Composite materials with a metal matrix. For operation at higher temperatures, metal matrices are used.
Metal CMs have a number of advantages over polymer ones. In addition to a higher operating temperature, they are characterized by better isotropy and greater stability of properties during operation, higher erosion resistance.
The plasticity of metal matrices gives the structure the necessary viscosity. This contributes to the rapid equalization of local mechanical loads.
An important advantage of metal CMs is the higher manufacturability of the manufacturing process, molding, heat treatment, formation of joints and coatings.
The advantage of metal-based composite materials is higher values of characteristics that depend on the properties of the matrix. These are, first of all, the tensile strength and modulus of elasticity in tension in the direction perpendicular to the axis of the reinforcing fibers, compressive and bending strength, plasticity, and fracture toughness. In addition, composite materials with a metal matrix retain their strength characteristics to higher temperatures than materials with a non-metal base. They are more moisture resistant, non-flammable, have electrical conductivity. The high electrical conductivity of metal CMs protects them well from electromagnetic radiation, lightning, and reduces the risk of static electricity. The high thermal conductivity of metal CM protects against local overheating, which is especially important for products such as rocket tips and wing leading edges.
The most promising materials for matrices of metal composite materials are metals with a low density (A1, Mg, Ti) and alloys based on them, as well as nickel, which is currently widely used as the main component of heat-resistant alloys.
Composites are obtained by various methods. These include the impregnation of a fiber bundle with liquid aluminum and magnesium melts, plasma spraying, the use of hot pressing methods, sometimes followed by hydroextrusion or billet rolling. When reinforcing with continuous fibers "sandwich" compositions consisting of alternating layers of aluminum foil and fibers, rolling, hot pressing, explosion welding, and diffusion welding are used. The casting of bars and pipes reinforced with high-strength fibers is obtained from the liquid metal phase. The fiber bundle continuously passes through the molten bath and is impregnated under pressure with liquid aluminum or magnesium. When leaving the impregnation bath, the fibers are combined and passed through a spinneret, forming a rod or tube. This method ensures the maximum filling of the composite with fibers (up to 85%), their uniform distribution in the cross section, and the continuity of the process.
Materials with aluminum matrix. Materials with an aluminum matrix are mainly reinforced with steel wire (SAS), boron fiber (VKA) and carbon fiber (VKU). As a matrix, both technical aluminum (for example, AD1) and alloys (AMg6, V95, D20, etc.) are used.
The use of an alloy (for example, B95) hardened by heat treatment (quenching and aging) as a matrix gives an additional effect of strengthening the composition. However, in the direction of the fiber axis, it is small, while in the transverse direction, where the properties are determined mainly by the properties of the matrix, it reaches 50%.
The cheapest, fairly effective and affordable reinforcing material is high-strength steel wire. Thus, the reinforcement of technical aluminum with a wire made of VNS9 steel with a diameter of 0.15 mm (σ in = 3600 MPa) increases its strength by 10-12 times with a fiber volume content of 25% and by 14-15 times with an increase in the content to 40%, after which temporary resistance reaches 1000-1200 and 1450 MPa, respectively. If a wire of smaller diameter, i.e., greater strength (σ in = 4200 MPa) is used for reinforcement, the tensile strength of the composite material will increase to 1750 MPa. Thus, aluminum reinforced with steel wire (25-40%) significantly surpasses even high-strength aluminum alloys in terms of basic properties and reaches the level of the corresponding properties of titanium alloys. The density of the compositions is in the range of 3900-4800 kg/m 3 .
Strengthening of aluminum and its alloys with more expensive fibers B, C, A1 2 Oe increases the cost of composite materials, but some properties are more effectively improved: for example, when reinforced with boron fibers, the elasticity modulus increases a 3-4 times, carbon fibers help reduce density. Boron weakens little with increasing temperature, so compositions reinforced with boron fibers retain high strength up to 400-500 ° C. A material containing 50 vol.% of continuous high-strength and high-modulus boron fibers (VKA-1) has found industrial application. In terms of elasticity modulus and tensile strength in the temperature range of 20-500°C, it surpasses all standard aluminum alloys, including high-strength ones (B95), and alloys specially designed for operation at high temperatures (AK4-1), which is clearly shown in Fig. 13.35. The high damping capacity of the material ensures the vibration resistance of structures made from it. The density of the alloy is 2650 kg/m 3 and the specific strength is 45 km. This is significantly higher than that of high-strength steels and titanium alloys.
