*3.4. Crack Formation*

In the present investigation, detailed data were obtained on the formation and development of cavity behavior, suggesting that plastic-controlled growth dominated the cavity interlinkage and coalescence. This was expected to result in crack formation and to influence the superplastic elongation-to-failure of the studied 5A70 alloy.

Figure 8a–h illustrates the testing results of the EBSD close to the fracture locations with different temperatures ranging from 400 to 550 ◦C, and the strain rate was 1 × <sup>10</sup>−<sup>3</sup> <sup>s</sup>−1. Correspondingly, Figure 8i–l shows the morphology tested by the SEM at 5 mm from the superplastic fracture location.

Figure <sup>8</sup> shows the EBSD analyses of the 5A70 alloy deformed at different temperatures at . *<sup>ε</sup>* = 1 × <sup>10</sup>−<sup>3</sup> <sup>s</sup>−<sup>1</sup> after superplastic fracture. The color of each grain was coded by its crystal orientation based on the [001] inverse pole figure, as seen in Figure 8a. There was an aggregation of a large number of ultrafine grains near the small cavities, in addition, the cavity interlinkage and coalescence were

precisely identified via supporting microscopy evidence. Nevertheless, new ultrafine grains occurred in limited regions and generated near initial deformed grains, indicating that dynamic recrystallization had occurred. Figure 8a,c,f,g shows that the microstructure consisted mainly of a grain size larger than 10 μm. It can be clearly seen in Figure 8a,c,e,g that the grain size of the FG 5A70 alloy increased with the increased temperature and the superplastic tensile deformation, while the grain sizes were 9.60, 11.78, 13.32 and 21.16 <sup>μ</sup>m. At 400 ◦C and 1 × <sup>10</sup>−<sup>3</sup> <sup>s</sup>–1, dynamic recrystallization occurred without obvious grain growth. However, in this work the final grain structure had an average recrystallized grain size of less than 15 μm at 400–500 ◦C, revealing that the 5A70 alloy had a strong ability to inhibit grain growth during superplastic deformation. At 550 ◦C, the content of Mg-rich phase particles decreased and the abnormal grain growth could be inhibited due to the reduction of the pinning effect during dynamic recrystallization. Therefore, the plastic deformation of the grain growth in the superplastic tensile direction dominated the grain growth and led to crack formation caused by cavity coalescence, as shown in Figure 8g.

**Figure 8.** *Cont*.

**Figure 8.** The Grain structures and the superplastic fracture morphologies at initial strain rate <sup>1</sup> <sup>×</sup> <sup>10</sup>−<sup>3</sup> <sup>s</sup>−<sup>1</sup> with different temperatures: 400 ◦C (**a**,**b**,**i**), 450 ◦C (**c**,**d**,**j**), 500 ◦C (**e**,**f**,**k**) and 550 ◦C (**g**,**h**,**l**).

In Figure 8i–l it can be seen that the cavities nucleated tightly attached to the particles, the cavity growth interlinked in the superplastic tensile state, indicated by red arrows, and the cavities coalesced, indicated by the yellow arrows. Comparing Figure 4a,c,e and Figure 8a,c,e, it is suggested that with the continuous precipitation of Mg-rich phase particles at the grain boundaries, the new cavities nucleated continuously in the superplastic tensile state. Meanwhile, the cavity area fractions in Figure 8i–k of 1.79, 4.07 and 8.66% increased with temperatures of 400, 450 and 500 ◦C, respectively. There was a large amount of cavity nucleation and growth along with the accumulation of tensile deformations, as shown in Figure 8i–l. Due to the decrease of second phase particles, it was clearly found that the increase in the grain sizes had a certain effect in terms of promoting the absorption of small cavities at <sup>550</sup> ◦C and 1 × <sup>10</sup>−<sup>3</sup> <sup>s</sup><sup>−</sup>1, while the cavity area fraction was 5.66% as shown in Fgiure 8l. Based on this research, it can be stated that cavity nucleation, growth, interlinkage and coalescence, along with the accumulation of tensile deformation, cause crack formation, resulting in the superplastic fracture of FG 5A70 alloys.
