| As the heat-treatable alloys, Al-Mg-Si-Cu alloys have the medium strength, good corrosion resistance, excellent formability and low density. Therefore, they have occupied main markets of Al alloys in the world. And, the excellent macro-performance is inseparable from the microstructure of alloy, which is always bound with crystal structure and phase transition of some nano-precipitates. It is a fundamental approach and effective way to optimize the performance by controlling the microstructure of Al-Mg-Si-Cu alloys and the structure, size, shape and distribution of these nano-precipitates in nanometer and atomic scale.In this paper, the conventional transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SADP) and other some technical means are used to research the Al-Mg-Si-Cu 6005A alloy extruded profile. The work mainly focuses on the microstructure and texture of as-quenched 6005A alloys, age-hardening and phase transformation in aging process, microstructure of precipitates and their influence on macro-performance of alloy. The aims are to understand deeply the microstructure and performance of 6005A alloy at nanoscale and further to explore new technologies and ideas. The main results can be described as follows:(1) 6005A alloy extrusion profile shows some fibrous grains with 50?m, and there are a large number of Mg2Si phases, a few AlFeMnMgSi phases with Fe and AlCrMnMgSi with Mn and Cr; After solution treatment at 550?for 1h, these Mg2Si phases can be re-dissolved, however, these phase containing Fe, Mn and Cr can not be eliminated; The main texture are brass {110}<112> and recrystallization cubic {100}<001>, after solution and aging treatment, the main texture components are not changed obviously.(2) 6005 A alloy can reach its peak hardness about 120Hv when it is aged at 175?for 12h. After the peak-aging, the?" phases can be existed in Al matrix for a long time so that this alloy presents an obvious anti over-aged softening capability until aging for 90h, when the hardness starts to decrease rapidly. The alloy can reach its peak hardness about 113Hv aged at 200?for 4h. After peak-aging, the?" phases can rapidly disappear so that the alloy gives an obvious over-aged softening behavior. In addtion, the alloy is also heated to different temperatures at 10?min-1 and displays two different peaks at 100?and 250?, respectively. The two peaks can be formed because of the precipitation of some clusters and GP zone and?" phase, respectively. The?" phase is the mainly strengthening phase. And, it is also found that the valley should be related to some smaller clusters solution at 160?. In addition, the alloy can reach the peak about 127Hv at 18h by secondary aging treatment. And after interruption aging treatment, these GP zone are more dispersed in matrix compared to the microstructure aging at 175?for 30min so as to provide more nucleation sites for P" phase.(3) Accornding to binding energy calculation and phase diagrams analyses, Si atoms in Al-Mg-Si-Cu alloys pay a very important role to control the numbers of Mg-Si co-cluster and GP zones, and then lead to different age-hardening behaviors. Together with the TEM observations, the precipitation sequence of 6005A alloys may be described as:SSSS?Si-v pairs, Mg-V pairs and Mg clusters?Si clusters and dissolution of Mg clusters?Mg atoms segregate into existed Si clusters?Mg/Si co-clusters?GP zones?metastable?"?metastable?' and Q'?stable?and Q.(4) HRTEM studies show that the?" phase has a C-centered monocline structure with a=1.516nm, b=0.405nm, c=0.674nm and?=105.26°; the?' phses has hexagonal structure with a=0.715nm, c=0.405nm and?=120°; Q' phase is also believed as hexagonal structure with a=1.032nm, c=0.405nm and y=120°.(5) Three main precipitates (?",?' and Q') have 12 variants with Al matrix in 6005 A alloy, respectively. And their orientation relationships with Al matrix can be described as: (010)?"//{100}Al, [001]?"//<310>Al and [100]?"//<230>Al; (0001)?'/{100}Al, [2110]?'/<310>Al and [1210]?'//<110>Al; (0001)Q'//{100}Al, [2110]Q'//<510>Al and [1210]Q'//<110>Al.(6) An investigative strategy can be proposed by HRTEM technology together with Transition Matrix calculation, diffraction patterns simulation and stereographic projection. And it should be used to any alloys in researching orientation relationships and analyzing complicated MADP. Based on this investigative strategy, two SADP models can be established at peak-aged and over-aged stage for 6005A alloy, respectively. Further, some "cross-shaped" diffraction streaks, which are always observed in precipitation process, can be reasonably explained by the two SADP models:these "cross-shaped" diffraction streaks appeared in peak-aged stage come from some±1-order diffraction spots of variants 5-12 of?" phase under the [304]?" and [106]?" zone axis, and in over-aged stage, they should mainly come form the diffraction spots of variants 5-12 of Q' phase under the [1450]Q. and [3210]Q' zone axis.(7) TEM and SADP analysis show that the?",?'and Q' phases have three different zone axis along the [001]Al direction of Al matrix:[010]?", [304]?" and [106]?" for?" phases; [0001]?', [1450]?' and [5410]?' for?' phases; [0001]Q', [1450]Q' and [3210]Q' for Q' phases. Based on these precipitate behaviors, the detailed HRTEM structural information have been characterzed for these lying and embeded?",?'and Q' phases on (001)Al plane. Further, some moire fringes appeared on?'and Q' phases can also been explained reasonably. Simultaneity, it is also found that some inside dislocations existed in precipitates make some strain-fields released so that the numbers of interface dislocations are different from the theoretical calculation.(8) HRTEM studies show that the P" phase is coherent with the Al matrix and has 2-dismentions coherent strain field; the?' phase is semi-coherent with the (200)Al plane and incoherent with the (020)Al plane of Al matrix; the Q'phase is basically coherent with the (200)Al plane and semi-coherent with the (020)Al plane of Al matrix. According to the coherency difference, the strengthening effect of three main precipitates is also different. Therefore, from the point of view in strain field, the strengthening effect of precipitates on alloys can be described to be:?" phase> Q' phase>?' phase.(9) According to the theory of interface controlled growth, it is found that the?" phase is basically coherent with the Al matrix and has a slow coarsening speed, and it is easier to grow into needle-shaped; the?' and Q' phases are semi-coherent and basically coherent with Al matrix along their long-axis, respectively, however, they are incoherent with Al matrix along their short axis. Therefore, some lath-shaped morphologies may be more favorable to?' and Q' phases. However, more incoherent?' phase will allow the particle to grow larger in cross-section comparing with Q' phase. |
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