| Al-Cu-Li-Mg alloys have been widely used in aerospace due to their advantages of low density,high specific strength,and good corrosion resistance.In recent years,with the further development of aerospace and other fields,these industries demand higher requirements on the properties of Al-Cu-Li-Mg alloys.Al-Cu-Li-Mg alloys are heat-treatable and can be strengthened by precipitation strengthening,solution strengthening,strain strengthening,and grain-boundary strengthening.The Precipitation strengthening is the most effective one.Especially,the precipitation of nano-scale strengthening phases can significantly improve the mechanical properties of the alloys.Therefore,it is of considerable guiding significance to study the precipitation behavior of the nano-scale precipitates,and then to clarify the microstructure evolution of the precipitates during artificial aging for the preparation of a new generation of Al-Cu-Li-Mg alloys with excellent properties.Using state-of-the-art characterization techniques,such as high angle annular dark-field(HAADF)-scanning transmission electron microscopy(STEM)imaging,three-dimensional electron tomography(3DET),and threedimensional atom probe(3DAP),in association with mechanical tests and other experiments,this dissertation systematically studied the effects of natural aging and pre-deformation on precipitation behaviors of the precipitates,and discussed the related hardening mechanism in an Al-Cu-Li-Mg alloy sheet with low Cu/Li ration(Al-2.95Cu-1.55Li-0.57Mg-0.18Zr(wt.%)).The microstructure evolution of the precipitates and their relationship with properties were clarified.The main contents and conclusions of this dissertation are summarized as follows:(1)To determine the optimal aging temperature of the alloy,the effects of aging temperatures on the mechanical properties and precipitation behaviors were studied.Compared with 140℃,180℃,200℃ peak-aged samples,the mechanical properties of the samples aged at 160℃ are better.A large amount of uniformly distributed T1 phase,a small amount of S phase and GPB(GuinierPreston-Bagaryatsky)zone are precipitated at 160℃.(2)The effects of pre-deformation on the precipitation behaviors of Al-CuLi-Mg-Zr alloy aging at 160° have been studied,and the relative strengthening mechanism has also been discussed.T1 precipitates,S precipitates,and a little of GPB zones are precipitated in the peak-aged sample with T6 treatment.The introduction of 5%pre-deformation can facilitate the precipitation of successive composite precipitates and T1 precipitates along the dislocations or sub-grain boundaries,but,inhibit the precipitation of GPB zones and S precipitates in the subsequent artificial aging.When the pre-deformation level increases from 5%to 15%,the number density of the successive composite precipitates and T1 precipitates increases,and the aspect ratio of T1 precipitates decreases.With the increase of pre-deformation level,the contributions of strain strengthening to the strength of alloy increases gradually,while the contributions of precipitation strengthening increase first and then decrease.(3)Successive composite precipitates are discovered.The morphology of the successive composite precipitates is similar to that of the S phase,which has a continuous lath-like morphology.But the microstructures of the successive composite precipitates are significantly different from that of the S phase,the former include Cu-Mg clusters,rod-like S phase,lath-like S phase,conventional GPB zones,Li-GPB zones,and so on.(4)The Mg-rich and Cu-Mg clusters forming in the natural aging can modify the precipitation behaviors of precipitates at 160℃,and cause two strengthening peaks in the hardness curve of the alloy.The Mg-rich and Cu-Mg clusters promote the precipitation of large numbers of GPB zones in the early artificial aging stage.The precipitation of GPB zones and T1 precipitates increases the hardness and leads to the first strengthening peak.Subsequently,the metastable GPB zones dissolve,and the hardness decreases.The dissolution of GPB zones provides Cu,Mg,and accompanied with other solute elements for the precipitation of other strengthening precipitates.With the aging time prolonging,the volume fraction of T1 precipitates and the number of lath-like S precipitates increase,which increases the hardness of the alloy,and the second peak value appears.When the aging time continuously exceeds,some of S precipitates coarse,and the hardness decreases again,After that,the alloy appears over-aging.(5)A heter-nucleation mechanism of the T1-phase induced by natural aging is discovered,which can significantly promote the precipitation of the T1-phase.A large number of fine and uniformly δ’ phases precipitate upon natural aging,and the δ’ phases could rapidly dissolve and transform into core-shell-like clusters with Li-rich core and Cu-rich shell in the early stage of subsequent artificial aging at 180℃.These core-shell-like clusters can then serve as the heterogeneous nucleation sites for the T1-phase to facilitate the nucleation and precipitation of T1-precipitates.(6)The heter-nucleation mechanism of the T1-phase is sensitive to the temperature of artificial aging.When aging at 140℃,the diffusion coefficients of atoms are so slow that the δ’ precipitates retain the original crystal structures and are difficult to transform into core-shell-like clusters,which inhibits the heterogeneous nucleation of T1-phase precipitates.When aging at 200℃,the diffusion coefficients of atoms are accelerated,and the δ’ precipitates can quickly dissolve and transform into core-shell-like clusters,promoting the heterogeneous nucleation of T1 precipitates. |
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