Studying On Age Hardening Behaviors Of Al-Zn-Mg-Cu Alloys With EET Theory

Al-Zn-Mg-Cu alloys are widely used in the aerospace, automotive, machinery and other industries with good physical properties and mechanical properties. Main alloying elements and their contents and the aging sequence have an important effect on the microstructure and mechanical properties of Al-Zn-Mg-Cu alloys. With solution-aging treatment, microstructure observation, hardness test and the calculation and analysis of the valence electron structures of the matrix and precipitates, the effects of Zn/Mg and the Cu content on microstructure and age hardening behaviors in Al-Zn-Mg-Cu alloys were studied, then the micromechanism of precipitates nucleation and age hardening behaviors were analyzed.It is showed that the peak aging time of Al-6.2Zn-2.3Mg is obvious shorter than that of Al-5.0Zn-3.0Mg. The hardening rate of Al-6.2Zn-2.3Mg in early aging is bigger than that of Al-5.0Zn-3.0Mg. With aging at 120℃, it appears double peaks of age hardening curve in Al-6.2Zn-2.3Mg while the characteristic of double peaks in Al-5.0Zn-3.0Mg is not obvious. With aging at 120℃, the age hardening curves have not the characteristic of double peaks in Al-6.2Zn-2.3Mg-1.5Cu and Al-6.2Zn-2.3Mg-3.5Cu, the hardness of alloys increases fast in early aging and it changes little in the middle and later aging. With aging at 160℃, the hardness of Al-6.2Zn-2.3Mg-x Cu changes greatly with the increasing of the aging time, and the range ability of hardness increases with the increase of Cu content.It is found with EET calculation that the micromechanism of aging with what kind precipitating sequence of Al-Zn-Mg-Cu alloys is the atom segregation formation of solid solution in the matrix in early aging. The reason of aging with η precipitating sequence lies in that the acting force of Zn-Mg is the strongest and their combination is the most stable, therefore large amounts atom clusters of α-Al-Zn-Mg prior form and it provides the condition of GP zone formation with η precipitating sequence. The reason that there exist two precipitating sequences of η and T in Al-Zn-Mg-Cu alloys lies in that except large amounts atom clusters of α-Al-Zn-Mg prior form in early aging, α-Al-Mg-Zn-Mg-Al compound solid solution with Zn center form because of the strong acting force of Al-Mg, and it provides the condition of GP zone formation with T precipitating sequence.It is found with EET calculation that the acting force of Zn-Mg is the strongest and that of Mg-Cu is the second among the ternary solid solution α-Al-Mg-Cu of Al-Zn-Mg-Cu alloys. When Cu content is lesser, α-Al-Zn-Mg-Cu-Mg-Zn-Al compound solid solution formed in early aging provides the condition of GP zone with Cu, therefore η′ and η with Cu form. When Cu content is more, the formation of α-Al-Mg-Cu solid solution provides the condition of GPB zone with S precipitating sequence.