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Review

A Review of Mo-Si Intermetallic Compounds as Ultrahigh-Temperature Materials

by
Liang Jiang
1,*,
Bin Zheng
2,
Changsong Wu
2,
Pengxiang Li
2,
Tong Xue
2,
Jiandong Wu
2,
Fenglan Han
2 and
Yuhong Chen
2
1
School of Mechanical Electrical Engineering, North Minzu University, Yinchuan 750021, China
2
School of Material Science and Engineering, North Minzu University, Yinchuan 750021, China
*
Author to whom correspondence should be addressed.
Processes 2022, 10(9), 1772; https://doi.org/10.3390/pr10091772
Submission received: 1 August 2022 / Revised: 24 August 2022 / Accepted: 28 August 2022 / Published: 3 September 2022
Browse Figures
Review Reports Versions Notes

Abstract

:
Mo-Si compound-based ultrahigh temperature structural material (UHTM) is a new high-temperature structural material with great potential due to its high melting point, high hardness, and suitable density. As an important material to meet the demands of advanced high-temperature structural applications in the range 1200–1600 °C, in oxidizing and aggressive environments, it has attracted the attention of researchers in aerospace, energy, chemical industry, machinery, mechanical metallurgy, and other fields. This study could provide a reference for the research and application of Mo-Si intermetallic compounds as UHTMs. Based on their crystal structure and properties, we summarized the preparation, limitations, and modification methods of Mo-Si intermetallic compounds and reviewed the research progress in their toughness reinforcement and medium-temperature resistance improvement. Based on the literature review, there has been a certain level of progress in the research on modification of Mo-Si intermetallic compounds worldwide. However, both the high brittleness at room temperature and insufficient strength at high temperature have not been solved yet.

1. Introduction

Traditional single-crystal nickel-based alloys are used for the application at ~1100 °C with hot spots of ~1200 °C, which is ~90% of their melting point. Using complex cooling systems and thermal barrier coatings, these materials can exist in the hottest regions of a turbine engine where temperatures can approach 1500 °C. However, the necessity for coatings and forced-air cooling greatly reduces the efficiency gained from operating at the higher temperatures, as shown in Figure 1. Thereby, the heat resistance of nickel-based superalloys has not been increased significantly in recent years. Thus, alternative materials have to be developed in order to achieve a significant increase in operating temperatures (Table 1) [1,2,3]. Ultrahigh-temperature material refers to a single material or combination of materials that can be used at temperatures above 1600–2000 °C, including refractory metals, ceramic matrix composites, and modified C/C composites. Ultrahigh-temperature material has high strength, high-temperature oxidation resistance, and high-temperature corrosion resistance, which can be used in aircraft nose cones, the wing leading edge, the rocket nozzle, the combustion chamber, the hot end of the engine, and other key parts or components. From the aspect of use, ultrahigh-temperature materials mainly include ultrahigh-temperature structural materials and ultrahigh-temperature protection materials. Ultrahigh-temperature structural materials require high-temperature ablative and oxidation resistance of materials, but also require materials with good high-temperature mechanics and other comprehensive properties. The rapid development of modern technologies and industries has put forward more stringent performance requirements for structural materials in ultrahigh-temperature and harsh environments. Thus, research on new ultrahigh-temperature structural materials (UHTMs) has become essential. At present, the UHTMs under study still have numerous limitations. Ceramics with higher melting point matrix composites are brittle and difficult to toughen [4,5]. Although carbon/carbon composites could maintain high strength at 2200 °C, their high-temperature oxidation resistance is poor, and their production and processing are expensive [6,7]. Compared with the above-mentioned UHTMs, intermetallic compounds have the excellent properties of metal-based and ceramic-based composites and the high-temperature oxidation resistance that carbon/carbon composites lack, which make them a UHTM with great potential. The melting point (Tm) of newly designed alloys must be above 2000 °C since the current working environment temperature for new-generation UHTMs has reached 1600 °C (0.8 Tm) [8]. Among the intermetallic compounds, the refractory metal silicide (Table 2) could meet this requirement and could offer excellent high-temperature oxidation resistance. Refractory metal Mo has a high melting point, low density, low expansion coefficient, high elastic modulus, high wear resistance, and embrittlement resistance in oxidative/corrosive environments [9]. Therefore, research on Mo-Si series intermetallic compounds has attracted much attention.
This article reviews the research progress of Mo-Si-based intermetallic compounds as UHTMs, and summarizes the fabrication and modification strategies of Mo-Si series compounds. It has been found that MoSi2 has excellent high-temperature oxidation resistance, while Mo5Si3 and Mo3Si have good creep and corrosion resistance. However, limitations, such as fragility at room temperature, pesting oxidation at medium temperature, and low strength at high temperature, seriously hinder their application as high-temperature structural materials. Due to the intrinsic brittleness of Mo-Si, the alloying has limited improvement in the mechanical aspect. Thus, it is hard to use Mo-Si series compounds for the components fabrication. Grain refinement is a vital technique which benefits the strength, toughness, and oxidation resistance of Mo-Si-based alloys. However, there is still no clear mechanism used to control the grain size, and thus the strength and toughness of alloys. The fabrication of Mo-Si-based UHTMs has two main strategies; one is an external reinforcing agent addition operation and the other is an in situ synthesis process, both of which have the designated advantages.
Mo-Si intermetallic compounds as UHTMs have sparked the interest of scholars since the 1950s. They have excellent physical and chemical properties under high temperature and superior toughness and oxidation resistance compared to silicon-based ceramics and carbon/carbon composite materials. The Mo-Si system includes three intermetallic compounds: MoSi2, Mo5Si3, and Mo3Si, which have different performance characteristics. Due to excellent high-temperature oxidation resistance and R’ characteristics (strength not decreasing with the increase of temperature in a higher temperature range), MoSi2 has been widely used in the preparation of various heating elements and high-temperature anti-corrosion coatings. However, performance limitations such as high brittleness at room temperature, pesting (accelerated severe oxidation and powdering) at medium temperature, and insufficient strength at high temperature have severely restricted their application as UHTMs [10,11]. In 1951, Maxwell [12] proposed MoSi2 as a structural material and believed that the brittleness at low temperature was the main problem hindering its application. Subsequently, many researchers have made a certain level of progress in modifying MoSi2 through alloying, compounding, and microstructure control methods but they still cannot overcome the brittleness at room temperature and insufficient strength at high temperature. Thus, researchers turned to Mo5Si3, which had a higher Mo content. In addition to a higher melting point and excellent mechanical properties, the creep resistance of Mo5Si3 is several orders of magnitude higher than that of MoSi2. However, its oxidation resistance at high temperatures is poor, and it is more brittle at room temperature. Mo3Si has the least silicon among the three. It has good creep resistance but the highest relative density, poor oxidation resistance at high temperatures, serious intrinsic brittleness, and thermal expansion anisotropy; thus, it is rarely studied [13,14]. This study provides a detailed and comprehensive review of Mo-Si series intermetallic compounds. Based on the crystal structure and performance characteristics, the limitations of Mo-Si series intermetallic compounds as UHTMs were analyzed, and the modification and preparation methods were reviewed. The research trend of Mo-Si series intermetallic compounds was prospected, in the hope of providing a theoretical basis for developing new UHTM systems based on the Mo-Si binary system.

