Rare Earth Elements Enhanced the Oxidation Resistance of Mo-Si-Based Alloys for High Temperature Application: A Review

: Traditional refractory materials such as nickel-based superalloys have been gradually unable to meet the performance requirements of advanced materials. The Mo-Si-based alloy, as a new type of high temperature structural material, has entered the vision of researchers due to its charming high temperature performance characteristics. However, its easy oxidation and even "pesting oxidation" at medium temperatures limit its further applications. In order to solve this problem, researchers have conducted large numbers of experiments and made breakthrough achievements. Based on these research results, the effects of rare earth elements like La, Hf, Ce and Y on the microstructure and oxidation behavior of Mo-Si-based alloys were systematically reviewed in the current work. Meanwhile, this paper also provided an analysis about the strengthening mechanism of rare earth elements on the oxidation behavior for Mo-Si-based alloys after discussing the oxidation process. Furthermore, the research focus about the oxidation protection of Mo-Si-based alloys in the future was prospected to expand the application field.


Introduction
As the world population increases, the problem of global resource shortage has become increasingly prominent. It is well known that in addition to waste recycling, improving energy utilization and exploring new energy are effective methods to solve resource problems, and are also the main trend of future scientific and technological development [1][2][3]. Nowadays, the development of new energy is overwhelming. As a new high temperature structural material, the Mo-Si-based alloy is expected to replace the nickel-based alloy and play an important role in turbine engine and industrial furnace components [4][5][6][7].
A large number of studies have pointed out that Mo-Si-based alloys have outstanding high temperature performance characteristics, such as moderate density, strong electrical and thermal conductivity, ultra-high melting point, high thermal impact resistance, etc., which has been widely used in various industries [8][9][10][11].
However, these alloys also have some inherent defects that limit their generalization as structural materials oxidation behavior of Mo-Si-B alloys.

Effects of La element addition
Compared with pure Mo-Si alloys, adding La element can significantly optimize the microstructure of these alloys by the means of reducing the grain size and making the intermetallic particles disperse more evenly, thus improving the fracture toughness, bending and compressive strength significantly [71][72][73][74]. Based on the existing studies of Mo-12Si-8.5B alloy (at.%) [75][76][77][78][79], we further analyzed the actions of doping La or La2O3 second phase on the oxidation behavior.
Zhang et al. [80] prepared Mo-12Si-8.5B (at.%, abbreviated as MSB) samples added with different contents of La2O3 through arc-melted and spark plasma sintered methods, and the specific contents were presented in Table 1. Fig. 1 (a) gives the XRD patterns of MSB+xLa2O3 samples (x=0, 0.3, 0.6, 1.2 wt.%). It can be seen that all the samples consist of Mo5SiB2, Mo3Si and α-Mo three phases, which is consistent with the phase diagram of isothermal Mo-Si-B composites [76]. At the same time, it also reveals that even if La2O3 is added will not affect the phase composition of samples. Figs. 1 (b-e) are micrographs of the four samples prepared, where the white regions are α-Mo phase, and the black regions are Mo5SiB2/Mo3Si phases dispersed in the α-Mo matrix. It can also be found from micrographs that the grain size of α-Mo and Mo5SiB2/Mo3Si will be reduced after adding La2O3, in which the α-Mo size change is more pronounced, whereas the decrease of each phase size is not sensitive to the La2O3 mass fraction. Moreover, the distribution of Mo5SiB2/Mo3Si phases is more uniform after doping La2O3. This is because parts of La2O3 can be used as nucleation sites, which leads to the increase of nucleation density. On the other hand, La2O3 particles play a "pinning" role on the α-Mo boundary to inhibit its grains growth. The results in Table 1 further reveal the effect of La2O3 on grain size. This is because adding La makes the sample cross section present a loose and porous oxide layer structure when oxidized at 1200 ℃ ( Fig. 2 (e)). Meanwhile, as oxidation temperature rises to 1300 ℃, a large number of cracks and holes are observed on the sample surface ( Fig. 2 (f)), which provides a pathway for O2 internal diffusion and MoO3 volatilization. In contrast, even if undoped sample is oxidized at 1300 ℃ for 72 h, a continuous and compact oxidation scale can be still observed in its cross section ( Fig. 2 (g)) [88]. Thus, undoped sample has better antioxidant properties in high-temperature environments.

