1 preface
As early as 1950s, the skins, frames and engine casings of airplanes and missiles copied by China had already adopted rare-earth magnesium alloys. After 1970s, with the rapid development of rare earth industry in China, the development and application of aviation rare earth entered a new stage of independent development. New rare-earth magnesium alloys, aluminum alloys, titanium alloys, high-temperature alloys, nonmetallic materials, functional materials and rare-earth motor products are also gradually popularized and applied in fighter planes, attack planes, helicopters, unmanned aerial vehicles, civil aircraft and missile satellites.
Rare Earth Materials and Their Applications in Aviation Industry
2. 1 rare earth magnesium alloy
Rare-earth magnesium alloys have high specific strength, and have broad application prospects in reducing aircraft weight and improving tactical performance. Rare-earth magnesium alloys developed by aviation industry corporation of china (hereinafter referred to as AVIC) include cast magnesium alloys and wrought magnesium alloys, with brands exceeding 10, and many of them have been used in production with stable quality. For example, ZM6 cast magnesium alloy with neodymium as the main additive element has been widely used in helicopter rear gearbox, fighter wing rib, lead plate of 30KW generator rotor and other important parts. BM25 is a rare-earth high-strength magnesium alloy jointly developed by AVIC and Nonferrous Metals Company, which has replaced some medium-strength aluminum alloys and has been applied to attack aircraft. During the Eighth Five-Year Plan period, in order to expand the popularization and application of rare earth magnesium alloys, the application of rare earth magnesium alloys in medical engineering was also carried out. At present, this material is undergoing medical biological experiments, and it is expected that rare earth magnesium alloy will be used as the bonding material of artificial bone to replace the existing metal fixture, which will reduce the surgery for patients to take out the fixture twice and open up a new and broad application world.
Rare earth cast magnesium alloy is mainly used for long-term use below 200 ~ 300℃, with good high-temperature strength and long-term creep resistance. The solubility of various rare earth elements in magnesium is different, and the order of increase is lanthanum, mixed rare earth, cerium, praseodymium and neodymium. Its good influence on mechanical properties at room temperature and high temperature also increases. ZM6 alloy with neodymium as the main additive element developed by AVIC has not only high mechanical properties at room temperature after heat treatment, but also good instantaneous mechanical properties and creep resistance at high temperature, which can be used for a long time at room temperature or 250℃. With the appearance of a new type of corrosion-resistant cast magnesium alloy containing yttrium, cast magnesium alloy has been neglected by foreign aviation industry again in recent years.
After adding a proper amount of rare earth metal into magnesium alloy, the fluidity of the alloy can be increased, the porosity can be reduced, the air tightness can be improved, and the hot cracking and porosity can be significantly improved, so that the alloy still has high strength and creep resistance at the high temperature of 200 ~ 300℃. 2.2 Rare Earth Titanium Alloy
In the early 1970s, Beijing Institute of Aeronautical Materials (hereinafter referred to as Institute of Aeronautical Materials) replaced some aluminum and silicon in Ti-A 1-Mo titanium alloy with rare earth metal cerium (Ce), which limited the precipitation of brittle phase and improved the thermal stability and strength of the alloy. On this basis, a cast high temperature titanium alloy ZT3 containing cerium with good properties was developed. Compared with similar alloys in the world, it has certain advantages in heat resistance and process performance. The compressor casing made of it is used in WPI3 engine. The weight of each aircraft is reduced by 39 kg, the thrust-to-weight ratio is increased by 1.5%, and the processing procedures are reduced by about 30%. The obvious technical and economic benefits have been achieved, and the blank of titanium casing for 500℃ aero-engine in China has been filled. The results show that there are fine cerium oxide particles in the microstructure of ZT3 alloy containing cerium. Cerium combines with some oxygen in the alloy to form refractory and high hardness rare earth oxide particles Ce203. These particles hinder the dislocation movement during the deformation of the alloy and improve the high temperature properties of the alloy. Cerium captures some gas impurities (especially on grain boundaries), so it can strengthen the alloy while maintaining good thermal stability. This is the first attempt to apply the point strengthening theory of insoluble solute to casting titanium alloy.
