MORPHOLOGY OF A PLANER PRECIPITATE AND IT S ORIENTATION RELATIONSHIP TO THE MATRIX IN AN AL-1.0 MASS-PERCENT-MG2SI-0.4 MASS-PERCENT-SI ALLOY

The morphology of planer precipitates and the orientation relationship s between the precipitates and the matrix in an Al-1.0 mass%Mg2Si-0.4 mass%Si alloy were investigated by transmission electron microscopy (T EM). Some equilateral triangular precipitates and many polygonal preci pitates were observed by a scanning electron microscope. TEM observati on of the precipitates and the extracted precipitates from the matrix by thermal phenol method shows that the equilateral triangular precipi tates are thin plates. The electron diffraction patterns taken from th ese precipitates were analyzed as a silicon that has a diamond structu re (a = 0.543 nm). Energy dispersive X-ray spectroscopic (EDS) analysi s measured at the extracted precipitates showed only Si peak. Some str aight boundaries exist in the equilateral triangular precipitate. It w as observed by high resolution electron microscopy that the boundaries are plane defects such as stacking faults. Electron diffraction spots taken from this precipitate show the forbidden reflection. Such spots are caused by stacking faults. Therefore, the equilateral triangular precipitate is silicon. Orientation relationships between the precipit ate and the matrix is as follows; (001)Alparallel-to (111)Si and the a ngle between [100]Al and [1BAR10]Si is 10 degrees. Three sides of the triangular Si precipitate are parallel to the direction of [110]Si in (111)Si.