What is a superalloy?

Thread Content

Metal materials that possess the ability to resist creep over long periods of time, along with high endurance strength and excellent corrosion resistance, when subjected to both stress and high temperatures (typically above 600–650 degrees Celsius), are known as heat-resistant alloys or superalloys. Commonly used ones include iron-based alloys, nickel-based alloys, cobalt-based alloys, as well as chromium-based alloys, molybdenum-based alloys, and other alloys. Superalloys are important materials for manufacturing components that operate at high temperatures, such as those in gas turbines and jet engines. Superalloys are a type of alloy used in high-temperature environments subjected to severe mechanical stresses as well as oxidation and corrosion. With the development of technology, superalloys have gradually taken on six relatively distinct categories. I. Deformable superalloys: Deformable superalloys are alloys that can be processed through thermal and cold deformation; they can operate within a temperature range of -253 to 1320°C. They possess excellent mechanical properties as well as high strength and toughness, along with superior oxidation and corrosion resistance. Based on their heat treatment processes, they can be divided into solution-strengthened alloys and age-hardened alloys. 1. Solid solution strengthened alloys have a service temperature range of 900–1300°C, with a maximum oxidation resistance temperature of 1320°C. For example, the GH128 alloy has a tensile strength of 850 MPa and a yield strength of 350 MPa at room temperature ； At 1000°C, the tensile strength is 140 MPa and the elongation is 85%; at 1000°C under a stress of 30 MPa, the endurance life is 200 hours with an elongation of 40%. Solid solution alloys are generally used to manufacture components such as combustion chambers and casings for aeronautical and aerospace engines. 2. Age-hardening alloys can be used at temperatures ranging from -253 to 950°C, and are typically employed in the manufacture of structural components such as turbine disks and blades for aviation and aerospace engines. The alloy used to manufacture turbine discs operates at temperatures ranging from -253 to 700°C, and it is required to possess good strength at both high and low temperatures as well as fatigue resistance. For example, the GH4169 alloy achieves a maximum yield strength of 1000 MPa at 650°C ； The temperature at which the alloy used to manufacture the blades can reach 950°C; for example, the GH220 alloy has a tensile strength of 490 MPa at 950°C, and its endurance life at 940°C is greater than 40 hours with a tensile strength of 200 MPa. Deformed superalloys primarily supply structural forgings, discs, rings, rods, sheets, tubes, strips, and wires to the aerospace, aviation, nuclear energy, and petroleum industries. II. Cast superalloys: Cast superalloys refer to a category of superalloys that can be, or can only be, shaped using casting methods. Its main features are: 1. It has a wider range of compositions; since there is no need to consider its formability, the design of the alloy can focus on optimizing its performance in practical applications. In the case of nickel-based superalloys, by adjusting the composition, the γ’ content can be increased to 60% or higher, enabling the alloy to maintain excellent properties at temperatures up to 85% of its melting point. 2. Wider range of application areas: Thanks to the special advantages of casting methods, high-temperature alloy castings with almost net shape or without waste material, and featuring any complex structure or shape, can be designed and manufactured according to the requirements of the parts’ usage. Based on the operating temperature of the casting alloys, they can be divided into the following three categories: Category 1: Isotropic crystal casting superalloys used at temperatures ranging from -253 to 650°C. These alloys exhibit good overall performance over a wide temperature range, particularly maintaining both strength and ductility even at low temperatures. The K4169 alloy, which is widely used in aeronautical and aerospace engines, has a tensile strength of 1000 MPa at 650°C, a yield strength of 850 MPa, and a tensile plasticity of 15% ； The endurance life at 650°C and a stress of 620 MPa is 200 hours. It has been used to manufacture diffuser casings in aeroengines, as well as various complex structural components for pumps in rocket engines. Category 2: Isotropic crystal cast superalloys used at 650–950 °C. These alloys exhibit high mechanical properties and resistance to thermal corrosion at high temperatures. For example, for the K419 alloy, at 950°C, the tensile strength is greater than 700 MPa and the tensile ductility is greater than 6% ； At 950°C and after 200 hours, the ultimate strength limit is greater than 230 MPa. Such alloys are suitable for use in aeroengine turbine blades, vanes, and monolithic turbines. Third category: Directionally solidified columnar and single-crystal superalloys used at 950–1100°C. These alloys exhibit excellent overall properties as well as good resistance to oxidation and thermal corrosion within this temperature range. For example, the durable life of the DD402 single-crystal alloy at 1100°C and a stress of 130 MPa is greater than 100 hours. This is the turbine blade material with the highest operating temperature in China, suitable for manufacturing first-stage turbine blades in new high-performance engines. With the continuous improvement of precision casting technology, new special processes are also emerging. Technologies