MoSi2 has a high melting point of up to 2030 ° C, a low density (6.24 g / cm 3 ) and excellent high temperature oxidation resistance as well as good thermal conductivity and electrical conductivity, so it is considered to be Ni-based superalloy (use temperature <1100 °C) and the third generation of ultra-high temperature structural materials after the second generation of high temperature TiAl alloy. However, MoSi2 also has some fatal shortcomings that hinder its practical application. In addition to high temperature brittleness (tough-brittle transition temperature of 900~1000 °C), high temperature strength below 1300 °C, especially creep resistance is low, MoSi2 will accelerate oxidation at 400~600 °C. Eventually it will turn from a dense body to a powder, causing catastrophic destruction of the material. This phenomenon of low-temperature oxidation is called Pest phenomenon in academia, and it is a difficult point of research and development of this material as a practical high-temperature structural material. At present, the research on the mechanism of Pest phenomenon of MoSi2 suggests that the volatilization of Mo oxide leads to discontinuity and non-density of SiO2 film; the sample itself is not dense enough or cracks, so that oxygen atoms can enter the interior of the material quickly; impurity elements O, N are in the grain boundary Priority diffusion, etc., these factors lead to rapid oxidation of the material. Based on the above analysis, the current research ideas for overcoming the Pest problem of MoSi2 are as follows: improving the purity of MoSi2; increasing the density of materials; adding elements with stronger affinity to Si and O, reducing internal oxidation The volume expands to suppress the Pest effect; the high temperature pre-oxidation forms a dense SiO2 film, forms a glass protective layer on the surface of the material, reduces the oxidation rate, and the like. The experiments show that the Mo powder and the Si powder mixture are synthesized in situ by the spark plasma sintering method to prepare the highly dense MoSi2, which is higher than the MoSi2 prepared by directly sintering the MoSi2 powder. As a result, the former performs in the Pest phenomenon temperature zone. The oxidation resistance is significantly better than the latter, which indicates that the density of the material has an important influence on its oxidation resistance. In addition, SiC particle reinforced MoSi2 matrix composites can be prepared by in-situ synthesis process. The composite has a density of up to 99.5%, the interface is direct atomic bonding, and no amorphous layer exists. Due to the high density of the material, low porosity and no cracks, the diffusion rate of O into the material through these defects is greatly reduced, so that the nucleation and growth rate of the powder are significantly slowed down, and the oxidation is effectively slowed down. After the material was oxidized at 500 ° C for 1000 hours, the Pest phenomenon did not occur. The SiC particle reinforced MoSi2 matrix composite not only has good oxidation resistance, but also has a fracture toughness of 25%~46% higher than that of a single MoSi2; the compressive flow stress at 1000~1400 °C is significantly higher than that of a single MoSi2; The compression creep performance at 1200~1400°C is also significantly improved. It has also been reported that a Mo-Si powder containing La2O3 is prepared by mechanical alloying and sintered to form a La2O3 toughened MoSi2 composite material. This material did not undergo a Pest phenomenon because the surface formed a dense SiO2 protective film. MoSi2 alloyed with Al has good oxidation resistance at 400 °C, 600 °C, and 700 °C, and the quality changes are less. The reason is that a continuous 3Al3O2•2SiO2 protective film is formed on the surface of the material; However, in the oxidation process at 500 °C, the Pest phenomenon occurred because the volatile MoO3 phase was formed in large quantities at this time, and the formation of the 3Al3O2•2SiO2 protective film was destroyed. The MoSi2 co-alloyed by Al and Nb was 500. After oxidation for 50 hours at °C, the contents of MoO3 and Mo9O26 in the material did not change much, and Al2O3 appeared, and its oxidation resistance was improved.
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A development difficulty of a new generation of ultra-high temperature structural material MoSi2
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