Effects of Hf element addition
Extensive experiments have reported that adding Hf/HfB2 to Mo-Si-B composites can clearly improve their performance features, such as high temperature strength, high temperature stability, creep resistance, fracture toughness, etc. [91][92][93][94][95]. Potanin et al. [96] discussed the oxidation behavior of MoB-HfB2-MoSi2 composites at 1200 ℃ in detail. The composition of each alloy is illustrated in Table 2, where the difference between X342 and X341 samples is that the former presents two-level structure (TLS), while the latter presents single-level structure (SLS). The microstructures of studied samples is depicted in Figs. 3 (a-c), on the whole, the three samples all contain MoB and MoSi2 phases. The difference is that X341 and X342 samples also have additional HfSiO4 and HfB2 phases and their grain sizes are finer than that of X0 sample. Fig. 4 gives the oxidation kinetics curves of the three samples, it can been observed that the weight increases of X341 and X342 samples are more obvious because adding HfB2 can make the samples generate HfSiO4 (6.97 g/cm 3 ) and HfO2 (9.68 g/cm 3 ), whose specific weights are greater than SiO2 (2.36 g/cm 3 ) [97]. By the way, the weight gain of X341 sample is smaller than that of X342 sample owing to the fact that X342 sample presents special arrangement and finer grain size. sample is due to the dissolution of hafnium. Furthermore, Six-1HfxO2 is generated because hafnium and silicon are equivalent elements leading to the incorporation of hafnium into the lattice of silica, as shown in Fig. 5. It is worth noting that the X342 sample oxide scale exhibits denser structure is caused by the existence of high wetting angle makes SiO2-B2O3 melt can shrink HfSiO4 particles together, resulting in the formation of smooth and compact oxide films [98].
To summarize, addition of HfB2 to X0 sample can produce HfSiO4/Six-1HfxO2 particles dispersed in oxide film, and even form an HfSiO4 interlayer. It has been proved that the HfSiO4 and ZrSiO4 particles have similar effects. On the one hand, they can promote the healing of cracks and holes in borosilicate scale [99,100], on the other hand, they can both act as barriers and HfSiO4 particles can also increase the crystallization temperature of amorphous scale [101]. Therefore, adding HfB2 can enhance the alloy antioxidant effects. The research results of Sciti et al. also confirmed this conclusion [97].   (a)  Reproduced with permission [96]. Copyright 2019 Elsevier. respectively. It can be seen that the mass loss of MSB1 increases after the addition of Ce (Figs. 6 (c, d)). Even so, MSBCe exhibits shorter transient oxidation periods as compared to MSB1, and its steady-state stage curve almost presents a horizontal trend, revealing that MSBCe is more effectively protected than MSB1.

Effects of Ce element addition
At the same time, the microstructural morphology of two oxidized alloys is shown in Fig. 7. During oxidation at 900 ℃, one-layered oxide film (i.e. Mo-oxide film) and flowing glassy phase are observed on the surface of both alloys (Figs. 7 (a, d)). The difference is that the glassy phase in MSBCe flows faster due to the presence of Ce, hence MSBCe presents a smaller mass variation at 900 ℃ ( Fig. 6 (b)). However, after oxidation at 1100 ℃ for 24 h, the oxide-layer structure of both alloys has changed distinctly, namely SiO2 layer is formed on top of the Mo-oxide layer (Figs. 7 (b, e)). When the oxidation temperature reaches 1300 ℃, B2O3 has begun to evaporate from the oxide scale, resulting in increased viscosity and weak fluidity of the scale, which leads It is well known that adding Al to Mo-Si-B systems may lead to the failure of alloy oxidation protection owing to the formation of mullite [110][111][112][113][114], which is also verified by the mass loss curves of Al-doped alloy in Figs.

