| 7xxx series (AlZnMgCu) aluminum alloys are among the age-hardening aluminum alloy, widely used as structural materials in aviation and aerospace industry, transportation, and other military or civil sectors. Nonetheless, there are still many problems to be solved about their microstructures, such as the structures of the hardening precipitates and their evolution in morphology and in overall distribution upon thermal ageing. Also, it is not clear about the contribution of these factors to the mechanical properties of the alloy. In addition, about the precipitate free zone (PFZ), there are still many ambiguous conclusions that need to be clarified, such as their contribution to the mechanical properties, and their width change during ageing, as well as their formation mechanism.An Al-6.47%Zn-1.63%Mg-2.05%Cu alloy was used in the present study. All samples were solution-treated and then quenched in water to room temperature. After that, one-step aging treatments were carried out in an oil bath furnace. In order to understand their microstructure evolution of the hardening precipitates formed in grains and at grain boundaries and the evolution of PFZs, micro-Vickers hardness, tensile test, conductivity measurement,optical microscopy (OM) and scanning / transmission electronic microscopy analysis (SEM / TEM) were carried out. The obtained main results are as follows:(1) An intermediate phase between theη’ phase and theηphase, when aged at 120℃. This intermediate phase with a hexagonal structure can be termed theη-precursor, since its a lattice parameter (a= 0.496nm) is the same as that of theη’ phase and its c lattice parameter (c=4d111Al=0.935nm) is approximately the same as that of theηphase (c=0.860nm) but smaller than theη’ phase (c=6d111Al =1.406nm). Theη-precursor is a newly discovered precipitate phase, which has never been reported proor to the present study. It is found that theη-precursor significant effect on hardness and PFZs.(2) Microstructure evolution of the targetting alloy aged at 120, 130, 140 and 150℃, respectively, was systematically studied using TEM in combination with hardness test. It was found that the alloy aged at 120℃for 72h has the maximum hardness up to 202HV. Correspondingly, its tensile strength is 708MPa, yield strength 691MPa and elongation 14%. Three types of precipitates, GP zone,η’ phase andη-precursor , all are observed in the peak aged samples at different temperatures. (3) The evolution of PFZs in the samples aged at 150℃has been studied in detail by means of TEM and scanning transmission electron microscopy high-angle annular dark field (STEM-HAADF) imaging. It has been found that PFZs formed in the samples aged at near 150℃. And the higher the ageing temperature, the wider the PFZs. The width of the PFZ does not change with increasing ageing time at the same ageing temperature. However, along with the increasing of ageing time, the successive precipitates lying at grain boundaries evolve into distributional larger precipitates. Combining the Scanning Transmission Electron Microscopy (STEM) with energy dispersive X-ray analysis (EDX), information about distributions of solute elements were detected. The solute elements are poor in the PFZ, and rich in the precipitates at grain boundaries, especially for zinc. With high resolution HAADF observation, it was found that the width of the PFZs in the vicinity of small-angle grain boundaries is smaller than that in the vicinity of high angle grain boundaries. Also, the precipitates formed on small angle grain boundaries are theη-precursor other thanηphase at high angle grain boundaries. |
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