2. Mo-Si Series Intermetallic Compounds

Three ordered compounds are included in the Mo-Si binary phase diagram, i.e., MoSi2, Mo5Si3, and Mo3Si, as shown in Figure 2 [15]. MoSi2 is a stoichiometric compound (Table 3) with high thermal stability, wear resistance, and oxidation resistance. However, it is prone to phase transition at 1900 °C, shifting its crystal structure from C11b to C40. Mo5Si3 is a nonlinear compound with a wide composition range (2 to 3 at%), a certain toughness and reinforcement potential, and high thermal stability. The same composition will melt at 2180 °C. Mo3Si is formed by the peritectic reaction at 2050 °C: L + Mo → Mo3Si [16]. Christensen [17] adopted the neutron scattering method to study Mo3Si single crystals and found that Mo3Si was a linear compound with a stable cubic A15 structure. Rosales [18] studied the stoichiometric ratio and mechanical properties of Mo3Si and found that its single-crystal composition is close to Mo-24 at% Si, which is not consistent with Mo-25 at% Si in the phase diagram. Therefore, Mo3Si is not believed to be a strict linear compound.

2.1. Crystal Structure and Performance Characteristics

Table 4 shows some of the crystallographic and physical parameters of the three Mo-Si series intermetallic compounds. The three intermetallic compounds have different crystal structures, which grant them significantly different physical and chemical properties. The Mo-Si series intermetallic compounds have different crystal structures, and the crystal structure of the compounds is shown in Figure 3. The Mo-Si series intermetallic compounds have both metal and covalent bonds. Mo and Mo are combined by metal bonds, Si and Si are combined by covalent bonds, and the bonds combining Mo and Si have the characteristics of the two bonds. The metal bonds grant the Mo-Si series intermetallic compounds a certain level of plasticity and good electrical and thermal conductivity. The covalent bonds enhance the atomic bonding force of the Mo-Si intermetallic compounds, slowing down the diffusion between components and giving them a high melting point, high hardness, and good creep resistance [19]. With the increase of the Si content, the number of covalent bonds increases, and the number of metal bonds decreases. Thus, the high-temperature oxidation resistance and corrosion resistance of the compounds improve significantly, but the melting point decreases and room temperature brittleness increases [20,21,22]. In addition, as the long-range ordered structure of Mo-Si intermetallic compounds inhibits the generation of cross-slip, the possibility of crack initiation is reduced during cyclic loading, which grants them good fatigue resistance [23].
Table
Table 4. Mo-Si intermetallic phase crystallography and physical parameters [24,25,26].
Table 4. Mo-Si intermetallic phase crystallography and physical parameters [24,25,26].
Alloy TypeCrystal StructureMelting Point
/°C		Density/
(g·cm−3)		Hardness/
GPa		CET/
K−1		Fracture Toughness
/(MPa·m1/2)		BDTT/
°C
MoSi2C11b-C4020306.2410.58.1 × 10−62.9900–1000
Mo5Si3D8m21808.1912.855.2 × 10−63.2-
Mo3SiA1520258.9711.489.0 × 10−63.51400
MoSi2 has a higher melting point (2030 °C), a density 30% lower than traditional nickel-based superalloys, a higher elastic modulus second only to WSi2 in metal silicide, a lower coefficient of thermal expansion (8.1 × 10−6 °C/K−1), better thermal conductivity (2020 °C), higher brittle-to-ductile transition temperature (900–1000 °C), and ductility of the metal and hardness and brittleness of ceramics above and below the brittle-to-ductile transition temperature, respectively, due to impurity content and the microstructure [27]. At room temperature, the hardness (10.5 GPa) and fracture toughness (2.9 MPa·m1/2) of MoSi2 are poor but can increase as the Si content decreases. Mo5Si3 has a stable D8m crystal structure that includes many Mo-Si bonds, a unique body-centered cubic lattice, a lattice constant a/c ≈ 2, no close-packed plane, Mo-Mo and Si-Si bonds that only appear in the [011] orientation, and Mo-Si bonds with a strong bonding force that grants greater room temperature brittleness. Among the Mo-Si binary intermetallic compounds, Mo5Si3 has the highest melting point (2180 °C) and a bulk elastic modulus K to shear modulus G ratio equivalent to Mo. According to the Pugh rule [28], it is an alloy with a high K/G ratio and a certain toughening potential, but its thermal expansion coefficient is severely anisotropic, which makes it easy to cause surface cracks [29,30]. Mo3Si has a stable A15 crystal structure, a melting point of 2025 °C, a density of 8.97 g/cm3, and a low thermal expansion coefficient (9.0 × 10−6 °C/K−1). It also has a high brittle-ductile transition temperature (1400 °C), with oriented hardness. When the crystal orientation is [100], the crystal is the hardest; when the crystal orientation is [110], the crystal is the softest [31]. Rosales et al. [32] investigated the fact that the microhardness is a function of crystal orientation and Mo3Si single crystals have the hardest crystal orientation of (100), followed by (111) and (110) planes, with a hardness of 1460, 1448, and 1425 kg/mm2, respectively.
Processes 10 01772 g003 550
Figure 3. Crystal structure of the Mo-Si intermetallic compounds: (a) MoSi2 C11b type; ref. [33]; (b) MoSi2 C40 type; ref. [34]; (c) Mo5Si3; ref. [35]; (d) Mo3Si [36].
Figure 3. Crystal structure of the Mo-Si intermetallic compounds: (a) MoSi2 C11b type; ref. [33]; (b) MoSi2 C40 type; ref. [34]; (c) Mo5Si3; ref. [35]; (d) Mo3Si [36].
Processes 10 01772 g003

