Superalloys
Roundup
1. Introduction
Supralloy is a kind of metal material that can work for a long time above 600 °C and under certain stress conditions, with excellent high temperature strength, good oxidation resistance and thermal corrosion resistance, good fatigue performance, fracture toughness and other comprehensive properties, mainly used in the manufacture of aviation, ships and industrial gas turbine turbine blades, guide vanes, turbine disks, high-pressure compressor discs and combustion chambers and other high-temperature parts, but also used in the manufacture of space vehicles, rocket engines, nuclear reactors, Petrochemical equipment and energy conversion equipment such as coal conversion. Its superalloy has become an irreplaceable key material for the hot end components of military and civilian gas turbine engines.
2. Category
According to the matrix elements, it can be mainly divided into iron-based superalloys, nickel-based superalloys and cobalt-based superalloys. According to the preparation process, it can be divided into deformed superalloy, cast superalloy and powder metallurgy superalloy. According to the strengthening method, there are solid solution strengthening type, precipitation strengthening type, oxide dispersion strengthening type and fiber strengthening type.
3. Development
Development processSince the late 30s of the 20th century, Britain, Germany, the United States and other countries have begun to study superalloys. During World War II, in order to meet the needs of new aircraft engines, the research and use of superalloys entered a period of vigorous development. In the early 40s, the United Kingdom first added a small amount of aluminum and titanium to the 80Ni-20Cr alloy to form a γ' phase (gamma prime) for strengthening, and developed the first nickel-based alloy with high temperature strength. During the same period, in order to meet the needs of the development of turbochargers for piston aero engines, the United States began to use Vitallium cobalt-based alloy to make blades.
In addition, the United States has developed Inconel nickel-based alloys to make the combustion chamber of jet engines. Later, in order to further improve the high temperature strength of the alloy, metallurgists added tungsten, molybdenum, cobalt and other elements to the nickel-based alloy, increased the content of aluminum and titanium, and developed a series of grades of alloys, such as "Nimonic" in the United Kingdom, "Mar-M" and "IN" in the United States; In cobalt-based alloys, nickel, tungsten and other elements are added to develop a variety of superalloys, such as X-45, HA-188, FSX-414, etc. Due to the lack of cobalt resources, the development of cobalt-based superalloys is limited.
In the 40s, iron-based superalloys were also developed, and grades such as A-286 and Incoloy901 appeared in the 50s, but due to poor high-temperature stability, the development has been slower since the 60s. The Soviet Union began to produce nickel-based superalloys of the "ЭИ" grade around 1950, and later produced the "ЭП" series of deformed superalloys and the ЖС series of cast superalloys. China began trial production of superalloys in 1956, and gradually formed "GH" series of deformed superalloys and "K" series of cast superalloys. In the 70s, the United States also used new production processes to manufacture directional crystallization blades and powder metallurgy turbine discs, and developed superalloy parts such as single crystal blades to meet the needs of increasing the temperature of aircraft engine turbine imports.
Various superalloy materials
1. Intermetallic high-temperature materials
Intermetallic high-temperature materials are a class of light-specific gravity high-temperature materials with important application prospects recently researched and developed. For more than ten years, the basic research, alloy design, process flow development and application research of intermetallic compounds have matured, especially in the preparation and processing technology, toughening and strengthening, mechanical properties and application research of Ti-Al, Ni-Al and Fe-Al materials.
Ti3Al-based alloy (TAC-1), TiAl-based alloy (TAC-2) and Ti2AlNb-based alloy have the advantages of low density (3.8~5.8g/cm3), high temperature and high strength, high rigidity, excellent oxidation resistance, creep resistance, etc., which can reduce the weight of structural parts by 35~50%. Ni3Al-based alloy, MX-246 has good corrosion resistance, wear resistance and cavitation resistance, showing excellent application prospects. Fe3Al-based alloy has good oxidation and wear resistance, high strength at medium temperature (less than 600 °C), low cost, and is a new material that can partially replace stainless steel.
2. Environmental superalloy
In many areas of the civil industry, the component materials in service are exposed to high temperature corrosive environments. In order to meet the needs of the market, according to the use environment of the material, a series of superalloys are classified.
