Rare earths have been widely used in the field of petrochemical, metallurgical machinery, electronics and information, light industry and agriculture, national defense, energy, environmental protection and new high-tech materials. In the field of high-temperature oxidation, it is known that rare earths can improve the high-temperature oxidation resistances of Fe-Cr, Ni-Cr and Fe-Ni-Cr alloys. Thereinto, the rare earths Y and Ce have the most significant effect .
Li et al.  studied the effects of trace rare earth Ce on high-temperature oxidation behavior of the stainless steel 00Cr17NbTi, and found that adding Ce can reduce the oxidation rate of the alloy and improve the adhesion properties of the oxide film. Ecer et al.  studied the effects of Ce on the oxidation behavior of Ni-50Cr alloy, and obtained a similar conclusion. Gao et al.  found that trace rare earth Y or Ce in maraging steel can reduce the oxidation mass gain in air by 20 times. Zhang et al.  studied the effect of different Ce contents on the oxidation behavior of Ni-33Al-28Cr-5.5Mo-0.5Hf alloy at 1,100°C in air, indicating that adding certain amounts of Ce significantly improves the high-temperature oxidation resistance of the alloy, but excessive addition deteriorates its oxidation performance. Ye et al.  studied the synergistic effect of Si and Ce on high-temperature oxidation behavior of nickel-chromium alloy, and found that adding Si and Ce in Ni-Cr alloys can promote the formation of Cr2O3 and improve its adhesion for the rare earth Ce promotes the inner oxidation and further brings about the “pinning effect” of SiO2. Rare earths can also improve the adhesion properties of Al2O3 oxide film, and thereby improves the oxidation resistances of the alloys taking Al2O3 as the protective oxide scale .
Besides directly adding rare-earth elements to alloys, coating CeO2 on alloy surface can also play similar roles: Seal et al.  coated AISI-316, -321 and -304 stainless steel with 2.1 μm of CeO2, Nguyen et al.  coated CeO2 in Fe-Cr-Al alloy, and Jin et al.  coated nano-sized CeO2 on pure chromium with sol-gel method.
The roles of rare earths on improving high-temperature oxidation resistances of alloys can be concluded as the following: (1) Grain refinement and the resulting enhancement of the formation rate of protective oxide film [2, 6, 9, 10]; (2) generating rare-earth oxides and promoting the stability of the protective oxide film (especially Cr oxide) [3, 4]; (3) the segregation of rare-earth oxides in grain boundaries to improve the interfacial energy and reduce the “short-circuit diffusion” ratio of alloy elements [4, 8]; (4) changing the mass transfer way from the outward diffusion of metal elements to the inward diffusion of oxygen to improve the adhesion of protective oxide film by the internal oxidation and the resulting “pinning effect” [2–4, 10]； (5) refining oxide film and through creep releasing the thermal stress generated during oxidation, which protects oxide scales from spalling [2, 9, 10].
The effect of rare earths on oxidation behavior of alloys has been researched much as mentioned above, which however, mostly focus on adding trace rare earths to Cr- or Al-containing alloys. In recent years, our group has studied the effects of relatively high content of Y on Fe-Si and Cu-Si alloys and drawn meaningful conclusions [11, 12]. In this paper, the effect of relatively high content of Ce, especially the resulting grain refinement and “pinning effect” on high-temperature oxidation behavior of Fe-Si alloys, is studied, which is expected to improve the high-temperature oxidation theory of alloys.
The experimental Fe-Si-Ce alloy ingots of about 50 g with nominal composition of Fe-Si-xCe (x = 0, 0.5 and 5.0, mass fraction) were prepared from high-purity metals by repeated arc-melting over five times under a Ti-gettered argon atmosphere using non-consumable tungsten electrodes from appropriate amounts of pure Fe (purity > 99.8%), Si (purity > 99.9%) and Ce (purity > 99.9%). The alloy ingots were subsequently annealed in argon atmosphere at 1,073 K for 24 h to remove the residual mechanical stresses and achieve a better equilibration of the alloys. The actual average compositions of the alloys after heat treatment are chemically analyzed to be Fe-3.07Si-0Ce, Fe-2.91Si-0.47Ce and Fe-3.05Si-4.92Ce, respectively.
After mechanically ground and polished, the surfaces of the Fe-3Si-0.5Ce and Fe-3Si-5.0Ce alloys were analyzed under scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDX) attachment, indicating that the two alloys are both duplex alloys, as shown in Figure 1(a) and (d). Figure 1(b) and (c) is the EDX analysis of the region A and point B shown in Figure 1(a), where the dark phase is determined to be rich in Fe, whose average composition is Fe-1.74Si-0.45Ce, while the light phase is rich in Si and Ce, whose average composition is Fe-4.87Si-8.40Ce. Figure 1(e) and (f) is the EDX analysis of the regions C and D shown in Figure 1(d), where the average compositions are Fe-1.53Si-0.57Ce (dark area) and Fe-4.72Si-27.63Ce (light area). The above results indicate that the Ce segregation is more serious in Fe-3Si-5.0Ce than that in Fe-3Si-0.5Ce.
