*2.2. Methods of Studying the Microstructure and Mechanical Properties*

The mechanical properties of the experimental alloys (ultimate strength and relative elongation) were determined in accordance with ASTM B557M-15 on an Instron 5982 testing machine (Instron, Norwood, MA, USA). In each experiment, 4 samples were tested. Each experiment was repeated twice.

Microstructural studies were carried out on a Carl Zeiss-brand Imager.Z2m AXIO universal research motorized microscope (Carl Zeiss, Microscopy GmbH, Göttingen, Germany).

Quantitative analysis of the microstructure (average length, area of eutectic silicon and α-Al dendrites) was carried out using the specialized program ImageExpert Pro 3.7, version 3.7.5.0, NEXSYS, (Moscow, Russia) over three images per each sample. The photographs were processed through the use of the following operations: changing the size of the photograph, selecting the scale, binarization, determining the object of study by color, etc. For image processing, the ImageExpert Pro 3.7 program uses built-in algorithms (the methods correspond to the ASTM E112-10 international standard).

To measure the size of the α-Al dendrite, the secondary dendrite arm spacing (SDAS) was determined. The SDAS was evaluated (as reported in [32]) by measuring thirty dendrites for each sample using three images at 50× magnification.

The microstructure and elemental composition and distribution of modifying elements in the structure were studied using a Phenom XL scanning electron microscope (SEM) with an integrated energy-dispersive spectrometry (EDS) system (Phenom-World BV, Eindhoven, Netherlands). To determine the elemental composition at a point and according to area, the method of quantitative X-ray spectral microanalysis was used using special Phenom Element Identification v. 3.8.0.0 software. To carry out X-ray mapping, the Elemental Mapping software was used.

#### *2.3. Methods of Chemical and Thermal Analysis*

The chemical (elemental) composition of the samples was studied using a CCD-based Q4 TASMAN-170 spark optical emission spectrometer. The Q4 TASMAN-170 spectrometer was controlled from a desktop computer using special QMatrix software version 3.8.1 (Bruker Quantron GmbH, Kalkar, Germany).

The actual rubidium content was obtained using an ICAP 6300 inductively coupled plasma atomic emission spectrometer (ICP-AES) (Thermo Fisher Corporation, Cambridge, UK).

Thermal analysis of the alloys' solidification was carried out using a Netzsch DSC404 F3 Pegasus differential scanning calorimeter (Netzsch-Geratebau GmbH, Bavaria, Germany). The samples were subjected to exposure at room temperature for 24 h before the test. The analyzed sample and the platinum standard were placed with thermocouples (Pt–Rh) in platinum crucibles and put into the heating chamber of the installation. The tests were carried out in an inert argon atmosphere. NETZSCH Proteus® v. 7.1 software was used to process the data obtained (Netzsch-Geratebau GmbH, Bavaria, Germany).

#### *2.4. Porosity Investigation Method*

The porosity of the alloys was evaluated on a five-level standard scale in macrosections of samples cut from castings in sandy-clay molds in accordance with ISO 10049:2019(E) [33]. To disclose the pores, the sample was sanded and etched in a 20% NaOH solution in accordance with ISO 10049:2019(E).