1. Superalloy master alloy series
2. Anti-corrosion superalloy plates, rods, wires, belts, tubes and forgings
3. High strength, corrosion-resistant superalloy bar, spring wire, welding wire, plate, strip, forgings
4. Glass corrosion resistant series products
5. Environmental corrosion resistance, hard surface wear-resistant superalloy series
6. Special precision casting parts (blades, booster turbines, turbine rotors, guides, instrument joints)
7. Centrifuge for glass wool production, high-temperature shaft and accessories 8, cobalt-based alloy heat-resistant pad and slide rail for billet heating furnace
9. Valve seat
10. Cast "U" shaped resistance strip
11. Centrifugal casting pipe series
12. Nanomaterial series products
13. Light specific gravity high temperature structural materials
14. Functional materials (expansion alloy, high temperature and high elastic alloy, constant elastic alloy series)
15. Biomedical materials series products
16. Electronic engineering target series products
17. Power unit nozzle series products
18. Si Tai Li alloy wear plate
19. Ultra-high temperature oxidation corrosion furnace roller, radiant tube.
3. Deformed superalloy
Deformation superalloy refers to a type of alloy that can be processed by hot and cold deformation, with a working temperature range of -253~1320 °C, good mechanical properties and comprehensive strong and toughness indicators, and high oxidation and corrosion resistance. According to its heat treatment process, it can be divided into solid solution strengthened alloy and aging strengthened alloy.
A. Solid solution strengthening alloy
The use temperature range is 900~1300 °C, and the maximum oxidation resistance temperature is 1320 °C. For example, GH128 alloy has a tensile strength of 850MPa at room temperature and a yield strength of 350MPa; The tensile strength at 1000°C is 140MPa, the elongation is 85%, and the lasting life of 1000°C and 30MPa stress is 200 hours and the elongation is 40%. Solid solution alloys are generally used to make aviation and aerospace engine combustion chambers, receivers and other components.
B. Aging strengthening alloy
The use temperature is -253~950 °C, which is generally used to make turbine discs and blades of aviation and aerospace engines. The working temperature of the alloy for making the turbine disc is -253~700 °C, which requires good high and low temperature strength and fatigue resistance. For example: GH4169 alloy, the highest yield strength at 650°C is 1000MPa; The alloy temperature of the blade can reach 950 °C, for example: GH220 alloy, the tensile strength of 950 °C is 490MPa, and the durability of 940 °C and 200MPa is more than 40 hours.
Deformed superalloys mainly provide structural forgings, cakes, rings, bars, plates, pipes, strips and wires for aerospace, aviation, nuclear energy, petroleum civil industries.
4. Casting superalloy
Casting superalloys refer to a class of superalloys that can or can only be molded by the casting method. Its main features are:
1. With a wider range of composition, because it is not necessary to take into account its deformation processing performance, the design of the alloy can focus on optimizing its use performance. For nickel-based superalloys, the composition can be adjusted to 60% or more to maintain excellent properties at temperatures up to γ 85% of the alloy's melting point.
2. It has a broader application field Due to the special advantages of the casting method, according to the use needs of the parts, the design and manufacture of near-final or no margin of superalloy castings with arbitrary complex structure and shape.
According to the use temperature of the casting alloy, it can be divided into the following three categories:
The first category: equiaxed crystal casting superalloys used at -253~650 °C, which have good comprehensive properties in a wide range of temperatures, especially at low temperatures, can maintain strength and plasticity without decreasing. For example, K4169 alloy, which is used in large amounts in aviation and aerospace engines, has a tensile strength of 1000MPa at 650°C, a yield strength of 850MPa, and a tensile plasticity of 15%; The long life at 650°C, 620MPa stress is 200 hours. It has been used to make diffuser receivers in aero engines and complex structural parts for various pumps in aerospace engines. The second category: equiaxed crystal casting superalloys used at 650~950 °C, which have high mechanical properties and thermal corrosion resistance at high temperatures. For example, K419 alloy, at 950 °C, the tensile strength is greater than 700MPa and the tensile plasticity is greater than 6%; At 950°C, the 200-hour lasting strength limit is greater than 230MPa. This type of alloy is suitable for use as aero engine turbine blades, guide vanes and cast turbines.
The third category: directional solidification column crystal and single crystal superalloy used at 950~1100 °C have excellent comprehensive properties and oxidation resistance and thermal corrosion resistance in this temperature range. For example, DD402 single crystal alloy has a lasting life of more than 100 hours under stress of 1100°C and 130MPa. This is the highest temperature turbine blade material in China, which is suitable for making first-stage turbine blades for new high-performance engines.
With the continuous improvement of precision casting process technology, new special processes are constantly emerging. Fine grain casting technology, directional solidification technology, CA technology of complex thin-walled structural parts, etc. have greatly improved the level of casting superalloys, and the application range has been continuously improved.
5. Powder metallurgy superalloy
The superalloy powder product is manufactured by atomizing the superalloy powder, forming by hot isostatic pressing or hot isostatic pressing and then forging into a production process. Using powder metallurgy process, due to the fine powder particles, fast cooling rate, so that the composition is uniform, no macro segregation, and the grains are fine, the hot working performance is good, the metal utilization rate is high, the cost is low, especially the yield strength and fatigue performance of the alloy are greatly improved.
