The hottest powder metallurgy high entropy alloy w

2022-10-14
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Powder metallurgy high entropy alloys will be more useful

"high entropy alloys have high strength, high hardness, magnetic properties and other properties, and have excellent application effects in corrosion resistance, high temperature resistance, wear resistance and so on. They can be used in the manufacture of welding materials, hydrogen storage materials, corrosion-resistant materials and so on." On July 25, Liu Bin, director of the Institute of high temperature structural materials of the Research Institute of powder metallurgy of Central South University, introduced the performance characteristics, preparation methods and forming methods of powder metallurgy high entropy alloys in the nonferrous metal cloud class hosted by the China Nonferrous Metals Society and undertaken by Central South University, and prospected the development and application of powder metallurgy high entropy alloys

high entropy alloys may have ideal excellent properties

in the traditional concept, the more kinds of metals added to the alloy, the easier it is to embrittle. However, high entropy alloys containing multiple metals will not embrittle. Scholars believe that high entropy alloys may have many ideal excellent properties. Therefore, as a new material, foam granulator and high entropy alloy have received a lot of attention in the field of material science and engineering

Liu Bin introduced that high entropy alloys have four core effects, including high entropy effect in thermodynamics - single-phase solid solution structure, serious lattice distortion effect in crystal structure, diffusion hysteresis effect in dynamics, cocktail effect - excellent comprehensive performance. For example, lattice distortion improves the strength of high entropy alloys, making them have excellent mechanical properties; Nano twinning induced high entropy alloy has excellent fracture toughness, which has excellent application potential in low temperature environment; The excellent high temperature mechanical properties and diffusion hysteresis effect lead to the structural stability of high entropy alloy at high temperature. At the same time, it is found that the high entropy alloy has stable structure, low and stable thermal expansion, low thermal conductivity and good superconductivity under high dose irradiation. In addition, the high entropy alloy has excellent corrosion resistance. In the environment of high concentration sulfuric acid, hydrochloric acid, nitric acid and other corrosive solutions, it is more corrosion resistant than traditional stainless steel

"at present, the most studied high entropy alloys are 3d transition high entropy alloys (Fe, Co, Cr, Ni, Mn, etc.) and refractory series high entropy alloys (W, Mo, Ta, Nb, V, etc.)." Liu Bin said

five preparation methods of high entropy alloy powder

in recent years, with the gradual maturity of related technologies such as additive manufacturing, powder metallurgy high entropy alloy has gradually entered the research field of academia and industry. Compared with traditional casting high entropy alloys, powder metallurgy high entropy alloys can give better play to the performance advantages of high entropy alloys and achieve more possibilities in different fields

Liu Bin introduced that compared with traditional casting high entropy alloys, powder metallurgy high entropy alloys are easy to obtain alloy materials with uniform composition and fine structure in terms of microstructure, and can prepare high melting point materials (including tungsten based irradiation resistant alloys, refractory high entropy high temperature components, high entropy cemented carbides, etc.), high-performance coatings (including superhard, wear-resistant, irradiation resistant, corrosion resistant, etc.), Complex components can also be prepared by additive manufacturing technology (laser powder feeding LMDF, selective laser melting SLM, electron beam powder bed EBM). In addition, the near net shape technology can realize the low-cost preparation of high entropy alloys, solve the problem of high preparation cost of high entropy alloys, and promote the rapid application of high entropy alloys

"at present, there are mainly five preparation methods for high entropy alloy special powder." Liu Bin introduced. The first is the mechanical alloying (MA) method. In the high-performance ball mill, the metal or alloy powder will repeatedly produce cold welding and fracture through the long-term fierce impact and collision between the powder particles and the grinding ball, resulting in the atomic diffusion in the powder particles, so as to obtain the alloyed powder. "This method is applicable to almost all high entropy alloys, but the prepared powder has insufficient sphericity, is easy to produce impurities, and some media will produce alloying element pollution, which is not convenient for use in the next step of forming and preparation." Liu Bin pointed out in particular

the second is tightly coupled inert gas atomization (GA). This method is to melt the alloy into liquid through the smelting furnace, flow into the spray tray through the conduit, and then disperse the liquid through the air outlet to form small droplets, which will become spherical powder after cooling. This preparation method has the advantages of low cost, fine particle size and good sphericity, but it is not suitable for processing refractory (upper limit temperature is about 1800 ℃) or high activity high entropy alloys

how to prepare high entropy alloy powder with high activity and high purity? Liu Bin introduced the third preparation method - electrode induction melting and atomization (Eiga). This method is to melt the high entropy alloy material into an electrode, put it into an induction coil to heat it to form a liquid flow, and then shock the liquid flow through high-pressure gas, and finally get the gasified powder. Liu Bin introduced that this preparation method has no crucible pollution, and the prepared powder has fine particle size and low impurity content