2.2. High-Temperature Performance and Issues in Application

Table 5 shows oxidation resistance of high melting point silicides. The Mo-Si series intermetallic compounds have high melting points, high hardness, and good high-temperature performance, which earned them extensive attention from researchers worldwide. Although Mo-Si series intermetallic compounds are expected to be developed as a new high-temperature structural material, they could serve as high-temperature alloys. There are still some unsolved issues in their chemical applications, and limitations such as brittleness at room temperature, pesting at medium temperature, and insufficient strength at high temperature, and limitations originating from the preparation and processing technology hinder the practical application of Mo-Si intermetallic compounds as high-temperature structural materials.
At room temperature, the crystal structure of MoSi2 has low symmetry and an insufficient openable slip system, resulting in higher strength and hardness but insufficient toughness. According to the traditional dislocation theory, dislocation energy is proportional to the Burgers vector b2, and the most likely slip systems in MoSi2 are {013}<100>, {110}<110>, and {110}1/2<111>. In actual research [32,33,34,35], slip systems such as {011}<100>, {110}<111>, {010}<100>, {023}<100>, and {013}<331> were also found. However, it changes with the conditions, which is much more complicated than the theoretical slip system. Different temperatures or stress states affect the number of slip systems. A reduced slip coefficient causes dislocation plugging, making MoSi2 more brittle at room temperature. When the temperature is above 1000 °C, the shear stress strongly depending on the <100> direction slipping causes the decrease of the high-temperature strength of MoSi2 [36].
At 1350 °C, SiO2 is glassy and forms a weak second phase or liquid phase at the grain boundary, which intensifies the slip and aggregation of the grain boundary and reduces the high-temperature creep resistance and strength. As the temperature continues to rise above 1500 °C, the strength of MoSi2 drops sharply below 100 Mpa, and the plasticity increases the elongation by 5% [38]. With a complex crystal structure and strong bonding force between atoms, Mo5Si3 has low fracture toughness at room temperature (3.2 Mpa·m1/2) but excellent creep resistance at high temperature. At 1000 °C, the creep resistance is equivalent to a single-crystal nickel-based superalloy. Mo3Si has oriented hardness and cracking behavior and a high brittle-ductile transition temperature of 1400 °C, where its strength is affected by the Si content and strain rate [31,38].
Figure 4 [39,40] shows the oxidation curves of Mo-Si intermetallic compounds at different temperatures. MoSi2 suffers strong oxidation and powdering at 500 °C, but the oxidation is significantly weakened when the temperature reaches 600 to 1400 °C. The possible reactions during the oxidation of MoSi2 are as follows:
2MoSi2 + 7O2 → 2MoO3 + 4SiO2
5MoSi2 + 7O2 → Mo5Si3 + 7SiO2
2MoSi2 + 5O2 → 2MoO3 + 4SiO
Reaction (1) mainly occurs in the temperature range of 400 to 600 °C. Mo and Si atoms are oxidized at the same time to generate whisker-like MoO3 and cluster-like SiO2. Meanwhile, no protective film can be formed on the surface of the compound since part of the MoO3 volatilizes. However, severe pesting occurred. Figure 4 [41] shows the morphological changes of the MoSi2 samples under different oxidation temperatures and oxidation times. The MoO3 whiskers generated at 500 °C severely damaged the continuity of the SiO2 protective film, causing severe pulverization and fragmentation to the MoSi2 samples [42]. Reaction (2) mainly occurs when the temperature is above 800 °C, forming a non-exfoliated and dense SiO2 oxide layer on the compound surface that hinders the progress of the oxidation reaction. The oxidation layer has a certain level of fluidity and can be found in cracks and holes. The SiO2 film formed at the limitations has a certain self-healing effect. Reaction (3) mainly occurs at high temperatures above 1800 °C. At this time, oxygen atoms accelerate the diffusion, generate volatile SiO and MoO3, and rapidly deteriorate the SiO2 protective film. Thus, the oxidation resistance is lost. Figure 5a [39] shows that the casting MoSi2 material has a higher oxidation rate and more severe pesting than those prepared by single-crystal and hot isostatic pressing. The reason is the many holes, cracks, and other limitations in the casting material. Oxidation reaction and volume expansion promote crack propagation following the enrichment of oxygen atoms there, which increases the oxidation of the matrix. Some studies [43,44] have shown that pesting is not an intrinsic characteristic of MoSi2 but is mainly attributed to the material’s microstructure, purity, and the surrounding environment [45,46,47]. Meschter [39] considered that when the density is greater than 98%, MoSi2 materials without microcracks or stress will not show pesting. Figure 5b [40] shows that when Mo5Si3 is at a lower temperature, Mo and Si atoms are oxidized to form oxides such as MoO3 and SiO2, which slightly increases the sample weight. When the temperature is over 750 °C, the generated MoO3 begins to volatilize continuously. At 800 °C, catastrophic pesting oxidation similar to MoSi2 occurs, and a large amount of oxide peels off, resulting in a significant reduction in sample weight. At 900 °C, a SiO2 oxide layer with a certain level of fluidity is formed, which improves the oxidation resistance of Mo5Si3 and significantly reduces the oxidation rate. When the temperature is above 1100 °C, the oxidation rate increases sharply to about 2400 times the oxidation rate at 900 °C, and the sample weight also shows a sharp downward trend. Rosales [25] found that pesting oxidation also occurs in Mo3Si at 1000 °C for several hours.
The high melting point and high brittleness at room temperature make it difficult to prepare and process Mo-Si series intermetallic compounds with conventional methods [41,42,43]. Generally, the intermetallic compounds prepared by the melting and casting method have limitations such as microcracks, shrinkage porosity, and coarse grains [44]. When the directional solidification method is used, the Mo-Si series often have better thermal conductivity [45,46,47]. The temperature difference could easily cause thermal cracks in the transition parts, making the structure prone to stress and deformation after solidification. The hot-pressing method tends to produce oxidation due to the long holding time and thermal stress after cooling. Moreover, the possible interfacial reactions impair the overall performance of the material. There are many methods to prepare refractory metal silicide, each of which has certain limitations. The relatively complicated preparation process and the difficult control of structure composition impede the improvement of alloy properties. Advanced preparation and processing technologies are the prerequisites for high-performance materials. To prepare excellent high-temperature materials, we must optimize the preparation and processing technologies.

