## 3 Results and Discussion
### High Temperature Properties
#### 3.2.3. Macrostructure and Microstructure  
Figure 12 shows the macrostructures of unstable and stable regions of the cross-sectioned areas under the process conditions identified in the processing map. Figure 12a-c represents the plastic unstable regions, and it can be seen that the compression of the unstable shapes appeared as looking at the shape of the specimen. During the compression, friction with the die and heat exchange caused a barreling phenomenon, and the Dead Zone was observed on the top and bottom of the specimen and the Severe Plastic Deformation region was observed in the center. In addition, it can be seen that the flow localization band was created at an angle of 45\({}^{\circ}\), and the flow localization band was generated by the insulation conditions caused by the deformation of the high-temperature compression and the local reduction of flow stress due to the low thermal conductivity of the Ti-6 Al-4V alloy. As the temperature increases and the strain rate decreases, the strength of the band decreases, and cracks are likely to occur at an angle of up to 45\({}^{\circ}\)[30]. In practice, cracks occurred along the flow localization band as shown in Figure 12c, and the unstable regions defined in Figures 5 and 11 can be confirmed. In Figure 12d,e, however, it can be seen that a relatively uniform deformation occurred in the Dead Zone and Severe Plastic Deformation region in a macrostructural manner at a temperature above the \(\beta\)-transus of the AMS4928 alloy used in this study. To observe this in detail, the microstructures under the same conditions are shown in Figures 13 and 14. First, Figure 13 shows the same Widmanstatten structure as the initial structure by compressing it at a high-temperature under the temperature below the \(\beta\)-transus. Figure 13a-c represents the regions belonging to the Dead Zone, and about 10 \(\upmu\)m of recrystallized grains that could be observed in the initial structure were observed. In addition, in the case of Figure 13c, it can be seen that the \(\beta\) phase fraction increased because it was compressed at a high-temperature of 850 \({}^{\circ}\)C and was relatively close to the \(\beta\) phase transformation region. In Figure 13d-f, it is possible to observe long elongated microstructures at the Severe Plastic Deformation region. Figure 6 represents that the flow stress has opposite values depending on the conditions of the fast strain rate and temperature, even though the layered interval becomes thinner and the grains become more elongated. In Figure 13d,e with the same temperature conditions, the flow caused by diffusion under the slow strain rate determines the overall strain rate. As the space  
Figure 11: Complex processing map showing the stable and unstable deformation region (\(\epsilon=0.6\)); (a) [PERSON]’s instability criterion, (b) [PERSON]’s instability criterion. Shaded area means the unstable region. between layers in the \(\alpha\)-phase is thin, the area of the \(\alpha/\beta\) interface is widened. Then, the diffusion through the interface is advantageous, and accordingly, it is confirmed that the deformation is easily performed even under low stress. In accordance with this principle, it can be found that the stress required for the deformation is further reduced because the grain of Figure 13f has a thinner layered space. In addition, as the lamellae kink is observed, there is a possibility of plastic instability [31]. Figure 12: Macrostructures of the cross-sectioned areas according to each condition after high-temperature compression of Ti-6 Al-4V alloy; (**a**,**d**) 800 \({}^{\circ}\)C, \(1\times 10^{-2}\)/s, (**b**) 800 \({}^{\circ}\)C, \(1\times 10^{0}\)/s, (**c**) 850 \({}^{\circ}\)C, \(1\times 10^{-2}\)/s, (**d**) 1050 \({}^{\circ}\)C, \(1\times 10^{-2}\)/s, (**e**) 1100 \({}^{\circ}\)C, \(1\times 10^{-2}\)/s.