Effects of Y element addition
It has been sure that adding Y element can significantly prolong the service life of Mo-based alloys due to the fact that Y will exhibit higher oxygen affinity than Mo [115,116]. Moreover, Y can also inhibit ion diffusion in grain boundaries and decrease the oxide-scale growth rate [117][118][119]. The presence of Y improves the adhesion between oxide film and substrate, thus improving the alloyed oxidation resistance [120,121].
Therefore, researchers try to add Y element to Mo-Si-B alloys to obtain materials with better performance.
After comparing the variation of Y-doped and Y-free Mo-9Si-8B (at.%) samples in the oxidation behavior at 650-1400 ℃, Majumdar et al. [122][123][124] found that all samples presented a trend of transient mass increase followed by continuous rapid decrease at 650 ℃ ( Fig. 8 (a)). Among all the samples, the 2at.% Y-doped sample had the minimum mass loss at 750-1000 ℃ (Figs. 8 (b-e)), and the 0.2at.% Y-doped sample presented the lowest mass loss at 1100 ℃ ( Fig. 8 (f)). This is because the addition of Y can produce stable Y6MoO12 and Y5Mo2O12 oxides at the initial period of oxidation, which can inhibit the generation and vaporization of MoO3, leading to a decrease in the mass loss of the samples containing Y.  9 (b, d)). It is worth noting that the thickness of inner MoO2 scale for 0.75at.% Y-doped sample is significantly thinner after oxidation at 1100 ℃ for 72 h (Fig. 9 (c)). Because SiO2 will present viscous flow to cover holes and cracks on the alloy surface during the temperature exceeds 965 ℃ [125,126], which prevents further oxidation of the substrate. Fig. 9 (e) shows the thickness changes of SiO2 and MoO2 layers for 0.2at.% Y-dpoed sample at 1100 ℃ and 1200 ℃, which further supports the above analysis results. When the oxidation temperature is higher than 1200 ℃, a thin yttrium-silicate (Y2Si2O7) scale is observed on the outer surface of Y-doped samples (Figs. 10 (a-d)), and the thickness of yttrium-silicate film gradually increases as the oxidation temperature raises (Figs. 10 (c, e)) [123]. It has been proved that the outer yttrium-silicate film is conducive to the alloy oxidation protection through preventing SiO2 from forming volatile silicon hydroxide in humid conditions above 1200 ℃ [127][128][129]. Similar studies have been reported by Gorr    Reproduced with permission [123]. Copyright 2013 Springer Nature.
There is no doubt that alloying with Zr has a great impact on the antioxidant ability of Mo-Si-B materials.
This is because the addition of Zr may produce polymorphic ZrO2 or monomorphic ZrSiO4, which mainly depends on the oxidation temperature. Among them, the ZrSiO4 can act as an obstacle phase, which is beneficial to improve the alloy oxidation behavior; whereas the ZrO2 will expand in volume at high temperatures (>1200 ℃), which destroys the integrity of SiO2 scale so that it loses the protective effect [131,132]. Therefore, inhibiting the formation of ZrO2 phase is essential to improve the alloy oxidation resistance.
Because the 0Zr-0Y sample has formed dense protective SiO2 films during the oxidation, which avoids the sample sustained mass loss ( Fig. 12 (a)), whereas the addition of Zr causes the SiO2 scale to become loose and porous due to the formation of ZrO2, and the porous structure provides channels for O2 inward diffusion, thus accelerating the sample oxidation ( Fig. 12 (b)). It is encouraging that further adding 0.3at.% Y can effectively prevent the rapid mass loss of 1Zr-0Y sample. As can be seen from Fig. 12 (c), ZrSiO4 rather than ZrO2 appears on the sample surface after the addition of Y, thus eliminating the adverse effect of Zr doping. Meantime, the Y-Mo-rich oxide is also observed around ZrSiO4 phase. EDS analysis shows that the Y/Mo atomic ratio of this oxide is about 1/2, revealing that the oxide may be Y2Mo4O15. Again, the XPS spectra also presents that the oxide has nearly the same Mo 3d and Y 3d bonding energies as Y2Mo4O15 gauged through You et al. [136], which further verifies the above inference (Figs. 11 (b, c)). What's more, the 1Zr-0.3Y sample surface also forms a uniformly dense outer Y2Si2O7 scale with the increase of oxidation time, which provides a better protection effect than 0Zr-0Y sample (Fig. 12 (d)). It has been observed from the cross-section enlarged Figs.
12 (e, f) that Y diffuses outward with the metastable Y2Mo4O15 as the carrier and produces Y2Si2O7 after a series of reactions at the top of SiO2 scale, which will be accumulated and compressed to form the outer Y2Si2O7 layer. Therefore, 1Zr-0.3Y sample presents the best antioxidant performance among the three samples.