3. Modification of Mo-Si Series Intermetallic Compounds

3.1. Alloying Research

Alloying could achieve toughness reinforcement and performance optimization of Mo-Si series intermetallic compounds. Alloying mainly includes microalloying and macroalloying. Microalloying changes the crystal structure of the matrix or the configuration and energy of crystal limitations by doping a small amount of alloying elements, thus improving the physical and chemical properties of the alloy. Macroalloying adds many alloying elements by adjusting the alloy composition ratio or introducing the plastic phase to achieve excellent performance. Studies have found that some alloying elements have a significant impact on the alloy’s properties. For example, alloying elements with a greater affinity for Si and O, such as B, Al, Cr, Zr, Ge, Ti, Ta, and Y, can reduce or suppress low-temperature pesting oxidation. During pesting oxidation and fragmentation, Al, Cr, and B elements can significantly improve the anti-oxidation performance. Al can form an amorphous Mo-Si-Al-O phase at cracks, voids, and other limitations, and the oxidation resistance properties of Mo3Si can be significantly improved by adding 15 mol% Al alloying [48,49]. Cr can form a composite oxide film composed of Cr2(MoO4)3, MoO3, and SiO2 on the alloy surface, which effectively inhibits low-temperature pesting oxidation [50,51]. Adding a small amount of B (<2 wt%) can form a new phase of Mo5SiB2, where B and Si elements could easily form a dense oxide film that significantly improves the high-temperature oxidation resistance and increases the alloy’s low-temperature strength [29,52,53,54,55]. Figure 6 [56] shows the surface morphology of Mo-Si-B alloys with different B contents after isothermal oxidation for 30 h. When the B content is low, a rough and discontinuous borosilicate glass phase (oxide layer) forms on the surface of the alloy after high-temperature oxidation, and some oxide particles concentrate on the surface. In addition, a few limitations, such as microcracks and voids, can also be observed. With the increase of B content, the fluidity of the borosilicate glass phase increases, and a smooth and continuous oxide layer gradually forms on the surface.
Meanwhile, the rough silicon-rich layer (SiO2) decreases, and limitations such as cavities and microcracks gradually disappear, which significantly improves the oxidation resistance and performance. The addition of alloying elements such as Cr, Zr, Nb, Co, C, Ta, and V can improve the room temperature toughness of the compound. Akinc [17] found that introducing Co or C into MoSi2 can form a new phase of CoMoSi or Mo5Si3C hexagonal crystal structure and improve the symmetry of the crystal structure, thereby improving the fracture toughness. C can also form with Si at the grain boundary β-SiC phase and increase the hardness. Sharif [57] showed that Al and Nb, alloy elements that stabilize the crystal structure of C40, can reduce the stacking fault energy and increase the width of the stacking fault region, thus reducing Peierls stress (the critical cut required to move a single bit fault in an ideal crystal) that increases dislocations’ mobility. Alloying elements such as W, Nb, Re, and Er [58,59,60] can improve the high-temperature mechanical properties. Specifically, W elements reduce the creep rate of MoSi2-based composites to increase their high-temperature strength [61]. Re and Nb form high-temperature deformation mechanisms with their change dominated by the dislocation climbing and the viscous slip of the dislocation, which hardens solid solutions [57,61,62]. Adding Nb also strengthens the Mo3Si matrix [63]. At the time of writing, the alloying method has achieved certain results in room temperature toughening, high-temperature reinforcement, and oxidation resistance improvement. However, due to the intrinsic brittleness of Mo-Si series intermetallic compounds, the alloying method can only improve their mechanics to a limited extent, thus it is still not widely used in the preparation of mechanical parts.

