**2. Materials and Methods**

The studied 5A70 alloy (correspond to GB/T 3190-2008 (China)), with a chemical composition of Al-5.72Mg-0.60Mn-0.058Cu-0.20Fe-0.080Si-0.095Zr-0.020Zn-0.043Ti (wt.%), was fabricated using continuous casting. A differential scanning calorimeter (DSC) experiment was conducted using the processed sample on the SDT-Q600 thermo-analytical instrument (TA Instruments Inc., New Castle, PA, USA), and the curve was obtained at a heating rate of 5 ◦C/min for the 5A70 alloy immediately after solutionizing and natural aging. Meanwhile, the phase-transition temperature of the Al-5.7Mg alloy ranged from 255–575 ◦C, according to the Al-Mg binary phase diagram shown in Figure 1b [32]. The DSC curve in Figure 1a shows two endothermic peaks. The result of the first endothermic peak clearly indicates the onset of incipient solid solution at 556 ◦C, and the solid solution temperature of the β phase particles was 581 ◦C. The second endothermic peak demonstrates that the melting temperature of 5A70 aluminum alloy was 631 ◦C.

The prepared experimental alloy ingot was homogenized and annealed at 450 ◦C for 40 h. Then, the ingot alloy was rolled into a billet with a rectangular normal direction plane of 255 mm × 255 mm at 380 ◦C. A billet with a plate shape (200 mm × 200 mm × 25 mm) was processed from the state of the extruding ingot, and the natural aging treatment was ≥240 h. The size and distribution of the precipitated phases in the smelted and forged processes were controlled to promote nucleation

during dynamic recrystallization. The billet was subjected to 14 passes of cold rolling on a 350 mm reversing cold mill. In addition, full recrystallization at 340 ◦C was performed for 30 min using a GS-2-1200 box-type resistance furnace when the sheet was 10 and 5 mm thick. Ultimately, a 2 mm thick fine-grained 5A70 alloy superplastic sheet was obtained. Further details of the rolling process and the full recrystallization system were reported in a previous work [33].

**Figure 1.** Differential scanning calorimeter (DSC) results of the 2 mm thick fine-grained 5A70 alloy (**a**) and the binary Al-Mg phase diagram (**b**).

Specimens with 8 mm gauge lengths and 4 mm gauge widths were machined along the parallel rolling direction. Superplastic tensile tests were performed on an AG-250KINC Instron machine (Shanghai Gold Casting Instrument Analysis Co., Ltd., Shanghai, China) with a microprocessor control pad in an NV63-CV high temperature furnace. The tests were performed at 400, 450, 500, and 550 ◦C, and the initial strain rates were 5 × <sup>10</sup>−3, 1 × <sup>10</sup>−3, and 5 × <sup>10</sup>−<sup>4</sup> <sup>s</sup>−<sup>1</sup> in air conditions. In addition, type-K thermocouples were used to detect the furnace temperature, which was controlled within approximately ±2 ◦C along the entire gauge length during the tests. The specimen was insulated at 340 ◦C for 10 min for full recrystallization; moreover, the furnace can control the temperature equilibrium during the heating process. The same heating rate, 21 ◦C/min, was utilized to reach the target temperature within 10 min and held for 2 min for stabilization.

Microstructural characterization and analysis of the 5A70 alloy were carried out using a Jeol-7100 (JEOL Ltd., Tokyo, Japan) transmission electron microscope (TEM), and a 600FEG (FEI Corporation, Hillsboro, OR, USA) scanning electron microscope (SEM). The average grain size was found using the linear intercept method with the OIM software (6.2.0 x86 version, EDAX Inc., Draper, UT, USA). The boundary orientation was measured by utilizing pixel-to-pixel measurements. To carry out the structural characterization in the SEM, specimens were cut into 5 mm sections near the superplastic fracture surface.

### **3. Results**