Calculations have shown that the replacement of the V95 alloy with a titanium alloy in the manufacture of an aircraft wing spar with reinforcing elements from VKA-1 increases its rigidity by 45% and saves about 42% in weight.
Aluminum-based composite materials reinforced with carbon fibers (CFC) are cheaper and lighter than materials with boron fibers. And although they are inferior to the latter in strength, they have close specific strength (42 km). However, the manufacture of composite materials with a carbon hardener is associated with great technological difficulties due to the interaction of carbon with metal matrices during heating, which causes a decrease in the strength of the material. To eliminate this disadvantage, special coatings of carbon fibers are used.
Materials with a magnesium matrix. Materials with a magnesium matrix (MCM) are characterized by a lower density (1800–2200 kg/m3) than those with aluminum, with approximately the same high strength of 1000–1200 MPa and, therefore, a higher specific strength. Wrought magnesium alloys (MA2, etc.) reinforced with boron fiber (50 vol.%) have a specific strength > 50 km. The good compatibility of magnesium and its alloys with boron fiber, on the one hand, makes it possible to manufacture parts by impregnation with little or no subsequent machining, on the other hand, it provides a long service life of parts at elevated temperatures. The specific strength of these materials is enhanced by the use of alloys alloyed with light lithium as a matrix, as well as by the use of lighter carbon fiber. But, as mentioned earlier, the introduction of carbon fiber complicates the technology of already low-tech alloys. As is known, magnesium and its alloys have low technological ductility and a tendency to form a loose oxide film.
Composite materials based on titanium. When creating titanium-based composite materials, there are difficulties caused by the need to heat to high temperatures. At high temperatures, the titanium matrix becomes very active; it acquires the ability to gas absorption, interaction with many hardeners: boron, silicon carbide, aluminum oxide, etc. As a result, reaction zones are formed, the strength of both the fibers themselves and composite materials as a whole decreases. And, in addition, high temperatures lead to recrystallization and softening of many reinforcing materials, which reduces the strengthening effect of reinforcement. Therefore, to strengthen materials with a titanium matrix, a wire made of beryllium and ceramic fibers of refractory oxides (A1 2 0 3), carbides (SiC), as well as refractory metals with a high modulus of elasticity and a high recrystallization temperature (Mo, W) is used. Moreover, the purpose of reinforcement is mainly not to increase the already high specific strength, but to increase the elastic modulus and increase operating temperatures. Mechanical properties of titanium alloy VT6 (6% A1, 4% V, the rest A1), reinforced with Mo, Be and SiC fibers, are presented in table. 13.9. As seen from. table, the most effective specific rigidity increases when reinforced with silicon carbide fibers.
Reinforcement of the VT6 alloy with molybdenum wire helps to maintain high values of the modulus of elasticity up to 800 "C. Its value at this temperature corresponds to 124 GPa, i.e., decreases by 33%, while the tensile strength decreases to 420 MPa, i.e. more than 3 times.
Composite materials based on nickel. Heat-resistant CMs are made on the basis of nickel and cobalt alloys reinforced with ceramic (SiC, Si 3 Ni 4 , Al 2 O 3) and carbon fibers. The main task in creating nickel-based composite materials (NBC) is to increase operating temperatures above 1000 °C. And one of the best metal hardeners that can provide good strength at such high temperatures is tungsten wire. The introduction of tungsten wire in an amount of 40 to 70 vol.% into the nickel-chromium alloy provides strength at 1100°C for 100 hours, respectively, 130 and 250 MPa, while the best unreinforced nickel alloy, designed for operation under similar conditions, has a strength of 75 MPa. The use of wire from tungsten alloys with rhenium or hafnium for reinforcement increases this figure by 30-50%.
Composite materials are used in many branches of industry, primarily in aviation, rocket and space technology, where reducing the mass of structures while increasing strength and rigidity is of particular importance. Due to their high specific strength and rigidity characteristics, they are used in the manufacture of, for example, horizontal stabilizers and aircraft flaps, propeller blades and helicopter containers, jet engine bodies and combustion chambers, etc. The use of composite materials in aircraft structures has reduced their weight by 30-40% , increased the payload without reducing the speed and range.
At present, composite materials are used in power turbine building (turbine blades and nozzle blades), automotive industry (car bodies and refrigerators, engine parts), mechanical engineering (body and machine parts), chemical industry (autoclaves, tanks, tanks), shipbuilding, (hulls of boats, boats, propellers), etc.
The special properties of composite materials make it possible to use them as electrical insulating materials (organic fibers), radio-transparent fairings (glass fibers), plain bearings (carbon fibers) and other parts.