3.2. Compounding Research

The toughening phase of MoSi2 includes continuous fiber (ceramic fiber, refractory metal fiber) and discontinuous fiber (whisker, particle). Petriovic summarized the results of recent studies on MoSi2 structural composites and pointed out that the toughening effects in descending order are continuous fiber, ductile metal, ZrO2 phase transition, and second phase whisker or particle (Table 6). Introducing strong and toughening phases can reinforce the Mo-Si series intermetallic compounds in a much simpler and more effective way than increasing the intrinsic toughness of intermetallic compounds [58]. The strong and tough second phase absorbs the deformation energy before fracture, effectively retaining the high-temperature performance of the intermetallic compounds and improving the low-temperature brittleness. When the Mo-Si series composite material is introduced into the second phase for compounding, the second phase with a high melting point, low density, good thermodynamic stability, sound physical and chemical compatibility, and similar thermal expansion coefficient with the matrix should be selected. Moreover, the form, quantity, size, and distribution of the second phase need to be reasonably controlled [62]. The second phase mainly includes high melting point ductile metal phases (Nb, Mo, Ti, Ta, W, etc.) and high melting point ceramic reinforcing phases (SiC, Si3N4, ZrO2, etc.) [64]. The ductile metal phase can improve the room temperature brittleness to a certain extent, but it will reduce the high-temperature strength of the composite material. The ceramic reinforced phase has certain advantages. For example, it will not undergo interfacial reaction within a certain temperature range, and its strengthening effect is related to the thermodynamic compatibility with that of the matrix and its physical and chemical properties. The effects of several different single composite additions, including Mo5Si3, SiO2, CaO, and TiC, all in particulate form, were investigated as an elastically hard, strong, brittle second phase which can potentially strengthen MoSi2 at high temperatures and impart low-temperature toughness by crack deflection processes (Table 7). Table 8 shows the physical and chemical properties of common strong and toughening phases and the performance changes after compounding. Massive experimental studies have shown that [36,65,66,67,68] the morphology of the second phase, mainly including fibers, whiskers, and particles, also greatly impacts the strengthening and toughening effect. Among the three types, continuous fiber metal wire has the best toughening effect. However, the interface reaction and poor oxidation resistance severely restrict its application as a reinforcement. Although discontinuous whiskers and particles are inferior in toughening and strengthening effects, their preparation is simple with isotropic material. Monolithic MoSi2 material has been found to be exceptionally wear-resistant in abrasive environments, with a wear mechanism similar to that of oxide ceramics. Moreover, the wear resistance of MoSi2 can be further improved with the addition of second phases [69,70].
At present, the reconsideration of Mo-Si series intermetallic compounds mainly focuses on the toughening and strengthening of MoSi2-based composites. Figure 7a,b [13,59,60,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96] show the toughening and strengthening of MoSi2-based composites. The room temperature toughness and high-temperature strength of MoSi2-based composites have been continuously improved. MoSi2-Si3N4-SCS SiC (SCS series: SiC fiber made by CVD deposited on continuous carbon fiber) composite material, with its room temperature fracture toughness of 35 MPa·m1/2, meets the requirements of 10 MPa·m1/2 and 15 MPa·m1/2 in the engineering and aerospace fields [72]. The Mo5Si3-Mo3Si composite strong and tough phases include Al2O3, Si3N4, and MoSi2. Among them, Al2O3 is considered one of the important strengthening phases of Mo5Si3 due to its high melting point, low density, excellent high-temperature creep resistance, and similar thermal expansion coefficient. Chen [88] introduced the Al2O3 phase into Mo5Si3, significantly improving the strength, wear resistance, and fracture toughness of Mo5Si3 composite matrix materials. However, the MoSi2-Mo5Si3 composite material synthesized by the in situ method has better compressive mechanical properties than single-phase MoSi2 and Mo5Si3 [97]. As shown in Table 8, the introduction of different single strong and tough phases can achieve certain toughening and strengthening effects, but the improvement of mechanical properties is still limited. In recent years, some researchers have used synergistic technology to prepare multiphase composite materials with better mechanical properties, such as MoSi2-Si3N4-SiC and MoSi2-WSi2-SiC, and this technology has gradually become a new research trend. Mason [98,99] introduced two phases of SiC and ZrO2 to MoSi2 simultaneously and found that they have a compound synergistic effect. SiC can reduce the influence of the intergranular coarseness of ZrO2 on the bending strength, while ZrO2 can reduce the heat expansion mismatch stress between SiC and MoSi2, with the high-temperature compressive performance of the alloy significantly improved. Jiang [100] synthesized MoSi2-ZrB2-SiC multiphase composite material by in situ combustion. Under the combined action of multiple toughening mechanisms, microcrack bridging, crack deflection, and trans granular fracture, the room temperature brittleness was significantly improved, with the fracture toughness increasing by about 73% and the Vickers hardness reaching 14.7 GPa. Figure 8 [101] shows the typical cracking behavior of MoSi2-ZrB2-SiC multiphase composites with different compositions. In Figure 8a,d [101], the cracks are deflected at the ZrB2 grains, while trans granular fractures appear in the SiC grains. Due to the tensile residual stress in the MoSi2 grains, multiple open cracks appear, as shown in Figure 8b,c, increasing the fracture toughness of the material [101]. Hebsur [72] used the powder cloth method to prepare MoSi2-Si3N4-SiCf material with excellent oxidation resistance. The introduction of Si3N4 can reduce the thermal expansion coefficient of the material and improve the oxidation resistance, with SiC lifting the fracture toughness to 35 MPa·m1/2. The synergistic effect of composites can overcome some shortcomings caused by the introduced phases, and each phase has interaction or supplementation, further improving the compressive performance of composite materials, which has gained widespread attention in composite research. Through precise control of the sintering process for the microwave reaction, Xu [81] et al. successfully prepared a 20% SiC-reinforced MoSi2 composite with a fracture toughness of 8.17 MPa·m1/2. Nie [87] et al. prepared ZrO2-SiC/MoSi2 composite ceramics by powder metallurgy. With the increase of ZrO2 and SiC content, fracture toughness of ceramics at the high temperature gradually increases. As the amount of ZrO2 and SiC increases to 20% and 10%, respectively, the fracture toughness of ceramics reaches the maximum value of 21.881 MPa·m1/2. C. Hochmuth [93] et al. studied the structure and creep rate of zirconium-based alloys obtained from spark plasma sintering. It is revealed that the creep rate of the Mo-9Si-8B-2Zr alloy is one order of magnitude lower than that of the alloy without zirconium under the same grain size. Olha Kauss [13] et al. claimed that the Mo3Si alloy has a higher creep response than Moss and Mo-Si-B alloys, and a lower creep response than single-phase Mo5SiB2 alloys via compression creep tests in different stress and temperature ranges.