In addition, in the investment casting process of titanium alloy, Aeronautical Materials Institute has developed stable and cheap yttrium oxide sand and powder by using special mineralization treatment technology after years of research, which has reached a good level in specific gravity, hardness and stability to titanium liquid, but shows greater superiority in the performance regulation of shell slurry. The outstanding advantage of making titanium castings with yttrium oxide shell is that titanium alloy castings can be made thinner than tungsten surface process under the condition that the casting quality and process level are equivalent to those of tungsten surface process. At present, this process has been widely used to manufacture various aircraft, engines and civil castings.
2.3 Rare Earth Aluminum Alloy
Compared with foreign nickel alloys, HZL206, a heat-resistant cast aluminum alloy containing rare earth, developed by AVIC, has superior mechanical properties at high temperature and room temperature, reaching the advanced level of similar alloys abroad. It has been used in pressure valves of helicopters and fighter planes, and its working temperature can reach 300℃, replacing steel and titanium alloys. The weight of the structure has been reduced and has been put into mass production. The tensile strength of rare earth Al-Si superalloy ZL 1 17 at 200 ~ 300℃ is higher than that of German piston alloys KS280 and KS282, and its wear resistance is 4 ~ 5 times that of ordinary piston alloy ZL 108, with small linear expansion coefficient and good dimensional stability. It has been used in aviation accessories KY-5, KY-7 air compressor and. The addition of rare earth elements can obviously improve the microstructure and mechanical properties of aluminum alloy. The mechanism of rare earth elements in aluminum alloy is as follows: dispersion distribution is formed, and fine aluminum compounds play a significant second phase strengthening role; The addition of rare earth elements plays the role of degassing and purification, thus reducing the number of pores in the alloy and improving the properties of the alloy; Rare earth aluminum compound, as a heterogeneous nucleus refined grain and * * * crystal phase, is also a modifier; Rare earth elements promote the formation and refinement of iron-rich phase and reduce the harmful effects of iron-rich phase. With the increase of rare earth addition, the solid solution of Fe in α-A 1 decreases. It is also beneficial to improve strength and plasticity.
2.4 Incomplete Rare Earth Materials
Rare earth organic potting compound XZ- 1 has been used in eight kinds of electromagnet products such as fuel electromagnetic switch and hydraulic electromagnetic switch in high performance oil control system. Because of its low cost and simple construction, it can replace epoxy potting compound in large quantities and has good economic benefits. The anti-aging rubber coating KF- 1 was successfully developed, which solved the long-standing problem of short service life of aircraft fuel tanks. The application of KF- 1 prolongs the service life of aircraft fuel tank from 3 ~ 5 years to 15 ~ 20 years, improves the service performance and obtains remarkable technical and economic benefits. MCrAIY coating containing Y2O3 is the third generation composition designable coating for hot-end parts of engines such as turbine blades and guide vanes, and has been applied in foreign high-performance and long-life engines. The Institute of Aeronautical Materials has successfully developed this kind of coating series by magnetron sputtering deposition process and multi-arc ion plating technology, and its thermal corrosion resistance and comprehensive performance have reached the advanced level of similar coatings abroad. The coating series has been selected by turbine blades and guide blades of superalloys, directionally solidified alloys, single crystal alloys and Ni-A 1 based alloys, and has been used as high-temperature oxidation-resistant coatings on advanced engines and ground gas turbines. Y2O3 plays a "pinning" role between the coating and the matrix alloy in this series of coatings, which significantly improves the adhesion between the coating and the matrix.
