The microstructure of Al-Si-Mg alloys strongly depends on their chemical composition and on the heat treatment that is performed during production. The influence of solution heat treatment and the additions of Er and Zr on the microstructure of gravity cast A356 (Al-7Si-0.4Mg) are investigated. The as-cast microstructure is characterized by means of grain size measurements, morphology of eutectic Si as well as morphology, area fraction and chemical composition of the intermetallic compounds. The morphology of eutectic Si is found to be unstable with respect to high temperature exposure during solution heat treatment. The evolution mechanisms are described using thermodynamic and kinetic models for the investigated alloys and validated using optical and SEM micrographs. The effects of high temperature exposure during solution heat treatment on the amount and morphology of the intermetallic compounds is also characterized using optical and SEM micrographs.
The micro-structural changes of an austenitic steel with high-nitrogen content have been investigated after heat treatments at 850• C. Micro-chemical composition of heat-induced structures was analyzed using XPS, multipoint AES and photoemission spectro-microscopy. Before the XPS, AES and photoemission spectro-microscopy analyses, the samples were cleaned in situ by Ar ion sputtering. The results show that the discontinuous precipitation of Cr 2 N from the nitrogen supersaturated γ s phase is accompanied by a net flux of nitrogen toward the transformed zones, thus confirming that the principal force behind boundary migration is the nitrogen concentration gradient between the γ and γ s phases. When γ and γ s reach the same nitrogen concentration, the precipitation stops even if the transformation of the matrix has not yet been completed.
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