3.3. Research on the Preparation of Single Crystals and Ultrafine Crystals

The fusion casting method, the most commonly used method for preparing intermetallic compounds worldwide, is prone to various shortcomings, such as coarse grains in the solidification stage. The preparation of single-crystal alloys can eliminate grain boundaries, reduce drawbacks such as cracks and voids, significantly reduce the use of grain-boundary-strengthening elements that can lower the melting point, increase the use temperature of the alloy, and perform solution treatment at higher temperatures. It can improve the alloy strength, creep resistance, and other properties. Single-crystal alloys have intrinsic anisotropy, and single-crystal superalloys with specific orientations, which have excellent compressive properties, can be prepared according to the anisotropy. Umakoshi [33] found that MoSi2 single crystals still have high strength at 1000–1500 °C, especially [001]-oriented MoSi2 single crystals, which have much higher yield strength than single crystals of other orientations. When the temperature is above 1400 °C, the [001]-oriented single crystal prepared by the suspension furnace method shows higher yield strength than the single crystal prepared by other methods [102]. Figure 9a [102] shows the change of yield strength of [001]-, [021]-, and [771]-oriented MoSi2 single crystals with the temperature. The yield strength of [001]-oriented MoSi2 single crystals is much higher than that of [021]- and [771]-oriented single crystals. At 1000–1400 °C, the yield strength of [021]- and [771]-oriented single crystals decreases with the increase of the temperature, and the strain rate has little effect. At 900–1300 °C, [001]-oriented MoSi2 single crystal does not show an obvious yield phenomenon. At 1300–1600 °C, the yield strength of [001]-oriented MoSi2 single crystal decreases significantly with the increase of the temperature, and the strain rate has a greater influence. Figure 9b [102] shows the change of critical shear stress (CRSS) of different slip systems with the temperature. It can be seen that there are {013}1/2 <331>(A), {011} <100>(B), {110}1/2 <111>(C), and {013} <100>(D). This kind of slip system is easy to start. In addition, some slip systems with larger CRSS show the plastic anisotropy of MoSi2 single crystal [102]. Obert et al. [60] studied the mechanical properties of Mo5Si3 single crystal and found that the activation energy for creep is 510 KJ/mol−1, the stress index is 6, and the thermal expansion coefficient is anisotropic in a and c directions. Moreover, the elastic modulus of Mo5Si3 single crystal has a smaller anisotropy trend compared with that of other silicides, and its yield deformation behavior is closely related to the crystal orientation. Mo3Si single crystal has the yield strength of 400 MPa at 1400 °C, with the strength affected by the Si content and strain rate [31].
Grain refinement, an essential means of strengthening and toughening Mo-Si series intermetallic compounds, also plays an important role in the toughening and strengthening of brittle materials, such as ceramics and metal silicide. Refining the grains can improve the strength, toughness, and oxidation resistance of the material. The Hall–Petch equation can explain the phenomenon that the strength of polycrystalline alloys increases with grain refinement. However, there is still no mature theory to quantitatively explain the effect of grain size on the strength and toughness of alloys [103]. Theoretical deduction and practical research show that the greater the degree of grain refinement, the larger the grain boundary surface area, with the total deformation of the alloy more evenly distributed in massive grains, reducing stress concentration and inhibiting microcracks. The deflection and reflection of cracks will further delay crack propagation, improving the plastic deformation ability of the material. Karch et al. and Bohn et al. [104,105] found that when the crystal grains are refined to the nanometer level, intermetallic compounds or ceramics that hardly deform can be deformed or super plastically deformed. Through grain refinement, the concentration of the oxide film-forming elements ubiquitous in the grain boundary of the Mo-Si series intermetallic compound can also be increased. The smaller oxide-crystal nucleus distance will promote the lateral growth of the crystal nucleus and continuous compactness, with the formation of oxide film significantly improving the oxidation resistance. In addition, nanocrystals, with a larger coefficient of thermal expansion than ordinary crystals, can eliminate growth stress and part of thermal stress, and improve the adhesion of the oxide film layer on the substrate’s surface, to heighten the oxidation resistance [106].

4. Preparation Process of Mo-Si-Based UHTMs

The preparation of advanced and high-performance composite materials requires materials with excellent performance and scientific and advanced preparation technology. Mo-Si series intermetallic compounds, which are brittle at room temperature and have a high melting point, are difficult to prepare and process by conventional methods. At present, the preparation process of Mo-Si-based ultrahigh-temperature structural materials is mainly divided into two categories. The first is the composite preparation with external reinforcement, such as casting method, directional solidification method, and laser cladding, etc. The second is in situ synthesis preparation, such as self-spreading high-temperature synthesis, mechanical alloying, in situ reaction sintering, and XDTM. The main preparation methods of Mo-Si-based ultrahigh-temperature structural materials and their advantages and disadvantages are shown in Table 9. Composite materials prepared by adding reinforcing agents are prone to shortcomings, such as coarse reinforcing particles, low interfacial bonding strength, and thermodynamic instability. However, composite materials prepared by the in situ synthesis technology that generate one or more reinforcing phases in the matrix through physical and chemical reactions have small reinforcing phase particles, stable thermodynamic properties, and high interface bonding strength without pollution.

5. Application of Mo-Si Series Intermetallic Compounds

As a new high-temperature structural material, Mo-Si series intermetallic compounds have the following four structural applications: (1) as the matrix of Mo-Si-based composite materials; (2) as the reinforcing agent of structural ceramic composites to improve the mechanical properties of the materials at high temperature; (3) because of its high melting point and excellent high-temperature oxidation resistance, it becomes a high-temperature welding material for structural ceramics; (4) used as high-temperature anti-oxidation coating for refractory metals and carbon-based materials.
With the development of aviation, aerospace, chemical industry, metallurgy, transportation, national defense, and other industries, the application of Mo-Si series intermetallic compounds as high-temperature structural materials will be more and more widely seen.

6. Summary

Mo-Si series intermetallic compounds have impressively excellent high-temperature performance as one of the most promising high-temperature structural materials. The present review article aimed to provide an overview of the critical challenges, and the recent breakthroughs in oxidation resistance, creep resistance, and damage tolerance of Mo-Si intermetallic compounds. The design and development of new materials for ultrahigh-temperature applications are invariably a competition between achieving excellent oxidation resistance and creep/fatigue strength at service temperatures and maintaining adequate ductility and toughness at both low and high temperatures. Unfortunately, the microstructural requirements to achieve acceptable behavior in all categories are generally mutually exclusive. This is a particularly difficult problem in Mo-Si intermetallic compounds where the microstructures for optimal oxidation resistance, creep strength, and damage tolerance (strength and toughness) are so contradictory. Much progress has been made in recent years in the development of high-temperature structural Mo-Si intermetallic compounds. There are, however, still two key issues for the future development of these materials. The first is to continue to improve the low-temperature fracture toughness, by both composite and alloying approaches. The second key issue is to improve the oxidation and creep resistance of these materials.

Author Contributions

Conceptualization, L.J. and C.W.; investigation, L.J., C.W., B.Z. and P.L.; writing—original draft preparation, C.W.; writing—review and editing, C.W., L.J., F.H., Y.C., T.X. and J.W.; funding acquisition, L.J. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the Ningxia Natural Science Foundation (2022AAC03254); the National Science Foundation (51964001); the Key Research and Development Project of Ningxia Hui Autonomous Region (2020BDE03001; 2020BCE01001; 2021BEG01003).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Informed consent was obtained from all subjects involved in the study.