Rare earth additives also play an important role in chemical heat treatment. Because rare earth elements have specific electronic structure and high chemical activity, they have significant activation in chemical heat treatment, and have obvious effects on improving the microstructure and properties of the infiltrated layer and increasing the infiltration rate. The conventional carburizing, nitrocarburizing and adding rare earth additives in 3 10 plant of AVIC general plant were compared. Preliminary experimental study shows that the permeability can be increased by 30%. The nitrocarburizing hardness Hv of rare-earth high-speed steel can be increased from 933 to 946, and the increase range is 1350 ~ 1478. The method of chemical heat treatment with rare earth elements is simple and has no special requirements for equipment. It is of great significance to improve product weight and save energy, and has good popularization and application value.
2.5 Rare Earth Permanent Magnet Materials
Rare earth permanent magnet materials have developed very rapidly and have been widely used in many fields, becoming an important material basis for contemporary new technologies. Since 1980s, samarium-cobalt alloy has been used as rare earth permanent magnet motor. Product types include servo motors, drive motors, automobile starters, ground military motors, aviation motors, etc. Some products are exported. The main characteristics of Sm-Co permanent magnet alloy are as follows: (1) demagnetization curve is basically a straight line, and its slope is close to anti-permeability, that is, the recovery straight line is approximately coincident with demagnetization curve; (2) High coercivity and strong demagnetization resistance; (3) It has a very high maximum magnetic energy product; (4) The reversible temperature coefficient is very small and the magnetic temperature stability is good. Because of the above characteristics, rare earth samarium cobalt permanent magnet alloy is especially suitable for open circuit, pressure situation, demagnetization situation or dynamic situation, and is suitable for manufacturing small parts.
Made by AVIC 125 factory 160LY? .2 The permanent magnet DC torque motor adopts NdFeB (NTP200/64) magnetic steel. Using NdFeB permanent magnet instead of SmCo permanent magnet reduces the cost and improves the performance. The QZDM0 1-H rare earth permanent magnet shallow car starter produced by this factory adopts NdFeB magnetic steel, which is a rare earth deceleration starter. The use of rare earth magnetic steel makes the starter small in size, high in efficiency, large in output torque and fast in starting speed. The temperature coefficient of domestic SmCo permanent magnet needs to be improved, the high temperature stability and corrosion resistance of NdFeB permanent magnet need to be further improved, and bonded NdFeB permanent magnet is still in the research and development stage.
The development of permanent magnet materials has gone through ferrite stage (magnetic energy product 4.6MGOe), Ainico alloy stage (magnetic energy product 1 1.5MGOe), SmCo stage (magnetic energy product 3 1.0MGOe) and NdFeB stage (magnetic energy product 43MGOe). The successful development of Ti-Fe-B-RE permanent magnet materials has made headphones, speakers, stepping motors and coreless motors ultra-miniaturized. Permanent magnet NdFeB is used in the 1000cc automobile engine of American General Motors Company, which reduces the weight and volume of the engine by 40 ~ 50% and 45% respectively. If the service temperature of the material can be increased, it will open up a broader application prospect for the material.
3 the role of rare earth elements in the development of aviation materials
The role of rare earth elements in the development of aviation materials is determined by their properties. The atomic radius of rare earth elements is larger than that of common metals such as Al and Mg, so the solid solubility of rare earth elements in these metals is extremely low, and solid solutions can hardly be formed. Due to the high chemical activity of rare earth elements, rare earth elements are extremely active in chemical reactions and easily react with gases (such as oxygen), nonmetals (such as sulfur) and metals to generate corresponding stable compounds; Most of these newly formed compounds have high melting point, low density and stable chemical properties. The role of rare earth elements in metals can be summarized as follows:
(1) Reduce the harmful effects of nonmetallic impurities. Hydrogen is a harmful impurity in steel and aluminum alloy. When hydrogen dissolved in liquid metal solidifies, it precipitates in atomic state and aggregates into molecules, which leads to hydrogen-induced defects such as intergranular crack, porosity and pinhole, and brings serious harm to casting, plastic processing and properties. The experiment shows that adding a proper amount of rare earth (0. 1 ~ 0.3%) to aluminum and its alloy can obviously reduce the hydrogen content, reduce the harm of hydrogen and improve the properties of the alloy. Its chemical reaction formula is as follows:
4/3[RE]+2[O]→2/3RE203 (solid)
[RE] ten [H]→REH (solid)
RE (bottle) 10 MnS (solid) →RES (solid) +Mn (bottle)
The rare earth compounds produced by the reaction have high melting point and light specific gravity, and float in the slag. And their tiny particles become heterogeneous nuclei during the crystallization of aluminum.
