The progress of aerospace materials depends on the following three factors: ① the new discovery of material science theory: for example, the aging strengthening theory of aluminum alloy leads to the development of hard aluminum alloy; The directional arrangement theory of rigid molecular chains of polymer materials leads to the development of aramid organic fibers with high strength and high modulus. (2) The progress of material processing technology: for example, the ancient casting and forging technology developed to directional solidification technology and precision forging technology, thus making high-performance blade materials practical; With the development of the design and technology of composite reinforced fiber ply, it has the best characteristics in different stress directions, which makes the composite material "designable" and opens up a broad prospect for its application. The achievements of new technologies such as hot isostatic pressing (HIP) and superfine powder manufacturing technology have created a new generation of aerospace materials and parts with brand-new properties, such as powder metallurgy turbine disks and high-performance ceramic parts. (3) the progress of material performance testing and nondestructive testing technology: modern electronic optical instruments can observe the molecular structure of materials; The testing device for mechanical properties of materials has been able to simulate the load spectrum of aircraft, and the nondestructive testing technology has also made rapid progress. Material performance testing and nondestructive testing technology are providing more and more detailed information, providing material performance data closer to actual use conditions for aircraft design, and providing testing means to ensure product quality for production. A new type of aerospace material can only be applied to aircraft when these three aspects are developed to a mature stage. Therefore, all countries in the world give priority to space materials. In 1950s, China established Beijing Institute of Aeronautical Materials and Beijing Institute of Aerospace Materials Technology, engaged in the application research of aerospace materials.
Introduction/kloc-industrial revolution in europe in the 1960s in 0/8 greatly developed the textile industry, metallurgical industry and machine manufacturing industry, thus ending the era when human beings could only challenge the sky with natural materials. 1903, the Wright brothers in the United States made the first plane with a piston aero-engine. At that time, the materials used were wood (47%), steel (35%) and cloth (18%), and the flight speed of aircraft was only 16 km/h, 1906. German metallurgists invented hard aluminum which can be strengthened by aging, making it possible to manufacture aircraft with all-metal structure. In the 1940s, the carrying capacity of all-metal aircraft was greatly improved, and the flying speed was over 600 km/h. A series of superalloys developed on the basis of alloy strengthening theory improved the performance of jet engines. The successful development and application of titanium alloy in 1950s played an important role in overcoming the "thermal barrier" problem of wing skin. The performance of the aircraft has been greatly improved, and the maximum flight speed has reached three times the speed of sound. The German V-2 rocket, which appeared in the early 1940s, only used ordinary aviation materials. After 1950s, the theory of material ablation thermal protection appeared, and ablative materials were successfully developed, which solved the thermal protection problem of ballistic missile warheads. Since the 1960s, with the continuous improvement of the performance of aerospace materials, some aircraft components have adopted more advanced composites, such as carbon fiber or boron fiber reinforced epoxy resin matrix composites and metal matrix composites, in order to reduce the structural weight. Returning spacecraft and space shuttle will encounter aerodynamic heating process when they re-enter the atmosphere, which takes much longer than ballistic missile warheads, but the heating speed is slow and the heat flow is small. The problem of heat protection can be solved by using special materials such as carbon-carbon composite ceramic heat insulation tiles with better oxidation resistance.
Classified aircraft has developed into a highly integrated product of machinery and electronics in the 1980s. It needs to use a wide variety of advanced structural materials and functional materials with various properties such as electricity, light, heat and magnetism. According to different users, aerospace materials can be divided into aircraft materials, aero-engine materials, rocket and missile materials and spacecraft materials. According to the chemical composition of materials, they can be divided into metal and alloy materials, organic nonmetallic materials, inorganic nonmetallic materials and composite materials.
Material conditions Many parts made of aerospace materials often need to work under extreme conditions such as ultra-high temperature, ultra-low temperature, high vacuum, high stress and strong corrosion. Due to the limitation of weight and accommodation space, some parts need to complete equivalent functions under normal conditions with minimum volume and mass. Some parts need to run in the atmosphere or outer space for a long time, and it is impossible to stop to check or replace parts, so they must have high reliability and quality assurance. Different working environments require aerospace materials to have different characteristics.
The basic requirements of high specific strength and specific stiffness for aircraft materials are: light weight, high strength and good stiffness. Reducing the structural weight of the aircraft itself means increasing carrying capacity, improving maneuverability, increasing flight distance or range, and reducing fuel or propellant consumption. Specific strength and stiffness are important parameters to measure the mechanical properties of aerospace materials.
