Investigation of mechanical and corrosion properties of an Al-Zn-Mg-Cu alloy under various ageing conditions and interface analysis of η′ precipitate

The mechanical and corrosion properties under various ageing treatment conditions were investigated in an Al-6.0Zn-2.3Mg-1.8Cu-0.1Zr (wt.%) alloy. The results showed that the retrogression and re-ageing (RRA) were capable of providing higher strength and improved corrosion resistance in comparison with the conventional T6 and T74 ageing. The optimised ageing process had been found to be 120 degrees C/24 h + 180 degrees C/60 min + 120 degrees C/24 h for the experimental alloy. The results obtained from the high resolution transmission electron microscopy (HRTEM) interface analysis revealed that a semi-coherent stress field between the eta' precipitate and the Al matrix was critical in controlling the strength of the Al-Zn-Mg-Cu alloy heat-treated under different conditions. Furthermore, Transition Matrix calculation showed that the eta' phases had only two zone axes: [(1) over bar2 (1) over bar3]eta' and [10 (8) over bar(2) over bar 23](eta)', which were parallel to the [112](Al) zone axis, when being precipitated from the Al matrix. Therefore, the orientation relationships between the eta' precipitates and the Al matrix under the [112](Al) zone axis could be described as: [(1) over bar2 (1) over bar3](eta')//[112](Al);(1 (2) over bar 12)(eta')//(1 (1) over bar)(Al) and [10 (8) over bar(2) over bar3](eta')//[112](Al);(1 (2) over bar 12)(eta')//(11 (1) over bar)(Al). Consequently, a new diffraction pattern model from eta' precipitates in two variants under the [112] Al zone axis had been established, which was in a good agreement with the experimental data. (C) 2015 Elsevier Ltd. All rights reserved.