*2.1. Material Preparation*

Two alloys with a low Mg/Si ratio: 0.3 for Al0.3Mg1Si and 0.38 for Al0.5Mg1.3Si were chosen for the study. Both alloys were alloyed with small additions of Zr and Sc in the following combinations: Al-0.3Mg-1Si-XSc-YZr and Al-0.5Mg-1.3Si-XSc-YZr alloys, where X = 0, 0.1, 0.2, 0.3 wt. % and Y = 0, 0.05, 0.10, 0.15 wt. %. Its chemical composition are shown in Table 1.

Melting of alloys was carried out in a medium-frequency induction furnace using a graphite crucible, the melt weight was 4–5 kg. The weight of the cast ingot was 3 kg. The following materials were used as a charge for the alloy: aluminum (purity 99.8%), magnesium (99.9%), Al-2%Sc master alloy, Al-12%Si master alloy, and Al-5%Zr master alloy. The casting temperature was 720–740 ◦C. Before pouring the molten metal into the casting mold, it was refined with a carnallite flux introduced at the rate of 5 g per 1 kg of charge. After that, the metal was poured into a steel mold at a uniform pouring time of 40 s. The obtained ingot size was 135 mm × 30 mm × 200 mm and its cooling speed was 2 ◦C/s. After solidification, the ingot was removed from the mold and cooled in water.

### *2.2. Material Characterization*

Micro-hardness tests were carried out on a digital stationary hardness tester according to the micro-Vickers method on the HV-1000 model (test forces 0.2452 N). In order to exclude the influence of grain boundaries and large intermetallic compounds, measurements were made in the grain center. Measurements of microhardness by indentation were carried out in accordance with GOST 9450-76 [33].


**Table 1.** Chemical composition of the investigated alloys.

The grain structure of the samples was examined using a Carl Zeiss Axiovert-40 MAT optical microscope (Carl Zeiss AG, Oberkochen, Germany). The preparation of microsections included cutting out samples, mechanical grinding, polishing, as well as electropolishing in a fluoroboric electrolyte of the following composition: boric acid—11 g,hydrofluoric acid—30 mL, distilled water—2200 mL. For each sample, the mean grain size was measured by the linear intercept method.

Intermetallic particles were investigated using a JEOL 6390A SEM. The sample preparation technique consisted of mechanical grinding, polishing and electropolishing. Electropolishing was carried out at a temperature of 85–110 ◦C and a voltage of 10–30 V in an electrolyte of the following composition: 500 mL of H3PO4; 300 mL H2SO4; 50 g CrO3; 50 mL H2O. Study of the chemical composition of structural components by energy dispersive spectroscopy using the X-Max 80T detector in the energy range 0–10 keV (the energy resolution of the detector is 122 eV).

For effective phase counting, the image was made with maximum contrast until the appearance of "noise" points. The ImageJ program (1.52 u, National Institutes of Health, Wayne Rasband (NIH), Bethesda, MD, USA) was used to process the image to the desired contrast of the aluminum matrix and secondary phases.

The study was carried out on the Tecnai G2 F20 S-TWIN TMP transmission electron microscope with a thermal-field cathode at accelerating voltage of 200 kV. The chemical composition of the fine dispersion participles was studied by energy-dispersive spectroscopy (EDS) method using the X-Max 80T detector in the energy range 0–10 keV. The energy resolution of the detector is 122 eV.

The sample preparation for the study was carried out in the following sequence: cutting out samples with a diameter of 3 mm and a thickness of 0.3 mm on the Art's electric discharge machine, mechanical thinning on SiC paper from both sides, electrolytic polishing on a Tenupol-5 installation in a nitrogen-alcohol electrolyte. Polishing mode: −38 ◦C, 16 V.

#### *2.3. Phase Diagram Calculation*

The polythermal sections of phase diagrams was calculated using the Thermo-Calc 2016a software. Thermodynamic database TCAL4 (Thermo-Calc Software Al-based alloy database, Stockholm, Sweden, Version 4.0) was used [34].
