| Compared with ordinary steels, Al-Zn-Mg-(Cu) alloys may have a similar strength (600-800MPa) with less than a half of weight, and therefore have been widely applied as important lightweight structural materials in high-speed train and aircraft industry and defence equipments. Pure aluminum is very soft, and its yield strength is less than100MPa. However, with addition of a small amount of alloy elements, such as Zn, Mg and Cu, a great number of nano-sized precipitates can be formed in Al matrix after proper aging treatments. Under an externally applied load, these precipitates hamper the dislocation motion in Al matrix, strengthening Al-Zn-Mg-(Cu) alloys. Owing to their samll size, the single-nanometer sized precipitates are very difficult to be accurately characterized for their structures by conventional methods, such as X-ray diffraction, electron diffraction and electron microscopy imaging without a sufficient resolution. Hence, the problems about their structures and their evolutions have thus far not yet been adequately solved. Recently, the devolopment of atomic-resolution electron microscopy has made it feasible to fully understand the evolutions of these precipitates well at the atomic scale.Using advanced image-corrected and probe-corrected electron microscopies, this dissertation aims to determine the atomic structures and evolutions of these precipitates by means of quantitative image analysis, exit-wave function reconstruction, Z-contrast imaging, in association with the first principle calculations. The determined precipitate structures and their evolution mechanisms have been used to understand the relationship of "Composition-Structure-Process-Property" in the alloy system. We demonstrate that the width of precipitate free zone can be controlled by the design of alloy compositions, which can be interpreted by precipitates nucleation. It is also shown that the mechanical properties and corrosion resistance can be improved by optimizing the heat treatment processes on the basis of understanding the phase transformation rules.The main contents and conclusions of this dissertation are summarized as below:We revisted the decomposition of supersaturated solid solution of7150alloy by means of traditional transmission electron microscopy (TEM) with selected area electron diffraction (SAED). It was found that there exists an intermediate hexagonal phase, termed as η precursor (ηp) phase, with lattice parameters a=3d112Al=0.496nm and c=4d111Al=0.935nm. The ηp phase is semi-coherent with the Al matrix and exhibits a well-defined crystallographic orientation relationship with the matrix (0001)ηp//{111}Al,[1120]ηp//<112>Al,(1010)η/(110)Al. The3-dimensional morphology of ηp particle was reconstructed via TEM observation from three different projections.Two state-of-the-art atomic resolution imaging techniques have been used to study the ηp-structure. With the help of image simulation, its experimental structure model was suggested as:a=b=4.96A, c=8.86A, α=β=90°, γ=120°. Then, the DFT calculations were employed to refine its energetically favorable structure, and the results show that the refined ηp-structure has a hexagonal lattice with a=5.00A, c=9.27A, belonging to the space group P6(No.174). The ηp-structure may initiate with a composition of Mg1-x(Zn,Cu)4-yAl1+x (0<x<1/3,0<y<0.2), evolving towards its metastable composition of MgZn4Al. Structurally, ηp-structure will smoothly transform into η-structure after Mg atoms replacing all of Al atoms suggested by DFT relaxation. The r\p precipitates are distinguished from η-phase, though their structural similarity and continuity. This is due to the facts that the ηp precipitates upon forming acts as an important strengthening phase in Al-Zn-Mg-(Cu) alloys, and keeps a certain orientation relationship with the matrix.We studied the nucleation of ηp precipitates using advanced imaging techniques at atomic level. It was found that ηp precipitates nucleate at GP ηp zones, and finally evolve into stable η-phase, through a dynamic transformation far away from equilibrium state. The GPηp zone, nucleating from the formation of a stable double-panel, is featured by their characteristic thickness of7atomic-layers. The GPηp zone can evolve in composition and structure under the guideline of its stable double-panel, untill a matured GPηp zone forms. After that, solute atoms gather on one side or on two sides of GPηp zones to grow in thickness, evolving into a10-atomic-layer transition phase prior to the ηp-phase. As more Zn atoms enter the transition structure, a misfit dislocation loop shall form accompanying the formation of the ηp-structure. Our study shows that the evolution process of ηp precipitates at atomic level is following GPnp zonesâ†'ηpâ†'-η phase, which is confirmed to be enegetically reasonable.We determined the structure of η’ phase and revisted the evolution process of η’ precipitates by advanced imaging techniques introduced in Charpter3. The results showed that the η’-precipitates with a composition of Mg2Zn5-xAl2+x (x>0) have a structure that is very similar to the model proposed by Li et al.. The revised structure has a trigonal lattice with a=4.96A, c=14.03A, belonging to the space group P321(No.150). The reported decomposition process of supersaturated solid solution in Al-Zn-Mg-(Cu) alloys has been confirmed by XRD and TEM results to be:clusters (GPI zones)â†'GPII zones (GPη’)â†'η’â†'η. Although the atomic structure of η’phase has been known, the evolution from η’to η is still not clear.Based on the updated knowledge of phase transformations obtained above, we studied the influence of alloy compositions on precipitation. It was shown that there are two possible decomposition paths depending upon the (Zn+Cu)/Mg atomic ratios in the alloys. We demonstrated that the width of precipitate free zones can be controlled by adjusting alloy compositions. It was also shown that with an improved RRA treatment better mechanical properties and corrosion resistance can be achieved. |
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