The Morphology and Formation of Precipitates in Al–Ge Alloys

The morphology and orientation relationship of Ge precipitates in quenched and aged (QA) Al–0.04 to 1.20 at% Ge alloys and in the same composite alloys slowly cooled to room temperature (SC), have been investigated using transmission electron microscopy. The morphology of precipitates in the QA specimen depends on Ge concentration; octahedral shape in Al–0.04 at% Ge and plate- and rod-like ones in Al–0.4 to 1.20 at% Ge are predominant. Particularly, the observed octahedral precipitate with pure Ge phase has the anomalous orientation relationship: (001)_Ge ∥ (111)_Al and [11-0]_Ge ∥ [11-0]_Al. In the SC specimen, a spherical precipitate with amorphous Ge phase has been observed. It is suggested that the nucleation of precipitates originates from vacancy aggregates in the QA specimen, and from Ge–vacancy complexes in the SC specimen, respectively.