FGH95 powder metallurgy superalloy, tensile strength 1500MPa at 650°C; The durable life under 1034MPa stress is more than 50 hours, which is a kind of coil powder metallurgy superalloy with the highest strength level under 650°C working conditions. Powder metallurgy superalloys can meet the requirements of engines with high stress levels, and are the material of choice for high-temperature parts such as turbine discs, compressor discs and turbine baffles of engines with high thrust-to-weight ratio.
Strength-enhancing process for superalloys
1. Solution strengthening
Adding elements different from the atomic size of the matrix metal (chromium, tungsten, molybdenum, etc.) causes distortion of the matrix metal lattice, and elements that can reduce the error energy of the stacking layer of the alloy matrix (such as cobalt) and elements that can slow down the diffusion rate of the matrix elements (tungsten, molybdenum, etc.) are added to strengthen the matrix.
2. Precipitation strengthening
Through aging treatment, the second phase (γ', γ", carbides, etc.) is precipitated from the supersaturated solid solution to strengthen the alloy. The γ' phase is the same as the matrix, both are face-centered cubic structures, the lattice constant is similar to the matrix, and is co-lattice with the crystal, so the γ phase can be uniformly precipitated in the form of fine particles in the matrix, hindering the dislocation movement, and producing a significant strengthening effect. The γ' phase is an A3B type intermetallic compound, A represents nickel, cobalt, B represents aluminum, titanium, niobium, tantalum, vanadium, tungsten, and chromium, molybdenum, iron can be both A and B. A typical γ' phase in nickel-based alloys is Ni3 (Al, Ti). The strengthening effect of the γ' phase can be enhanced by:
(1) increase the number of γ' phases;
(2) Make the γ' phase and the matrix have an appropriate degree of mismatch to obtain the strengthening effect of colattice distortion;
(3) Adding niobium, tantalum and other elements increases the reversed-phase domain boundary energy of the γ' phase to improve its ability to resist dislocation cutting;
(4) Add cobalt, tungsten, molybdenum and other elements to improve the strength of the γ' phase. The γ" phase is a body-centered tetragonal structure, and its composition is Ni3Nb. Due to the large mismatch between the γ" phase and the matrix, it can cause a large degree of colattice distortion, so that the alloy obtains high yield strength. However, above 700 °C, the strengthening effect is significantly reduced. Cobalt-based superalloys generally do not contain γ phases and are reinforced with carbides.
3. Grain boundary strengthening
At high temperatures, the grain boundaries of the alloy are weak links, and the addition of trace amounts of boron, zirconium and rare earth elements can improve the grain boundary strength. This is because rare earth elements can purify grain boundaries, and boron and zirconium atoms can fill grain boundary vacancies, reduce grain boundary diffusion rate during creep, inhibit the accumulation of grain boundary carbides and promote the spheroidization of grain boundary second phase. In addition, adding an appropriate amount of hafnium to the cast alloy can also improve the strength and plasticity of the grain boundaries. It can also be heat treated to form a chain distribution of carbides at the grain boundary or cause curved grain boundaries to improve plasticity and strength.
4. Oxide dispersion strengthening
By powder metallurgy method, fine oxides that remain stable at high temperature are added to the alloy in a diffuse distribution state, so as to obtain a significant strengthening effect. Commonly added oxides are ThO2 and Y2O3. These oxides strengthen the alloy by impeding dislocation motion and stabilizing dislocation substructures.
Manufacturing process of superalloy
Superalloys that do not contain or contain little aluminum and titanium are generally smelted by electric arc furnaces or non-vacuum induction furnaces. If the superalloy containing aluminum and titanium is smelted in the atmosphere, the element burning loss is not easy to control, and the gas and inclusions enter more, so vacuum smelting should be used. In order to further reduce the content of inclusions, improve the distribution of inclusions and the crystalline structure of the ingot, a duplex process combining smelting and secondary remelting can be used. The main means of smelting are electric arc furnace, vacuum induction furnace and non-vacuum induction furnace; The main means of remelting are vacuum self-consumption furnace and electric slag furnace.
Solid solution strengthening alloys and alloy ingots containing low aluminum and titanium (the total amount of aluminum and titanium are about 4.5%) can be forged and opened; Alloys containing aluminum and titanium are generally extruded or rolled blanks, and then hot-rolled into materials, and some products need to be further cold-rolled or cold-drawn. Alloy ingots or cakes with larger diameters need to be forged with a water press or fast forging hydraulic press.
Alloys with a high degree of alloying and not easy to deform, are currently widely used in precision casting molding, such as casting turbine blades and guide blades. In order to reduce or eliminate grain boundaries perpendicular to the stress axis in cast alloys and reduce or eliminate looseness, directional crystallization processes have been developed in recent years. This process is to grow the grains in one crystal direction during the alloy solidification process to obtain parallel columnar crystals without transverse grain boundaries. The primary process condition for achieving directed crystallization is to establish and maintain a sufficiently large axial temperature gradient and good axial heat dissipation conditions between the liquid and solid phases. In addition, in order to eliminate all grain boundaries, it is also necessary to study the manufacturing process of single crystal blades.