"at present, the fourth popular and common method for preparing high entropy alloy powder is rotating electrode atomization (PREP)." Liu Bin said that this method is to melt the tip of the sample through a large amount of current between the tungsten electrode and the electrode of the high entropy alloy sample. At the same time of melting, the electrode rotates at a high speed, and the molten liquid is thrown out by centrifugal force to form small droplets, which form powder after cooling. Liu Bin pointed out, "this preparation method has no crucible pollution, and the prepared powder has good sphericity and low impurity content, which is suitable for the preparation of high activity powder."

"if you encounter a high entropy alloy with high melting point, you can use the plasma spheroidization method. This kind of high entropy alloy powder prepared by the plasma spheroidization method has good sphericity and uniform composition." Liu Bin introduced the fifth preparation method. In this method, the high entropy alloy is instantly melted in the plasma at a high temperature of 8000~10000 degrees Celsius to form a liquid, which forms droplets through the action of surface tension, and then forms a powder after the droplets are cooled

five forming methods of powder metallurgy high entropy alloy

after preparing high entropy alloy powder, the next step is to prepare and form high entropy alloy powder. Liu Bin introduced the forming method of powder metallurgy high entropy alloy

the first is SPS (discharge plasma sintering) method, which realizes plasma sintering through pulse current and upper and lower die pressing. This method has the advantages of fast temperature rise, low sintering temperature and high density. It is suitable for the manufacture of small materials such as nano materials, composite materials and ceramic materials. The finished products produced have high strength but low plasticity

the second is powder extrusion. In this method, after the high entropy alloy powder is placed in the mold, the gas is extracted and sealed to form a vacuum green body, and the green body is placed on a preheated extruder for extrusion to obtain a finer and more uniform alloy finished product. The yield strength of the finished product is higher than that of the finished product prepared by traditional casting method, and the comprehensive mechanical properties are better. "At the same time, this method is also applicable to the preparation of composite materials." Liu Bin fixed the frequency for several times. In particular, when preparing composite materials, the oxide phase with dispersion distribution reaching nano scale can be formed in situ by hot extrusion, and finally the alloy materials with good high-temperature mechanical properties and irradiation properties can be obtained, but they are not suitable for refractory high entropy alloy materials

the third is hot isostatic pressing (hip) forming method. Liu Bin introduced that after the powder is made into a green body by this method, it is deformed by hot isostatic pressing machine at high temperature and high pressure. The alloy produced has dense structure, isotropic effect, high material utilization rate, short production cycle, and can realize near net shape

"research shows that high entropy alloy coating has excellent application potential in wear resistance." Liu Bin introduced the fourth method of preparing high entropy alloy coating, including thermal spraying (atmospheric plasma spraying, supersonic flame spraying), plasma or laser cladding (applicable to composite materials), etc

in addition, the fifth method, additive manufacturing method, can also be used to form high entropy alloy powder, including selective laser melting method and laser powder feeding method. The solution is to clean the buffer valve (applicable to the manufacture of large components), electron beam powder bed method (applicable to the preparation of materials with high hot cracking tendency), etc

Liu Bin believes that powder metallurgy high entropy alloys show good application and development prospects in many fields. Among them, in the field of irradiation, high entropy alloys can be used to manufacture reactor cladding tubes, the first wall of fusion reactors, divertor and other materials; In marine, petrochemical, aerospace and other fields, high entropy alloys can be used to manufacture high-temperature, wear-resistant, corrosion-resistant and other high-performance coatings

at the same time, Liu Bin suggested that in the next step, based on the idea of material genetic engineering design, the high-throughput preparation and characterization technology of powder metallurgy can be used to realize the rapid screening of high entropy alloy components and shorten the research and development cycle of new alloys; Give full play to the advantages of powder metallurgy process flexibility and component supersaturation, and design heterostructure materials with particle strengthening and toughening composite structure, layered structure, gradient structure, etc; It can study the additive manufacturing technology of aerospace, biomedical and other complex high entropy alloy components and difficult to form high entropy alloy components, and promote the application and development of high entropy alloys

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