## 2 Experimental Procedures  
The studied alloy had chromium and aluminum contents of 13% and 4%, respectively. Vacuum induction melting was used to prepare a 20 kg FeCrAl alloy ingot comprising high-quality commercial metals (purity > 99.95%). The alloy ingot underwent homogenization at 1200 \({}^{\circ}\)C for a duration of 5.5 h, followed by hot forging into a plate, with the final forging temperature set at 1050 \({}^{\circ}\)C. The plate had a thickness of approximately 35 mm. Subsequently, the forged plate underwent hot rolling at 790 \({}^{\circ}\)C, achieving an approximate 70% reduction in overall thickness. Ultimately, the hot-rolled plate was subjected to heat treatment at 850 \({}^{\circ}\)C for a duration of 1.5 h. The chemical composition of the Fe-13 Cr-4 Al alloy was measured with inductively coupled plasma optical emission spectrometry (ICP-OES) and was determined to be 78.20 wt.% iron, 13.10 wt.% chromium, 3.94 wt.% aluminum, 1.93 wt.% molybdenum, 0.63 wt.% niobium, 0.40 wt.% tantalum, and 0.05 wt.% yttrium.  
Cylindrical specimens, with a gauge length of 25 mm and a gauge diameter of 5 mm, were prepared for high-temperature tensile testing according to the ASTM E8/E8M standard [32]. The dimensions of the tensile specimens are shown in Figure 1. The applied tensile loading axis of each specimen coincided with the rolling direction of the alloy plate. Given that the normal cladding temperature of a light water reactor core is between 300 and 420 \({}^{\circ}\)C, it was important to investigate the mechanical behaviors of alloy in this temperature range. However, the cladding temperature is expected to be significantly higher in accidental conditions. For this reason, a wider testing temperature range (200-800 \({}^{\circ}\)C) was chosen to expand our understanding of the mechanical properties and corresponding fracture mechanisms of the investigated alloy. Uniaxial tensile tests were performed using a hydraulic material-testing machine (MTS, Eden Prairie, MN, USA) within a temperature range of 200 to 800 \({}^{\circ}\)C, at a constant strain rate of 10\({}^{-3}\) s\({}^{-1}\). During tension, according to the ASTM E8/EBM standard [32], the specimens were first strain-controlled with an extensometer to record strain signals for determining their yield properties. Afterward, it switched to displacement mode to record displacement signals until fracture, while the force signal was continuously recorded throughout the whole process. Each specimen was acclimated to the test temperature for approximately 20 min in an unstrained state before tensile loading to ensure uniform temperature throughout the gauge length. During the tensile process, the temperature was controlled within \(\pm\)2 \({}^{\circ}\)C. To maintain data consistency, at least two specimens were tested under each loading condition, with the majority of conditions having three specimens tested, given the high cost of material preparation and the limited availability of specimens.  
Metallographic specimens were prepared to determine the initial microstructure of the Fe-13 Cr-4 Al alloy. The specimen was gradually ground with 400-2000 grit silicon carbide papers, polished with a 1 mm diamond paste, and etched with a ferric chloride solution (5 g FeCl\({}_{3}\) + 10 g H\({}_{2}\)O + 10 mL HCl) for 3-5 s. An optical microscope (OLYMPUS GX71, Olympus-IMS, Tokyo, Japan) and a field-emission scanning electron microscope (FE-SEM, TSCAN MIA3 LMH, TESCAN, Brno, Czech Republic) equipped with energy-dispersive spectroscopy (EDS) were employed to examine the initial microstructure of the FeCrAl alloy. After the tensile tests, the fracture morphologies of the broken specimens were analyzed again using FE-SEM to determine the fracture mode.