CRYSTAL-STRUCTURE OF ROD-SHAPED PRECIPITATES IN AL-1.0 MASS-PERCENT-MG2SI ALLOY

The rod-shaped precipitates in an Al-1.0 mass%Mg2Si alloy were observed by a transmission electron microscope in order to clarify about their crystal structure. The micro-beam diffraction (MBD) patterns obtained from the direction parallel to the longitudinal direction of precipitates show that there are rotating relationships between the matrix [100] direction and the longitudinal directions of the precipitates. The rotating axis is parallel to the longitudinal direction of the precipitates. When the precipitates were observed from the direction normal to their longitudinal direction and parallel to the [100] or [31O] matrix directions, three kinds of selected area diffraction patterns are obtained. These diffraction patterns are explained by the rotating relationships between the matrix and the precipitates. Based on the results in this study, the crystal structure of this precipitate is proposed as follows: The crystal system is H.C.P. whose unit cell contains four Mg and two Si atoms (Mg/Si=2.0) and its lattice parameters are a=0.407 nm and c=0.405 nm. The electron diffraction patterns observed are in agreement with the computer simulated patterns based on the crystal structure of the rod-shaped precipitate.