificial aging in Al-Mg-Si alloys,and this effect varies with alloy composition,artificial aging temperature and natural aging time.Although numerous explanations have been proposed in the past,there is still no model that can conclusively describe the natural aging effects.Using transmission electron microscopy,we systematically studied the effects of natural aging on the precipitation behavior during artificial aging in Al-Mg-Si alloys.The relationships between the precipitate microstructure and the age hardening response have been established.Furthermore,the different natural aging effects have been explained based on the varied effect of natural aging clusters on the nucleation of precipitates.The negative natural aging effect on the age-hardening potential upon artificial aging occurred in the “normally-β"-hardened” alloys is related with a reversal of precipitate type from the β"-phase to the β’-phase(when the Si-content in the alloys is relatively low)or with a coarsening of precipitate microstructure(when the Si-content in the alloys is relatively high).Whereas the positive natural aging effect occurred in the “normally-β’-hardened” alloys is related with a reversal of precipitate type from the β’-phase to the β"-phase.For the “normally-β"-hardened” alloys,even if there is natural aging before artificial aging,homogeneous nucleation of precipitates still dominates over heterogeneous nucleation of β"-phase on natural aging clusters.With increasing natural aging time,the solute super-saturation will be lowered continuously due to the formation of natural aging clusters,leading to the gradual reversal of precipitation sequence(when the Si-content in the alloys is relatively low)or to the gradual coarsening of precipitate microstructure(when the Si-content in the alloys is relatively high).For the “normally-β’-hardened” alloys,the kinetics of homogeneous nucleation is slow.With increasing natural aging time,the kinetics of homogeneous nucleation will be further slowed down and more and more nuclei of β"-phase among natural aging clusters will form,such that homogeneous nucleation of β’-phase becomes more and more difficult while heterogeneous nucleation of β"-phase on natural aging clusters becomes more and more favorable,leading to the gradual reversal of precipitation sequence. |
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