Data Availability Statement

The data presented in this study are available on request from the authors.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Core power versus turbine inlet temperature for selected gas turbine engines; ref. [3]. (b) The role of high-temperature materials in the historical evolution of the turbine blade surface temperature in aeroengines. (c) The ranges of operating temperature and strength/weight ratio for different classes of materials [4].
Figure 1. (a) Core power versus turbine inlet temperature for selected gas turbine engines; ref. [3]. (b) The role of high-temperature materials in the historical evolution of the turbine blade surface temperature in aeroengines. (c) The ranges of operating temperature and strength/weight ratio for different classes of materials [4].
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Figure 2. Mo-Si binary phase diagram [15].
Figure 2. Mo-Si binary phase diagram [15].
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Figure 4. Oxidation of Mo-Si intermetallic compounds: (a) isothermal oxidation of MoSi2 with different preparation processes in the range of 40 to 1400 °C; ref. [39]; (b) isothermal oxidation of Mo5Si3 in the temperature range of 800 to 1200 °C [40].
Figure 4. Oxidation of Mo-Si intermetallic compounds: (a) isothermal oxidation of MoSi2 with different preparation processes in the range of 40 to 1400 °C; ref. [39]; (b) isothermal oxidation of Mo5Si3 in the temperature range of 800 to 1200 °C [40].
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Figure 5. Variation of MoSi2 sample morphology with oxidation temperature and oxidation time: (a) unoxidized; (b) 400 °C, 23 h; (c) 450 °C, 23 h; (d) 500 °C, 21 h; (e) 550 °C, 70 h; (f) 600 °C, 100 h; (g) 675 °C, 20 h [41].
Figure 5. Variation of MoSi2 sample morphology with oxidation temperature and oxidation time: (a) unoxidized; (b) 400 °C, 23 h; (c) 450 °C, 23 h; (d) 500 °C, 21 h; (e) 550 °C, 70 h; (f) 600 °C, 100 h; (g) 675 °C, 20 h [41].
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Figure 6. Morphology of Mo-Si-B alloys with different B content after isothermal oxidation for 30 h: (a,b) Mo-Si-5B; (c,d) Mo-Si-8.5B; (e,f) Mo-Si-17B [56].
Figure 6. Morphology of Mo-Si-B alloys with different B content after isothermal oxidation for 30 h: (a,b) Mo-Si-5B; (c,d) Mo-Si-8.5B; (e,f) Mo-Si-17B [56].
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Figure 7. The toughening and strengthening of MoSi2 matrix composites: (a) fracture toughness change of MoSi2 matrix composites in 1986–2000; (b) creep resistance change of MoSi2 matrix composites in 1986–2000; (c) fracture toughness change of MoSi2 matrix composites in 2000–2021; (d) creep resistance change of MoSi2 matrix composites in 2000–2021 [13,59,60,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96].
Figure 7. The toughening and strengthening of MoSi2 matrix composites: (a) fracture toughness change of MoSi2 matrix composites in 1986–2000; (b) creep resistance change of MoSi2 matrix composites in 1986–2000; (c) fracture toughness change of MoSi2 matrix composites in 2000–2021; (d) creep resistance change of MoSi2 matrix composites in 2000–2021 [13,59,60,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96].
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Figure 8. Typical cracking behavior of ZrB2-MoSi2-SiC composites with different compositions: (a) ZMS-2; (b) ZMS-4; (c) ZMS-8; (d) ZMS-10 [101].
Figure 8. Typical cracking behavior of ZrB2-MoSi2-SiC composites with different compositions: (a) ZMS-2; (b) ZMS-4; (c) ZMS-8; (d) ZMS-10 [101].
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Figure 9. Curves of yield strength and critical shear stress (CRSS) versus the temperature for MoSi2 single crystals with different orientations at 10−4/s, 10−5/s strain rate: (a) curves of yield strength versus temperature MoSi2 with [001], [021], and [771] orientations; (b) critical shear stress (CRSS) versus the temperature for different slip systems [102].
Figure 9. Curves of yield strength and critical shear stress (CRSS) versus the temperature for MoSi2 single crystals with different orientations at 10−4/s, 10−5/s strain rate: (a) curves of yield strength versus temperature MoSi2 with [001], [021], and [771] orientations; (b) critical shear stress (CRSS) versus the temperature for different slip systems [102].
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Table
Table 1. Potential materials system application at high temperatures.
Table 1. Potential materials system application at high temperatures.
Temperature Range (°C)Materials SystemMain Limitation (s)
650–1200Nickel aluminides
Molybdenum disilicides 		Damage tolerance and creep resistance
Damage tolerance
1200–1400Molybdenum disilicides
Niobium silicides 		Damage tolerance and creep resistance
Damage tolerance
1400–1800Ceramic matrix composites Damage tolerance
1800–2000Carbon–carbon compositesOxidation resistance
Table
Table 2. Structure and properties of some silicides with melting points over 2000 °C.
Table 2. Structure and properties of some silicides with melting points over 2000 °C.
Alloy TypeCrystal StructureMelting Point/°CDensity/(g·cm−3)
Ta5Si3D81250513.40
Nb5Si3D8124847.16
W5Si3D8m137014.5
Zr5S i3D8823275.99
Mo5Si3D8m21808.24
V5Si3D8m21505.27
Ti5Si3D8821304.32
TaSi2C4022209.10
WSi2C11b21609.86
MoSi2C11b20306.24
MoSi3A1520258.97
ZrSi-21505.94
Table
Table 3. Mo-Si intermetallic phase crystallography and physical parameters.
Table 3. Mo-Si intermetallic phase crystallography and physical parameters.
Alloy TypeTheoretical Si Concentration (at.%)Actual Si Concentration (at.%) Consistent
Mo3Si2524 [18]slightly off-stoichiometric
Mo5Si337.537.5–40off-stoichiometric
MoSi266.766.7stoichiometric
Table
Table 5. Oxidation resistance of high melting point silicides [37].
Table 5. Oxidation resistance of high melting point silicides [37].
