Study on the high-temperature oxidation behavior of Mg-Ce binary alloys

The aim of current research is to study the oxidation resistance of binary magnesium alloys containing Ce at high temperatures to provide a theoretical foundation for the application of magnesium alloys which are applied at high-temperature conditions. The oxidation properties of binary magnesium alloys containing different Ce content at high temperatures have been studied. The thermogravimetric results show that a small amount of Ce (0.2, 0.4 wt. %) addition can improve the oxidation resistance of magnesium alloys at high temperatures. Continuous-distributed Mg12Ce compounds in the two test alloys with high Ce content (≥0.7 wt. %) provided channels to the internal oxidation and thus deteriorated the oxidation resistance. MgO and CeO2 were the primary components which are detected in the oxide films. CeO2 was mainly detected at the inner layer of the film.


Introduction
Magnesium alloy was regarded as the lightest metal structure material and the "green engineering material".Magnesium alloy was widely used in mechanical and electronic industries where weight plays an important role.However, Mg is a chemically active metal and easily reacts with oxygen, especially at high temperatures [1].Loose and unprotected MgO film is apt to form on the surface of magnesium alloy, which seriously restricts its application in high-temperature environments such as automobile engine boxes, aircraft parts, etc [2].Alloying is a convenient method to optimize the oxidation behavior of magnesium alloys.For now, various elements have been attempted to alloy in magnesium to develop novel magnesium alloys.
Researchers have found that alloying with Ca, Gd, Be and other alloying elements reflected a very positive influence on the enhancement of oxidation resistance of magnesium alloys [3,4].The development of anti-oxidation magnesium alloys by adding Ce element also attracted extensive attention.The Ce-dissolved films could prevent further oxidation of the alloys.Ce can also prevent oxidation of ternary magnesium alloy due to the third element effect [5].Besides, unlike Y and Gd elements, the addition of Ce in magnesium alloy cannot deteriorate their properties, especially in Mg-Al alloys.However, when the Ce element can no longer be solidly dissolved into the matrix (Ce exceeds 0.25 wt.%), the oxidation rate of Ce-containing magnesium alloys is accelerated.Especially, oxidation behavior of binary magnesium alloys containing Ce has not been analyzed and characterized thoroughly at elevated temperatures, which is extremely important to carry out systematic research.
In this work, oxidation resistance of binary magnesium alloys with different Ce contents was evaluated.The weight change kinetics of the test alloys were evaluated using thermogravimetric analysis between 450 °C and 550 °C.The compositions and morphologies of oxide products were analyzed and characterized.

Materials
The binary magnesium alloys containing Ce for this work were processed by pure magnesium (99.99 wt.%) and Mg-20wt.% Ce alloy.The original materials were melt at 740 °C under the protection of a CO 2 (99.5 vol%) and SF 6 (0.5 vol%) mixed gas.The homogeneous melts were cooled down to room temperature in the air in steel moulds.

Oxidation methods
The experimental samples were processed into original pieces with a thickness of 0.5 mm and a diameter of 5 mm for the thermogravimetric experiment (TGA, Mettle 1100LF, Switzerland) and blocks for isothermal oxidation experiments (3 mm × 10 mm × 10 mm).The samples for the thermogravimetric experiment were heated up to 450 °C, 500 °C, and 550 °C and lasted for 2 hours in dry air.The heating rate was 30 °C per minute.A longer oxidation test was processed in a muffle furnace at 500 °C and lasted for 6 hours.

Characterization
The microstructure of as-cast alloys and the morphologies of oxidized samples was analyzed using a scanning electron microscope (SEM).X-ray photoelectron spectroscopy (XPS) measurement was employed to analyze the electronic state of the oxidation products.The sputtering rate of XPS depth profiling was 0.2 nm/s.The compounds of the alloys and oxidation products were determined by X-ray diffraction (XRD).