Composite materials with a ceramic matrix. For the highest operating temperatures, ceramics are used as the matrix material. Silicate (SiO 2), aluminosilicate (Al 2 O 3 - SiO 2), aluminoborosilicate (Al 2 O 3 - B 2 O 3 - SiO 2) materials, refractory oxides of aluminum (Al 2 O 3), zirconium are used as ceramic matrices (ZrO 2), beryllium (BeO), silicon nitride (Si 3 N 4), titanium (TiB 2) and zirconium (ZrB 2) borides, silicon (SiC) and titanium (TiC) carbides. Composites with a ceramic matrix have a high melting point, resistance to oxidation, thermal shock and vibration, and compressive strength. Ceramic CMs based on carbides and oxides with additions of metal powder (< 50об. %) называются cermets . In addition to powders for reinforcing ceramic CM, metal wire from tungsten, molybdenum, niobium, heat-resistant steel, as well as non-metallic fibers (ceramic and carbon) are used. The use of a metal wire creates a plastic frame that protects the CM from destruction when the brittle ceramic matrix cracks. The disadvantage of ceramic CM reinforced with metal fibers is low heat resistance. CMs with a matrix of refractory oxides (can be used up to 1000°C), borides and nitrides (up to 2000°C), and carbides (over 2000°C) have high heat resistance. When reinforcing ceramic CMs with silicon carbide fibers, a high bond strength between them and the matrix is achieved in combination with oxidation resistance at high temperatures, which makes it possible to use them for the manufacture of heavily loaded parts (high-temperature bearings, seals, rotor blades of gas turbine engines, etc.). The main drawback of ceramics - the lack of plasticity - is compensated to some extent by reinforcing fibers that inhibit the propagation of cracks in ceramics.
Carbon-carbon composite . The use of amorphous carbon as a matrix material and fibers of crystalline carbon (graphite) as a reinforcing material made it possible to create a composite that can withstand heating up to 2500°C. Such a carbon-carbon composite is promising for astronautics and atmospheric aviation. The disadvantage of the carbon matrix is the possible oxidation and ablation. To prevent these phenomena, the composite is coated with a thin layer of silicon carbide.
Carbon matrix, similar in physical and chemical properties to carbon fiber, provides thermal stability of CCCM
The most widely used are two methods for producing carbon-carbon composites:
1. carbonization of the polymer matrix of a preformed carbon fiber blank by high-temperature heat treatment in a non-oxidizing environment;
2. vapor deposition of pyrocarbon, formed during the thermal decomposition of hydrocarbons in the pores of the carbon fiber substrate.
Both of these methods have their advantages and disadvantages. When creating UCCM they are often combined to give the composite the desired properties.
Carbonization of the polymer matrix. The carbonization process is a heat treatment of a carbon fiber product to a temperature of 1073 K in a non-oxidizing environment (inert gas, coal filling, etc.). The purpose of heat treatment is to convert the binder into coke. In the process of carbonization, thermal destruction of the matrix occurs, accompanied by weight loss, shrinkage, the formation of a large number of pores, and, as a result, a decrease in the physico-mechanical properties of the composite.
Carbonization is carried out most often in retort resistance furnaces. A retort made of a heat-resistant alloy protects the product from oxidation by atmospheric oxygen, and the heating elements and insulation from getting volatile corrosive pyrolysis products of the binder onto them and ensures uniform heating of the reaction volume of the furnace.
The mechanism and kinetics of carbonization are determined by the ratio of the rates of dissociation of chemical bonds and recombination of the resulting radicals. The process is accompanied by the removal of evaporating resinous compounds and gaseous products and the formation of solid coke enriched with carbon atoms. Therefore, in the carbonization process, the key point is the choice of temperature-time regime, which should ensure the maximum formation of coke residue from the binder, since the mechanical strength of the carbonized composite depends, among other things, on the amount of coke formed.
The larger the dimensions of the product, the longer the carbonization process should be. The rate of temperature rise during carbonization is from several degrees to several tens of degrees per hour, the duration of the carbonization process is 300 hours or more. Carbonization usually ends in the temperature range 1073-1773 K, corresponding to the temperature range of transition of carbon to graphite.
The properties of CCCM largely depend on the type of initial binder, which is used as synthetic organic resins that give a high coke residue. Most often, phenol-formaldehyde resins are used for this purpose due to their manufacturability, availability of low cost, the coke formed in this process has high strength.
