## 3 Results and Discussion
### Identification of Serrations  
In this study, it is crucial to examine the serrated flow behavior observed in the stress-strain curves at 300 and 400 \({}^{\circ}\)C and to explore its correlation with the mechanical properties. According to the literature [23; 36; 37; 38], four types of serrations were typically found in stress-strain curves. Serration types differ from one another in how the stress rises and falls in relation to the general stress level, typically viewed as a smooth engineering stress-strain curve before serration occurs. Specifically, type-A serrations are distinguished by a periodic pattern where there is a sharp increase in stress, followed immediately by an abrupt drop to below baseline levels, while the remainder of the stress-strain curve remains smooth. In contrast, type-C serrations are characterized by an initial sudden drop in stress below the starting point of the stress-strain curve, followed by a rapid increase back to the overall stress level. Type-B serrations, which frequently coexist with either type-A or type-C serrations, are primarily marked by continuous stress fluctuations around the average line of the stress-strain curve. In addition, type-D serration often occurs at high temperatures with minimal work hardening, and they can be identified by the presence of plateaus or steps in the stress-strain curve. As mentioned before, significant serrations were evident in the stress-strain curves of the Fe-13 Cr-4 Al alloy at intermediate temperatures (300-400 \({}^{\circ}\)C). Figure 10 shows the  
Figure 9: SEM pictures of the fracture surfaces of specimens at (**a,b**) 600 \({}^{\circ}\)C, (**c,d**) 700 \({}^{\circ}\)C, and (**e,f**) 800 \({}^{\circ}\)C.  
servation characteristics at 300 and 400 \({}^{\circ}\)C in detail. At the initial stage of the stress-strain curve at 300 \({}^{\circ}\)C, there was a rapid increase in stress of approximately 7.3 MPa, as illustrated in Figure 10a. This rapid increase was attributed to the locking of mobile dislocations. Once the peak stress was attained, it promptly returned to the general stress level. This sudden drop in stress was primarily caused by the unpinning of dislocations. Such behavior observed at the initial portion of the stress-strain curve is indicative of a typical type-A serration. In addition to type-A serrations, type-B serrations with small stress oscillations became dominant in the latter stage of plastic deformation. Consequently, the deformation behavior at 300 \({}^{\circ}\)C is characterized by a combination of type-A and type-B serrations, referred to as type A + B. When the deformation temperature reached 400 \({}^{\circ}\)C, type-C serrations became predominant, as displayed in Figure 10b. In the initial part of the engineering stress-strain curve, the stress initially exhibited a sudden fall (\(\Delta\sigma\approx\) 11.8 MPa), followed by a steady increase back to its general level. This pattern of behavior is characteristic of a typical type-C serration. Moreover, following the appearance of type-C serrations, small-amplitude type-B stress oscillations were observed, indicating the presence of a mixed serration pattern, known as type B + C, which combines features of both type-B and type-C serrations. As strain increased, type-B serrations progressively diminished, while type-C serrations featuring significant stress drops became more prevalent. Consequently, the serration pattern at 400 \({}^{\circ}\)C is characterized as type-B + C, with a dominance of type-C serrations.