Preparation and Research Progress of High Entropy Alloys
At present, the research of high entropy alloys is mostly focused on the fields of bulk, powder, coating, film, etc., and the research in other fields is less and lacks a unified classification. In this paper, based on the current research progress of high entropy alloys, all kinds of high entropy alloys studied are divided, the principle of element selection is introduced, the preparation methods of high entropy alloys are summarized, the research institutions, research forms, research contents, etc. of high entropy alloys are reviewed, the application prospect of high entropy alloys is prospected, and it is proposed that the current research on high entropy alloys is less on mechanism, incomplete on performance, and unsystematic on thermal stability A series of scientific problems such as optimization of coating preparation process parameters, design of light high entropy alloy, expansion of research field, etc., and solutions are given, which have certain guiding significance for the expansion of research direction in the future application field of high entropy alloy.
The traditional alloy system is dominated by one metal element, and different types of alloys are formed by adding a small amount of other different elements. At present, aluminum alloys dominated by aluminum, magnesium alloys dominated by magnesium, and titanium alloys dominated by titanium have been developed and applied  . With the deepening of alloy research, some scholars have developed binary base intermetallic compound alloys or new amorphous alloys based on 1~2 metal elements  . However, they all adopt the traditional alloy design concept, and improve the performance by adding a specific small amount of alloy elements. Too many kinds of alloy elements will generate many compounds, especially brittle intermetallic compounds, which will increase the brittleness of the alloy and limit the development of the alloy in the multi-element direction. Therefore, the fewer the alloy elements, the better, and the fewer the elements will lead to the reduction of the required properties of the alloy. The good compatibility of the design of the alloy element types and proportions with the alloy properties is particularly important.
In 1995, Ye Junwei et al. [3-5] broke through the traditional concept of material design and put forward a new alloy design concept based on amorphous alloys, called High Entropy Alloys (HEAs). High entropy alloy is a kind of alloy with high mixing entropy, which is formed by alloying more than 5 elements in equal or near equal atomic ratio to form solid solution. Because of the high entropy effect in thermodynamics, lattice distortion effect in structure, hysteresis diffusion effect in dynamics, and cocktail effect in performance  , it is easy to obtain solid solution phase, nanostructure, and even amorphous structure with high thermal stability. High entropy alloys have excellent properties that traditional alloys cannot have at the same time, such as high strength, high hardness, high wear resistance, high oxidation resistance, and high corrosion resistance, It has become one of the three hotspots with the greatest development potential in recent years, and has high academic research value.
According to the current research progress of high entropy alloys, this paper divides the types of high entropy alloys, summarizes the selection principles and preparation methods of alloy elements, summarizes the research institutions, research forms and research contents of high entropy alloys, looks forward to the application prospects of high entropy alloys, puts forward the scientific problems of high entropy alloys, and gives the corresponding solutions to the scientific problems.
1. Classification of high entropy alloys
Table of Contents
- 1. Classification of high entropy alloys
- 2. Preparation of high entropy alloy
- 2.1 Design and selection of high entropy alloy composition
- 2.2 Preparation method of high entropy alloy
- 2.2.1 Preparation method of high entropy alloy block
- 2.2.2 Preparation method of high entropy alloy powder
- 2.2.3 Preparation method of high entropy alloy coating
- 2.2.4 Preparation method of high entropy alloy film
- 2.2.5 Preparation method of high entropy alloy foil
- 2.2.6 Preparation method of high entropy alloy matrix composite
- 3. Research status of high entropy alloy
- 4. Analysis of application prospects and research problems of high entropy alloys
- 5. Conclusion
1.1 Metallic high entropy alloy
According to the current research progress, the metals used to prepare metal like high entropy alloys mainly include Mg, Al in the third cycle; Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn in the fourth cycle; Zr, Nb, Mo, Sn of the fifth cycle; Hf, Ta, W, Pb in the sixth cycle, as well as metalloid elements Si, B, etc. According to the different characteristics of these elements, high entropy alloys with “cocktail” performance are formed according to different ratios, including light high entropy alloys, high entropy alloys of refractory metals, etc. According to the summary of current literature, the high entropy alloys studied and prepared are mainly AlCrFeCoNiCu system, but also VNbMoTaW system composed of refractory metals and high entropy alloys of other metal systems [7-10] . According to statistics, the addition frequency of different metals in all types of high entropy alloy systems studied at present is shown in Figure 1. It can be seen from Figure 1 that Al, Ti, Cr, Fe, Co, Ni, Cu and other elements are widely used in the research of high entropy alloys.
