The constituent of precipitations phases of aged melt-spun AlMgCu ribbons was characterized by high-resolution transmission electron microscopy and microhardness test. The cooling rate of as melt-spun ribbon was estimated to be 1.60×105 K/s from the empirical relation. The samples were aged at 200 ＆#176;C for 16 h after solution treatment. Two precipitation phases, i.e. Al2CuMg and abnormal amorphous SiO2 were identified in the T6 melt-spun AlMgCu ribbon. The crystal structure and stoichiometric composition of Al2CuMg phase are in good agreement with the reference results [WANG et al （2007; 2005）]. The combined experiments show that the formation of abnormal amorphous SiO2 appears to be associated with the higher cooling rate in melt-spinning process and has no significant effect on the peak hardness.
The precipitation sequence of η（MgZn2） phase along low-angle grain boundaries in Al-Zn-Mg-Cu alloy was investigated by examining samples aged at 135 ℃ for various times from 5 min to 6 h. High resolution transmission electron microscopy （HRTEM） observations and energy dispersive X-ray spectroscopy （EDX） analysis indicate that the precipitation sequence of η phase along low-angle grain boundaries should be supersaturated solid solution （SSS）→vacancy-rich clusters （VRC）→GP Ⅱ zones→η＇→η. Based on the theory of non-equilibrium grain boundary segregation （NGS） and non-equilibrium grain boundary co-segregation （NGCS）, the excessive solute elements gradually segregate to the grain boundaries by the diffusion of the solute-vacancy complex during aging treatment. The grain boundary segregation plays an important role in the nucleation and growth of VRC, GP Ⅱ zones, η＇ phase as well as η phase.