Phenol-formaldehyde resins have certain disadvantages. Due to the polycondensation nature of their curing and the release of volatile compounds, it is difficult to obtain a uniform dense structure. The amount of shrinkage during carbonization of phenol-formaldehyde binders is greater than for other types of binders used in the production of CCCM, which leads to the appearance of internal stresses in the carbonized composite and a decrease in its physical and mechanical properties.
More dense coke is provided by furan binders. Their shrinkage during carbonization is less, and the coke strength is higher than that of phenol-formaldehyde resins. Therefore, despite a more complex curing cycle, binders based on furfural, furfurylidene acetones, and furyl alcohol are also used in the production of CCCM.
Coal and oil pitches are very promising for obtaining a carbon matrix due to the high carbon content (up to 92-95%) and high coke number. The advantages of pitch over other binders are the availability and low cost, the exclusion of the solvent from the technological process, the good graphitization of coke and its high density. The disadvantages of pitches include the formation of significant porosity, deformation of the product, the presence of carcinogenic compounds in their composition, which requires additional security measures.
Due to the release of volatile compounds during the thermal degradation of the resin in carbonized plastic, significant porosity occurs, which reduces the physical and mechanical properties of CCCM. Therefore, the stage of carbonization of carbon fiber completes the process of obtaining only porous materials that do not require high strength, for example, low-density CCCM for heat-insulating purposes. Usually, to eliminate porosity and increase density, the carbonized material is re-impregnated with a binder and carbonized (this cycle can be repeated several times). Re-impregnation is carried out in autoclaves in the "vacuum-pressure" mode, i.e., the workpiece is first heated in a vacuum, after which a binder is supplied and an overpressure of up to 0.6-1.0 MPa is created. When impregnating, solutions and melts of binders are used, and the porosity of the composite decreases with each cycle, so it is necessary to use binders with a reduced viscosity. The degree of compaction during re-impregnation depends on the type of binder, coke number, porosity of the product and the degree of filling of the pores. With an increase in density during re-impregnation, the strength of the material also increases. This method can be used to obtain CCCM with a density of up to 1800 kg/m 3 and higher. The method of carbon fiber carbonization is relatively simple, it does not require complex equipment, and it provides good reproducibility of the material properties of the resulting products. However, the need for repeated compaction operations significantly lengthens and increases the cost of obtaining products from CCCM, which is a serious disadvantage of this method.
Upon receipt of the UCCM by method of deposition of pyrocarbon from the gas phase a gaseous hydrocarbon (methane, benzene, acetylene, etc.) or a mixture of hydrocarbon and diluent gas (inert gas or hydrogen) diffuses through the porous carbon fiber frame, where, under the influence of high temperature, the hydrocarbon is decomposed on the heated surface of the fiber. The precipitated pyrolytic carbon gradually creates connecting bridges between the fibers. The deposition kinetics and the structure of the obtained pyrolytic carbon depend on many factors: temperature, gas flow rate, pressure, reaction volume, etc. The properties of the obtained composites are also determined by the type and content of the fiber, and the reinforcement scheme.
The deposition process is carried out in vacuum or under pressure in induction furnaces, as well as in resistance furnaces.
Several technological methods for obtaining a pyrocarbon matrix have been developed.
With the isothermal method the workpiece is placed in a uniformly heated chamber. The uniformity of heating in the induction furnace is ensured with the help of a fuel element - a susceptor made of graphite. Hydrocarbon gas is fed through the bottom of the furnace and diffuses through the reaction volume and the billet; gaseous reaction products are removed through the outlet in the furnace cover.
The process is usually carried out at a temperature of 1173-1423 K and a pressure of 130-2000 kPa. Reducing the temperature leads to a decrease in the rate of deposition and an excessive lengthening of the duration of the process. An increase in temperature accelerates the deposition of pyrolytic carbon, but in this case the gas does not have time to diffuse into the bulk of the workpiece and pyrolytic carbon is deposited on the surface. The duration of the process reaches hundreds of hours.
The isothermal method is usually used for the manufacture of thin-walled parts, since in this case the pores near the surface of the product are mainly filled.
For volumetric saturation of pores and obtaining thick-walled products, non-isothermal method, which consists in creating a temperature gradient in the workpiece by placing it on a heated mandrel or core or by directly heating it with current. Hydrocarbon gas is supplied from the lower temperature side. The pressure in the furnace is usually equal to atmospheric. As a result, pyrocarbon deposition occurs in the hottest zone. The cooling effect of the gas flowing over the surface at high speed is the main way to achieve a temperature gradient.
Increasing the density and thermal conductivity of the composite leads to the displacement of the temperature front of deposition, which ultimately ensures the volumetric compaction of the material and the production of products with high density (1700-1800 kg/m3).