Fig.1 Addition frequency of various elements from all kinds of HEAs
1.2 Composite high entropy alloy
The introduction of fine dispersed hard ceramic phase can further enhance the mechanical properties of multipartite high entropy alloys. Common reinforcing phases are  : ceramic reinforcing phases TiC, TiB, TiB2, B4C; Intermetallic compounds TiAl, Ti3A1, Ti5Si3; Oxide Al2O3, R2O3 (R is rare earth element) and nitride AlN, TiN, etc.
Wang Yanping  prepared a TiC reinforced multi-component high entropy alloy matrix composite consisting of AlCrFeCoNiCu-10% (volume fraction, the same below) TiC and CrFeCoNiCuTi-10% TiC through the in-situ self generation method improved by “high-temperature self propagation melting casting”, which further improved the mechanical properties of the alloy. Sheng Hongfei  prepared TiC reinforced multi principal component composite Al0.5CoCrCuFeNi-y% TiC (y=5,10,15) by in-situ self generation method to discuss its microstructure and properties.
2. Preparation of high entropy alloy
2.1 Design and selection of high entropy alloy composition
The properties of the material are related to its structure, which depends on the thermodynamic entropy, enthalpy, electronegativity, atomic radius, etc. of the elements that make up the alloy. Therefore, alloy elements with high mixing entropy, similar electronegativity and small difference in atomic radius should be selected to promote the formation of solid solution. The general principle for the formation of alloy solid solution is Hume Other principle  , that is, there are at least five components to obtain high mixing entropy, the alloy mixing enthalpy is in the range of – 40~10kJ/mol, and the maximum atomic radius difference is less than 12%. With the progress of research, some scholars summarized and proposed the thermodynamic parameters Ω ≥ 1.1 and atomic radius parameters in the design of high entropy alloys δ ≤ 6.6%, electronegativity difference value Δχ And empirical criterion rules such as VEC requirements for valence electron concentration parameters that can be used to judge alloy phase structure [15, 16] .
Based on the idea of “required performance oriented design” of alloy, Hume Other principle or thermodynamic parameter Ω and atomic radius parameter can be followed δ、 Electronegativity difference value Δχ、 High entropy alloy elements are selected for empirical criteria calculation such as valence electron concentration parameter VEC. Common elements of different high entropy alloys are shown in Figure 1, mainly including Al, Ti, Cr, Fe, Co, Ni and Cu, sometimes supplemented by Si, B and other elements. The high entropy effect, lattice distortion effect, delayed diffusion effect and cocktail effect of each element atom are fully exerted to form a solid solution phase with simple crystal structure, so that the alloy has the required properties.
2.2 Preparation method of high entropy alloy
At present, there are many preparation methods of high entropy alloys. This paper classifies the preparation methods of high entropy alloys based on the idea of “studying the specimen morphology”. At present, the research samples of high entropy alloys mainly include bulk, powder, coating, film, foil and composite high entropy alloys. Figure 2 shows the classification of research and preparation methods of samples with different morphologies.
Fig.2 Preparation technology of HEAs
2.2.1 Preparation method of high entropy alloy block
The preparation methods of bulk high entropy alloy mainly include vacuum melting method  and powder metallurgy method  . Vacuum melting is the most widely used method for preparing high entropy alloys. In [4,5] , Ye Junwei et al. prepared AlCrCoNiCu bulk high entropy alloy by the method of “vacuum arc melting+copper mold casting”. The powder metallurgy method has the characteristics of low temperature sintering, avoiding segregation, and higher material utilization than the traditional casting method. Qiu Xingwu et al.  prepared CrFeNiCuMoCo high entropy alloy by powder metallurgy and studied its structure and properties.
2.2.2 Preparation method of high entropy alloy powder
The preparation method of high entropy alloy powder is mainly mechanical alloying  , which is easy to obtain nanocrystalline or amorphous particles with uniform structure and component distribution. Varlakshmi et al. [21,22] of Madras Institute of Technology in India prepared CuNiCoZnAlTi, AlFeTiCrZnCu series high entropy alloys by this method, and studied their structures and properties. Wei Ting et al.  prepared AlFeCrCoNi high entropy alloy by this method and studied its properties at different annealing temperatures.