Powder metallurgy process, mainly used to produce precipitation-strengthened and oxide-dispersion-strengthened superalloys. This process can make cast superalloys that generally cannot be deformed obtain plasticity or even superplasticity.
The properties of the comprehensive treatment of superalloys are closely related to the structure of the alloy, and the structure is controlled by metal heat treatment. Superalloys generally require heat treatment. Precipitation-strengthening alloys are usually solution-treated and aged. Solution-strengthening alloys are only solution-treated. Some alloys undergo one or two intermediate treatments before aging. Solution treatment is first to make the second phase dissolve into the alloy matrix, so that the γ, carbide (cobalt-based alloy) and other enhanced phases can be uniformly precipitated during aging treatment, and secondly, to obtain the appropriate grain size to ensure high temperature creep and durable performance.
The solution treatment temperature is generally 1040~1220 °C. At present, the widely used alloy is mostly processed at 1050~1100 °C before aging treatment. The main function of intermediate treatment is to precipitate carbides and γ films at grain boundaries to improve the grain boundary state, and at the same time, some alloys also precipitate some γ phases with larger particles to form a reasonable match with the fine γ phases precipitated during aging treatment. The purpose of aging treatment is to uniformly precipitate the supersaturated solid solution γ phase or carbide (cobalt-based alloy) to improve high temperature strength, and the aging treatment temperature is generally 700~1000 °C.
Substance Application
Superalloy refers to a class of metal materials based on iron, nickel and cobalt that can work for a long time under high temperature and certain stress above 600 °C; And it has high high temperature strength, good oxidation and corrosion resistance, good fatigue performance, fracture toughness and other comprehensive properties. Superalloy is a single austenitic structure, which has good microstructure stability and reliability of use at various temperatures
Based on the above performance characteristics, and the high degree of alloying of superalloys, also known as "super alloys", it is an important material widely used in aviation, aerospace, petroleum, chemical industry and ships. According to the matrix elements, superalloys are divided into iron-based, nickel-based, cobalt-based and other high-temperature alloys. The use temperature of iron-based superalloys can generally only reach 750~780 °C, and for heat-resistant parts used at higher temperatures, nickel-based and refractory metal-based alloys are used. Nickel-based superalloys occupy a particularly important position in the entire field of superalloys, which are widely used to manufacture aerojet engines and the hottest end components of various industrial gas turbines. If the durable strength of 150MPA-100H is the standard, and the current maximum temperature that nickel alloys can withstand is 1100 °C, and nickel alloys are about 950 °C, iron-based alloys (850 °C), that is, nickel-based alloys are correspondingly higher than 150 °C to 250 °C. So people call nickel alloy the heart of the engine. At present, in advanced engines, nickel alloys have accounted for half of the total weight, not only worm gear blades and combustion chambers, but also turbine discs and even the last few stages of compressor blades have begun to use nickel alloys. Compared with ferroalloys, the advantages of nickel alloys are: higher working temperature, stable structure, less harmful phase and large anti-oxidation and corrosion ability. Compared to cobalt alloys, nickel alloys can operate at higher temperatures and stresses, especially in moving blade applications.
The above advantages of nickel alloys are related to some of their own superior properties. Nickel is a face-centered cube, the structure is very stable, and there is no allotrope transformation from room temperature to high temperature; This is important for the selection of substrate materials. It is well known that austenitic tissue has a range of advantages over ferritic tissue.
Nickel has high chemical stability, almost no oxidation below 500 degrees, and is not affected by warm gas, water and some salt aqueous solutions at school temperature. Nickel dissolves very slowly in sulfuric acid and hydrochloric acid, while it dissolves quickly in nitric acid.
Nickel has a large alloying ability, and even the addition of more than ten alloying elements does not appear harmful phases, which provides potential possibilities for improving the various properties of nickel.
Although the mechanical properties of pure nickel are not strong, the plasticity is excellent, especially at low temperatures, the plasticity changes little.
Trends
The development trend of superalloys is to further improve the working temperature of the alloy and improve the ability to withstand various loads at medium or high temperatures, and extend the life of the alloy. As far as turbine blade materials are concerned, single crystal blades will enter the practical stage, and the comprehensive performance of directional crystalline blades will be improved.
In addition, it is possible to fabricate hollow vanes with multi-layer diffusion connections from quenched alloy powders to accommodate the need to increase gas temperatures. In the case of guide vanes and combustion chamber materials, it is possible to use oxide-dispersion-reinforced alloys to significantly increase the operating temperature. In order to improve corrosion resistance and wear resistance, protective coating materials and processes for alloys will also be further developed.S
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