High resolution transmission electron microscopy of precipitate microstructures of two-step aged Al-1.6%Mg2Si alloy

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 similar to 423 Ii 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 Ii 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 Ii for 60 lis, the primary G.P. zones with the diameter of similar to 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 Ii aging, while multi layer G.P. zones continuously transform into random-type precipitate or act as effective nucleation sites for the random-type precipitates.