High temperature ablative high strength and tough molybdenum alloy
Solid rocket engine is the most important extended range power system of ammunition, and its nozzle is usually faced with instantaneous ultra-high overload impact, thousands of K high temperature, high speed, high pressure gas and particles generated by high energy extrusion agent for ten seconds, which requires the nozzle throat liner material to have excellent high temperature ablation resistance and room temperature strength.
At present, the main materials suitable for the manufacture of rocket engine nozzle lining are: SiC, Si3N4, ZrC, pyrolytic graphite, polycrystalline graphite and other ceramics and graphite, as well as molybdenum alloy, tungsten alloy, tantalum alloy, niobium alloy and other refractory metal materials. Among them, the thermal shock resistance of ceramic lining is poor, and the high temperature erosion resistance of graphite lining is poor, which can not meet the requirements of solid rocket motor. Tungsten alloy has excellent high temperature ablative resistance, but tungsten alloy has poor strength and toughness at room temperature, and it is easy to crack under the impact of high emission overload. Tantalum alloy has high room temperature toughness and high temperature ablation resistance, but tantalum alloy is a rare precious metal and expensive. Molybdenum and its alloys have high melting point (2620℃), good erosion resistance, and the material density is moderate (10.2g/cm3), and the earth is rich in storage, cheap, is the preferred material for solid rocket engine nozzles and other heavy parts.
The existing rare earth molybdenum materials such as molybdenum lanthanum alloy have good room temperature strength and toughness, such as when used as nozzle throat lining materials, they can meet the requirements of instantaneous ultra-high overload impact use, but their high temperature structure stability is poor, and they can only meet the scouring of high temperature, high speed, high pressure gas and particles for microseconds to a few seconds working time. Its throat is prone to serious ablation, which has become one of the keys to restrict the improvement of range extension efficiency of solid rocket engines.
At present, the common molybdenum alloy throat lining materials mainly include: adding an appropriate amount of La2O3 and other rare earth oxide particles to molybdenum to improve its room temperature strength and toughness (rare earth molybdenum); Or to add TiC, ZrC and other carbide particles to molybdenum to improve the recrystallization temperature and high temperature mechanical properties of molybdenum alloy; Or to add Ti, Zr, C to molybdenum in situ to form TiC, ZrC and other carbide particles to improve its strength and toughness; However, after carbide and carbon exceed a certain content, it is easy to react with molybdenum to form Mo2C brittle phase, resulting in its room temperature strength, toughness and high temperature performance is limited. In order to produce molybdenum alloy with high strength and toughness at room temperature and high temperature ablation resistance, the measures usually taken in the power field are mainly: adding second phase particles such as rare earth oxides or carbides to molybdenum to improve the homogeneity of molybdenum alloy and increase the density of molybdenum alloy.
Adding proper amount of second phase particles to molybdenum is an effective method to improve the strength and toughness of molybdenum alloy at room temperature and high temperature ablation resistance. However, adding trace amounts (such as 0.2 ~ 1.5wt %) of HfC, TaC, TiC and other carbide particles to molybdenum alloy, although it can increase the high temperature ablative performance of molybdenum alloy, but the improvement of the strength and toughness of molybdenum alloy at room temperature is limited, and the resulting molybdenum alloy is easy to crack under the impact of ultra-high overload of 10,000 g or more. It has seriously affected the reliable working requirements of solid rocket motors. If the carbide content is further increased, it will also reduce the strength and toughness of molybdenum alloy, mainly because: (1) due to the high brittleness of carbide, the increase in its content leads to an increase in the brittle phase of molybdenum alloy, thereby reducing the fracture toughness and strength of the alloy; (2) The increase of carbide content is easy to react with molybdenum matrix to produce Mo2C brittle phase, and the increase of brittle phase decreases the fracture toughness and strength of the alloy; (3) The melting point of carbide is higher than that of molybdenum (2620℃), requiring higher sintering activation energy, with the increase of carbide content, the degree of bonding between carbide particles is weak, and cracks are more likely to occur under loading conditions, so that the alloy strength and other properties decline. So far, there have been no reports of molybdenum alloys that simultaneously add La2O3 and other rare earth oxide particles and carbide particle components.
