Observation of precipitates in Al-1wt%Mg₂Si base alloys by analytical electron microscope

Various kinds of precipitates were observed in Al-1%Mg₂Si base alloys by high resolution transmission electron microscopy (HRTEM). Analysis of elements was performed simultaneously by the attached energy dispersive X-ray spectroscope (EDX). HRTEM observation showed that a needle like precipitate in Al-1 wt%Mg2Si alloy had the hexagonal array of bright spots, corresponding to a hexagonal unit cell. The following orientation relationship with the matrix was obtained: (001)_m∥(0001)i. In a Mn addition alloy, a needle like precipitate was devided into several regions by the different array of bright spots. Thus a simple orientation relationship with the matrix was not obtained. In an excess Si alloy, some small precipitates with high Si content had been already coexisted among needle like precipitates at peak aged condition. In a Mn addition alloy, spherical precipitates with high Mn content were observed on the rod like precipitates which consist of Mg and Si at the early stage of aging at 400°C. At the later stage of aging, large flake or square shape precipitates containing Mn were observed after disappearence of rod like precipitates.