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Use of nickel-based alloys
Jul 03, 2018

The development of engine technology is related to the development of superalloys. In 1929, Meric, Bedford, and Pilling of the United States dissolved a small amount of titanium and aluminum in 80 nickel-20 chromium alloy, which increased the creep strength of the alloy, but this did not attract people's attention. In 1937, Heinkel, the German gas turbine engine of Hansvonohain, came out. Two years later, the UK also developed the Whittle turbojet. However, the hot end components of jet generators have high requirements for the high temperature and mechanical properties of the material. In 1939, British Mond Nickel Company first developed low-carbon and titanium-nickel alloy nimonic75, which is ready to be used as a turbine blade for Whittle engines. Soon, Nimonic80 alloy is available, its performance is superior, because the alloy contains aluminum and titanium, so the creep The variable performance is at least 50 ° C higher than Nimonic75.

The development of nickel-based superalloys consists of two parties: the design of the composition and the innovation of the process. In the early 1940s, vacuum smelting technology was initially applied. In the late 1950s, a series of casting alloys with good high temperature strength were developed by investment casting process. In the mid-1960s, better directional crystallization and development were developed. Single crystal superalloys and powder metallurgy superalloys; in order to comply with the needs of ships and industrial gas turbines, in the 1960s, a group of high-chromium-nickel alloys resistant to hot corrosion and tissue stability were developed. During these four decades, the use temperature of nickel-based alloys has increased from 700 ° C to 1100 ° C, with an average annual increase of about 10 ° C.

Nickel-based superalloys, especially nickel-based single crystal superalloys, have been widely used in advanced gas turbine blades and guide vanes. This alloy is strengthened by the precipitation of the γ' phase, and the Ni3Al-rich nickel γ'L12 structure has a coherent phase. Its excellent mechanical properties depend on the high volume fraction and fine dispersion in the matrix, as well as the content of solution strengthening elements (rare earth, chromium, molybdenum, tungsten, molybdenum, chromium, molybdenum, chromium, molybdenum, chromium) in the matrix. Therefore, it is a key issue for high temperature alloy structural stability. The coarsening of the γ' phase particles is detrimental to the alloy. For polycrystalline nickel-based superalloys, the u-700 has a long-lasting lifetime that is reduced by about 50% of the expected life due to sigma phase formation. Tian et al. found that due to the consumption of refractory elements, the μ phase of precipitation significantly reduces the creep resistance and the longevity of the superalloy.

Alloys that use a directional solidification process to eliminate all grain boundaries are called single-crystal superalloys. The primary use of single-crystal superalloys is to make turbine blades and guide vanes for advanced aeroengines, or hollow turbine blades and guide vanes with complex cavities, and The entire blade is a single crystal. DD6 single crystal alloy is the second generation of nickel-based single crystal superalloy, which has good casting performance, high temperature resistance, corrosion resistance, high strength, high hardness and good comprehensive performance. Compared with the first generation single crystal alloy, its temperature resistance is improved by about 30 ° C - 60 ° C. Widely used in engine turbine blades.

The difference between the DD6 single crystal alloy and the previous nickel-based superalloy is that ytterbium element, yttrium element and a small amount of lanthanum element which can improve the life and strength of the single crystal are added to the DD6 single crystal alloy.钽 can increase the γ/γ' mismatch, strengthen the γ' phase and improve its high temperature stability; a small amount of lanthanum, carbon and lanthanum can increase the strength of the small-angle grain boundary of the single crystal alloy, and the yttrium element significantly affects the single crystal Durable properties of the alloy: an appropriate amount of niobium can improve the long-lasting life of the single crystal alloy; and excessive niobium will reduce the stability of the alloy and reduce the longevity of the alloy.

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