2.2.3 Preparation method of high entropy alloy coating
At present, the preparation methods of high entropy alloy coating mainly include laser cladding, thermal spraying and cold spraying. Qiu Xingwu et al.  prepared Al2CrFeCoxCuNiTi system high entropy alloy coating on Q235 steel substrate by laser cladding to study its structure and performance; Liang Xiubing et al.  prepared FeCrNiCoCu (B) coating on Mg substrate by thermal spraying to analyze its structure and performance; Zhu Sheng et al.  prepared AlCrFeCoNi system high entropy alloy coating on Mg substrate by cold spraying to analyze its structure and performance.
2.2.4 Preparation method of high entropy alloy film
The preparation methods of high entropy alloy films mainly include magnetron sputtering, plasma based ion implantation and electrochemical deposition. Dolique et al.  of the University of Orleans in France prepared AlCoCrCuFeNi films by DC magnetron sputtering, and analyzed their structures and properties. Feng Xingguo  prepared (TaNbTiW) N nitride films by multi target magnetron sputtering and plasma based nitrogen injection, and Yao Chenzhong et al.  prepared NdFeCoNiMn amorphous nanocrystalline high entropy alloy films by electrochemical deposition and carried out relevant research.
2.2.5 Preparation method of high entropy alloy foil
At present, there is little research on the application of high entropy alloys in the welding field. According to the literature, Xu Jinfeng’s research group  of Xi’an University of Technology prepared TiFeCuNiAl and other systems of high entropy alloy foils on the surface of copper rolls by single roll rapid solidification method for welding or transition welding of titanium/steel weld joints.
2.2.6 Preparation method of high entropy alloy matrix composite
The preparation of metal type high entropy alloys can be divided into the above five categories, and the common preparation methods of high entropy alloy matrix composites are mainly self propagating high-temperature synthesis (SHS)  . Li Bangsheng’s research group is committed to the research of high entropy alloy matrix composites. In the literature  , Wang Yanping, his doctoral student, prepared AlCrFeCoNiCu-10% TiC, CrFeCoNiCuTi-10% TiC high entropy alloy matrix composites using the “SHS+melting casting” method, The structure and properties of the composite were studied.
The principles and characteristics of the above preparation methods are shown in Table 1.
Table.1 Classification, principle and characteristic of HEAs based on prepared samples’ shape
|Samples’ shape||Preparation method||Principle|
|Bulk||Vacuum melting||Use high temperature by the arc discharge generated between electrode and crucible as the heat source to melt metal in the vacuum, then condensing and forming in the crucible|
|Powder metallurgy||Use metal or non-metal mixed powder as raw material, then press-forming and sinter-ing to prepare products|
|Powder||Mechanical alloying||Powder particles impact with grinding ball in the high energy ball mill or grinding machine after a long period, produce cold welding and breaking repeatedly, then cause the atom of powder particles diffusing|
|Coating||Laser cladding||Use rapid solidification process produced by laser beam and fuse the raw materials in the surface of substrate|
|Thermal spraying||Heat materials to melt or semi-molten state, then spray them to the substrate’s surface|
|Cold spraying||Use high pressure gas to accelerate powder particles and impact substrate fastly to produce plastic deformation and accumulate step by step, finally form dense coating|
|Film||Magnetron sputtering||Use ions to impact the deposited material’s surface and sputter particles, then form a film on the substrate’s surface|
|Plasma based ion implantation||Based on magnetron sputtering, produce plasma through radio and microwave and make them accelerate in the electric field, finally inject to the surface of solid materials|
|Electrochemical deposition||Metal ions in the electrolyte are restored and deposited at the cathode under outer voltage|
|Foil||Single-roll rapid solidification||Heat and re-melt the master alloy block, then rush out of the nozzle and jet to the surface of copper roller which rotates fastly|
|Composite materials||Self-propogatinghigh temperaturesynthesis||Mix reinforced particles and raw material powder in certain proportion and press-forming, then light them in vacuum to produce exothermic chemical reaction in order to generate reinforced phase|
3. Research status of high entropy alloy
3.1 Research institutions
Many research institutions at home and abroad have carried out research work related to high entropy alloys. Taiwan [3-5] , China, has studied high entropy alloys earlier, and its research in the field of high entropy alloys is at the international leading level. After the concept of “high entropy alloy” was put forward, many units in mainland China began to study high entropy alloy. Professor Jiang Qing of Jilin University was the first to study high entropy alloy  . In addition, Tsinghua University, Beijing University of Science and Technology, Beijing University of Technology, Northwest University of Technology, Harbin University of Technology, Southeast University, Chongqing University, Sun Yat sen University, Guilin University of Electronic Science and Technology, Guangxi University Scholars from many scientific research schools and units, such as the Armored Corps Engineering College and the Beijing General Institute of Nonferrous Metals Research, have conducted in-depth research on the structure and properties of high entropy alloys and have made certain achievements.
