| High strength Al-Zn-Mg-(Cu) alloys have excellent mechanical properties, and therefore are widely used in aerospace, transport and construction industries. With proper chemical compositions, casting and manufacture processes, and heat treatments, the alloys can achieve comprehensive properties of high strength, toughness and corrosion resistance. Hence, understanding as accurate as possible the relations between the microstructure and the property and the relations between the microstructure and the manufacture process of the alloys have become more and more important in the study of the materials. With the development of high-resolution electron microscopy (HRTEM) and analytical electron microscopy, the precipitate evolution of aging hardenable Al alloys can be studied more precisely than ever at atomic scale. The focus ion beam (FIB) technique can accurately cut specimens of very local areas for TEM characterization. The atomic resolution HAADF-STEM technique can provide not only the local crystal structure information, but also the local chemical information. The electron backscattering diffraction (EBSD) technique can determine the orientations of various phases in the alloy, so as to better understand the mechanical properties and the corrosion mechanisms of the alloys. In terms of alloying itself, by optimizing alloy compositions and heat treatment processes, new strengthening precipitates microstructures can be discovered, therefore, giving guidance for alloy design. So, in this dissertation, by using these techniques mentioned above, the commercial alloys were studied in the first stage, with emphases on improving heat treatments, optimizing microstructure, and achieving better mechanical properties and corrosion resistance of the materials. In the second stage, a new high-strength alloy which is strengthened by T-phase precipitates was systematically proposed and investigated.The content of this dissertation is divided into four parts as described below in detail.(1) A critical three-stage solution heat treatment (SHT) was applied on the7055Al alloy. The effect of the SHT is compared with a single-stage SHT and a high temperature two-stage SHT. The electrochemical polarization curves and immersion testing in a NaCl aqueous solution show that:better corrosion resistance can be obtained by the critical three-stage SHT. This critical SHT can reduce the volume fraction of the secondary phases, and make the remaining particles to have a spherical shape, especially the coarse constituent particles, so as to decrease the original pitting sites. The achieved higher strength is due to more solute atoms dissolved into the matrix, therefore, giving higher solution strengthening effects. Furthermore, it is demonstrated the more dissolved atoms can yield a higher precipitation strengthening effect after usual aging treatment.In the spherical secondary phase particles (SSPPs), icosahedral (Al, Zn, Cu)49Mg32quasicrystal was observed. There are two requirements appropriate for the formation of the quasicrystal phased. One is to form the crystal approximant T-phase (Al, Zn, Cu)49Mg32which has a similar chemical composition with the quasicrystal phase. Another requirement is through quenching after the solution treatment to create a rapid cooling rate. This non-equilibrium condition makes the alloying atoms to solidify quickly and therefore formed a quasicrystal. There are three types of eutectic microstructures in the SSPPs:quasicrystal+Al, η+Al, T+Al. It was observed that the lath like Al7CuFe particles frequently act as heterogeneous nucleation sites for the SSPPs. Al2Cu and Mg2Si phases usually imbedded in the SSPPs. The unmelted S phase (Al2CuMg) particles shall become spherical or elliptical.(2) HRTEM was used to study the relationship between the precipitates morphology and the microhardness of the7055alloy. During aging at120℃, the first precipitated is the GPⅡ zones with an disk shape lying at the{111} Al planes. Because of the quick increase of precipitate volume fraction, number density and size in diameter, the hardness increases quickly at the early stage till20h. In this stage, the GP II zones keep its thickness unchanged. Once the GP Ⅱ zones begin to grow in the thickness direction, the phase transformation from GP Ⅱ zones to η-p or η’ begin, often accompanied by the decrease of hardness. When η’ or η-p grows in the diameter direction, the hardness increases again for the second time. When the precipitates become too large, the hardness decreases again. In summary, if the GP Ⅱ zones or η’ or η-p grow in the diameter direction, the hardness increases. On the contrary, the phase transformation from GP Ⅱ to η’, or to η-precursor shall lead to hardness decrease.(3) An Al-7.60Zn-2.55Mg(wt.%) alloy with a characteristically high Mg-to-Zn atomic ratio has been investigated for its strength and hardening precipitates in comparison with that of7150alloy. It is shown that this alloy yields a rather high strength upon thermal ageing. Interesting is that this high-strength alloy is hardened by the coherent polyhedral T-phase [(AlZn)49Mg32] particles and their early-stage precipitates, which is very different from that of other high-strength AA7xxx AlZnMg(Cu) alloys hardened by the disc-like precipitates of the η-type phases.(4) The intergranular corrosion (IGC) and micro structure of four commercial alloy with different aging treatment (the7055,7150,7075and7N01Al alloys) are investigated in comparison for their alloying compositions and alloying contents. Over aging increases the electrical conductivity and IGC resistance of alloy, decreases the corrosion potential, elevates the prone to corrosion, but decreases the corrosion current. The reason for these could be that the corrosion products prevent the self-corrosion. The IGC resistance of the Cu-containing alloy decreases with the increase of total element content, whereas the non-Cu alloy has the best inter IGC resistance. This is due to the present of S phase in the Cu-containing alloy. The exfoliation corrosion (EXCO) and stress corrosion cracking (SCC) of two alloys (2#,3#) with similar Zn and Mg content but with/without Cu are investigated. Cu can refine the grains during solidification, reduce the difference of corrosion potential between the grain boundary and the interior, an average corrosion is present; The non-Cu alloy has less nuclei than Cu-containing alloy during solidification, having coarser grains and larger difference of corrosion potential between the grain boundary and the interior, and therefore lead to more severe intergranular corrosion. The Cu-containing alloy has a higher strength and lower toughness, and produces crack earlier during the SCC experiment as compared with the non-Cu alloy. All SCC crack tips prorogate along the large grain boundary no matter the alloy contain Cu or not. |
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