Cu‐mold centrifugal cast processing is employed as a rapid solidification method for producing samples with and without Sc. The Al–Si–Mg and Al–Si–Mg–Sc alloy samples are exposed to direct aging treatments varying temperature and time to verify the microstructural changes. Both rapidly solidified samples and as‐aged samples are characterized by a number of methods, including optical microscopy, SEM–EDS, transmission Electron Microscopy (TEM)–EDS, TEM–HAADF, X‐ray diffraction (XRD), differential scanning calorimetry (DSC), and Vickers hardness. At first, the results point to a strong precipitate‐related hardening effect formed as a result of the Sc addition to the alloy. All samples containing Sc show a higher hardness value when compared to their respective treated samples without Sc. Second, when comparing the Al–10Si–Mg–0.4Sc alloy samples among themselves after being treated at different conditions, high temperatures, and excessive treatment times are recognized as detrimental to the hardness. This is due to the growth of larger Sc‐bearing precipitates of approximately 1 μm in size under such conditions, having lower efficiency in pinning dislocations during loading. The best aging condition is 255 °C for 60 min, which produces a very fine dispersion of Mg and Sc intermetallics (200 nm in size) with a peak hardness of 110 HV.
The effects of 0.4wt.% Sc addition on a typical Al-Si10-Mg alloy were systematically investigated in the present research. Samples with and without Sc produced refined dendritic arranged microstructures with sensitivity to the aging treatment after solidification, particularly in the case of the alloy without Sc. After being exposed to 300°C for 90 minutes, the dendritic spacing nearly doubled in the Al-10wt.%Si-0.45wt.%Mg samples. The rapidly solidified microstructures were constituted by the -Al dendritic phase surrounded by eutectic phases/intermetallics such as Si, Mg2Si and Al3Sc (in the case of the alloy containing Sc). 255°C and 300°C were deemed most appropriate temperatures for aging treatments, with four exposure times of up to 120 minutes tested for each alloy. The heat treatments allowed the Vickers hardness profiles to be plotted and compared. Moreover, in order to detect both Sc- and Mg- precipitates after aging, specific samples have been prepared for either SEM or TEM analyses. At first, the results pointed to a strong precipitate-related hardening effect formed as a result of the Sc addition to the alloy. All samples containing Sc showed a higher hardness value when compared to their respective treated samples without Sc. Secondly, when comparing the Al-10Si-Mg-Sc alloy samples among themselves after being treated at different conditions, high temperatures and excessive treatment times can become detrimental to the hardness. This was due to the growth of larger Sc-bearing precipitates of approximately 1 µm in size under such conditions, having less pronounced hardening effect. The best condition (255°C for 60 min) for as centrifuged samples in Cu-mold produced very fine dispersion of Mg- and Sc- intermetallics (~200 nm in size) with a peak hardness of 110 HV.
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