The Air Force Laboratory of Wright Patterson Air Force Base [33, 34] developed W-Nb Mo Ta, W-Nb Mo Ta-V, Ta Nb Hf Zr Ti series and other high temperature resistant high entropy alloys for high temperature load-bearing components and thermal insulation systems in the aerospace field. The University of Tennessee and Oak Ridge National Laboratory  prepared CoCrFeMnNi based high entropy series alloys; High entropy AlCoCrCuFeNi alloys were prepared by Helmholtz Center in Berlin, Germany. The phase structure and element distribution of high entropy alloys prepared by splash quenching and conventional crucible melting casting were compared  ; India Madras Institute of Technology prepared AlFeTiCrZnCu and CuNiCoZnAlTi nanostructured high entropy alloy powders by mechanical alloying method for related research [21, 22] ; AlCoCrCuFeNi thin films were mainly prepared by magnetron sputtering at the University of Orleans in France for research on high entropy alloys  .
3.2 Research methods
At present, most of the research on high entropy alloys is to prepare high entropy alloys or their composite material blocks, powders, coatings, films, etc. through different preparation methods for analysis and research, which can be summarized in three aspects:
(1) Within the changeable range, the microstructure and properties of high entropy alloys under different conditions can be analyzed and compared by changing the content of one or several elements. For example, in literature  , Liu Yuan analyzed the effect of changing the Al content on the properties of AlxCoCrCuFeNi high entropy alloys, and Xie Hongbo et al.  studied the effect of different Zr content on the microstructure and corrosion properties of AlFeCrCuZrx high entropy alloys.
(2) Some elements are added to analyze the effects of elements on the properties of high entropy alloys. For example, Li Rui  analyzed and compared the effects of different Mn and Mg contents on the properties of Mgx (MnAlZnCu) 100-x by adding Mn and Mg, and Xie Hongbo et al.  analyzed the effects of adding Al on the microstructure and friction properties of AlxFeCrCoCuV high entropy alloys.
(3) Change the process parameters or cooling rate to study the influence of different process parameters or cooling rates on the properties of high entropy alloy or optimize the properties of high entropy alloy through heat treatment, rolling or other mechanical treatment methods. For example, Qiu Xingwu et al.  studied the influence of different process parameters of laser cladding on the properties of Al2CoCrCuFeNiTi high entropy alloy coating by changing the laser power, scanning rate and spot size, Ma et al.  analyzed the effect of different cooling rates on the microstructure and mechanical properties of AlxSi0.2CrFeCoNiCu1-x high entropy alloy, and Wang Chong et al.  studied the effect of cold rolling on the microstructure and mechanical properties of Al10Cu25Co20Fe20Ni25 high entropy alloy.
3.3 Research contents
The research on high entropy alloy mainly focuses on the modeling and simulation of theoretical research (mainly material calculation) and the research on phase structure and microstructure morphology and properties in practical research.
3.3.1 High entropy alloy calculation and simulation modeling
The simulation of high entropy alloy plays an important role in the design, phase structure and performance prediction of high entropy alloy, which can provide a basis for experimental testing. At present, the calculation and simulation methods for high entropy alloys mainly include Density Functional Theory (DFT), Ab Initial Thermodynamics (AITD), Ab Initial Molecular Dynamics (AIMD), New PHACOMP, Calculation of Phase Diagram, CALPHAD, etc. For example, Zhang et al.  studied the adhesion and elastic properties of AlxCoCrCuFeNi system high entropy alloy by DFT method; Ma et al.  studied the thermodynamic properties and phase stability of CoCrFeMnNi system high entropy alloys by using Ab Initio Thermodynamics method, discussed the proportion of the influence of electron entropy, vibration entropy and magnetic entropy on the phase stability of high entropy alloys, and verified it through experiments; Gao et al.  predicted the structure and properties of AlxCoCrCuFeNi system high entropy alloy by using AIMD method; Guo et al; Zhang et al.  used CALPHAD method to enrich the thermodynamic data of high entropy Al Co Cr Fe Ni alloy system, and studied the effect of Al content on the phase stability of AlxCoCrFeNi system.