(2) Refine the grain and dendrite structure and improve the thermoplasticity. Rare earth can refine the as-cast structure of the alloy, make the dendrite network clearer, and thus improve the thermoplasticity of the alloy. The tiny solid particles of rare earth compounds provide heterogeneous nuclei or segregation at the crystallization interface, which hinders the growth of unit cells and provides better thermal conditions for the crystallization and refinement of molten steel.
(3) Change the shape and distribution of inclusions. Rare earth and impurities form compounds and precipitate at grain boundaries, which changes the existing mode of solid solution and reduces the number of inclusions.
(4) it has a strengthening effect. Adding rare earth elements to the alloy reduces the amount of hydrogen, oxygen and inclusions, and refines the grain and dendrite network. High-melting-point compounds generated by the interaction between rare earth and nonmetallic elements are dispersed in the matrix, and high-melting-point intermetallic compounds are generated by rare earth and metallic elements, which not only eliminates coarse massive structures, but also stabilizes grain boundaries, and both play a role in improving the strength of materials. (5) The introduction of rare earth improves the corrosion resistance and high temperature oxidation resistance of rare earth-containing alloy materials. The addition of rare earth elements has also been studied in casting, forging, welding, heat treatment and surface coating technology, many of which have achieved positive results, but the mechanism of rare earth elements in these thermal processes and parts needs further development and research.
4 application prospect of rare earth in aviation materials
Because of the large atomic radius of rare earth metals, it is easy to lose two S electrons in the outermost layer and one electron of 5d or 4f in the second layer, and become trivalent ions. Therefore, rare earth metals are extremely active in chemical reactions and easily react with other substances. Rare earth elements have the characteristic that electrons do not completely fill the 4f layer, which leads to various special properties such as magnetic, electrical and optical properties. These attractive properties and extensive potential uses of rare earth elements have aroused great concern and extensive research by aviation materials scientists. Recent research focuses on:
4. 1 rare earth ceramic materials
New progress has been made in the application of rare earth materials in aero-engines with high thrust-to-weight ratio. In recent years, China Aviation Group Corporation has carried out research on the application of rare earth in structural ceramics. Silicon nitride ceramics have excellent properties such as high high temperature strength, good thermal shock resistance and small high temperature creep, and are the most promising new structural ceramic materials for high thrust-to-weight ratio engines. Silicon nitride ceramics still follow the mechanism of liquid phase sintering, and some oxide additives need to be added to react with Si3N4 and SiO2 layers on the particle surface to generate liquid phase to promote sintering. The fracture toughness and strength of silicon nitride ceramics are not high when oxides such as A 1203 and MgO are introduced as sintering AIDS, while the fracture toughness and strength of silicon nitride ceramics are obviously improved at room temperature when rare earth oxides Y2O3, namely y2 O3-a 1203 or Y2O3-MgO, are introduced as sintering AIDS, but the high temperature performance is not good. In recent years, it has been found that the mechanical properties, especially the fracture toughness at high temperature, of materials are greatly improved with rare earth oxides Y203 and La203 as additives. The results show that the introduction of Y2O3 and La203 has an important influence on the growth behavior of β-Si3N4 grains in silicon nitride ceramics, thus affecting the structure and properties of silicon nitride ceramics. Choosing Y203 and La2O3 with proper proportion and content as additives can obtain β-Si3N4 grains with large axial ratio, which makes silicon nitride ceramics have self-toughening effect. Ceramics are brittle materials and generally cannot be used in structural parts. To overcome its brittleness. Fiber, whisker and other reinforcing components are usually introduced, but this makes it difficult for different forms of components to be evenly dispersed, which brings difficulties to the manufacturing process. At present, this problem is the key to limit the application of ceramic materials in high-tech fields. The introduction of rare earth oxides into ceramic powder can produce the effect of in-situ toughening, that is, self-toughening, during ceramic sintering, which just overcomes the manufacturing difficulties caused by the introduction of fibers and whiskers. Therefore, the