Microstructural characteristics and properties of spray formed Zn-rich Al-Zn-Mg-Cu alloy under various aging conditions

In this study, the single- and dual-aging characteristics of a spray formed plus extruded Zn-rich Al-Zn-Mg-Cu alloy were investigated through hardness and electrical conductivity curves, tensile tests, and transmission electron microscopy. The results show that the single-aged alloy presents dual-peak hardness with aging time, while the hardness gradually drops with the progress of the dual-aging process. Accordingly, the peak-aged alloy which corresponds to the maximum hardness during single-aging, demonstrates excellent strength but relatively low conductivity, whereas the strength of the dual-aged alloy steadily decreases and conductivity increases with the extension of aging time. The microstructure of the peak-aged alloy exhibits the high volume fraction of fine and uniformly distributed intragranular precipitates consisting mainly of eta phase, and displays continuous grain boundary precipitates (GBPs) with no obvious precipitation free zone (PFZ). In the dual-aging process, the preaging treatment produces high density of very fine intragranular precipitates comprising of GP-I zones/partly transformed to eta, which gradually grow and finally convert to equilibrium eta phase as the aging proceeds at second-step temperature, simultaneously, the GBPs show progressively more discontinuous distribution and widening PFZ. Besides, the corrosion susceptibility of the alloy is discussed with reference to the evolved microstructure and the conductivity values.