Table.2 shows the comparison of calculation and simulation modeling methods for the above types of high entropy alloys.
Table.2 Comparison of HEAs’ calculation and modeling method
|DFT||Research the multiparticle system’s ground-state properties using the distribution of electronic density as basic variables||
➢Most desirable and basic technique to tackle HEAs
➢Calculate the instantaneous forces acting on atoms
➢Provide computing framework for ab initio
|AITD||Deal with thermodynamics problems of alloys based on ab initio||
➢Predict behavior and thermodynamic properties of HEAs
➢Require much numerical computation
|AIMD||Begin simulation and calculation of alloys combining DFT with MD||
➢Predict dynamic, structural, diffusion constants at finite temperatures
➢Complex program and large calculation
|New PHACOMP||Use the d-electron concept to define the phase boundaries in terms of Md (metal d-level)||➢Simple calculation and prediction for phase diagrams|
|CALPHAD||Based on equipment data, select proper thermodynamic model and then choose algorithm to optimize thermodynamic parameter and calculate phase diagram||
➢Deal with phase equilibrium/diagram
➢Enrich thermodynamic database representing phase diagram and thermodynamic properties
3.3.2 Research on phase diagram, microstructure and properties of high entropy alloy
High entropy alloys have simple solid solution phase structure, which is generally fcc, bcc, hcp or a mixture of them. When observing the microstructure morphology, metallography, SEM and other methods are generally used to analyze the microstructure morphology. At the same time, three-dimensional probes or EDS are used to analyze the local element distribution of the microstructure.
In terms of performance research, it mainly focuses on mechanical properties, thermal stability, corrosion resistance, magnetic properties, etc.
(1) Mechanical properties
Mechanical properties include compressive properties, hardness and tensile properties. Compressibility test generally applies axial pressure to the sample to measure its strength and plasticity, and then draws the stress-strain curve to analyze the alloy’s compressibility, and sometimes analyzes the compression morphology; Hardness is an important indicator of mechanical properties of materials. Microhardness tester can be used to test the hardness of alloys. Tensile properties refer to tensile tests conducted according to national standards or non national standards, and then the tensile mechanical properties of alloys can be measured. For example, Wang Yanping  studied the effects of Mn, Ti, V on the compressive strength, plasticity and hardness. The results show that V can improve the yield strength, hardness and damping properties of the alloy. Ti can improve the hardness of the alloy, but reduce the plasticity of the alloy. The strength, hardness and plasticity of the alloy are reduced when Mn is added alone, and the alloy with Mn, Ti, V added at the same time has the highest strength; Dong et al.  prepared AlCrFe2Ni2 high entropy alloy and studied the tensile properties of the alloy at room temperature. The results show that the yield strength at room temperature of the alloy is 796MPa, the tensile strength is 1437MPa, and the elongation is 15.7%. The mechanical tensile properties of the alloy are excellent; Li et al.  put forward the design idea of “metastable dual phase high entropy alloy”, regulated and prepared as cast high entropy alloy Fe50Mn30Co10Cr10 with stronger, more ductile and more ductile mixture of fcc and hcp phase structures. The engineering strain tensile strength of the alloy is 900MPa, and the ductility is 60% higher than that of high-strength steel, realizing the integration of high strength and high toughness.
(2) Thermal stability
The research on thermal stability of high entropy alloy mainly refers to its ability to resist high temperature oxidation, which is mainly analyzed by measuring oxidation kinetics curve, XRD of oxide layer, surface morphology of oxide film, cross-section morphology of oxide film, etc. For example, Hong Lihua et al.  analyzed the oxidation resistance of Al0.5CrCoFeNi at different annealing temperatures; Zhang Hua et al.  studied the high-temperature oxidation resistance of three high entropy alloys Al0.5FeCoCrNi, Al0.5FeCoCrNiSi0.2, Al0.5FeCoCrNiTi0.5 at 900 ℃; Xie Hongbo et al.  studied the effect of Mn, V, Mo, Ti, Zr elements on the oxidation resistance of AlFeCrCoCux system high entropy alloy at different temperatures.
