**3. Results**

#### *3.1. Microstructure of Fe-Cr-Zr Alloy*

Figure 1 shows the XRD pattern of Fe-Cr-Zr alloy. The peak of Fe2Zr phase with the hexagonal polymorphs (C14/C36) could be observed besides the sharp diffraction peaks of α-Fe. It is confirmed that the microstructure of Fe-Cr-Zr alloy mainly compromises α-Fe and Fe2Zr phase.

**Figure 1.** XRD patterns of the Fe-Cr-Zr alloy.

As shown in Figure 2a, the SEM image reveals that the micrometer-sized Fe2Zr phases are nearly uniformly distributed in the α-Fe matrix, and the area fraction of Fe2Zr phases is ~9.5%. EDS point analysis shows that Fe2Zr phases are composed primarily of Fe and Zr, and low contents of Cr and W are present at the phase (Figure 2b). Figure 2c shows the bright-field TEM micrograph and corresponding selected area electron diffraction (SAED) pattern. The TEM micrograph shows the coexistence of light α-Fe and dark Fe2Zr phases, and the α-Fe and Fe2Zr phases are both in the micrometer-sized range. The SAED pattern (inset in Figure 2c) of Fe2Zr phases confirms the Laves structure. The high-resolution electron microscopy (HRTEM) image of α-Fe/Fe2Zr clearly shows that their interface is incoherent (Figure 2d).

#### *3.2. Oxidation Behavior in Air*

#### 3.2.1. Structure and Morphology of Oxide Scale

Figure 3 shows the SEM surface morphologies of Fe-Cr-Zr alloy after oxidation at 923 K for 10 min, 20 h, 500 h, and 1000 h, respectively. It is found that at the initial oxidation stage within 10 min, numerous oxide particles are distributed on the metal surface. However, an uneven oxide surface is formed (Figure 3a and inset), which is indicative of a nonuniform oxidation process. As the oxidation time is prolonged, the surface flatness of oxide scale is aggravated (Figure 3b–d). After oxidation of 1000 h, a number of pits could be found on the surface due to the nonuniform growth of oxide (arrows in Figure 3d and inset). The structure of oxide scale was identified by XRD after oxidation at 923 K for 1000 h, as shown in Figure 4. Single (Fe,Cr)2O3 phase is detected in the oxide scale, which indicates that Cr2O3 could not form in the Fe-Cr-Zr alloy with a Cr content of ~9 wt.%.

**Figure 2.** (**<sup>a</sup>**,**b**) SEM/EDS, (**c**) TEM, and (**d**) HRTEM image of the Fe-Cr-Zr alloy. (**b**) EDS point analysis of Fe2Zr phase. The inset in (**c**) represents the SAED patterns of Fe2Zr phase.

**Figure 3.** SEM surface morphologies of the Fe-Cr-Zr alloy oxidized in air at 923 K for (**a**) 10 min, (**b**) 20 h, (**c**) 500 h, and (**d**) 1000 h.

**Figure 4.** XRD patterns of the Fe-Cr-Zr alloy oxidized in air at 923 K for 1000 h.