Effects of rare earth elements on oxidation behavior of other Mo-Si alloys
Previous studies have pointed out that adding Nb to the Mo-Si-based materials can play a satisfactory effect in improving mechanical properties due to damaging the stability of Mo3Si phase [137][138][139]. However, the presence of Nb will lead to catastrophic oxidation of the material [140][141][142]. Inspired by the above study that adding Y can enhance the antioxidant properties of Zr-doped Mo-Si-B alloys, we further discussed the role of adding Y on the oxidation behavior of Nb-doped Mo-Si alloys.
Majumdar [143] used the nonconsumable arc-melted method to prepare the undoped and 0.5Y-doped Mo-26Nb-19Si samples (at.%), which simply referred to as Alloy1 and Alloy2, respectively. The microstructures of both samples are shown in Figs. 13 (a, e). It can be found that they are both composed of dark and bright areas. According to XRD analysis ( Fig. 14 (a)) and EBSD mappings (Figs.13 (b-d, f-h)), the dark and bright areas are (Mo, Nb)5Si3 and (Mo, Nb)ss phases, respectively. Moreover, Y2O3 particles are also observed on the Alloy2 grain boundaries. These particles can suppress the elongated grain growths, which results in the difference of microstructure morphology between the two samples. Meanwhile, Majumdar [143] also studied the oxidation process of Alloy2 at 1000 ℃ and 1300 ℃. It is established that the sample exhibits continuous linear mass loss when oxidized at 1000 ℃. When the oxidation temperature increases to 1300 ℃, the sample is oxidized more vigorously and loses its antioxidant capacity within 2 h of oxidation, as shown in Fig. 14 (b). Fig. 15 shows the cross-section and surface micrographs of the oxidized sample. It can be discovered that the Alloy2 surface has formed a thick oxide layer after oxidation at 1000 ℃ for 24h ( Fig. 15 (a)). As can be seen (a) from Fig. 15 (d), the oxide layer is mainly composed of MoO2, Nb2O5 and SiO2, wherein Nb2O5 can act as a channel for O2 internal diffusion due to the lack of protective action, which leads to rapid oxidation of the sample. In addition, the sample surface oxide film, which consists of Y2O3, Nb2O5 and SiO2, appears numerous cracks and holes during oxidation at 1300 ℃ for 2 h (Figs. 15 (b, c)), resulting in the loss of protection from oxidation. Therefore, adding Y to Mo-Si-Nb alloys cannot overcome the oxidizing problem.  Reproduced with permission [143]. Copyright 2018 Elsevier.

