*3.1. Superplastic Tensile Tests*

It is known that 5A70 aluminum alloy is non-heat treatable with the dissolution and melting temperature is 556 and 631 ◦C, respectively. Superplastic tensile tests focused on investigating the states of cavity in superplastic deformation of the studied 5A70 alloy. Superplastic tensile tests of FG 5A70 alloy were performed at an initial strain rate of 1 × <sup>10</sup>−<sup>3</sup> <sup>s</sup>−<sup>1</sup> and the temperatures ranged from 400 to 550 ◦C, according to relevant literature and thermal analysis experimental results [21]. The true stress–true strain results of the superplastic tensile tests are shown in Figure 1a. At a constant strain rate, the strain enhancement phenomenon of the materials was consistent with the general law of superplastic elongation characteristics and increased temperature. Meanwhile, three more tensile tests were carried out and the tensile deformations were unloading at the strain *ε* = 0.65, 1.40 and 2.65, when the superplastic tensile test temperature was 500 ◦C and the strain rate was 1 × <sup>10</sup>−<sup>3</sup> <sup>s</sup>−1. The intermediate state of the deformation structures was obtained using water quenching, and the engineering stress–engineering strain results are shown in Figure 1b.

**Figure 1.** True stress–true strain results of 5A70 aluminum alloy at 400–550 ◦C and 1 <sup>×</sup> <sup>10</sup>−<sup>3</sup> <sup>s</sup>−<sup>1</sup> (**a**), and different superplastic tensile strain stages, *<sup>ε</sup>* = 0.65, 1.40 and 2.65, at 500 ◦C and 1 <sup>×</sup> <sup>10</sup>−<sup>3</sup> <sup>s</sup>−<sup>1</sup> (**b**).

Figure 1a shows that there was no obvious steady-state flow stage in the superplastic tensile state of the 5A70 alloy, as was the case for other aluminum alloys with a different magnesium content [22,23]. When the strain rate was constant, the superplastic elongation-to-failure results of the FG 5A70 alloy enlarged with the increased temperatures were *δ* = 205%, 321%, 390% and 406%. At 550 ◦C, which is close to the dissolution temperature of precipitated phase particles. The reduction of the pinning effect during dynamic recrystallization because of the content of phase particles decrease. In addition, the strain hardening obviously occurred during the superplastic deformation due to the increased distortion of the grain growth, leading to an increase of true stress [24]. The strain hardening was attributed to dislocation sliding/climbing. Additionally, the dislocation density changed nonmonotonically with stable grain structure during the initial stage of the superplastic deformation. A high dislocation density at the beginning of the deformation at *T* = 400 ◦C with a high strain rate caused by grain adaptation—i.e., where the dislocation density increased rapidly and was plugged into the grain—formed dislocation walls/cells and led to increased true stress [25]. However, with the accumulation of deformations, the grain rotation occurred under shear stress and dislocations were absorbed by the grain boundaries, which led to the true stress remaining stable for a short period of time [26]. This is the reason for the true stress presenting in a step-up state. With the increasing temperature and the accumulation of superplastic deformation, the true stress increased and the strain hardening strengthened gradually and clearly.

Figure 1b demonstrates that in the initial stage of superplastic tensile, the engineering stress increases continuously and reaches its maximum at *ε* = 0.65. This is due to the large number of dislocations formed during superplastic deformation and the grain growth in high temperature and low strain rate. The maximum stress value, *σmax* = 3.75 MPa as shown in Figure 1b. When the strain reached *ε* = 1.40, the applied stress gradually decreased and the superplastic deformation reached a correspondingly stable region. The applied stress slowly decreased and eventually remained low value until the strain reached *ε* = 2.65. The data at different stages of superplastic tensile stress are provided by the test results in Figure 1b. In the last stage of the superplastic tensile, the stress value remains constant in Figure 1b and the corresponding true stress value increases continuously as shown in Figure 1a.
