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Due to limitations in operating temperature and high-temperature mechanical properties, C-103 alloy and niobium silicide anti-oxidation coatings have been unable to meet the evolving needs of spacecraft. To this end, our country has developed Nb521 (Nb-5W-2Mo-1Zr) niobium alloy. The alloy uses a high-temperature anti-oxidation protective coating of molybdenum silicide, which increases the operating temperature to about 1550°C, significantly reducing the flow rate of propellant used to cool the combustion chamber, thereby effectively increasing the specific impulse of the engine and becoming the current track in China. The second generation niobium alloy base material for the thrust chamber of control/attitude control engine refractory metal materials.

Nb521 niobium alloy adds W, Mo, Zr alloying elements and a small amount of C element to the niobium matrix. It further improves the room temperature and high temperature mechanical properties of the niobium alloy through a combination of solid solution strengthening and precipitation strengthening. It is a medium-grade niobium alloy. Strength plastic niobium alloy. The alloy can be cast into ingots through vacuum electron beam melting or vacuum electron beam + vacuum consumable melting, and bars and rods of various specifications can be prepared through hot extrusion, forging, cold forging, drawing and rolling. Forgings and plates. W and Mo elements are added to the alloy mainly because of their high melting point and similar atomic radii, which can form a substitutional solid solution to improve the creep resistance and high-temperature strength of the alloy matrix. Zr and C elements are added to the alloy because C easily forms a dispersed and precipitated strengthening phase with Zr and Nb, which plays a role in precipitation strengthening and can further improve the high-temperature strength of the alloy.

Usually, dispersion strengthening of niobium alloys is strengthened through stable, fine carbides, oxides and nitrides. This strengthening method is very effective in improving high temperature strength. To this end, the carbide strengthening phase of Nb521 alloy is studied as dispersed (Nb, Zr)C, ZrC and Nb2C phases. ZrC is a dispersed stable carbide phase, while Nb2C is a metastable carbide phase with different contents and sizes. Effect of Nb2C particles on the structure and properties of Nb-W-Mo-Zr-C niobium alloy. It is believed that adding Nb2C particles with a content of 0.4wt% and a size of about 5μm can maximize the high-temperature strength of the alloy.

Nb521 niobium alloy has good room temperature formability. Plate spinning can usually be used to prepare uniform transition nozzle extensions. However, most nozzles are obtained by machining from rods. The machining is difficult and the material utilization rate is low. relatively low. Therefore, studying the near net shape and additive manufacturing of Nb521 niobium alloy is an effective method to improve the utilization rate of the alloy material. Nano-Nb521 alloy powder was successfully prepared at room temperature using a high-energy ball mill and analyzed and characterized accordingly. The results show that the ball milling speed plays a decisive role. When the ball milling speed reaches 450 rpm, nano-powder with a grain size of 14nm can be obtained in 60 hours. The Nb521 alloy powder prepared by plasma rotary atomization method was studied. It was found that the alloy powder prepared by this method was mostly spherical and had a high degree of sphericity. The surface of the large particle powder was rough, while the surface of the small particle powder was smooth. The diameter distribution conforms to the standard normal distribution; through XRD and nanoindentation experimental analysis, the results show that only Nb diffraction peaks exist, and as the powder particle size decreases, the nanohardness and maximum load increase. Nb521 alloy was prepared by electron beam selective melting (EBSM), and the microstructural characteristics of the prepared sample and the ingot sample were compared. The results show that in the Nb521 sample prepared by the EBSM method, the precipitate content gradually increases from the top to the bottom, and point-like or rod-like precipitates of different morphologies are distributed inside the grains or along the grain boundaries, while in Nb521 alloy ingots Then there are needle-like precipitates with large aspect ratio. In the sample prepared by the EBSM method, the precipitated phases are mainly (Nb, Zr)C and Nb2C, and with the extension of the thermal equilibrium holding time, the elongated precipitated phases are partially broken, and the grains become fine and uniform.

In addition, the molybdenum silicide high-temperature anti-oxidation coating prepared from Nb521 alloy material has a certain self-healing ability, and its linear expansion coefficient is relatively close to that of Nb521 alloy material, so it has good anti-oxidation protection performance and bonding performance. In order to further improve the comprehensive performance of molybdenum silicide coating, two plasma spraying technologies (SAPS and SPS) were used to prepare a multi-phase MoSi2 coating on the surface of niobium-based alloy, and the melting index (i.e. M.I. value) of the plasma sprayed MoSi2 coating was defined. ) to characterize the multiphase effect of the coating, and found that the higher the M.I. value, the stronger the mechanical properties of the coating and the better the oxidation resistance. This study studied the preparation of MoSi2-based/NbSi2 double coatings on the surface of Nb521 alloy. After pre-oxidation at 1500°C for 10 hours, a continuous SiO2 barrier can be formed on the coating surface, which can effectively delay the hot corrosion between the MoSi2-based coating and Na2SO4 salt.

The mullite-modified MoSi2 (MM) coating, WSi2 and mullite jointly modified MoSi2 (WMM) coating prepared on the surface of Nb521 alloy and their anti-oxidation properties were studied. It was found that MM with 10 wt% mullite added The anti-oxidation performance of the coating is very good, and the anti-oxidation performance of the WMM coating is even better. The effective protection time (500 h) at 1500°C is at least 2.8 times that of the MoSi2 single coating (175 h) and the MM (346 h) coating. times and 1.5 times. In order to further develop ultra-high temperature niobium alloy coatings suitable for higher temperatures, a novel three-step method was used to prepare 10%ZrB2+5%YSZ modified Si-Mo-18%W coating with boride diffusion barrier layer. , the study found that the NbB2-Nb3B2 diffusion barrier coating can effectively protect the Nb521 alloy at 1850°C for more than 8 hours, while the coating life without the diffusion barrier layer is only 3.5 hours. The contribution to the superior coating performance is mainly due to the diffusion barrier layer and the formation of self-healing SiO2-B2O3-ZrSiO4-ZrO2 oxide scale. The diffusion barrier layer can effectively prevent the mutual diffusion between the coating and the substrate, and can reduce the inward consumption of silicon element. Thereby increasing the life of the coating.

After recent years of practice and application, Nb521 niobium alloy and its supporting high-temperature anti-oxidation coating have been successfully used in the attitude control/orbit control of the new generation of high-thrust launch vehicle gas-oxygen kerosene engine nozzles, Nigerian satellites, Sino-6, etc. Engines and Chinese lunar rover engines are currently the preferred materials for thrust chambers of refractory metal materials for domestic attitude control/orbit control engines.