Precipitation and hardening in Al-Si-Ge

The principal focus of this work is to explain precipitation in Al-lat.%Si- la%Ge. The microstructure is characterized using conventional and high reso lution 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 dem onstrated that all the precipitates start out, and remain multiply twinned throughout the aging treatment. Any twinned section of the precipitate no l onger maintains a low index interface with the matrix. and consequently goe s from a crystallographic to a spherical interface with the matrix. This ex plains the equiaxed shape of the Si-Ge precipitates. There is a variation i n 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 achie ve satisfactory hardness through conventional heat treatment. This result i s explained in terms of sluggish precipitation of the diamond-cubic Si-Ge p hase coupled with particle coarsening. (C) 2001 Elsevier Science B.V. All r ights reserved.