Effects Of Cu Content On Structures And Properties Of Al-Zn-Mg Alloys

Al-Zn-Mg based alloys are widely used in aerospace and civil industry for their ultrahigh strength. The improvements in strength and hardness of the alloys depend on the formation of a series of fine precipitates from the supersaturated solid solution (SSSS) during artificial aging treatment. In general, the precipitation sequence of Al-Zn-Mg based alloys during artificial aging treatment can be described as follows: SSSSâ†'G.P. zone (â… ,â…¡)â†'η’â†'η. It has been shown that the addition of Cu has a significant effect on the aging behavior of Al-Zn-Mg alloys. Previous studies showed that the addition of Cu can accelerate the precipitation kinetics from G.P. zones to η’phase, enhance age hardening in the very early stage of natural and artificial aging, and stabilize the η’ phase from changing into η phase. It has to be mentioned that whether Cu modifies the existing aging process and introduces additional precipitates is still uncertain.Four samples with different Cu contents were prepared in an induction furnace under an argon atmosphere. The as-cast ingots were first solution heat-treated at480℃for90min, and then quenched in ice-water. Subsequently the alloys were aging treated at140℃. The microstructural evolution and hardness variations of four Al-Zn-Mg based alloys with different Cu contents during aging at140℃have been systematically studied through a combination of hardness tests, TEM, SAED patterns and their simulations. The results show that:It was confirmed that the addition of Cu to Al-Zn-Mg alloy will significantly affect the aging behaviors of Al-Zn-Mg alloy. It has been shown that the hardness of the alloys with Cu content below3wt.%increases with increasing the Cu content. While the internal microstructures and precipitation process will be affected when the Cu content reaches4%, and this decreases the hardness of the alloy. The addition of Cu promotes the nucleation of G.P.-â…¡ zones and enhances the hardness of the alloys after quenching. A certain amount of Cu (≤3%) added to Al-Zn-Mg alloy accelerates the early hardening response and the transformation from the G.P.-â…¡ zone to the η’ precipitates, and improves the precipitation density of the η’ precipitates. Cu addition can also promote the formation of the η precursor and retard its transition to the stable phase. However, the addition of Cu above about3wt.%in alloy will decrease the accelerating effect of the transition from GP.-II zone to the η’ precipitates.The η phase formed in C4(Al-6.87wt.%-2.89wt.%-3.92wt.%) alloy is with the same crystal structure as that of the η phase in NC alloy and with the systematic extinction being ruined. The composition of the η phase in C4alloy is suggested to be Mg4Zn4Cu3Al through experimental analysis and theoretical calculations. The favored Mg and Zn substitutions by Cu and Al in the MgZn2structure have been discussed and the most possible atomic structure of the η phase have been proposed based on the calculated enthalpies of formation. The Zn atoms of the η phase in Al-Zn-Mg-(Cu) alloy will be replaced by Cu and Al atoms through increasing the Cu content, and during this congruent transformation process the systematic extinction can be ruined for certain composition range.