(3) Corrosion resistance
The research on corrosion resistance of high entropy alloys is relatively common, and almost every paper on the research of high entropy alloys has a research on corrosion resistance. The corrosion resistance of high entropy alloy can be studied through two aspects: ordinary immersion corrosion and electrochemical corrosion, by drawing corrosion kinetics curve (weight-loss method, depth method), potentiodynamic polarization curve, corrosion surface morphology analysis, corrosion product composition analysis and other methods. For example, Li Wei et al.  studied the electrochemical corrosion capacity of AlFeCuCoNiCrTix and compared it with 304 stainless steel. The results show that the corrosion rate of the alloy is low in 0.5mol/L H2SO4 solution; In 1mol/L NaCl solution, the corrosion rate of this alloy is equivalent to that of 304 stainless steel, but its pitting resistance is better than that of 304 stainless steel. Hong Lihua et al.  drew the Al0.5CoCrFeNi corrosion kinetics curve and analyzed the corrosion products, corrosion surface morphology and corrosion section morphology, and studied the high temperature corrosion resistance of high entropy alloy in 75% Na2SO4+25% NaCl solution at 800900 ℃; Dai Yi et al.  studied the electrochemical corrosion behavior of AlMgZnSnCuMnNix, and studied the influence of different Ni content on the corrosion resistance of AlMgZnSnCuMnNix by comparing the electrochemical corrosion potential.
(4) Magnetic properties
The room temperature magnetization curve and hysteresis loop can be measured by the physical property testing system for the magnetic properties of high entropy alloys, and then the magnetic behavior of the alloys can be analyzed. For example, Liu Liang  , a doctoral student of Professor Jiang Qing from Jilin University, studied the magnetic properties of FeNiCuMnTiSnx high entropy alloy. The results show that when x=0, the alloy is paramagnetic. With the increase of Sn content, the magnetic properties of the alloy also change from the initial paramagnetism to soft magnetism.
In addition to the above contents, the current research on high entropy alloys also includes the rules of grain growth, the influence principle of enthalpy and entropy on the formation of high entropy alloys, the research on the hydrophobicity of high entropy alloys, and other contents. For example, Liu et al.  studied the rules of grain growth of FeCoNiCrMn high entropy alloys; Otto et al.  studied the relationship between entropy and enthalpy on the phase stability of high entropy alloys by selecting one element with comparable crystal structure, size and electronegativity to replace another element; Dolique et al.  studied the wettability of AlCoCrCuFeNi high entropy alloy film with water. The results showed that the film with fcc, bcc structure has superhydrophobic effect and has the same value as the polymer PTFE, which makes it very promising that the high entropy alloy will replace the polymer PTFE in the future.
4. Analysis of application prospects and research problems of high entropy alloys
4.1 Prospect analysis of high entropy alloy application field
High entropy alloys play an important role in the research of new materials. High entropy alloys are widely used in industrial fields because of their high strength, high hardness, high wear resistance or high temperature softening resistance. Examples of current applications include: tools for high-speed cutting, various tools, molds, golf balls, fire-resistant frameworks of super tall buildings, corrosion-resistant high-strength materials for chemical engineering, aeroengines, ships, turbine blades, electronic devices, communications and other fields.
At present, the research on high entropy alloy is mainly from the aspects of specimen morphology, preparation technology, calculation simulation, microstructure, performance analysis, etc., and the performance research method is mainly from the three aspects described in Section 3.2. According to the summary of reading materials, the possible application and development fields of high entropy alloy in the future are:
- (1) The high entropy alloy is used as the transition layer of metal welding or direct welding solid forming, and is applied in the welding field as the welding material.
- (2) As a high temperature resistant and wear-resistant coating, it has the advantages of good thermal stability and wear resistance. At the same time, because it can show the characteristics of many elements, it can be used in biomedicine as a high wear-resistant coating.
- (3) As a corrosion-resistant material, it is used in deep-sea and other fields with high requirements for metal corrosion resistance, and its advantages of good corrosion resistance are brought into play.
