| Summary: | The effects of scandium (Sc) additions on the solidification microstructure and mechanical properties of Al–7Si–0.6Mg-0.25Ti alloy at different cooling rates have been systematically studied by thermal analysis and detailed SEM and TEM microstructure characterization of the castings. It is found that the addition of Sc has significant effects of refining the grain structure and modifying the Al–Si eutectics into fibrous structure. As a result, substantially improved strength and ductility in the as-cast state have been achieved in the Sc-added alloy. More importantly, the addition of Sc reduces the sensitivity of the alloy's strength and ductility to local cooling rates in the castings. The underlying mechanisms on the grain refinement, modification of Al–Si eutectics and strengthening by Sc addition have been studied. A new phase, DO22 structured (Al, Si)3(Ti, Sc), has been found to form in the liquid aluminium, acting as high potency nucleation sites of Al grains, which overcomes the Si poisoning effect on grain refinement for Al–Ti–B grain refiners.
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