such as fine-grain casting, directional solidification, and CA technology for complex thin-walled components have all contributed to **improving the quality of cast superalloys, thereby expanding their range of applications. III. Powder metallurgy superalloys: These are products made from superalloy powder, which is produced through a manufacturing process that involves the use of atomized superalloy powder, followed by hot isostatic pressing or, after hot isostatic pressing, forging. The powder metallurgy process is employed; due to the small size of the powder particles, the cooling rate is fast, resulting in a uniform composition without any macroscopic segregation. Additionally, the grain size is small, which gives good hot workability, high metal utilization, and low costs. In particular, the yield strength and fatigue performance of the alloy are significantly improved. FGH95 powder metallurgy superalloy, tensile strength at 650°C: 1500 MPa ； It has a endurance life of over 50 hours at a stress of 1034 MPa, making it the powder metallurgy superalloy with the highest strength level currently available for operation at 650°C. Powder metallurgy superalloys can meet the requirements of engines operating under high stress levels, and are the preferred materials for high-temperature components such as turbine disks, compressor disks, and turbine vanes in engines with high thrust-to-weight ratios. IV. Oxide Dispersion Strengthened (ODS) alloys are a special type of superalloy created using a unique mechanical alloying (MA) process; ultra-fine oxide dispersion strengthening phases (with sizes less than 50 nm) that remain highly stable at high temperatures are uniformly dispersed throughout the alloy matrix. Its alloy strength can be maintained even at temperatures close to the alloy’s melting point; it possesses excellent high-temperature creep resistance, superior high-temperature oxidation resistance, as well as resistance to carbon and sulfur corrosion. The three main ODS alloys that are currently available for commercial production are: the MA956 alloy, which can operate at temperatures up to 1350°C in an oxidizing atmosphere, and stands out as the best among high-temperature alloys in terms of resistance to oxidation as well as to carbon and sulfur corrosion. It can be used for the combustion chamber lining of aero engines. The MA754 alloy can be used at temperatures up to 1250°C in an oxidizing atmosphere, maintaining a fairly high strength at high temperatures as well as resistance to corrosion by neutral and alkaline glasses. It is now used to manufacture the rack rings and guide vanes of aeroengine guides. The tensile strength of the MA6000 alloy at 1100°C is 222 MPa, and its yield strength is 192 MPa ； At 1100℃, the endurance strength after 1000 hours is 127 MPa, the highest among superalloys, making it suitable for use in aircraft engine blades. V. Intermetallic high-temperature materials: Intermetallic high-temperature materials are a type of lightweight high-temperature material with significant application potential, which have been developed in recent times. Over the past decade or so, fundamental research on intermetallic compounds, alloy design, development of processing techniques, and applied research have matured, with remarkable achievements particularly in the preparation and processing technologies, toughening and strengthening, mechanical properties, and applied research of Ti-Al, Ni-Al, and Fe-Al-based materials. Ti3Al-based alloys (TAC-1), TiAl-based alloys (TAC-2), and Ti2AlNb-based alloys possess advantages such as low density (3.8–5.8 g/cm3), high strength at high temperatures, high stiffness, as well as excellent oxidation and creep resistance, enabling the reduction of the weight of structural components by 35–50%. The Ni3Al-based alloy MX-246 exhibits excellent corrosion resistance, wear resistance, and cavitation resistance, indicating great potential for application. Fe3Al-based alloys possess excellent oxidation and wear resistance, high strength at moderate temperatures (below 600°C), and low cost; they are a new material that can partially replace stainless steel. VI. Environmental superalloys: In many fields of the civil industry, the materials of components in use are exposed to high-temperature corrosive environments. To meet market demands, series of superalloys have been classified based on their application environments. 1. Superalloy master alloy series
2. Corrosion-resistant superalloy plates, bars, wires, strips, tubes, and forgings
3. High-strength, corrosion-resistant superalloy bars, spring wires, welding wires, plates, strips, and forgings
4. Products resistant to glass corrosion
5. Superalloy series with environmental corrosion resistance and hard surface wear resistance
6. Special precision casting parts (blades, boost turbines, turbine rotors, guides, instrument connections)
7. Centrifuges, high-temperature shafts, and accessories for glass wool production
8. Cobalt-based alloy heat-resistant pads and slides for steel billet heating furnaces
9. Valve seat rings
10. Cast “U”-shaped resistive strips
11. Centrifugal cast pipe series
12. Nanomaterial product series
13. Low-density high-temperature structural materials
14. Functional materials (expansion alloys, high-temperature high-elasticity alloys, constant-elasticity alloy series)
15. Biomedical material product series
16. Target materials for electronic engineering
17. Nozzle products for power plants
18. Stellite alloy wear-resistant sheets
19. Ultra-high temperature oxidation and corrosion-resistant furnace rolls and radiant tubes.