The isothermal method of obtaining CCCM with a pyrocarbon matrix is characterized by the following advantages: good reproducibility of properties; simplicity of technical design; high density and good matrix graphitization; Possibility of processing several products at the same time.
The disadvantages include: low deposition rate; surface deposition of pyrocarbon; poor filling of large pores.
The non-isothermal method has the following advantages: high deposition rate; the ability to fill large pores; volume seal of the product.
Its disadvantages are as follows: complex hardware design; only one product is processed; insufficient density and graphitization of the matrix; formation of microcracks.
3.4.4. High-temperature heat treatment (graphitization) of CCCM. The structure of carbonized plastics and composites with a pyrocarbon matrix after compaction from the gas phase is imperfect. The interlayer distance d 002, which characterizes the degree of ordering of the carbon matrix, is relatively large - over 3.44 10 4 µm, and the crystal sizes are relatively small - usually no more than 5 10 -3 µm, which is typical for two-dimensional ordering of basic carbon layers. In addition, during the production process, internal stresses can occur in them, which can lead to deformations and distortions of the product structure when these materials are used at temperatures above the temperature of carbonization or pyrocarbon deposition. Therefore, if it is necessary to obtain a more thermally stable material, its high-temperature processing is carried out. The final temperature of heat treatment is determined by the operating conditions, but is limited by the sublimation of the material, which proceeds intensively at temperatures above 3273 K. Heat treatment is carried out in induction furnaces or resistance furnaces in a non-oxidizing environment (graphite filling, vacuum, inert gas). The change in the properties of carbon-carbon materials during high-temperature heat treatment is determined by many factors: the type of filler and matrix, the final temperature and duration of heat treatment, the type of medium and its pressure, and other factors. At high temperatures, energy barriers in the carbon material are overcome, preventing the movement of multinuclear compounds, their attachment and mutual reorientation with a greater degree of compaction.
The duration of these processes is short and the degree of conversion is determined mainly by temperature. Therefore, the duration of high-temperature heat treatment processes is much shorter than in the case of carbonization or pyrocarbon deposition, and usually amounts to several hours. During high-temperature heat treatment of carbonized plastics, irreversible deformations of the product occur, gradual “healing” of defects. For well-graphitized pitch-based materials at temperatures above 2473 K, an intensive growth of three-dimensionally ordered carbon crystallites is observed up to the transition to a graphite structure. At the same time, in carbonized plastics based on poorly graphitized polymer binders, structural defects persist up to 3273 K, and the material remains in a non-graphitized structural form.
Fibrous composite metal materials.
Eutectic composite metal materials.
Composite metal materials formed by sintering.
Dispersion-strengthened materials on a metal matrix.
Composite materials on a metal matrix.
Lecture #2
Laminated reinforced plastics
Textolites- materials formed from layers of fabric impregnated with thermosetting synthetic resin.
Dubbed heads- laminates consisting of sheets of polyethylene, polypropylene and other thermoplastics, connected by a sublayer based on fabric, chemically resistant rubber, non-woven fibrous materials, etc.
Linoleum- polymer roll material for flooring - is a multi-layer or fabric-based KPM containing alkyd resins, polyvinyl chloride, synthetic rubbers and other polymers.
Getinaks- laminated plastic based on paper impregnated with thermosetting synthetic resin.
metal-plastic- a structural material consisting of a metal sheet, provided on one or both sides with a polymer coating of polyethylene, fluoroplastic or polyvinyl chloride.
Wood Laminates- materials obtained by "hot" pressing of blanks from wood (veneer) impregnated with synthetic thermosetting resins.
Topic: "COMPOSITE MATERIALS ON A METAL MATRIX"
The nomenclature of CMM is divided into three main groups: 1) dispersion-strengthened materials reinforced with particles, including pseudo-alloys obtained by powder metallurgy; 2) eutectic composite materials - alloys with directional crystallization of eutectic structures; 3) fibrous materials reinforced with discrete or continuous fibers.
Dispersion-hardened materials
If particles of the strengthening phase 1–100 nm in size, occupying 1–15% of the composite volume, are distributed in the CMM metal matrix, the matrix perceives the main part of the mechanical load applied to the CMM, and the role of the particles is reduced to creating effective resistance to the movement of dislocations in the matrix material. Such CMMs are characterized by increased temperature stability, as a result of which their strength practically does not decrease up to temperatures (0.7 ... 0.8) T pl, where T mp is the melting temperature of the matrix. Materials of this type are divided into two groups: materials formed by sintering and pseudomaterials.
