High-temperature alloys, also known as heat-resistant alloys or superalloys, are a class of metal materials capable of long-term operation in high-temperature environments and under certain stresses. They exhibit excellent resistance to high-temperature oxidation and hot corrosion, as well as superior high-temperature strength, fatigue strength, and creep fracture toughness. These alloys are primarily used in aerospace, energy, and marine turbine engines.
Classification of high-temperature alloys
1. According to the matrix material, they can be divided into three categories: iron-based, nickel-based, and cobalt-based.
(1) Iron-based high-temperature alloys are also called heat-resistant alloy steels. Heat-resistant alloy steels can be divided into martensite, austenite, pearlite, ferrite heat-resistant steels, etc., according to their normalizing requirements. Iron-based high-temperature alloys have a relatively low operating temperature (600~850℃), but their medium-temperature mechanical properties are good, which are equivalent to or better than similar nickel-based alloys. In addition, they are cheap and easy to deform during hot working. They are generally used in parts of the engine with lower operating temperatures, such as turbine discs, casings, and shafts.
(2) Nickel-based high-temperature alloys have the highest operating temperature (about 1000℃) and are widely used in the manufacture of the hottest parts of aviation jet engines and various industrial gas turbines, such as turbine blades, guide vanes, turbines, etc.
(3) Cobalt-based high-temperature alloys have good castability and weldability and can be used at high temperatures of 700~1050℃. It is mainly composed of cobalt, and its typical representative is K610, which contains more than 58% cobalt. Due to the high price and shortage of cobalt, it is rarely used at home and abroad. Existing brands include K640, K644, GH188, etc.
2. According to the preparation process, it can be divided into deformed high-temperature alloys, cast high-temperature alloys, and powder high-temperature alloys.
(1) Deformed high-temperature alloys
Deformed high-temperature alloys refer to high-temperature alloys that are made by cold and hot processing of ingots into various profiles or part blanks, and finally into hot-end parts. The key is that the alloy ingot can form. Compared with cast high-temperature alloys, deformed high-temperature alloys have a low degree of alloying. Therefore, the melting point is higher, the upper limit of the hot working temperature is higher, the alloy recrystallization temperature is lower, and the lower limit of the hot working temperature is lower. Therefore, the hot working range of deformed high-temperature alloys is wider than that of cast high-temperature alloys. According to the different matrix elements, deformed high-temperature alloys can be divided into iron-based deformed high-temperature alloys, nickel-based deformed high-temperature alloys, and cobalt-based deformed high-temperature alloys.
(2) Casting high-temperature alloys
Casting high-temperature alloys is a process that directly casts or directionally solidifies into parts after remelting the alloy ingots. Their development began in the 1940s. Casting high-temperature alloys no longer considers forging deformation performance. Precision casting methods or directional solidification processes can be used to cast hollow thin-walled blades with complex shapes and unobstructed inner cavities. Therefore, the total amount of elements in cast superalloys is significantly higher than in deformed superalloys. Solid solution strengthening elements include Re and Ru, while the content of the refractory metal W is increased (in some alloys, exceeding 10%). Precipitation-strengthening alloying elements, in addition to Al and Ti, also include Nb, Ta, Hf, and V.
Cast superalloys can be classified by solidification method into three categories: equiaxed cast superalloys, directionally solidified columnar superalloys, and single crystal superalloys. Single-crystal superalloys, a new type of superalloy, are formed by eliminating all grain boundaries through directional solidification. Metals are composed of individual crystals, hence the name single-crystal superalloy. Grain boundaries are areas within the metal where various distortions, defects, and impurities accumulate. While stronger at room temperature than within the crystal, they are susceptible to slip at high temperatures. When the strength of grain boundaries decreases at high temperatures, the metal's strength decreases. Therefore, eliminating grain boundaries through directional solidification yields single-crystal superalloys with excellent performance. Currently, almost all advanced engines utilize single-crystal alloy turbine blades or guide vanes.
(3) Powdered high-temperature alloys
As the working temperature of heat-resistant alloys becomes higher and higher, the number of strengthening elements in the alloys increases, and the composition becomes more complex, resulting in some alloys that can only be used in the cast state and cannot be deformed by hot working. In addition, the increase in alloying elements causes serious component segregation in nickel-based alloys after solidification, resulting in uneven structure and performance. The use of powder metallurgy technology to produce high-temperature alloys can solve the above problems. Because the powder particles are small and the cooling speed during powder making is fast, segregation is eliminated, and hot working properties are improved. The alloy that can only be cast is turned into a high-temperature alloy that can be hot-worked, and the yield strength and fatigue properties are improved. Powdered high-temperature alloys have created a new way to produce higher-strength alloys. Powdered high-temperature alloys are mainly used to manufacture turbine disks for high-thrust-to-advanced aircraft engines, and are also used to produce high-temperature hot-end components such as compressor disks, turbine shafts, and turbine baffles for advanced aircraft engines.
Application areas of high temperature alloys
1. Aerospace
(1) Combustion chamber
The combustion chamber is the area with the highest operating temperature among all engine components. When the gas temperature in the combustion chamber reaches 1500-2000℃, the temperature of the chamber wall alloy can reach 800-900℃, and locally up to 1100℃. In recent years, most of the high-temperature alloys used in the combustion chamber are solid solution-strengthened alloys. The alloys contain a large amount of solid solution strengthening elements such as W, Mo, and Nb. They have high temperature strength and good forming and welding properties. Representative grades include GH1140, GH3030, GH3039, GH3333, GH3018, GH3022, GH3044, GH3128, and GH3170.
(2) Guide vanes
The guide vanes are components that adjust the flow direction of the gas coming out of the combustion chamber. They are also called guide vanes. They are one of the parts of the turbine engine that are subject to the greatest thermal shock. Especially when the combustion in the combustion chamber is uneven or the operation is poor, the guide vanes are subjected to greater heat load. The operating temperature of the guide vanes of advanced turbine engines can reach 1100℃. The operating temperature of domestic guide vane alloys can reach 1000~1050℃. Representative high-temperature alloy precision casting alloys include K214, K233, K406, K417, K403, K409, K408, K423B, etc.
(3) Turbine blades
Turbine blades are the components with the most severe working conditions in aircraft engines. The working environment temperature is high. Typical grades of high-temperature alloy materials include GH4033, GH4037, GH4143, GH4049, GH4151, GH4118, GH4220, etc., which can be used in an environment of 750-950℃. When developing new engines and modifying old models, cast high-temperature alloys are used to manufacture turbine blades. Typical grades of cast alloys include K403, K417, K417G, K418, K403, K405, K4002, etc.
