Transmission Electron Microscope Observation of Grain Boundary Precipitates in an A1-1 mass% Mg₂Si Alloy

The crystal structure of grain boundary precipitates in an A1-1 mass% Mg₂Si alloy was investigated by transmission electron microscopy. At a peak aged condition where the intergranular fracture occurs, the diffraction pattern of hexagonal system can be obtained from the grain boundary precipitates. These precipitates may be considered as an intermediate phase from comparing the diffraction patterns with matrix precipitates. The grain boundary precipitates have the preferential orientation relationship with the matrix as {001}_m {0001}_p The equilibrium precipitate (Mg₂Si) is scarcely observed at the grain boundaries. The number of grain boundary precipitates decreases with increasing the misorientation of grain boundary, while the mean size of grain boundary precipitates increases with increasing misorientation.