The heat treatment method of the directionally solidified niobium-based alloy includes the following steps: 1) Place the directionally solidified niobium-based alloy in an inert gas environment and heat it to 1750 ~ 1850℃ for 5 ~ 20 hours; 2) With the furnace cooling down to 1000 ~ 1700℃, heat preservation for 2 ~ 10 hours. The heat treatment process significantly refines the grain size of the directionally solidified niobium-based alloy and increases the fracture toughness by about one time while maintaining the alloy strength demand (with a small decrease in strength). Therefore, the process optimizes the comprehensive mechanical properties of niobium-based alloy. Gas turbine engine plays a pivotal role in aerospace, energy power, transportation and other fields, and its energy utilization efficiency is closely related to the temperature of high-temperature structural materials. At present, nickel-based single crystal superalloy is widely used in turbine blade materials. The highest operating temperature has exceeded 1100° C, reaching 85% of the melting point of pure nickel.
Therefore, it is necessary to develop a new generation of high-temperature structural materials. Turbine blades work under harsh conditions of high temperature, high pressure and high corrosion. Blade materials must have a series of excellent comprehensive properties, including high temperature strength, low temperature plastic toughness, oxidation resistance, creep resistance, fatigue resistance, and machining molding. It is known that niobium-based alloys have high melting point and low density. Nb and Nb5Si3 are composed of Nb and Nb5Si3 phases. There are three isomers of Nb5Si3 phase :aNb5Si3 and Mb5Si3' Y Nb5Si3. Mb5Si3 and Y Nb5Si3 are stable phases at high temperature, and A Nb5Si3 is stable phase at low temperature. In the two phases of niobium-based alloy, Nb phase has better plasticity and Nb5Si3 phase has better high-temperature strength, creep resistance and oxidation resistance.
The combination of the two can obtain better comprehensive properties. Therefore, niobium-based silicon alloy is a candidate material to replace nickel-based superalloys and can be used for turbine blade and other hot-end components operating at 1200 ~ 1500°C. By adding alloying elements, the comprehensive properties can also be optimized. At present, there are mainly two systems of niobium-based alloys, including Nb-Si-Ti series of high toughness type and Nb-Si-Mo series of high strength type. The processing technology of niobium-based alloy includes arc melting, induction melting, directional solidification, hot extrusion, electron beam physical vapor deposition, etc. For the preparation of turbine blades, the directional solidification investment casting process is usually adopted to produce the parts by near-net forming.
