Transmission electron microscope observation of grain boundary precipitates in an Al-1 mass%Mg2Si alloy with excess magnesium

The crystal structure and preferential orientation relationship with the matrix of grain boundary precipitates in a Al-1 mass%Mg2Si alloy containing excess magnesium was investigated using transmission electron microscopy. A small number of equilibrium phases precipitated on a grain boundary at relatively high aging temperature. The equilibrium precipitates have some preferential orientation relationship with the matrix. At a peak aging condition where the intergranular fracture occurs, the diffraction pattern of a hexagonal system is obtained from the grain boundary precipitates. These precipitates are considered as an intermediate phase. The grain boundary precipitates have a preferential orientation relationship with the matrix as{001}m_?_?{0001}p. No equilibrium precipitate is observed at grain boundaries. The number of grain boundary precipitates decreases with increasing misorientation between two grain boundaries. The mean size of grain boundary precipitates increases with increasing misorientation.