Three-dimensional characterization of 'as-cast' and solution-treated AlSi12(Sr) alloys by high-resolution FIB tomography

In this study, the three-dimensional (3D) microstructure of different unmodified and Sr-modified Al-Si base alloys is characterized by a novel focused ion beam-energy dispersive spectroscopy (FIB-EDX) for tomography. Al-Si alloys containing >7% Si present a percolating Si network in the 'as-cast' condition. Modification treatment with the addition of small quantities of Sr produces the formation of an extremely fine eutectic architecture of coral-like morphology. The spheroidization and coarsening of the eutectic Si structure during solution treatment is studied in 2D cross-sections and 3D reconstruction. The connectivity of the 'as-cast' Si networks is lost rapidly when reheated to >400 deg. C, and the Si particles coarsen with holding time. Both aluminum and silicon phases are identified with a resolution of ∼60 x 75 nm2 in the image plane and ∼60-300 nm between each slice to reconstruct a FIB tomography of the eutectic Si in the 'as-cast' and solution-treated conditions. The combination of EDX element mapping with low-contrast secondary electron or backscattered electron imaging of a series of parallel cross-sections produced by FIB yield 3D geometrical parameters for the Si which differ from those determined by 2D metallography. The stereological data obtained from FIB tomography quantify the spheroidization of Si much better than those derived from 2D metallography