*3.1. Initial Microstructures*

The 2 mm thick, deformed, superplastic sheet was fully recrystallized at 340 ◦C for 30 min, while the total deformation was 90% (20–2 mm). The electron backscatter diffraction (EBSD) map of the studied alloy based on the inverse pole figure coloring (insert) is shown in Figure 2. The average grain size of the fully recrystallized structure was 7.80 μm, the fraction of the high angle grain boundaries (HAGBs, θ ≥ 15◦) was 0.96, and the average orientation angle was 38.0◦ in the rolling direction (RD) plane, as shown in Figure 2a,d,e. Similarly, Figure 2b,d,e show the recrystallization structure along

the normal direction (ND) plane. The average grain size was 8.4 μm, the HAGBs were 0.51, and the average of the orientation angle was 19.4◦. Figure 2c,d,e show the recrystallization structure along the transverse direction (TD) plane. The average grain size was 9.3 μm, the HAGBs were 0.85, and the average of the orientation angle was 35◦. The GBS can frequently take place during superplastic deformation because of the fine-grained structure and high fraction of the HAGBs in the initial microstructures.

Grain fragmentation followed by rolling deformation of new grains rapidly occurred in the original grain interiors after full recrystallization, as shown in Figure 2a–c. The new grains exhibited ultrafine equiaxed structures along the original boundaries. The grain size distributions were uniform over all planes, and most of the grain sizes ranged from 5 to 10 μm. However, the low angle grain boundaries (LAGBs, θ = 2–15◦) were as high as 57.4% in the ND plane whenever there was only 8.3 and 14.7% in the RD and TD planes, respectively. In summary, the larger the grain orientation angle and the higher the grain boundary index, the smaller the fine-grained size (average grain size = <sup>√</sup><sup>3</sup> 7.8 <sup>×</sup> 8.4 <sup>×</sup> 9.3 <sup>=</sup> 8.48 <sup>μ</sup>m).

**Figure 2.** EBSD results of the grain size in the RD plane (**a**), ND plane (**b**), and TD plane (**c**); the grain size distribution (**d**); and the grain orientation angle (**e**) of the 2 mm thick fine-grained 5A70 alloy.
