Materials Science Forum, Vol.396-4, 905-910, 2002
Clustering and precipitation in Al-Si-Ge and Al-Si-Ge-Cu alloys
Atom probe tomography and high resolution electron microscopy revealed that in the very early stage of aging Al-Si-Ge alloys, clustering of Si and Ge atoms takes place followed by precipitation at structural defects. Electron microscopy observations demonstrated that precipitates are multiply twinned throughout the aging treatment. Any twinned section of the precipitate no longer maintains low-index interfaces with the matrix, and consequently goes from a crystallographic to a spherical interface with the matrix. This explains the equiaxed shape of the Si-Ge precipitates. CALPHAD calculation predicted and X-ray microanalysis confirmed a decreasing concentration of Ge within the equilibrium precipitate as the aging temperature was increased. In the quaternary AlSi-Ge-Cu alloy, Si-Ge particles quickly nucleate and grow during elevated temperature aging (the y are detected after as little as 30 min. at 190degreesC). The Si-Ge particles then act as nucleation sites for theta' precipitates, resulting in a peak aged microstructure consisting of a dense distribution of theta ' attached to Si-Ge.