Nickel-based alloys are a class of alloys that possess high strength and certain resistance to oxidation and corrosion at high temperatures of 650–1000℃. Based on their main properties, nickel-based alloys can be further subdivided into heat-resistant nickel-based alloys, corrosion-resistant nickel-based alloys, wear-resistant nickel-based alloys, precision nickel-based alloys, and shape memory nickel-based alloys. High-temperature alloys are classified according to their matrix materials into iron-based high-temperature alloys, nickel-based high-temperature alloys, and cobalt-based high-temperature alloys. Nickel-based high-temperature alloys are usually simply referred to as nickel-based alloys.
Origin and Development
The research and development of nickel-based alloys began in the late 1930s. The UK first produced Nimonic 75 (Ni-20Cr-0.4Ti) in 1941. To improve creep strength, aluminum was added, resulting in the nickel-based alloy 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 successively developed nickel-based alloys. The development of nickel-based alloys encompasses two aspects: improvements in alloy composition and innovations in production technology. For example, the development of vacuum melting technology in the early 1950s created conditions for purifying nickel-based alloys with high aluminum and titanium content, significantly improving their strength and operating temperature. In the late 1950s, the increasing operating temperature of turbine blades placed higher demands on the high-temperature strength of alloys. However, high strength made deformation difficult or even impossible, leading to the development of a series of casting alloys with good high-temperature strength using precision casting technology. In the mid-1960s, the performance of directionally solidified and single-crystal high-temperature alloys, as well as powder metallurgy high-temperature alloys, was improved. To meet the needs of marine and industrial gas turbines, since the 1960s, a series of high-chromium nickel-based alloys with good high-temperature corrosion resistance and stable microstructures have been developed. From the early 1940s to the late 1970s, over approximately 40 years, the operating temperature of nickel-based alloys increased from 700℃ to 1100℃, an average increase of about 10℃ per year. Today, the operating temperature of nickel-based alloys exceeds 1100℃. From the initially simple Nimonic 75 alloy to the recently developed MA6000 alloy, which boasts a tensile strength of 2220 MPa and a yield strength of 192 MPa at 1100°C, its creep strength at 1100°C/137 MPa is approximately 1000 hours, making it suitable for aero-engine blades.
The Role of Various Metals in Nickel-Based Alloys
For a specific nickel-based alloy, numerous variables exist in a particular environment, including: concentration, temperature, ventilation, liquid (gas) flow rate, impurities, wear, and circulating process conditions. These variables can lead to various corrosion problems. Nickel and other alloying elements can address these issues. Metallic nickel maintains an austenitic face-centered cubic structure before reaching its melting point. This provides freedom for the ductile-brittle transition and significantly reduces manufacturing problems caused by the coexistence of other metals. In the electrochemical sequence, nickel is more inert than iron but more reactive than copper. Therefore, in reducing environments, nickel is more corrosion-resistant than iron but less corrosion-resistant than copper. Adding chromium to nickel imparts oxidation resistance to the alloy, resulting in a variety of alloys with excellent corrosion resistance to both reducing and oxidizing environments. Compared to stainless steel and other iron-based alloys, nickel-based alloys can accommodate a wider variety of alloying elements in the solid solution state while maintaining good metallurgical stability. These properties allow for the addition of various alloying elements to nickel-based alloys, enabling their widespread application in a variety of corrosive environments.
Common elements in nickel-based alloys include:
Nickel (Ni): Provides metallurgical stability, improves thermal stability and weldability, enhances resistance to reducing acids and caustic sodas, and improves resistance to stress corrosion cracking, especially in chloride and caustic soda environments.
Chromium (Cr): Improves oxidation resistance, high-temperature oxidation resistance, sulfidation resistance, and resistance to pitting and crevice corrosion.
Molybdenum (Mo): Improves resistance to reducing acid corrosion, enhances resistance to pitting and crevice corrosion in chloride-containing aqueous solutions, and increases high-temperature strength.
Iron (Fe): Improves high-temperature carburizing resistance, reduces alloy costs, and controls thermal expansion. Copper (Cu): Improves resistance to reducing acid corrosion (especially sulfuric acid).