Nickel-based high-temperature alloys

Nickel-based high-temperature alloys (hereinafter referred to as nickel-based alloys) were developed in the late 1930s. In 1941, Britain first produced the nickel-based alloy Nimonic 75 (Ni-20Cr-0.4Ti); in order to improve creep strength and add aluminum, developed Nimonic 80 (Ni-20Cr-2.5Ti-1.3Al). The United States in the mid-1940s, the Soviet Union in the late 1940s, and China in the mid-1950s also developed nickel-based alloys. The development of nickel-based alloys includes two aspects: the improvement of alloy composition and the innovation of production process. early 1950s, the development of vacuum melting technology created the conditions for refining nickel-based alloys containing high aluminum and titanium. In the late 1950s, due to the increase in the working temperature of turbine blades, the alloy is required to have higher high-temperature strength, but the strength of the alloy is high, it is difficult to deform, or even can not be deformed, so the use of investment casting process, the development of a series of casting alloys with good high-temperature strength. mid-1960s to develop better performance of directional crystallization and single crystal high-temperature alloys As well as powder metallurgy high temperature alloys. In order to meet the needs of ships and industrial gas turbines, since the 1960s also developed a number of high-chromium nickel-based alloys with good thermal corrosion resistance and stable organization. In about 40 years from the early 1940s to the end of the 1970s, the working temperature of nickel-based alloys increased from 700 ℃ to 1100 ℃, an average annual increase of about 10 ℃

Composition and properties
Nickel-based alloys are the most widely used high-temperature alloys, high-temperature strength of the highest class of alloys. The main reason, one is that nickel-based alloys can dissolve more alloying elements, and can maintain good organizational stability; secondly, the formation of co-lattice ordered A3B-type intermetallic compound γ'[Ni3(Al, Ti)] phase as the strengthening phase, so that the alloy can be effectively strengthened to obtain higher high-temperature strength than iron-based high-temperature alloys and cobalt-based high-temperature alloys; thirdly, nickel-based alloys containing chromium have a better resistance to oxidation and gas corrosion than iron-based high-temperature alloys. Alloys have better oxidation resistance and gas corrosion resistance. Nickel-based alloys contain more than ten elements, of which Cr mainly plays the role of oxidation and corrosion resistance, while other elements mainly play a strengthening role. According to their strengthening effect can be divided into: solid solution strengthening elements, such as tungsten, molybdenum, cobalt, chromium and vanadium; precipitation strengthening elements, such as aluminum, titanium, niobium and tantalum; grain boundary strengthening elements, such as boron, zirconium, magnesium and rare earth elements, etc. (see metal strengthening).

Nickel-based high-temperature alloys are solid-solution strengthened alloys and precipitation strengthened alloys according to the strengthening method.

Solid solution reinforced alloys
Has a certain high temperature strength, good resistance to oxidation, thermal corrosion, cold and thermal fatigue resistance, and good plasticity and weldability, etc., can be used in the manufacture of higher operating temperatures, not subject to stress (a few kilograms per square millimeter force , such as the combustion chamber of gas turbines.

Production process
Nickel-based alloys, especially precipitation-reinforced alloys contain high levels of aluminum, titanium and other alloying elements. They are usually melted in vacuum induction furnaces and remelted by vacuum self-consumption furnaces or electroslag furnaces. Thermal processing using forging, rolling process, for high-alloyed alloys, due to poor thermoplasticity, the use of extrusion open billet after rolling or direct extrusion process with soft steel (or stainless steel) package. Casting alloys usually use vacuum induction furnace melting master alloy, and vacuum remelting - precision casting method into parts.

Deformation alloys and some casting alloys require heat treatment, including solution treatment, intermediate treatment and aging treatment, to Udmet 500 alloy, for example, its heat treatment system is divided into four stages: solution treatment, 1175 ℃, 2 hours, air cooling; intermediate treatment, 1080 ℃, 4 hours, air cooling; primary aging treatment, 843 ℃, 24 hours, air cooling; secondary aging treatment, 760 ℃. 16 hours, air-cooled. To obtain the required tissue state and good overall performance.