The phases and microstructures
The backscattered scanning electron (BSE) graphs and XRD results of the alloys are demonstrated in Figure 1.Because solid solubility of the Ce element in the magnesium matrix is relatively low, the second phase exists in the four components as seen in the figure.The white granular second phase and α-Mg were the primary components in Mg-0.2Ce alloy.As Ce content increased, for the other three alloys, some continuously distributed secondary phases were found in the grain and along the grain boundary.The content of secondary phases also increased with increased Ce content.XRD results are shown in Figure 1

Air oxidation
To facilitate the investigation of the oxide films of test alloys, isothermal oxidation tests were proceeded on the block samples at 500 ºC for 6 hours.The macrographs of oxidized alloys are shown in Figure 3.
For Mg-0.2Ce and Mg-0.4Ce alloys, smooth and intact films were formed on their surfaces.However, once the content of Ce in the alloys exceeded 0.4 wt.%, oxide nodules appeared on the surface after oxidation.Mg-1.0 Ce alloy was severely oxidized with several oxide nodules on its surface.The formation of oxide nodules implies that Mg-0.7Ce and Mg-1.0Ce alloys cannot resist oxidation for a long time at 500 ºC.A possible reason for the phenomenon is that internal oxidation was carried out at the surface layer and subcutaneous layer.FESEM analysis was carried out in order to dig a little deeper into the morphology of the oxidized Mg-Ce alloys.Figure 4(b) presents micrographs of the oxidized alloys.As observed from Figure 6, the films of the alloys consisted of flake-like oxides.The oxide film is loose on Mg-0.2Ce alloy, and some tiny pores are existing among the flakes.Thus, Oxygen atoms could diffuse through the pores and react with the Mg matrix, which leads to further oxidation.The flake-like oxides became dense with increased Ce content, and the number of tiny pores decreased obviously.The difference in the morphology is consistent with the weight change curves.Hence, the compactness of film will affect the oxidation resistance of test alloys.
Results of TGA curves and surface micrographs suggested that with the increase in Ce content, the oxidation resistance of test alloys is improved at elevated temperatures.Nevertheless, as reflected in the macrographs (Figure 3), when Ce content exceeds 0.4 wt.%, oxide nodules were formed (Mg-0.7Ceand Mg-1.0Ce).To further analyze this phenomenon, cross-sectional BSE analysis and corresponding EDS-linear scan proceeded, as Figure 5 reflected.Because of the low content of Ce, bright white particles were not obviously observed on the surface layer, which indicates that the content of CeO 2 is very little in the film (Figure 5a).A thin and discontinuous bright oxide layer was formed on the surface layer of Mg-0.4Ce (Figure 5b).EDS linear scanning result indicates that the bright layer is enriched with Ce element, which is in accord with the BSE image.Because the Ce-enriched layer is thin, it can be inferred that a small amount of CeO 2 existed in the film.MgO is mainly detected at the inner layer, and CeO 2 was mainly formed at the outmost of the film.Unlike Mg-0.4Ce alloy, for the two test alloys with higher Ce content, serious internal oxidation occurred as shown in Figures 5(c) and 5(d).The internal oxidation of Mg-1.0Ce alloy is more serious than that of Mg-0.7Ce alloy, which also explains the formation of protrusions in Figure 3. Since the low solid solubility of Ce element in Mg matrix, Ce mainly exists as secondary phases and is continuously distributed along grain boundaries or in the matrix.Meanwhile, secondary phases would not dissolve into the matrix as the oxidation experiment progresses.Therefore, continuous-distributed second phases existed on the surface throughout the whole oxidation process, and caused the internal oxidation to spread along the secondary phases into the matrix.This phenomenon is also proved by EDS linear scanning.Thus, there were more oxide nodules formed on Mg-1.0Ce alloy after oxidation.Among the four ingredients, the addition of 0.4 wt.% Ce in magnesium is the most beneficial to elevate the oxidation resistance.

XPS analyses
To elucidate the surface electronic state of the oxidation products, XPS analysis was proceeded on the oxidized Mg-0.4Ce alloy.The variation of atomic percentage of Mg, O, Ce, and C elements along depth direction was shown in Figure 6(a).Since the outermost layer of the sample was exposed to the atmosphere, the content of the C atom is the highest at the outermost layer.As sputtering time increased, the content of Mg and O did not change significantly, and Ce content increased slightly.However, due to the low content of Ce, the content of Ce in the film is also small, that is, a large amount of MgO and a few compounds containing Ce were the primary components in the film.Ce element is prone to existing in the inner layer of the film.
Figure 6(b) shows the XPS depth sputtering high-resolution spectra for different elements of the oxidized Mg-0.4Ce alloy.Two peaks were observed in Mg 2p spectrum.The one peak at 49.7 eV is identified as metallic Mg, and another peak at about 50.5 eV implies the formation of Mg hydroxide, Mg oxide, or Mg carbonate [6].The peak at 50.5 eV gradually shifted to a lower binding energy peak as sputtering time went on, which can be ascribed to the existing components such as hydroxide and carbonate at the outermost surface.There are three peaks shown in the Ce 3d spectrum.The two peaks can be identified as Ce 3d3/2 (918 eV and 902 eV), while the peak at about 884 eV was Ce 3d5/2, confirming the presence of CeO 2 .Before sputtering, Ce was not detected, and the peak strength gradually increased as the sputtering time extended, which indicates that CeO 2 existed at the inner layer of the film.The oxygen spectrum shows the binding energy peaks are centered at about 532 eV and 530 eV that were assigned to the Mg oxide/hydroxide/carbonate and CeO 2 , respectively, also indicating the formation of CeO 2 in the film.The carbon spectrum reflects that Carbon existed only on the outermost surface.Although TGA and isothermal oxidation experiments were carried out for 2 hours and 6 hours, respectively, some limitations still existed.Due to the limitation of experimental conditions, the exposure times were not long enough.The experimental results only reflected the oxidation behavior of test alloys at the setting temperatures for a relatively short time.In the next step, further experiments with a longer exposure time at high temperatures are necessary.

