HRTEM observation of G.P. Zones and metastable phase in Al-Mg-Si alloys

A high resolution transmission electron Microscopy (HRTEM) was performed by the microstructure analyses of a Al-1.6mass%Mg₂Si alloy in order to elucidate the characteristic two-step aging behavior of Al-Mg-Si alloys. The optical conditions (accelerating voltage, defocus and crystallographic orientations ) for the HRTEM observation of fine precipitates were determined. In the alloy aged at 343-423 K for 60ks, the fine G.P.zones with mono layer or multi layer structures could be found. When the specimens containing such G.P.zones are finally aged at 473K 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 results in the increasing of 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 -1nm 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 precipitate results in the decreasing of hardness, exhibiting negative effect of the two-step aging. Based on the microstructure analysis, 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. There were no β" and β' phase at peak aged condition in this quasi-binary alloy.