Strengthening mechanism of rare earth elements
According to the above research, it can be determined that the improvement of oxidation behavior of Mo-Si-based alloys by rare earth elements is mainly achieved through the following three ways. First, optimizing the microstructure of the alloy is caused by refining grains or distributing phase compositions uniformly, which contributes to the rapid formation of oxide scale [80,96,103,144]. Second, producing stable rare earth oxides, these oxides are dispersed in scale and act as obstacle phases or diffusion barriers, which is conducive to suppressing the MoO3 volatilization and O2 inward diffusion [59,122,123]. Third, forming an additional rare earth oxide layer, thus further improving the antioxidant capacity [96,135]. Fig. 16 shows a schematic diagram of the oxidation process for rare earth element doped and undoped Mo-Si-based alloys at medium-high temperatures, which is helpful to further understand the strengthening mechanism of rare earth elements. It can be seen that the alloy with finer grain size can be prepared after adding rare earth elements like La, which will affect the oxidation behavior to some extent. Overall, the oxidation process of the two kinds of alloys can be divided into two stages: initial and stable oxidation stages.
During the initial oxidation stage, a discontinuous SiO2 scale is formed on the surface of alloy without rare earth doping, which cannot effectively isolate oxygen. As a result, the alloy is oxidized violently and forms a Mo-oxide (MoO2 and MoO3) layer below the SiO2 scale. Among them, MoO3 is highly volatile, which leads to a severe mass loss of the alloy and leaves some holes and cavities on the surface [145]. Fortunately, SiO2 gradually increases and flows to heal these holes and cavities as the oxidation time increases, thus facilitating the formation of continuous SiO2 scale [146]. During the stable oxidation stage, the complete scale can provide sufficient protection for the substrate due to the effective restriction of O2 internal diffusion, resulting in the reduction of oxygen pressure inside the alloy. Obviously, low oxygen partial pressure inhibits the continuous generation of MoO2, and the original MoO2 will continue to oxidize to produce MoO3 and slowly volatilize so that the Mo-oxide interlayer becomes thinner [147]. Meanwhile, the substrate below MoO2 layer has been oxidized selectively, leading to the emergence of internal oxidation zone [122], as shown in Fig. 16 (a). In contrast, the alloy doped with rare earth can generate rare earth oxides such as La2O3, Y6MoO12, Y5Mo2O12, etc. in the initial oxidation stage. These stable oxides, on the one hand, promote the formation of continuous SiO2 scale. On the other hand, they fill holes in the scale to eliminate the shortcut of O2 inward diffusion and MoO3 volatilization, so that the alloy can enter the stable oxidation stage faster. In addition, a double-layer protective oxide film (i.e. Y2Si2O7-SiO2 or SiO2-HfSiO4 duplex scales) is formed on the alloy surface during the stable oxidation stage, providing more effective protection against oxidation, as shown in Fig. 16 However, it is disappointing that sometimes the addition of rare earth elements may even lead to the deterioration of alloy oxidation behavior in high temperature environments. For example, adding La to the Mo-Si-B system above 1100 ℃ has leaded to its accelerated oxidation attribute to the formation of large amounts of cracks and holes [60]. Therefore, the challenges ahead remain severe.

Conclusion and outlook
This paper comprehensively reviewed the role of rare earth elements on the oxidation behavior of Mo-Sibased alloy. Based on the thorough study about the oxidation process of Mo-Si-based alloy, the strengthening mechanism of various rare earth elements such as La, Hf, Ce and Y was summarized. The addition of La to Mo-Si-B alloys can make grains become finer to promote the rapid formation of continuous boroilicate scales; Meantime, producing stable La-containing oxides with a "pinning" effect like 3La2O3·MoO3, La2O3 and However, it is noteworthy that adding rare earth elements do not always improve the antioxidation ability of Mo-Si-based alloys in practice. For example, adding Y to the Mo-Si-Nb system cannot prevent its catastrophic oxidation; The addition of La also causes deterioration in the oxidation behavior of Mo-Si-B alloys above 1000 ℃. Therefore, a further search for other oxidation protection methods is necessary. Some research schemes worth exploring in the future are listed below, hoping to help solve the problems encountered in the practical application of Mo-Si-based alloys. Before using, preoxidation treatment at an appropriate temperature can obtain protective silica scales on the alloy surface, thus effectively inhibiting the inward diffusion of O2 and obviously extending the service life of alloy. Processing of preceramic polymers is a very promising method owing to its simple operation and low cost. Preceramic polymers, which mainly includes many siliconcontaining ceramic precursors like polysilazanes, polycarbosilanes and polysiloxanes, can be decomposed into