3.2.2. True Strain-True Stress

Figure 4 shows the superplastic tensile results for the true strain-true stress of the fine-grained 5A70 alloy at 400–550 ◦C. At a constant temperature, the strain rate enhancement phenomenon of the materials was consistent with the general law of superplastic elongation characteristics and increased strain rate. The peak applied stress can be observed without the steady-state flow stage, which was the typical variation of the true stress-true strain behavior of the Al-Mg alloys [34–36]. The strain hardening was attributed to dislocation sliding/climbing. In addition, the dislocation density changed non-monotonically with the stable grain structure during the initial stages of superplastic deformation. A high dislocation density at the initial deformation for *T* = 400 ◦C with a high strain rate was caused by grain adaptation, i.e., the dislocation density increased rapidly and plugged in the grain, forming dislocation walls/cells and leading to an increased true stress. However, with the accumulation of deformation, the grain rotation occurred under shear stresses, and dislocations were absorbed by the grain boundaries, which led to the true stress remaining stable over a short period of time [37]. This is the reason that the true stress presented a step-up state. As the temperatures increased and the superplastic deformation accumulated, the true stress increased and the strain hardening gradually enhanced. At 500 and 550 ◦C with 5 × <sup>10</sup>−<sup>4</sup> <sup>s</sup>−1, superplastic *<sup>δ</sup>* values achieved 398 and 437% due to the strong reduction of the unstable superplastic flow. However, the effect of diffusion creep was continuously strengthened as the deformation accumulated. The strong pinning effect declined

because of the dissolution of the precipitated second phase particles, and the decreasing of the pinning effect led to abnormal grain growth during superplastic tensile. In addition, the completion of the grain rotation at the final tensile stage, which cannot balance the shear stress caused by the true stress, continually increased, as shown in Figure 4b.

**Figure 4.** True strain-true stress results for the fine-grained 5A70 alloy at different temperatures: 400–450 ◦C (**a**) and 500–550 ◦C (**b**) with strain rates: 5 <sup>×</sup> <sup>10</sup><sup>−</sup>3–5 <sup>×</sup> <sup>10</sup>−<sup>4</sup> <sup>s</sup><sup>−</sup>1.
