An AM60 magnesium alloy nanocomposite reinforced with 1 wt % of AlN nanoparticles was prepared using an ultrasound (US) assisted permanent-mould indirect-chill casting process. Ultrasonically generated cavitation and acoustic streaming promoted de-agglomeration of particle clusters and distributed the particles throughout the melt. Significant grain refinement due to nucleation on the AlN nanoparticles was accompanied by an exceptional improvement in properties: yield strength increased by 103%, ultimate tensile strength by 115%, and ductility by 140%.Although good grain refinement was observed, the large nucleation undercooling of 14 K limits further refinement because nucleation is prevented by the formation of a nucleation-free zone around each grain. To assess the industrial applicability and recyclability of the nanocomposite material in various casting processes, tests were performed to determine the effect of remelting on the microstructure. With each remelting, a small percentage of effective AlN nanoparticles was lost, and some grain growth was observed. However, even after the third remelting, excellent strength and ductility was retained. According to strengthening models, enhanced yield strength is mainly attributed to Hall-Petch strengthening caused by the refined grain size. A small additional contribution to strengthening is attributed to Orowan strengthening.

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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.

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