Materials formed by sintering contain finely dispersed particles of oxides, carbides, nitrides and other refractory compounds, as well as intermetallic compounds, which, when forming CMM do not melt and do not dissolve in the matrix. The technology for forming products from such CMMs belongs to the field of powder metallurgy and includes the operations of obtaining powder mixtures, pressing them in a mold, sintering the resulting semi-finished products, deformation and heat treatment of blanks.
Aluminum Matrix Materials. CMs with an aluminum matrix that have found application are mainly reinforced with steel wire, boron and carbon fibers. Both technical aluminum (for example, AD1) and alloys (B95, D20, etc.) are used as a matrix.
Dispersion-hardened steels contain oxides as reinforcing components: Al 2 O 3, TiO 2, ZrO 2, etc.
CMM on cobalt matrix contain thorium oxide as a dispersed additive, on magnesium matrix- own oxides.
Copper Based Materials, hardened with oxides, carbides, nitrides, acquire heat resistance, which is combined with the high electrical conductivity of the copper matrix. Such CMMs are used to make electrical contacts, roller welding electrodes, sparking tools, etc.
Nickel based KMM, filled with thorium oxide and hafnium oxide, are designed to operate at temperatures above 1000 ° C and are used in aircraft construction, power engineering, and space technology.
Pseudo-alloy - dispersion-strengthened CMM, consisting of metallic and metal-like phases that do not form solutions and do not enter into chemical compounds. The technology of forming pseudo-alloys belongs to the field of powder metallurgy. The final operations for obtaining pseudo-alloys are impregnation or liquid-phase sintering of moulds.
Impregnation consists in filling the pores of a mold or a sintered blank made of a refractory component with a melt of a low-melting component of a pseudo-alloy. Impregnation is carried out by immersing the porous preform into the melt.
The nomenclature of pseudo-alloys includes mainly materials for tribotechnical purposes.
W-Cu and W-Ag tungsten-based pseudo-alloys combine high hardness, strength and electrical conductivity. They are used to make electrical contacts. Pseudo-alloys based on molybdenum (Mo - Cu) and nickel (Ni - Ag) and others have the same purpose.
Eutectic CMMs are alloys of eutectic or similar composition, in which the reinforcing phase is oriented fibrous or lamellar crystals formed in the process of directional crystallization of the metal matrix.
The technology for the formation of eutectic CMMs consists in the fact that the sample is pulled out of the melt at a constant rate, subjecting it to continuous cooling. The shape of the crystallization front depends on the drawing speed and heat exchange conditions, which are controlled by the structural elements of the mold.
F iber materials. The technology for forming fibrous CMMs includes methods of pressing, rolling, co-drawing, extrusion, welding, spraying or deposition, and impregnation.
"Hot" pressing (pressing with heating) is used to obtain CMM, the initial matrix material of which is powders, foils, tapes, sheets and other metal semi-finished products. They and reinforcing elements (wire, ceramic, carbon or other fibers) are laid in a certain order on a press plate or in a mold and then pressed when heated in air or in an inert atmosphere.
The rolling method processes the same components as pressing.
The joint drawing method is as follows. Holes are drilled in the blank from the matrix metal, into which reinforcing rods or wire are inserted. The billet is heated and its compression and drawing are carried out, which is completed by annealing.
The extrusion method produces products in the form of rods or pipes reinforced with continuous and discrete fibers. The starting material of the matrix are metal powders,
The nomenclature of fibrous CMM includes many materials on matrices of aluminum, magnesium, titanium, copper, nickel, cobalt, etc.
This type of composite materials includes materials such as SAP (sintered aluminum powder), which are aluminum reinforced with dispersed particles of aluminum oxide. Aluminum powder is obtained by spraying molten metal, followed by grinding in ball mills to a size of about 1 micron in the presence of oxygen. With an increase in the duration of grinding, the powder becomes finer and the content of aluminum oxide in it increases. Further technology for the production of products and semi-finished products from SAP includes cold pressing, pre-sintering, hot pressing, rolling or extrusion of sintered aluminum billet into the form of finished products that can be subjected to additional heat treatment.
SAP-type alloys are used in aviation technology for the manufacture of parts with high specific strength and corrosion resistance, operating at temperatures up to 300–500 °C. Piston rods, compressor blades, shells of fuel elements and heat exchanger tubes are made from them.
Reinforcement of aluminum and its alloys with steel wire increases their strength, increases the modulus of elasticity, fatigue resistance and extends the temperature range of the material.