Conclusion
According to the experimental investigations and the results of oxide film analysis, the following conclusions are obtained: (1) A small amount of Ce (0.2, 0.4 wt.%) addition is beneficial for improving oxidation resistance of magnesium at test temperatures.
(2) On account of the low solid solubility of Ce element in magnesium, continuous-distributed Mg 12 Ce phases in the two test alloys with high Ce content (≥0.7 wt.%) deteriorated the oxidation resistance.Oxygen diffused into the matrix along the secondary phases, resulting in severe internal oxidation.
(3) MgO and CeO 2 were the primary products in the oxide products of Mg-0.4Ce.CeO 2 was mainly formed at the inner layer of the film.
(b).The granular and continuously distributed second phases in Mg-Ce alloys is Mg 12 Ce.

Figure 1 .
Figure 1.(a) BSE images and (b) XRD patterns of as-cast Mg-Ce alloys.

3. 2
Oxidation kineticsFigure2reflects the comparison of the weight change curves of test alloys oxidized at different temperatures for 2 hours respectively.There was no significant weight change observed at 450 °C.There is a slight loss of weight of the alloys during the whole heating stage when tested at 500 °C, which can be ascribed to Mg(OH) 2 dehydration and moisture evaporation.The weight changes of the tested alloys occur according to parabolic law, which suggested that the matrix was protected by the formed oxide products of the alloys at this testing temperature in 2 hours.In Figure2(c), as experimental temperature raised to 550°C, the four alloys exhibited different oxidation behavior as observed from the weight change curves.The weight gain of the samples is decelerated within 2 hours with increased Ce content.For Mg-0.2Ce and Mg-0.4Ce alloys, their weight started to increase significantly at about 40 and 60 minutes after the initial stage of the experiment, respectively.The curves of Mg-0.2Ce and Mg-0.4Ce alloys conform to linear law in the second half of the test, while the curves of Mg-0.7Ce and Mg-1.0Ce alloy were unnoticeable as the experiment progressed.This result suggested that for the test Mg-Ce alloys, the oxidation resistance enhanced with Ce content increased within 2 hours at 550 °C.

Figure 2 .
Figure 2. Weight change curves of binary magnesium alloys containing Ce oxidized in dry air for 2 hours between 450 ºC and 550 ºC.

Figure 3 .
Figure 3. Macrographs of oxidized Mg-Ce alloys.Figure4(a) reflects the XRD results of the oxidized products.Both MgO and CeO 2 peaks were detected, which suggests that MgO and CeO 2 were the primary oxidation products of the test alloys.The detected α-Mg is the matrix.The intensity of the MgO peak is high, which implies that the main oxide in oxide products is MgO.

Figure 6 .
Figure 6.(a) The variation of atomic percent of Mg, Ce, O, and C elements along depth direction and (b) depth profiling spectra of the oxidized Mg-0.4Ce alloy examined by XPS.Although TGA and isothermal oxidation experiments were carried out for 2 hours and 6 hours, respectively, some limitations still existed.Due to the limitation of experimental conditions, the exposure times were not long enough.The experimental results only reflected the oxidation behavior of test alloys at the setting temperatures for a relatively short time.In the next step, further experiments with a longer exposure time at high temperatures are necessary.

Table 1 .
Table 1 reflects the chemical components of the four ingots, in which the contents of Ce were tested.Chemical components of the test alloys (wt.%).