3.3.3. Effect of the Fine-Grained Structure on Cavity Growth

The above analyses showed that the fine-grained structure played an important part in the superplastic diffusion growth and plastic-controlled growth. Figure 7a showed that fully dynamic recrystallization occurred during the superplastic tensile deformation and the grain size increased, while the result for *ε* = 2.65 illustrated that the grain size fraction of *d* = 20–30 μm was clearly much higher than *ε* = 0.65 and 1.40. It is well-know that the glide plane of the face-centered cubic crystal of FG 5A70 alloy is {1 1 1}, and the grain boundary slip direction is <1 1 0> [40]. Meanwhile, in order to satisfy the balance of grain-to-grain deformation and the balance of the reaction stress in the superplastic

tensile state, the GBS occurred mainly in the sliding direction. This clearly clarified the grain rotation and the GBS in the superplastic tensile state of the FG 5A70 alloy, as illustrated in Figure 7b [41].

Figure 7b shows that the fraction of the grain boundary angle at 3.58◦ was 20.3% (*ε* = 2.65), which was greater than 5.41% (*ε* = 0.65) and 3.83% (*ε* = 1.40). Plastic-controlled growth dominated the cavity interlinkage during deformation because the fraction of large cavities increased obviously from *ε* = 0.65 to 2.65, as shown in Figure 7c. At *ε* = 2.65, the cavity area fraction of 20–200 μm2 was significantly higher than *ε* = 0.65 and 1.40. This demonstrated that the cavity coalescence formed a larger cavity, which was detrimental to the superplastic tensile state and eventually led to cracking.

**Figure 7.** Distribution of the strain due to grain boundary sliding in (**a**) the dynamic recrystallization grain size, (**b**) the grain boundary angle rotation and (**c**) the number of cavity fraction in different strain stages. The superplastic tensile condition was 500 ◦C and 1 <sup>×</sup> <sup>10</sup>−<sup>3</sup> <sup>s</sup>−<sup>1</sup> of fine-grained 5A70 alloy.
