## 3 Results and Discussions
### Macro- and Micrograph Results  
To identify the oxide appearance of the post-test samples, investigation of the macro- and micrographs was conducted. Figure 5 shows the macrographs of the post-test samples. Shown in Figure 5a-j, photos were captured after 2 h of isothermal test under 180 mL/min, \begin{table}
\begin{tabular}{c c c c c} \hline
**Oxygen Flow Rate** & **Final Mass** & **Peak Temperature** & **Breakaway** & \(\delta\) **at Breakaway** \\ **(mL/min)** & **Gain (\%)** & **(\({}^{\circ}\)C)** & **Time (s)** & **(\(\mu\)m)** \\ \hline
20 & 14.24 & 1001.86 & 1473 & 47.90 \\
40 & 15.12 & 1003.17 & 1341 & 50.14 \\
60 & 17.89 & 1004.44 & 1387 & 52.25 \\
80 & 25.08 & 1005.81 & 1242 & 54.50 \\
100 & 27.80 & 1033.81 & 1386 & 66.17 \\
120 & 26.75 & 1038.88 & 1430 & 71.65 \\
140 & 26.02 & 1048.33 & 1395 & 78.94 \\
160 & 16.17 & 1062.71 & 1672 & 81.78 \\
180 & 9.48 & 1076.14 & 3079 & 94.81 \\
200 & 11.24 & 1096.42 & 6109 & 131.12 \\ Average (20–140) & 21.84 & 1019.47 & 1379 & 57.10 \\ Average (20–200) & 18.98 & 1037.16 & 2051 & 72.92 \\ \hline Alumina tube & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \\ (Dummy test) & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \\ \end{tabular}
\end{table} 
Table 1: Summary of final mass gain, peak temperature, breakaway time, and oxide thickness at breakaway with various oxygen flow rate conditions for 2 h oxidation test at 1000 \({}^{\circ}\)C.  
Figure 4: Results of the temperature escalation in the early stage of the oxidation reaction: 0 to 150 s. (**a**) Reproducibility check of the temperature: 20 mL/min, and 200 mL/min. (**b**) History of the temperature escalation; oxygen flow rate from 20 to 200 mL/min. while those in Figure 5k,l were obtained after 4 h tests under 200 mL/min, respectively. At a first glance, there are numerous cracks on the external surfaces of the samples, and the color of the oxides appears to be light until 140 mL/min (Figure 5a-g). On the other hands, cracks are not observed, and a dark-colored oxide is observed for 180 and 200 mL/min conditions (Figure 5i,j, respectively), while a mixture of light- and dark-colored oxides were observed with no apparent cracks for 160 mL/min. The change in oxide colors from dark to light coincided with the kinetic transition, which was reported in previous studies [8]. Moreover, the cracks are generated when the accumulated stresses in the oxide scale are released progressively during the oxide phase transition from tetragonal to monoclinic [3,7,9]. These descriptions support the change in oxide color from dark to light in both 180 and 200 mL/min cases, as oxygen exposure time is increased from 2 to 4 h, which can be assumed as the occurrence of the breakaway (Figure 5i-l). Additionally, the investigation of the macrographs is in good agreement with the previously reported TG mass gain. Figure 6 shows microphotography by an optical microscope (OM). For the 2 h photos, the oxidation formation was characterized by the formation of dense columnar oxides at the outermost layer, and porous oxides underneath the dense oxide layer were formed until 140 mL/min (Figure 6a-g). Additionally, large radial cracks in the oxide layer indicate the breakaway. These oxide formations were detected in [1,3,7], where the breakaway temperature range existed (typically ~1050 \({}^{\circ}\)C). In contrast, a different oxide formation was identified from 160 mL/min. Small amounts of porous oxides and some cracks in the dense columnar oxide layer at 160 mL/min were observed. This seems to be a transition region and corresponds to previous mass-gain and macrograph results. These differences are more apparent for 180 and 200 mL/min (Figure 6i,j, respectively). Only thick, dense oxides are observed. As expected, the porous oxides were observed in both 180 and 200 mL/min for 4 h post-test samples (Figure 6k,l, respectively); however, there were still the dense oxide layers without the porous oxides in other points.