Specific strength =/
Specific stiffness =/where [kg2][kg2] is the strength, elastic modulus and specific gravity of the material.
In addition to static load, aircraft will also be subjected to alternating load caused by take-off and landing, engine vibration, high-speed rotation of rotating parts, maneuvering flight and sudden wind, so the fatigue performance of materials has also received great attention.
The high temperature environment experienced by aircraft with excellent high and low temperature resistance is caused by aerodynamic heating, engine gas and solar radiation in space. Planes have to fly in the air for a long time, and some fly at speeds as high as 3 times the speed of sound. The high-temperature materials used should have good high-temperature endurance strength, creep strength and thermal fatigue strength, high oxidation resistance and thermal corrosion resistance in air and corrosive media, and structural stability for long-term work at high temperatures. The gas temperature of rocket engine can reach above 3000[2oc] and the injection speed can reach more than Mach 10. In addition, solid rocket gas is mixed with solid particles. When the ballistic missile warhead re-enters the atmosphere, the speed can reach Mach 20 or above, the temperature can reach tens of thousands of degrees Celsius, and sometimes it is eroded by particle clouds. Therefore, the high-temperature environment involved in aerospace technology often includes both high-temperature and high-speed airflow and particle erosion. Under this condition, it is necessary to use the physical properties of materials, such as heat of fusion, vaporization heat, sublimation heat, decomposition heat, combination heat and high-temperature viscosity, to design high-temperature ablative materials and cooling materials to meet the requirements of high-temperature environment. Solar radiation will cause the surface temperature alternation of satellites and spacecraft operating in outer space, which is generally solved by temperature control coatings and heat insulation materials. The formation of low temperature environment comes from nature and low temperature propellant. When an airplane flies at subsonic speed in the stratosphere, the surface temperature will drop to about -50[2oc], and the severe winter in various regions of the polar circle will make the ambient temperature of the airport drop below -40[2oc]. In this environment, it is required that metal parts or rubber tires are not fragile. Liquid rocket uses liquid oxygen (boiling point-183[2oc]) and liquid hydrogen (boiling point -253[2oc]) as propellants, which puts forward more harsh environmental conditions for materials. In this case, some metal materials and most polymer materials will become brittle. Only by developing or selecting suitable materials, such as pure aluminum and aluminum alloy, titanium alloy, low temperature steel, polytetrafluoroethylene, polyimide, perfluoropolyether, etc., can the bearing capacity and sealing problem of the structure be solved at ultra-low temperature.
Aging resistance and corrosion resistance: the influence of various media and atmospheric environment on materials is corrosion and aging. The contact media of aerospace materials are aircraft fuel (such as gasoline and kerosene), rocket propellant (such as concentrated nitric acid, nitrogen tetroxide and hydrazine), various lubricants and hydraulic oil. Most of them have a strong corrosive or expansive effect on metal and nonmetal materials. The aging process of polymer materials will be accelerated by the mold produced by sunlight, wind and rain erosion in the atmosphere and long-term storage in underground humid environment. Corrosion resistance, aging resistance and mildew resistance are good characteristics that aerospace materials should have.
Adapting to the space environment, the influence of space environment on materials is mainly manifested in high vacuum (1.33x10 [55-1] pa) and cosmic ray irradiation. When metal materials are in contact with each other in high vacuum, because the surface is purified in high vacuum environment, the molecular diffusion process is accelerated and the phenomenon of "cold welding" appears; Non-metallic materials will accelerate volatilization and aging under high vacuum and cosmic ray irradiation. Sometimes this phenomenon will pollute the optical lens due to volatile matter deposition, and the sealing structure will fail due to aging. Space materials are generally selected and developed through ground simulation tests to adapt to the space environment.
Life and safety In order to reduce the structural weight of aircraft, it is considered as the goal of aircraft design to choose the smallest possible safety margin to achieve an absolutely reliable safety life. For short-time use aircraft such as missiles or launch vehicles, people strive to maximize the material properties. In order to make full use of material strength and ensure safety, the "damage tolerance design principle" is adopted for metal materials. This requires the material not only to have high specific strength, but also to have high fracture toughness. It is an important basis for design, production and use to measure the data of crack initiation life and crack propagation rate of materials and calculate the allowable crack length and corresponding life under simulated use conditions. For organic nonmetallic materials, natural aging and artificial accelerated aging tests are needed to determine the insurance period of their life. The failure mode, life and safety of composite materials are also an important research topic.