3.2.2. Cross-Sectional Morphology

Figure 5a shows a backscattered detector (BSE) image of the cross-sectional morphologies of oxide scale formed on Fe-Cr-Zr alloy after oxidation at 923 K for 10 min. The matrix/scale interface appears to roughen, and incorporated Fe2Zr phase could be found at the oxidation front due to their white contrast compared to the oxide scale (black arrows in Figure 5a). EDS point analysis in the vicinity of the incorporated Fe2Zr phase reveal that ~5.2 at.% of O is detected at the incorporated phase (Figure 5b), and concentrations of Fe, Zr, and Cr of the incorporated phase are similar to the composition of Fe2Zr phase obtained in the matrix (Figure 2b). In contrast, a much higher concentration of O (~59.1 at.%) is detected in the gray scale around the incorporated phase, and the atomic percent is consistent with the stoichiometric ratio of (Fe,Cr,Zr)2O3 oxide (Figure 5c). Besides, a low concentration of O (~5.9 at.%) is present in the α-Fe adjacent to the gray scale (Figure 5d). Therefore, the incorporated Fe2Zr phase in the oxide scale close to the matrix/scale interface exhibits delayed oxidation.

**Figure 5.** (**a**) BSE images of cross-sectional morphologies of Fe-Cr-Zr alloy after oxidation in air at 923 K for 10 min. (**b**–**d**) EDS spectrums for point 1, point 2, and point 3 marked in (**a**).

After oxidation for 20 h, the average thickness of the oxide scale is ~10 μm, and the incorporation of dark phase in the inner part of white scale could be observed by optical

microscope (OM) (Figure 6a). BSE image shows that Fe2Zr phase are observed in the inner part of oxide scale and some of the phase in the scale are much smaller than those in the matrix (arrows in Figure 6b), which might be resulted from the gradual oxidation of Fe2Zr phase. EDS point analysis at the Fe2Zr phase sites in the scale reveals that the ratio of Fe to Zr corresponds to about 2 while the measured oxygen concentration is ~33.5 at.% (Figure 6c), indicating that the Fe2Zr phase are oxidized. By contrast, the oxide scale surrounding the Fe2Zr phase having an oxygen concentration of 59.3 at.% (Figure 6d), is found to be fully oxidized into (Fe,Cr,Zr)2O3. Therefore, it is concluded that the incorporated Fe2Zr phase in the oxide scale exhibit too low oxygen concentration to be fully oxidized.

**Figure 6.** (**a**) OM and (**b**) BSE images of polished cross-sectional morphologies of Fe-Cr-Zr alloy after oxidation in air at 923 K for 20 h. (**<sup>c</sup>**,**d**) EDS spectrums for point 1 and point 2 marked in (**b**).

OM image of the cross-sectional morphologies of scale after oxidation of 1000 h is shown in Figure 7a. A roughened surface is observed, which is corresponding to the uneven oxide surface morphology (Figure 3d). The incorporated dark phases are also observed in the inner part of the white scale. The BSE image reveals the presence of Fe2Zr phase in the inner part of oxide scale, and a significant decrease in number density of incorporated phase is observed from the oxide/matrix interface to the outer surface, as shown in Figure 7b. The compositional profiles by EDS point analysis show that the oxide scale has a duplex-layered structure (Figure 7c). The outer layer, with a thickness of ~20 μm, consists mainly of the 40Fe-60O (at.%), which is in line with the stoichiometry of Fe2O3. The inner layer, with a thickness of ~25 μm, consists mainly of the 30Fe-6Cr-3Zr-1W-60O (at.%), which is in line with the stoichiometry of (Fe,Cr,Zr)2O3. Within the inner layer, particles with higher concentration of Zr (20~26 at.%) and lower concentration of O (10~40 at.%) are the delayed oxidized Fe2Zr phase. In addition, the concave surface (arrows in Figure 7b) is in accordance with the pits on the oxide surface morphologies (arrows in Figure 3d). It is shown that the density of incorporated Fe2Zr phase is higher in the oxide scale beneath the concave surface than that beneath the convex surface (Figure 7b), which is reminiscent of the fact that the inhomogeneous cross-sectional morphology is related with the number density of the Fe2Zr phase.

**Figure 7.** (**a**) OM and (**b**) BSE images of polished cross-sectional morphologies of Fe-Cr-Zr alloy after oxidation in air at 923 K for 1000 h. (**c**) Compositional profiles with the distance across the oxide scale marked in (**b**).

#### *3.3. Oxidation Behavior in Stagnant Liquid Pb-Bi Eutectic*

Figure 8 shows the BSE cross-sectional image of oxide scale formed on Fe-Cr-Zr alloy after exposure to oxygen-saturated Pb-Bi eutectic at 823 K for 500 h, 1000 h, and 2000 h, respectively. As the exposure time increases from 500 h to 2000 h, the thickness of oxide scale increases from ~10 μm to ~30 μm. The outer layer of the oxide scale seems porous. In comparison, the inner layer of the oxide scale seems compact, and the incorporation of Fe2Zr phase in the inner layer is also observed. It could be found that the cavities are mainly present at the inner/outer layer interface of oxide scale and formed adjacent to the incorporated Fe2Zr phase (white arrows in Figure 8), which implies cavity nucleation around the Fe2Zr phase.

EPMA analysis of the oxide scale after exposure to oxygen-saturated Pb-Bi eutectic at 823 K for 1000 h is shown in Figure 9. In the matrix, the Fe-rich α-Fe and Zr-rich Fe2Zr phase could be clearly identified. The oxide scale formed on the Fe-Cr-Zr alloy exhibits a three-layered structure. An enrichment of Fe and O is observed in the outer oxide layer while Cr and Zr could not be detected in this layer, revealing that the outward diffusion of Fe from matrix to the external interface. The outer layer composed of the iron oxide is identified as magnetite (Fe3O4) according to the SEM/EDS [19], XRD [21] and glow discharge optical emission spectroscopy (GD-OES) [22]. The penetration of Pb/Bi into the formed magnetite layer could also be observed. In comparison with the outer oxide layer, the inner oxide layer is a Fe-Cr spinel which is enriched by Cr and depleted in Fe. An inhomogeneous distribution of O is present in the inner oxide layer, and the region with higher concentration of O is found exactly to be at the α-Fe site (black arrows in Figure 9). The lower concentration of O at the Fe2Zr phase site may be resulted from the delayed oxidation of the phase. The ratio of outer magnetite layer thickness on the inner Fe-Cr spinel layer thickness is ~1.2, which is in agreemen<sup>t</sup> with the oxidation results of ferritic/martensitic steels in liquid Pb-Bi eutectic [22–24]. An internal oxidation zone (IOZ) with a significant decreased concentration of O develops between the matrix and inner

oxide scale, and a relative homogeneous distribution of O exists in the vicinity of the inner oxide layer/IOZ interface.

**Figure 8.** BSE images of cross-sectional morphologies of Fe-Cr-Zr alloy after exposure to oxygensaturated Pb-Bi eutectic at 823 K for (**a**) 500 h, (**b**) 1000 h, and (**c**) 2000 h.

**Figure 9.** EPMA analysis of the cross-sectional area of Fe-Cr-Zr alloy after exposure to oxygensaturated Pb-Bi eutectic at 823 K for 1000 h.