- (4) Due to the low density and high strength of light weight high entropy alloy, it can be considered that light weight high entropy alloy can be used as repair materials or functional structural materials for aerospace and naval equipment.
4.2 Scientific problems of high entropy alloy research
Although many scholars have studied the calculation simulation, preparation process, microstructure and properties of high entropy alloys, there is still no scientific theoretical system for the research of high entropy alloys, and there are still many problems to be solved:
- (1) The research on its mechanism is still few. For example, the law of phase formation and transformation of high entropy alloys, the effect of atomic parameters such as mixing entropy, mixing enthalpy and atomic size on the formation of alloy solid solution. Professor Ye Junwei of Taiwan proposed that high entropy alloys are easy to form solid solutions because of their high mixing entropy, but atomic parameters such as mixing enthalpy and atomic radius will also play a role, and process conditions will also affect the formation and stability of phases.
- (2) The research on the properties of high entropy alloy is only limited to some conventional properties, such as hardness, wear resistance, corrosion resistance, and other properties, such as its creep properties at higher temperatures, such as thermal fatigue and combustion resistance, and the data are few.
- (3) There is no systematic study on the thermal stability of high entropy alloys, and there is no clear theoretical system on how to determine the thermal stability of different kinds of high entropy alloys. The long-term stability of different kinds of high entropy alloys at high temperatures is not clearly defined.
- (4) The process parameters of high entropy alloy coating are still in the experimental stage and have not yet been applied in practice. For example, the optimal selection of process parameters such as spot size, laser power, scanning rate and so on in the preparation of high entropy alloy by laser cladding requires continuous experimental testing to select process parameters. The solution of this problem can greatly promote the practical industrial application of high entropy alloy coating.
- (5) Design of high entropy alloys with specific properties. High entropy alloy also needs light weight. How to design the element and composition ratio reasonably and how to obtain high entropy alloy with superior performance need further research.
- (6) Research field of high entropy alloy. At present, the research on high entropy alloy mainly focuses on the preparation of high entropy alloy coating and film by different technologies or the preparation of high entropy alloy block sample separately to carry out its microstructure observation and performance analysis. The research on the expansion and application of high entropy alloy in other fields such as welding is less.
4.3 Solutions to scientific problems in high entropy alloy research
- (1) As the phase diagram determines the thermodynamic equilibrium and performance characteristics of materials, for the scientific research of high entropy alloys (1), (2), (3), it is necessary to study the phase formation and its laws of high entropy alloys. At present, there are many kinds of high entropy alloys studied, but few in-depth studies have been carried out on their corresponding phase diagrams. We can use the material genome idea  to combine experimental data and CALPHAD technology to summarize and divide the phase diagram genome of high entropy alloys, develop the thermodynamic database of high entropy alloys and continue to optimize and expand it, providing a basis for the mechanism and performance research of various high entropy alloys.
- (2) The “orthogonal experiment” or “artificial intelligence algorithm” is used to optimize or predict the process parameters of the high entropy alloy coating, and then the high entropy alloy coating with good performance is obtained in the experiment.
- (3) According to the idea of “low density, light weight and simple phase structure”, light weight and high entropy alloy with superior performance is further developed. For example, Khaled et al.  developed a single-phase low-density, high-strength and excellent high entropy alloy Al20Li20Mg10Sc20Ti30 that is “as light as aluminum and as strong as titanium”, and Zhang Yong et al.  developed a high-strength AlLiMgZn (Cu, Sn) system lightweight high entropy alloy and conducted related research.
- (4) Expand the research of high entropy alloy in welding and other related fields. It can further diversify the preparation forms of high entropy alloys, develop high entropy alloy welding wires, and provide a basis for the industrial application of high entropy alloys.
High entropy alloys provide the possibility for a large number of research and production of various high-performance alloys. Because of their high strength, high hardness, high wear resistance, high oxidation resistance, high corrosion resistance and other characteristics, their research in the field of materials science has high academic research and application value. Together with large amorphous and composite materials, they are called the three hot spots with the most development potential in the coming decades. Although some achievements have been made in the research of high entropy alloys, there are still some scientific problems to be solved. Further breakthroughs in these problems will be of great significance to the expansion of research contents and application fields of high entropy alloys.
Authors: Chen Yongxing, Zhu Sheng, Wang Xiaoming, Du Wenbo, Zhang Yao
Source: China High Entropy Alloy Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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