## 2 Materials and Methods  
The alloy with the following composition: Al-1.2 Mg-0.7 Si-1.0 Cu-0.1 Sc-0.2 Zr wt.%, was prepared using a graphite-fireday crucible in an inductive furnace (Interselt, SaintPetersburg, Russia). When the temperature reached 780 \({}^{\circ}\)C, the casting was carried out in a copper water-cooling mold with a cooling rate of \(\sim\)15 K/s. The temperature of the melt was measured using a chromel-alumel thermocouple. To prepare the melt, the following components were co-added: 99.99 Al, 99.95 Mg, Al-12 Si, Al-53.5 Cu, and Al-5 Zr, Al-2 Sc. The master alloys were placed in the molten Al in decreasing order of their solidus temperatures. The obtained ingots were annealed in two steps at 350 \({}^{\circ}\)C for 8 h and subsequently at 480 \({}^{\circ}\)C for 3 h. The annealing was carried out using a Nabertherm N30/65A air-forced convection furnace (Nabertherm GmbH, Lilienthal, Germany). The multidirectional forging (MDF) was carried out using a hydraulic press machine (Nordberg N3650E, Changshu Tongrun Auto Accessor Co., Ltd., Changshu, China) providing a force of 50 tons. The scheme of the MDF process is demonstrated in Figure 1. The forging process included several circles. Each circle involved three passes in mutually perpendicular directions. During the MDF, the deformation occurred in two directions. The sample shape remained unchanged at the end of each pass. After each pass, the samples were rotated through 90\({}^{\circ}\) to maintain multiaxial deformation. One pass provided a strain of 0.8 and the cumulative strain for a full circle was 2.4. The initial crosshead speed for each pass was 5 mm/min. The samples exhibited parallelepipedal shape with a size \(l\) in one dimension and a similar size \(a\) in two other dimensions (Figure 1). The true stress-true strain curves and structure evolution during forging were studied using small samples with \(l=16\) mm and \(a=8\) mm. The superplastic behavior was analyzed using bulk samples with \(l=56\) mm and \(a=28\) mm. The MDF procedure is described in detail in ref. [50]. The maximum number of MDF circles for the studied alloy was 6 (18 passes) that provided for a maximum cumulative strain of 14.4. The microstructure evolution was analyzed using a Tescan-VEGA3 LMH scanning electron microscope (Tescan Brno s.r.o., Kohoutovice, Czech Republic). The elemental distribution maps were obtained using an energy dispersive spectroscopy (EDS) detector (Oxford Instruments plc, Abingdon, UK). An electron backscatter diffraction (EBSD) maps were obtained using an HKL NordlysMax detector (Oxford Instruments plc, Abingdon, UK). The accelerating voltage and operating distance were 20 kV and 10-15 mm, respectively. The samples' SEM were preliminarily prepared by grinding and polishing in a Struers LaboPol machine (Struers APS, Ballerup, Denmark). The analysis of nanoscale dispersoids was performed using a JEOL JEM transmission electron microscope (JEOL, Tokyo, Japan). Disc-shaped specimens with a diameter of  
Figure 1: The scheme of the multidirectional forging procedure. 3 mm and a thickness of 0.18-0.22 were examined. The specimens were preliminarily electrochemically polished at 20 \(\pm\) 1 \({}^{\circ}\)C and 19 \(\pm\) 2 V in a 30% HNO\({}_{3}\) in methanol electrolyte using a Struers TenuPol-5 twinjet polishing machine (Struers APS, Ballerup, Denmark). The EBSD data were obtained with a scanning step of 0.15 \(\upmu\)m in the area of 250 \(\times\) 250 \(\upmu\)m, and the maps were analyzed using Aztec (EBSD) software (v.2.0, Oxford Instruments plc, Abingdon, UK). The high-angle grain boundaries (HAGBs) were identified as boundaries between the grains with a misorientation angle higher than 15\({}^{\circ}\). The low-angle grain boundaries (LAGBs) were identified as boundaries between the grains with a misorientation angle of 2-15\({}^{\circ}\), respectively.