SilicideOxidation Temperature (°C)Oxidizing Time (h)Mass Change
(mg cm−2)
MoSi212004+0.3
15004+1.3
WSi212004−17
15004−23
NbSi212006+40
TaSi212004+60
TiSi212004+22
Mo5Si315004−67
W5Si315004−205
Ta5Si315001+125
Ti5Si315002+32
Table
Table 6. Room-temperature fracture toughness of MoSi2-based composites [71].
Table 6. Room-temperature fracture toughness of MoSi2-based composites [71].
Type of ReinforcementHighest Fracture Toughness
(MPa·m−1/2)
Refractory metal (Nb, W, MO) wires>15
20 vol.% Ta particles10
20 vol.% ZrO, particles7.8
20 vol.% Sic whiskers4.4
20 vol.% SIC particles4.0
Polycrystalline MoSi23
Table
Table 7. Coefficients of thermal expansion (CTE) and melting temperatures Tm for MoSi2 and several potential particulate additions [72].
Table 7. Coefficients of thermal expansion (CTE) and melting temperatures Tm for MoSi2 and several potential particulate additions [72].
MaterialCTE (°C−1) @ 1000 °CTm (°C)
MoSi28.5 × 10−6 2030
TiC7.7 × 10−63250
Mo5Si36.7 × 10−6 (@ 500 °C)2180
HfC6.25 × 10−63890
HfB25.5 × 10−63250
SiO2 (vitreous)0.55 × 10−6 1710
CaO13.1 × 10−62570
MgO15.7 × 10−6 2800
Ce2014 × 10−62600
Table
Table 8. Common reinforcement and toughening phase and some physical and chemical properties.
Table 8. Common reinforcement and toughening phase and some physical and chemical properties.
PhaseCTE/K−1CompatibilityPerformance Variation after CompoundingMechanism
SiC [41,69,70]4.7–5.0 × 10−6CLimited toughening effect at low temperatures, improved strength and oxidation resistance of MoSi2 at high temperature, a great difference in thermal expansion coefficient causing cracking.Crack deflection; Dispersed phase strengthening
ZrO2 [70,71]8.8 × 10−6RThree times the improvement for fracture toughness of MoSi2 at room temperature, improved strength of MoSi2 at high temperatures, poor thermodynamic compatibility with MoSi2.Phase transformation toughening
Si3N4 [70,72]3.0 × 10−6CFracture toughness reaching 15 MPa·m1/2 at room temperature, creep rate of 10–8 s at 1200 °C, inhibiting pesting oxidation of MoSi2.Microbridge toughening
Al2O3 [64,70]8.3 × 10−6RFracture toughness improvement of MoSi2 at room temperature; poor thermodynamic compatibility with MoSi2, strength and fracture toughness improvement of Mo5Si3.Crack pinning; Fine grain strengthening
TiB2 [70,73]8.1 × 10−6CImproved strength of MoSi2 at high temperatures and improved toughness of MoSi2 at room temperature. Crack deflection
ZrB2 [70,73]8.3 × 10−6RImproved MoSi2 fracture toughness.Crack deflection; Microbridge toughening
TiC [66,74]7.4 × 10−6CImproved brittleness and strength of MoSi2 at room temperature.Enhanced dispersion
Nbf/Nbp [66,70]7.2 × 10−6RFibers can help to enhance MoSi2 fracture toughness 1–3 times, promote crack production, decrease strength at high temperature; particles help to increase fracture toughness by 20–30%.Plastic deformation; Refining and toughening
Tap/Wp [71]6.6/4.5 × 10−6-Limited improvement of MoSi2 fracture toughness, enhanced strength at high temperatures.Fine grain strengthening
Note: C stands for chemical compatibility, R stands for chemical incompatibility.
Table
Table 9. The preparation process of Mo-Si-based UHTMs.
Table 9. The preparation process of Mo-Si-based UHTMs.
Preparation MethodsMechanismsDisadvantages
Casting [107,108]Vacuum arc melting and high-temperature annealingProduces drawbacks such as cracks, dispersed shrinkage, shrinkage cavity, and coarse grains; the microstructure is hard to be controlled
Directional solidification [107]Establishes a specific temperature gradient to allow solidification along a specific orientationProduces thermal cracking in the transition part due to large temperature difference; produces stress and deformation after solidification
Mechanical alloying [107,109]Repeated ball milling, crushing, and cold welding to achieve complete alloying of the mixture.Powder pollution; the difficulty informing metastable alloy powder; the lack of flexibility in operation
Self-propagating high-temperature synthesis [107,109]The chemical reaction of the powder releases a lot of heat to make the reaction continueThe reaction is hard to be controlled; the density of the obtained material is low
Hot pressing; hot isostatic pressing [107,109]Simultaneous powder pressing and pressure sinteringRequires long forming and holding time; is prone to oxidation; produces thermal stress during the cooling process
Laser cladding [107,110]The high-energy density laser beam causes the material to melt rapidly and solidifyDifficulty in preparing large bulk material; high equipment requirement
Solid-state displacement reaction [111]In situ reaction for 2, 3 elements or compounds to form new compounds.Slow and expensive solid-state replacement reaction
Low vacuum plasma deposition [112]In the low vacuum environment, the melting powder deposits with the high-speed plasma flow hitting the substrateLimitations on the surface of the deposited film; complicated reaction; degraded quality of the film
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Jiang, L.; Zheng, B.; Wu, C.; Li, P.; Xue, T.; Wu, J.; Han, F.; Chen, Y. A Review of Mo-Si Intermetallic Compounds as Ultrahigh-Temperature Materials. Processes 2022, 10, 1772. https://doi.org/10.3390/pr10091772

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Jiang L, Zheng B, Wu C, Li P, Xue T, Wu J, Han F, Chen Y. A Review of Mo-Si Intermetallic Compounds as Ultrahigh-Temperature Materials. Processes. 2022; 10(9):1772. https://doi.org/10.3390/pr10091772

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Jiang, Liang, Bin Zheng, Changsong Wu, Pengxiang Li, Tong Xue, Jiandong Wu, Fenglan Han, and Yuhong Chen. 2022. "A Review of Mo-Si Intermetallic Compounds as Ultrahigh-Temperature Materials" Processes 10, no. 9: 1772. https://doi.org/10.3390/pr10091772

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