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°C/24h+180°C/60min+120°C/24h 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 η' 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 η' phases had only two zone axes: [1{combining overline}21{combining overline}3]_η' and [108{combining overline}2{combining overline}3]_η', which were parallel to the [112]_Al zone axis, when being precipitated from the Al matrix. Therefore, the orientation relationships between the η' precipitates and the Al matrix under the [112]_Al zone axis could be described as: [1{combining overline}21{combining overline}3]_η'//[112]_Al;(12{combining overline}12)_η'//(11{combining overline})_Al and [108{combining overline}2{combining overline}3]_η'//[112]_Al;(12{combining overline}12)_η'//(111{combining overline})_Al. Consequently, a new diffraction pattern model from η' precipitates in two variants under the [112]_Al zone axis had been established, which was in a good agreement with the experimental data.