Reinforcement with short fibers is carried out by powder metallurgy methods, consisting of pressing followed by hydroextrusion or rolling of blanks. When reinforcing with continuous fibers of sandwich-type compositions consisting of alternating layers of aluminum foil and fibers, rolling, hot pressing, explosion welding, and diffusion welding are used.
A very promising material is the composition "aluminum - beryllium wire", which implements the high physical and mechanical properties of beryllium reinforcement, and first of all, its low density and high specific rigidity. Compositions with beryllium wire are obtained by diffusion welding of packages from alternating layers of beryllium wire and matrix sheets. Aluminum alloys reinforced with steel and beryllium wires are used to make rocket body parts and fuel tanks.
In the composition "aluminum - carbon fibers" the combination of low density reinforcement and matrix allows you to create composite materials with high specific strength and rigidity. The disadvantage of carbon fibers is their fragility and high reactivity. The composition "aluminum - carbon" is obtained by impregnating carbon fibers with liquid metal or by powder metallurgy methods. Technologically, it is most simply feasible to pull bundles of carbon fibers through a melt of aluminum.
Composite "aluminum - carbon" is used in the design of the fuel tanks of modern fighters. Due to the high specific strength and rigidity of the material, the mass of fuel tanks is reduced by
thirty %. This material is also used for the manufacture of turbine blades for aircraft gas turbine engines.
GENERAL CHARACTERISTICS AND CLASSIFICATION
Traditionally used metallic and non-metallic materials have largely reached their structural strength limit. At the same time, the development of modern technology requires the creation of materials that work reliably in a complex combination of force and temperature fields, under the influence of aggressive media, radiation, deep vacuum and high pressures. Often, the requirements for materials can be contradictory. This problem can be solved by using composite materials.
composite material(CM) or composite is called a bulk heterogeneous system consisting of mutually insoluble components that differ greatly in properties, the structure of which allows you to use the advantages of each of them.
Man borrowed the principle of construction of CM from nature. Typical composite materials are tree trunks, plant stems, human and animal bones.
CMs make it possible to have a given combination of heterogeneous properties: high specific strength and rigidity, heat resistance, wear resistance, heat-shielding properties, etc. The spectrum of CM properties cannot be obtained using conventional materials. Their use makes it possible to create previously inaccessible, fundamentally new designs.
Thanks to CM, a new qualitative leap has become possible in increasing engine power, reducing the mass of machines and structures, and increasing the weight efficiency of vehicles and aerospace vehicles.
Important characteristics of materials operating under these conditions are specific strength σ in /ρ and specific stiffness E/ρ, where σ in - temporary resistance, E is the modulus of normal elasticity, ρ is the density of the material.
High-strength alloys, as a rule, have low ductility, high sensitivity to stress concentrators, and relatively low resistance to fatigue crack development. Although composite materials may also have low ductility, they are much less sensitive to stress concentrators and better resist fatigue failure. This is due to the different mechanism of crack formation in high-strength steels and alloys. In high-strength steels, a crack, having reached a critical size, then develops at a progressive rate.
In composite materials, another mechanism operates. The crack, moving in the matrix, encounters an obstacle at the matrix-fiber interface. Fibers inhibit the development of cracks, and their presence in the plastic matrix leads to an increase in fracture toughness.
Thus, the composite system combines two opposite properties required for structural materials - high strength due to high-strength fibers and sufficient fracture toughness due to the plastic matrix and the fracture energy dissipation mechanism.
CMs consist of a relatively plastic matrix material-base and harder and stronger components that are fillers. The properties of CM depend on the properties of the base, fillers and the strength of the bond between them.
The matrix binds the composition into a monolith, gives it a shape and serves to transfer external loads to reinforcement from fillers. Depending on the base material, CMs are distinguished with a metal matrix, or metal composite materials (MCM), with a polymer - polymer composite materials (PCM) and with a ceramic - ceramic composite materials (CMC).
The leading role in the strengthening of CMs is played by fillers, often referred to as hardeners. They have high strength, hardness and modulus of elasticity. According to the type of reinforcing fillers, CMs are divided into dispersion-strengthened,fibrous And layered(Fig. 28.2).
Rice. 28.2. Schemes of the structure of composite materials: A) dispersion-strengthened; b) fibrous; V) layered
Fine, uniformly distributed refractory particles of carbides, oxides, nitrides, etc., which do not interact with the matrix and do not dissolve in it up to the phase melting temperature, are artificially introduced into dispersion-hardened CMs. The smaller the filler particles and the smaller the distance between them, the stronger the CM. Unlike fibrous, in dispersion-strengthened CMs, the main bearing element is the matrix. The ensemble of dispersed filler particles strengthens the material due to the resistance to the movement of dislocations under loading, which hinders plastic deformation. Effective resistance to the movement of dislocations is created up to the melting temperature of the matrix, due to which dispersion-strengthened CMs are characterized by high heat resistance and creep resistance.
