Grain boundary precipitates in an Al-Mg-Si alloy with excess Si

The crystal structure of grain boundary precipitates in an Al-1 mass%Mg2Si alloy containing exess Si was investigated by transmission electron microscopy. At a peak aged condition where the marked intergranular fracture occurs, the same diffraction patterns are observed from both the grain boundary precipitates and the matrix precipitates. These precipitates may be considered as an intermediate phase from the analysis of the diffraction patterns. The equibrium precipitate (Mg2Si) is not observed on grain boundary. The size as well as number of grain boundary precipitates are almost independent of the misorientation, on the grain boundaries with misorientations larger than 10 degrees. While on grain boundaries with misorientation of 2 degrees, the size is much smaller and the number is anomalously larger as compared with those of the large angle boundaries.