Figure 2(a)–(c), respectively, shows the optical micrographs of the Fe-3Si-0Ce, Fe-3Si-0.5Ce and Fe-3Si-5.0Ce alloys after being treated with a metallographic etch. After statistical average, the grain-size sequence can be determined to be 512 μm (Fe-3.0Si-0Ce) > 374 μm (Fe-3.0Si-0.5Ce) > 46 μm (Fe-3.0Si-5.0Ce). The grain sizes of the Fe-3.0Si-0.5Ce and Fe-3.0Si-5.0Ce alloys are reduced by about 27% and 91%, respectively, compared to that of Fe-3.0Si-0Ce, which coincides with the description of the grain-size reduction by the rare-element Ce in Refs [2, 9].
The methods of preparing specimens for the oxidation tests and conducting the isothermal oxidation tests have been reported in Ref. , which are thus omitted in this paper.
Results and analysis
Figure 3 shows the kinetic curves of Fe-3.0Si-0Ce, Fe-3.0Si-0.5Ce and Fe-3.0Si-5.0Ce alloys when being oxidized at 1,173 and 1,273 K in pure oxygen, indicating that the addition of 0.5 mass% Ce noticebly decreases the oxidation mass gain of Fe-3.0Si alloys, while as the Ce content futher increases to 5.0 mass%, the mass gain increases slightly.
Compositions and morphologies of the oxide scales
Figure 4 shows the X-ray diffraction (XRD) spectra of Fe-3.0Si-xCe alloys (x = 0, 0.5, 5.0) after being oxidized at 1,173 and 1,273 K in pure O2 for 24 h. It can be seen that all of the three alloys form Fe2O3 at the two temperatures during oxidation. It is worth noting that the diffraction peaks of Fe2O3 are gradually weakened and widened in the XRD patterns as the sequence of Fe-3Si-0Ce, Fe-3Si-0.5Ce and Fe-3Si-5.0Ce alloys at both temperatures. According to the XRD theory and the grain size shown in Figure 2, the grain size of the oxides formed on grain-refined alloys also decreases, which coincides with Refs. [14, 15].
It can be preliminarily deduced that after 24-h oxidation at the two temperatures, the oxide scales on Fe-3Si-0Ce and Fe-3Si-0.5Ce alloys at least comprise Fe2O3 and SiO2. CeO2 is not detected and may be related to the low content of Ce. However, for Fe-3Si-5.0Ce alloy at both temperatures, CeO2 is formed in the scales.
The cross-sectional morphologies of Fe-3Si-0Ce oxide films after 24-h oxidation at 1,173 and 1,273 K in pure O2 were reported in Figure 5 of Ref. , indicating that the external oxidation is the main mass-gain mechanism, the oxide film formed at both temperatures includes SiO2 and Fe2O3, and the thickness of the oxide film formed at 1,273 K is larger than that formed at 1,173 K.
Figure 5 is the cross-sectional morphology of Fe-3Si-0.5Ce alloy after 24-h oxidation at 1,173 K and the corresponding EDX analysis of certain selected points. Figure 5(b)–(d) is the EDX analysis of the points A, B and C in Figure 5(a). It can be seen that the inner oxide layer (point A, Figure 5b) is rich in Si and Ce, indicating that the inner layer of the oxide film mainly consists of Ce and Si oxides. This is beneficial to improving the oxidation resistance of the alloy by impeding the penetration of oxygen. The middle oxide layer (point B, Figure 5c) has larger Fe content than the outer (point C, Figure 5d) and inner ones, indicating that the oxidation mechanism is the inward diffusion of oxygen, which makes some regions in the oxide scales possessing unoxidized or incompletely oxidized Fe.
Figure 6 is the cross-sectional morphologies of Fe-3Si-0.5Ce alloy after 24-h oxidation at 1,273 K and the corresponding EDX analysis of certain selected points. Figure 6(b)–(d) is the EDX analysis of the points D, E and F in Figure 6(a). It can be seen that similar to the element distribution in the oxide scale formed at 1,173 K, the inner oxide layer (point E, Figure 6c) is rich in Si and Ce, and some regions in the middle oxide layer (point D, Figure 6b) have higher Fe content than the inner and outer layers (point F, Figure 6d).
Comparing Figures 5 and 6, it can be seen from the total thickness of oxide scales that the scale formed at 1,273 K (8.2 μm) is greater than the one formed at 1,173 K (1.3 μm), which agrees with Figure 3(a). In addition, from Figures 5(b) and 6(b), it can be seen that the Si-rich oxides form near the Fe-3Si-0.5Ce alloy matrix, which is beneficial to improving the high-temperature oxidation resistance of the alloy.
