With the development of high-performance aerospace vehicles, the working temperature of the hot end parts of aero-engine has been increasing continuously, which has exceeded the limit temperature of nickel-based superalloy. Therefore, it is urgent to find new ultra-high temperature materials to replace the existing nickel-based superalloy. At the end of last century, the US Air Force proposed to develop materials that could be used at 1400℃, so that the thrust-to-weight ratio of the engine could reach 15 ~ 20. In the past 20 years, refractory metal silicides have attracted more and more attention because of their high melting point, low density and excellent high temperature performance. Among them, Nb-Si-based alloys (with densities of 6.6 ~ 7.2 g/cm3) have become the most promising new ultra-high temperature materials.
Nb-Si-based superalloy has high melting point, low density and excellent comprehensive performance. The target service temperature of Ni-based superalloy is 200 ~ 300℃ higher than that of Ni-based superalloy. It is expected to be applied to gas turbine engine blades working at 1300 ~ 1500℃ and hot end parts of transcendant ramjet of space aircraft. The Nb-Si based superalloy relies on Nb5Si3 phase of silicide to strengthen at high temperature, Nb-based solid solution phase to toughen at room temperature, and Cr2Nb phase of chromide to provide better high temperature oxidation resistance, so as to achieve the match of room temperature toughness, high temperature strength and high temperature oxidation resistance.
The research and development idea of Nb-Si based superalloy is mainly through adding ductile solid solution phase to form Nb-Si based solid solution /Nb5Si3 two-phase composite material, which can maintain excellent high temperature strength and improve its room temperature toughness.
The main alloying elements of Nb-Si based superalloy are Titanium, Aluminium, Chromium, Hafnium, Zirconium, Molybdenum, Vanadium, Tantalum, Stannum, Boron, Holmium and Dysprosium. The typical Nb-Si alloys studied at home and abroad are composed of Si 12 ~ 18at%, Ti 20 ~ 24at%, Cr 2 ~ 6at%, Al 2 ~ 6at%, Hf 2 ~ 8at%, and rare earth elements generally less than 5at%.
