Initial Aging of Al-Zn-Mg-Si and Al-Zn-Mg-Ge Quaternary Alloys

Abstract

Al-rich Al-Zn-Mg-Si and Al-Zn-Mg-Ge quaternary alloys were studied by the measurement of electrical resistivity. The results may be summarized as follows: (1) The isothermal aging curves of electrical resistivity in the Al-Zn-Mg-Si and Al-Zn-Mg-Ge quaternary alloys seemed to be composed of two stages when electrical resistivity was plotted against logarithm of aging time. (2) When Mg:Si=2:1, the maximum increase of resistivity, Δρ_M, in the Al-Zn-Mg-Si and Al-Zn-Mg-Ge quaternary alloys was nearly equal to that in Al-Zn binary alloy, with the same amount of Zn with quaternary alloys under the same aging condition. But when Mg:Si>2:1, Δρ_M, in the quaternary alloys was larger than that in the binary alloy. (3) The times, t_Ms, required to reach the maximum in electrical resistivity in the Al-Zn-Mg-Si and Al-Zn-Mg-Ge quaternary alloys were much longer than that in the Al-Zn binary alloy or Al-Zn-Mg ternary alloy. (4) It might be concluded that the increase of electrical resistivity was mainly dependent upon the spherical G.P.zones which were similar to those in Al-Zn binary alloy (Zn-G.P.zones), when Mg:Si=2:1. The clusters containing Mg and Si in the ratio of 2:1 ((Mg₂Si) clusters) might be formed during the aging. However, it seems that these clusters do not contribute to the increase of electrical resistivity. (5) The shape of isothermal aging curves and the very large value of t_Ms in the quaternary alloys may be explained, if it is assumed that (Mg₂Si) clusters may absorb more vacancies when clusters are formed and consequently the rate of growth of Zn-G.P.zones is decreased. (6) It seems that (Mg₂Si) clusters can absorb much more vacancies than those absorbed by isolated atoms of Mg and Si in solid solution.