Reinforcement in fibrous CM can be fibers of various shapes: threads, tapes, meshes of various weaves. Reinforcement of fibrous CM can be carried out according to a uniaxial, biaxial and triaxial scheme (Fig. 28.3, A).
The strength and stiffness of such materials is determined by the properties of the reinforcing fibers that take the main load. Reinforcement gives a greater increase in strength, but dispersion hardening is technologically easier to implement.
Layered composite materials (Fig. 28.3, b) are made up of alternating layers of filler and matrix material (sandwich type). The filler layers in such CMs can have different orientations. It is possible to alternately use layers of filler from different materials with different mechanical properties. For layered compositions, non-metallic materials are usually used.
Rice. 28.3. Fibrous reinforcement schemes ( A) and layered ( b) composite materials
DISPERSION-HARDENED COMPOSITE MATERIALS
During dispersion strengthening, the particles block the sliding processes in the matrix. The effectiveness of hardening, under the condition of minimal interaction with the matrix, depends on the type of particles, their volume concentration, as well as the uniformity of distribution in the matrix. Apply dispersed particles of refractory phases such as Al 2 O 3 , SiO 2 , BN, SiC, having a low density and a high modulus of elasticity. CM is usually produced by powder metallurgy, an important advantage of which is the isotropy of properties in different directions.
In industry, dispersion-strengthened CMs on aluminum and, more rarely, nickel bases are usually used. Characteristic representatives of this type of composite materials are materials of the SAP type (sintered aluminum powder), which consist of an aluminum matrix reinforced with dispersed particles of aluminum oxide. Aluminum powder is obtained by spraying molten metal, followed by grinding in ball mills to a size of about 1 micron in the presence of oxygen. With an increase in the duration of grinding, the powder becomes finer and the content of aluminum oxide in it increases. Further technology for the production of products and semi-finished products from SAP includes cold pressing, pre-sintering, hot pressing, rolling or extrusion of sintered aluminum billet into the form of finished products that can be subjected to additional heat treatment.
Alloys of the SAP type are satisfactorily deformed in the hot state, and alloys with 6–9% Al 2 O 3 are also deformed at room temperature. From them, cold drawing can be used to obtain foil with a thickness of up to 0.03 mm. These materials are well machined and have high corrosion resistance.
SAP grades used in Russia contain 6–23% Al 2 O 3 . SAP-1 is distinguished with a content of 6-9, SAP-2 - with 9-13, SAP-3 - with 13-18% Al 2 O 3. With an increase in the volume concentration of aluminum oxide, the strength of composite materials increases. At room temperature, the strength characteristics of SAP-1 are as follows: σ in = 280 MPa, σ 0.2 = 220 MPa; SAP-3 are as follows: σ in \u003d 420 MPa, σ 0.2 \u003d 340 MPa.
SAP type materials have high heat resistance and outperform all wrought aluminum alloys. Even at a temperature of 500 °C, their σ is not less than 60–110 MPa. Heat resistance is explained by the retarding effect of dispersed particles on the recrystallization process. The strength characteristics of SAP-type alloys are very stable. Long-term strength tests of SAP-3 type alloys for 2 years had practically no effect on the level of properties both at room temperature and when heated to 500 °C. At 400 °C, the strength of SAP is 5 times higher than the strength of aging aluminum alloys.
SAP-type alloys are used in aviation technology for the manufacture of parts with high specific strength and corrosion resistance, operating at temperatures up to 300–500 °C. Piston rods, compressor blades, shells of fuel elements and heat exchanger tubes are made from them.
CM is obtained by powder metallurgy using dispersed particles of silicon carbide SiC. The chemical compound SiC has a number of positive properties: high melting point (more than 2650 ° C), high strength (about 2000 MPa) and elastic modulus (> 450 GPa), low density (3200 kg / m 3) and good corrosion resistance. The production of abrasive silicon powders has been mastered by the industry.
Powders of aluminum alloy and SiC are mixed, subjected to preliminary compaction under low pressure, then hot pressing in steel containers in vacuum at the melting temperature of the matrix alloy, i.e., in a solid-liquid state. The resulting workpiece is subjected to secondary deformation in order to obtain semi-finished products of the required shape and size: sheets, rods, profiles, etc.