High resolution transmission electron microscopy (HRTEM) was performed on the micro-structure analyses of a two-step aged Al-1.6 mass%Mg2Si alloy in order to elucidate the characteristic two-step aging behavior of Al-Mg-Si alloys. The optimum conditions for the HRTEM observation of fine precipitates were determined, including accelerating voltage, defocus and crystallographic orientations. The alloy aged at 343∼423 K for 60 ks was found to produce fine G.P. zones with mono layer or multi layer structures. When the specimens containing such G.P. zones are finally aged at 473 K up to their maximum hardness conditions, a number of random-type precipitates, which are assumed to contain high concentration vacancies, are homogeneously formed. These precipitates result in the increased hardness, exhibiting so-called positive effect of the two-step aging. On the contrary, the alloy aged at 293 K for 60 ks, the primary G.P. zones with the diameter of ∼1 nm are formed with high number density. When this alloy is finally aged at 473 K up to a maximum hardness parallelogram-type precipitates are preferentially formed. These parallelogram-type precipitates result in the decreased hardness, exhibiting negative effect of the two-step aging. Based on the microstructure analyses it is concluded that both the primary and mono layer G.P. zones almost dissolve into the matrix at the beginning of 473 K aging, while multi layer G.P. zones continuously transform into random-type precipitate or act as effective nucleation sites for the random-type precipitates.
