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

• The effect of heat treatments on mechanical and corrosion properties was investigated.
• The RRA process could provide a good balance between mechanical and the corrosion properties.
• The η′ precipitates were vital for strengthening effect by HRTEM interface analysis.
• A diffraction pattern model from η′ precipitates was established under the [112]Al zone axis.

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/24 h + 180 °C/60 min + 120 °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 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: [1̅21̅3]η′ and [108̅2̅3]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: [1̅21̅3]η′//[112]Al;(12̅12)η′//(11̅)Al and [108̅2̅3]η′//[112]Al;(12̅12)η′//(111̅)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.

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