Enhanced tensile strength in an Al–Zn–Mg–Cu alloy via engineering the precipitates along the grain boundaries

An investigation was conducted to study the influence of peak-ageing and double ageing treatments in the precipitation along the different grain boundary orientations of an extruded Al–Zn–Mg–Cu alloy. The results display that the peak-aged grain boundary precipitates along the Low Angle Grain Boundaries (LAGBs) are mainly of θ'' with the existence of few η′ precipitates, while the η′ and θ'' precipitates are along the High Angle Grain Boundaries (HAGBs). The major grain boundary precipitates are mainly of θ'' precipitates along LAGBs and θ′, η′, and η along the HAGBs after the double ageing treatment. The variation in the precipitate types along different grain boundary orientations was attributed to the different diffusion rates of solutes along the grain boundaries. In addition, it is observed that the yield strength (YS) (∼ 690 MPa) of peak-aged Al–Zn–Mg–Cu alloy increased by ∼ 73% and ∼ 18% by comparison with the extruded and double-aged alloys, respectively. The double-aged alloy displays good elongation to fracture (∼16%) compared to the peak-aged alloy. Precipitation hardening contributes to the tensile strength of the alloy, and the improvement of the elongation owed to the growth of the grain boundaries precipitates and the corresponding absence of the precipitate-free zone area. The current study provides new insights in engineering the grain boundary precipitates to the required precipitate type and the corresponding mechanical properties.