Precipitation and hardening in Al–Si–Ge

The principal focus of this work is to explain precipitation in Al–1 at.%Si–1at%Ge. The microstructure is characterized using conventional and high resolution transmission electron microscopy, as well as energy dispersive X-ray spectroscopy. The first precipitates to come out of solid solution have a cube–cube orientation relationship with the matrix. High resolution TEM demonstrated that all the precipitates start out, and remain multiply twinned throughout the aging treatment. Any twinned section of the precipitate no longer maintains a low index interface 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. There is a variation in the stoichiometry of the precipitates, with the mean composition being Si–44.5 at.%Ge. It is also shown that in Al–Si–Ge it is not possible to achieve satisfactory hardness through conventional heat treatment. This result is explained in terms of sluggish precipitation of the diamond-cubic Si–Ge phase coupled with particle coarsening.