Figures 7(a) and 8(a) are the cross-sectional morphologies of Fe-3Si-5.0Ce alloy after 24-h oxidation at 1,173 and 1,273 K, respectively. Figure 7(b)–(d) is the EDX analysis of the points A, B and C in Figure 7(a), and Figure 8(b)–(d) is the EDX analysis of the points D, E and F in Figure 8(a). From Figure 7, it can be seen that the outer and middle layers of the oxide scale have remarkably high content of Fe, indicating the growing mechanism of the oxide scale is also the inward diffusion of oxygen, and the points A and C rich in Fe may be related to the Fe-rich phase there (see Figure 1). Oxygen penetrates into the oxide scales and internal oxides form, which can be demonstrated by the EDX analysis of point B, as shown in Figure 7(c). At 1,273 K, different regions (see the points D, E and F in Figure 8(a)) have similar compositions with those in the oxide scale formed at 1,173 K, but the depth of oxygen penetration into the alloy matrix along Ce-rich phase is smaller than that at 1,173 K, which may be related to the fact that at higher temperature, the Ce and Si oxides can rapidly form to inhibit the further inward diffusion of oxygen.
It can be obviously seen from Figures 7 and 8 that oxygen can penetrate into the alloy matrix to make the alloy, especially the Ce-rich phase, oxidized, and the oxides can tie into the alloy matrix like “nails” to increase the actual contact area between the oxide and matrix and extend the expanding distance of cracks at the interfaces. These are beneficial to improving the adhesion of the oxide scales and the oxidation resistances of the alloys.
According to Figure 2, the grain size of Fe-3Si-0.5Ce and Fe-3Si-5.0Ce alloys has been significantly reduced, whose most direct effect is the formation of large concentrations of grain boundaries within the alloys . Because of their rather open and disorder structure, grain boundaries allow a faster transport (short-circuit diffusion) of the alloying elements compared to normal bulk diffusion due to the chemical potential gradients . Another effect of grain-size reduction of the alloys is the formation of more nucleation sites for oxides and the reduction of the distance between them, which on the one hand reduces the grain size of the oxides formed on the alloy matrix to enhance the mass-transfer rate in them (see Figure 4), and on the other hand is beneficial to make oxides connected to form continuous and protective oxide scales of reactive elements and thus protects the alloy matrix from being further oxidized .
The grain refinement and the resulting faster mass transfer have both positive and negative effects on high-temperature oxidation behavior of the alloys . In the present study, positive effects include the enhancement of mass-transfer rate to promote the formation of protective SiO2 and CeO2. Although in this study single continuous oxide film of SiO2 or CeO2 does not form on the alloys, their concentration at the inner layer of the oxide scales can also inhibit the penetration of oxygen to some extent (see Figures 5–8) and is beneficial to enhancing the oxidation resistances. The negative effect in this study means that the grain refinement promotes not only the diffusion of reactive elements, but also the mass transfer of all other elements, which makes the mass gain of the alloys to increase rapidly. Overall, the positive effect of the Ce addition and the resulting grain refinement is predominant, so the oxidation resistance of Fe-3Si-0.5Ce and Fe-3Si-5.0Ce alloys is improved significantly (see Figure 3).
Moreover, it should be noted that the addition of 5.0 mass% Ce makes the alloy have slightly larger mass gain than that of 0.5 mass% Ce, which may be because the preferential oxidation of the excessive content of Ce exceeds its beneficial effects mentioned earlier. This is in agreement with the viewpoints in Ref. .
The addition of Ce changes the oxidation mechanism of the alloys. The Fe-3Si-0Ce alloy is mainly oxidized externally . However, in this study, internal oxidation mainly occurs in Fe-3Si-0.5Ce and Fe-3Si-5.0Ce alloys at 1,173 and 1,273 K, respectively, as shown in Figures 7 and 8. Oxygen penetrates into the oxide scales and preferentially reacts along Ce-rich phases. The formed internal oxides tie into the alloy matrix like “nails,” which on the one hand improves the adhesion of the oxide scales and on the other hand inhibits the growth of oxide grains , and this is called “pinning effect.”
As the Ce content increases, the grain sizes of Fe-3Si alloy and its oxide scales are significantly reduced to increase the grain boundary concentration in them, which promotes the short-circuit diffusion of the alloying elements and oxygen. In this process, the positive effect of grain-size reduction plays a more important role in affecting the oxidation behavior of the Ce-containing alloys, which is the main reason for the addition of Ce improving the oxidation resistances of Fe-3Si alloys.
The Ce addition changes the oxidation mechanism of Fe-3Si alloy from the outward diffusion of alloying elements to the inward diffusion of oxygen. The oxygen penetrates the oxide scales and reacts preferentially with the Ce-rich phase, and the formation of CeO2 and SiO2 at the inner layer of oxide scales inhibits the further penetration of oxygen to enhance the oxidation resistances of the Ce-containing alloys.
The preferential oxidation reaction of Ce-rich phase makes oxides tie into the alloy matrix and generates the “pinning effect” to enhance the adhesion of the oxide scales.
The excessive addition of Ce and its preferential oxidation exceeding its beneficial effects is the reason for the Fe-3Si-5.0Ce alloy having slightly larger mass gain than Fe-3Si-0.5Ce alloy.
We are grateful for financial support from the Liaoning Educational Committee (No. 2009A579 and No. L2012147).
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Published Online: 2015-04-17
Published in Print: 2016-02-01