introduction of rare earth oxides into ceramic materials will broaden the application prospects of ceramic materials in high-tech fields. In order to meet the business requirements, ASIC must be radiation-resistant and improve its reliability. At the same time, the development of integrated circuits and computer technology to higher circuit density and faster operation speed has promoted the development of ceramic substrates and their packaging to higher performance and finer technology. As a substrate material, it must meet the requirements of low dielectric constant, high thermal conductivity, high mechanical strength and thermal expansion coefficient matching with semiconductor chips. Compared with the traditional alumina (A 1203) substrate, aluminum nitride (AIN) multilayer substrate has higher thermal conductivity, which is suitable for high power consumption, high lead count and large-size chips, and has become the focus of aviation and military development in recent years. Using rare earth yttrium oxide (Y203) and calcium oxide as mixed additives can reduce the sintering temperature of aluminum nitride and promote sintering. The thermal conductivity of this doped aluminum nitride (AIN) ceramic is 260W/(m.K), which is suitable for high-density wiring, and its thermal resistance is only 1/4 of that of alumina package with the same structure and the same number of leads. This substrate has been used to package a multi-layer wiring array of a computer system with 65,438+0,800 input/output heads.
4.2 Rare Earth Permanent Magnet Materials
Rare earth permanent magnet material is the key material for preparing high performance microwave power tube-traveling wave tube. Modern military communication, radar, missile guidance, electronic warfare and so on all need various kinds of traveling wave tubes, which are characterized by wide working frequency (2 ~ 18 GHz) and high efficiency (up to 50%). The United States used a large number of high-performance broadband and high-power traveling wave tubes in the electronic jamming equipment, early warning aircraft, fire control radar and precision guidance system used in the Gulf War. The key to manufacture these high-power TWTs is the rare earth permanent magnet material with high magnetic energy product and low temperature coefficient. This material is also very important for realizing the high efficiency, miniaturization and lightweight of military motors and promoting the performance improvement of military computers. According to the actual situation of the development of rare earth permanent magnet materials in China, the main research and development directions of rare earth permanent magnet materials in the aerospace field in the future are: (1) SmCo permanent magnet materials with high stability; (2) NdFeB permanent magnet material with high working temperature; (3) Rapidly quenching NdFeB magnetic powder and bonded NdFeB permanent magnet materials; (4) new SmFeN permanent magnet materials; (5) The fourth generation rare earth permanent magnet material with low cost and high performance. 4.3 The working temperature of A 1-Cu-Mg-Fe-Ni series heat-resistant aluminum alloys LD7 and LD8 for rare earth aluminum alloy aviation shall not exceed 270℃, and the working temperature of Al-Cu-Mn series LYI6 or 202 1 shall not exceed 300℃. There is no aluminum alloy that can work at 350 ~ 400℃ except sintered aluminum powder. Sc can increase the recrystallization temperature of aluminum alloy to 450 ~ 550℃, and the * * * lattice precipitation phase A 13Sc, especially A 13(ScZr) compounded with Zr, has extremely high thermal stability. When heated at 350℃ or 450℃ for a long time, the growth rate of particle size is extremely slow, and the * * * crystal lattice can be maintained for a long time, which is the developing working temperature. At present, the high-strength and high-toughness aluminum alloys for aviation with the best comprehensive properties are A 1-Zn-Mg-Cu-Zr series 7075, 7 150 and 70 10. Zr replaces Mn and Cr, which significantly improves the hardenability of the alloy and is suitable for producing thick plates (≥75mm). However, this alloy has poor castability, and its strength and toughness in the thickness direction are not high enough. If 0. 1 ~ 0.2% Sr and Zr are added to form a * * * lattice precipitation phase A 13(ScZr), not only the strength is improved, but also the recrystallization temperature is increased. Particle A 13Sc inhibits the recrystallization of the alloy, obtains the non-recrystallized structure, plays a role in strengthening the substructure, and can improve the strength and toughness of the plate in the thickness direction. After full aging, the fatigue strength and fracture toughness (k1c. ) and stress corrosion resistance (SCR) have been significantly improved, and it is entirely possible to develop a new generation of ultra-high strength and high toughness aluminum alloy for rockets and aircraft.
