| Summary: | The current study focusses on the phase composition, solidification path, and microstructure evaluation of gravity cast Al-4Mg-0.5Si-<i>x</i>La aluminum alloy, where <i>x</i> = 0, 0.1, 0.25, 0.5, 0.75, and 1 wt.% La. A computational CalPhaD approach implemented in Thermo-Calc software and scanning electron microscopy technique equipped with electron microprobe analysis (EMPA) was employed to assess its above-mentioned characteristics. The thermodynamic analysis showed that the equilibrium solidification path of La-containing Al-Mg-Si alloys consists of only binary phases LaSi<sub>2</sub> and Mg<sub>2</sub>Si precipitation along with <i>α</i>-Al from the liquid and further solid-state transformation of this mixture into <i>α</i>-Al + Al<sub>11</sub>La<sub>3</sub> + Mg<sub>2</sub>Si + Al<sub>3</sub>Mg<sub>2</sub> composition. Scheil–Gulliver simulation showed a similar solidification pathway but was accompanied by an increase in the solidification range (from ~55 °C to 210 °C). Furthermore, microstructural observations were congruent with the calculated fraction of phases at 560 °C and related to <i>α</i>-Al + LaSi<sub>2</sub> + Mg<sub>2</sub>Si three-phase region in terms of formation of La-rich phase having both eliminating effect on the eutectic Mg<sub>2</sub>Si phase. Quantitative EMPA analysis and elemental mapping revealed that the La-rich phase included Al, La, and Si and may be described as Al<sub>2</sub>LaSi<sub>2</sub> phase. This phase shows a visible modifying effect on the eutectic Mg<sub>2</sub>Si phase, likely due to absorbing on the liquid/solid interface.
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