Introduction and Classification of Superalloys

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(1) Introduction and Classification of Superalloys Superalloys are a type of metal that can operate for extended periods at high temperatures above 600°C under certain stress conditions. What distinguishes superalloys from traditional metals and alloys is that they possess high strength in high-temperature operating conditions, as well as excellent oxidation and thermal corrosion resistance, good fatigue resistance and fracture toughness. They also maintain good structural stability and reliability at various temperatures; in the West, they are also referred to as superalloys. Superalloy materials are the preferred choice for aviation engines, occupying an irreplaceable position in the development of the modern aviation industry. Whether they are developed on a large scale directly determines the level of advancement of aviation equipment. In addition, superalloys are also widely used in the hot-section components of aerospace engines. With the development of industrialization, the demand for superalloy materials in the civilian high-end equipment industry is on the rise. Thanks to their properties of withstanding high temperatures, wear, and corrosion, superalloys are widely used in various civilian industries such as diesel and internal combustion engine turbocharging systems, gas turbines, energy and power generation sectors, petrochemical industries, and the glass and building materials industry. The proportion of superalloys used in these civilian industries has reached around 20%. Superalloys can be classified based on the method of material fabrication, the types of matrix elements, and the mechanisms used for alloy strengthening. 1) Based on the method of material fabrication, superalloys can be divided into wrought superalloys, cast superalloys (including conventional precision-cast alloys, directionally solidified alloys, single-crystal alloys, etc.), and powder metallurgy superalloys (including conventional powder metallurgy superalloys and oxide-dispersed strengthening superalloys ODS) ； 2) Based on the type of matrix element, superalloys can be classified into iron-based, nickel-based, cobalt-based, etc ； 3) Based on the type of alloy strengthening, superalloys can be divided into solid solution strengthened superalloys and age-hardening precipitation strengthened alloys. Superalloy grades are denoted by specified symbols and *** digits. For deformed superalloy grades, the prefix “GH” is used (“G” and “H” are the first letters of the pinyin for “high” and “superalloy” respectively), followed by four digits. “The first digit following the “GH” symbol indicates the classification code; that is: 1 – denotes iron-based alloys strengthened by solid solution ； 2 —— denotes age-hardening iron-based alloys ； 3 —— indicates nickel-based alloys strengthened by solid solution strengthening ； 4 —— denotes age-hardening nickel-based alloys ； 5 —— denotes cobalt-based alloys strengthened by solid solution strengthening ； 6 —— denotes age-hardening cobalt-based alloys. “The second, third, and fourth digits after the “GH” symbol indicate the alloy number. Superalloys are alloys that can withstand certain stresses at high temperatures of 600–1200°C, and possess oxidation or corrosion resistance. Based on the matrix element, they can be mainly classified into iron-based superalloys, nickel-based superalloys, and cobalt-based superalloys. Based on the manufacturing process, they can be divided into deformed superalloys, cast superalloys, and powder metallurgy superalloys. Based on the strengthening mechanism, there are solid solution strengthening, precipitation strengthening, oxide dispersion strengthening, and fiber strengthening types. Superalloys are primarily used to manufacture high-temperature components such as turbine blades, guide vanes, turbine discs, high-pressure compressor discs, and combustion chambers for aviation, naval, and industrial gas turbines. They are also used in the production of spacecraft, rocket engines, nuclear reactors, petrochemical equipment, as well as energy conversion devices for coal conversion. Nickel-based alloys are a type of superalloy that is widely used and possesses high strength at high temperatures. The main reason is that nickel-based alloys can dissolve a large amount of alloying elements while maintaining good structural stability ； Secondly, a coherently ordered A3B-type intermetallic compound γ’ phase can be formed as a strengthening phase, thereby effectively strengthening the alloy and enabling it to achieve higher high-temperature strength compared to iron-based and cobalt-based superalloys ； Thirdly, chromium-containing nickel-based alloys have better oxidation resistance and resistance to gas corrosion than iron-based superalloys. Nickel-based alloys contain more than a dozen elements; among them, Cr primarily serves to resist oxidation and corrosion, while the other elements mainly contribute to strengthening the alloy. Based on their mechanisms of strengthening, they can be classified as solid solution strengthening elements, such as tungsten, molybdenum, cobalt, chromium, and vanadium, etc ； Precipitation strengthening elements, such as aluminum, titanium, niobium, and tantalum ； Grain boundary strengthening elements, such as boron, zirconium, magnesium, and rare earth elements. (II) Technical challenges and innovations in superalloy materials: In high-temperature environments, the various rates of material degradation are accelerated; during use, issues such as structural instability, deformation and crack growth under the effects of temperature and stress, as well as oxidative corrosion on the material surface, can occur. The properties of superalloys, such as high-temperature resistance and corrosion resistance, mainly depend on their chemical composition and microstructure. Superalloy materials have a very complex composition, containing reactive elements such as chromium and aluminum, which render them chemically unstable in oxidizing or thermally corrosive environments. Additionally, the surfaces of parts manufactured through machining suffer from defects such as work hardening and residual stresses, which have a highly adverse effect on both the chemical and mechanical properties of these materials. Due to their high degree of alloying, superalloy materials are highly prone to component segregation, and this segregation has a significant impact on the microstructure and properties of both cast and wrought superalloys. These characteristics of superalloys determine the processing techniques that differ from those used for ordinary metal materials. The development of superalloys is a process of continuous improvement and innovation in alloy theory and production technologies, with material properties being enhanced through a combination of alloy strengthening and processing strengthening. Alloy strengthening includes alloy solid solution strengthening, grain boundary strengthening by second-phase reinforcements, etc ； Process intensification involves improving aspects such as smelting, solidification and crystallization, hot working, heat treatment, and surface treatment, in order to enhance the microstructure of the alloy. The production process of superalloys mainly includes three steps: melting, casting, and heat treatment. The manufacturing process has a significant impact on the mechanical properties of superalloy materials. The introduction of a new process often leads to a substantial improvement in the properties of these superalloys, enabling the development of new types of superalloys, which in turn drives the advancement of new generations of aircraft engines and airplanes. Older models of alloys can also have their processing improved to enhance their material properties. For example, the application of single-crystal turbine blades has significantly advanced the progress of aircraft engines. The turbine rotor blades of the F119 engine used in the F-22 are made of the third-generation single-crystal superalloy PWA1484; the maximum operating temperature of this material is around 1070°C. By using computational fluid dynamics to design and manufacture supercooled blades, the operating temperature of these turbine rotor blades can be increased to 1621–1677°C (compared to 1400°C for the F100 engine), which illustrates the important role of technological innovation in the development of materials. The preparation technologies and processes for superalloy materials are still undergoing continuous improvement and innovation. For example, the smelting process employs a three-stage approach of vacuum induction + electroslag remelting + vacuum consumable arc melting, and advanced melting control methods are used in vacuum consumable arc melting ； Improving the high-temperature strength of materials through directional solidification columnar grain alloy and single crystal alloy processing techniques ； The powder metallurgy method is used to reduce the segregation of alloying elements and improve the strength of the material, among other things. In addition, oxide-dispersed strengthening superalloys and intermetallic high-temperature materials are also continuously evolving and being innovated.