4.4 Rare Earth High Temperature Combination
Rare earth elements play an important role in improving the properties of superalloys. Superalloys are used in the hot-end components of aero-engines, but their oxidation resistance, corrosion resistance and strength decrease at high temperature limit the further improvement of aero-engine performance. Recent research shows that adding a small amount of rare earth to nickel-based alloys can improve the sulfide resistance, high temperature strength and thermoplasticity. Adding 0. 1 ~ 0.2% Y to cobalt-based alloy and adding Cu or Ce to nickel-based alloy can improve the corrosion resistance of materials by 10 times. In Ni-Cr alloy, rare earth can obviously improve the oxidation resistance of the alloy, such as adding 0.3% Y； to Ni-30Cr alloy; With the addition of 0.05% La and Ce, the service life of the alloy at 1200℃ and 1300℃ is 2970 hours and 6 13 hours, respectively, while the service life of the same nickel-chromium alloy without rare earth is only151at the above temperatures. Rare earth elements are closely related to the research and development of high-tech new materials. It is the historical mission of rare earth materials researchers to study the function and mechanism of rare earth elements in aviation materials and their influence on performance changes, so as to explore new aviation materials and develop high-tech products more widely. In recent years, people have focused on the study of the effect of rare earth on improving the properties of materials, but the mechanism of rare earth action has not been studied enough. In order to establish the application of rare earth in materials on a solid scientific basis and develop more and better new rare earth metal and nonmetal materials, it is necessary to systematically and deeply study the modification mechanism of rare earth on materials. Combined with rich rare earth elements (lanthanum, cerium, neodymium, ytterbium, dysprosium, scandium, etc. ) The systematic and in-depth research on these rare earths and materials science in China aims at opening up new application channels for effectively and rationally utilizing the characteristics of each rare earth, obtaining more patents on rare earth materials, and building China's rare earth materials on its own intellectual property rights.
During the "Seventh Five-Year Plan" and "Eighth Five-Year Plan" period, the development and application of aviation rare earth elements have done a lot of work to improve the application function of materials, prolong their service life and improve economic benefits through the role of rare earth elements in new materials. However, in the development and application of rare earth materials, there is still great potential to give full play to the functions of aviation rare earth materials, and we need to continue to make unremitting efforts to develop and further study and apply them. As the leading industry of China in the world, the share of rare earth in the international market is increasing year by year, and its position is becoming more and more important. We should seize the opportunity to accelerate the development and application of rare earths in aviation industry. To sum up, rare earth elements can strengthen metal materials, reduce the harmful effects of impurities, change the shape and distribution of inclusions, and improve corrosion resistance and oxidation resistance. Many rare earth magnesium alloys, aluminum alloys, titanium alloys, superalloys and functional materials for aviation have been developed, and good technical and economic benefits have been achieved in their applications. However, compared with the special role and potential use of rare earth in the development of aviation materials, these achievements can only be said to be a good start for the development of rare earth, which is also extremely disproportionate to our status as a rare earth power. In order to fully meet the needs of national economy and high-tech development, we should strengthen the basic theoretical research and engineering application of aviation rare earth materials in the future, and increase investment to further develop rare earth materials, accelerate the development of rare earth materials in China, establish material science with China characteristics and its engineering application system, and give full play to the advantages of rare earth resources in China.