The current high entropy alloys' studies are most in block, powder, coating, film and other areas. There are few studies of high entropy alloys in other areas and they are lack of unified classification. According to the current high entropy alloys' research situation, The paper has focused on the classification on all kinds of high entropy alloys having been researched, introduced the selecting principle of elements, summarized the preparation methods, reviewed the research institutions, research methods and research contents of high entropy alloys, prospected the application prospect of high entropy alloys, put forward a series of scientific problems of high entropy alloys, including less research on mechanism, incomplete performance research, unsystematic thermal stability study, preparation process parameters to be optimized, lightweight high entropy alloys' design, the expansion on the research field, etc, and the solutions have been given. Those have certain guiding significance for the expansion of the application of high entropy alloys subjects in the future research direction.

The refractory high-entropy alloys (RHEAs) usually form a multi-principal elements alloy with equal atomic ratio or near equal atomic ratio via adding a variety of high melting point elements, showing simple phase composition and excellent high temperature properties, and processing a broad application prospect in the field of superalloy. Based on the performance characteristics and preparation process of RHEAs, and from the perspective of the current situation and challenges in fabrication and forming, the property tuning methods and its research progress of RHEAs were summarized, as well as the achieved breakthrough and the facing dilemma of the additive manufactured RHEAs. A prospection on the composition design and optimization, material preparation and processing, and additive manufactured forming of RHEAs was also proposed.The following suggestions are put forward for the key research trend of RHEAs in the future: tuning phase composition and phases interface to overcome the strength-ductility trade-off of RHEAs, designing alloys by combining the mature traditional strengthening and toughening theory with the properties of RHEAs, modifying the formability and properties of RHEAs by drawing support from the processing characteristics of additive manufacturing technology, and investigating the servicing performance and failure mechanism in high temperature or multi-field coupling condition of RHEAs.

High-entropy alloy(HEA) coatings with high thermal stability and high temperature resistance of HEA coatings show a new attraction in the field of high temperature coating. The method of preparing HEA coating(HEAc) by laser cladding technology is one of the best preparation methods which can achieve superior performance. The latest research results of HEAc prepared by laser cladding technology were summarized from the view of component design, microstructure, annealing process and properties, high temperature oxidation resistance and other properties. The problems existing in the preparation of HEAc by laser cladding technology were analyzed. It was put forward that the scientific research system should be perfected from the aspects of component design, basic theory, performance law and processing technology in the future. The high-entropy alloy coatings with excellent performance are expected to be prepared.

The Al_0.1CoCrFeNi high-entropy alloy (HEA) was melted by vacuum magnetic levitation, and quasi-static tensile experiments were performed by using an INSTRON mechanical testing system. The crystal structure, surface morphology, composition, microstructure, hardness, and creep behavior of the samples before and after the experiment were analyzed by X-ray diffraction, optical microscopy, scanning electron microscopy, transmission electron microscopy, and nanoidentation. Results reveal that after tensile deformation, the alloy has an excellent strength-ductility combination, a significant strain-hardening effect, and an improved creep resistance. The fracture mode of sample is the typical microvoid accumulation fracture; there are a lot of microbands (the band width is about 200-300nm) inside the grains. The excellent strain-hardening ability is believed to be originated from the microband-induced plasticity effect during tensile loading.

With the improvement of alloy manufacturing level and the complexity of performance requirements, high-entropy alloys (HEAs) have gradually attracted great attention.At present, the research in the field of material processing mainly focuses on brazing and surface engineering.In the field of brazing, HEAs can be used as filler material for brazing at high temperature and low temperature, the empirical parameters related to high entropy were summarized. The application of the simulation and calculation methods such as first-principle method and calculation of phase diagram were described in the field of HEAs design for filler metals development. The latest research progress of HEAs fillers for brazing of nickel-based superalloys and dissimilar ceramics-metals, as well as low temperature packaging was introduced. The influence of welding process parameters on microstructure and properties of HEAs brazing joints was also analysed.In the field of surface engineering, the application direction and preparation methods of HEAs in film/coating were discussed. The research progress in high-temperature protective coating, hard protective layer and other application directions was summarized. At the same time, the problems existing in the research and application of HEAs in the fields of brazing and surface engineering were summarized. The future trends were put forward in order to decrease the melting temperature of HEAs filler, improve high temperature mechanical properties of welds, and develop the eutectic HEAs filler/coating.

Orthogonal test was carried out with ball-on-disc friction and wear tester in dry, deionized water, and simulated rain water under three loads (5, 10, 15N), respectively. The behavior of two materials at different loads under different environment was compared, the friction and wear properties of the alloys under the simulated service situation was explored. The composition of the samples was examined by X-ray diffraction (XRD). The contour of wear scars was detected by a three-dimensional surface profiler based on scanning white light interferometry. Optical electron microscope is used to observe the structure. The morphology of the worn surfaces were observed by scanning electron microscopy (SEM) and the wear mechanism was analyzed. Results show that due to the increase of Al, the body-centered cubic phase (bcc) substitutes the face-centered cubic (fcc) which attributed to the high hardness of Al_1.3CrCuFeNi₂ leading to good wear property. In dry condition, the wear mechanism are oxidation, adhesion, plastic deformation, and mild abrasive wear while in liquid, the abrasive is dominated along with oxidation and slight adhesive behavior.

FeCrNiCoCuAl_x (x=0, 1, 2, 3) high-entropy alloy (HEA) coatings were prepared on Q345 steel by laser cladding. The effects of Al content on microstructure and properties of FeCrNiCoCuAl_x HEA coatings were studied by XRD, SEM and erosion wear tests. The results show that the HEA coatings are mainly composed by simple FCC and BCC solid-solution phases. With the increase of Al addition, the microstructure is evolved gradually from FCC to BCC. And the hardness of the HEA coatings improves significantly and the maximum value is 580HV. In 3.5%NaCl solution, the corrosion current density of FeCrNiCoCuAl_x HEA coatings decreases firstly and then increases with the increase of Al addition. And the HEA coating has the best corrosion resistance at x=1. Meanwhile, the mass loss rate of HEA coating decreases in the erosion test when the impact angle is changed from 90° to 30°, exhibiting an erosion characteristic of ductile materials. The slurry erosion properties of the HEA coating increase with the increase of Al content, and the erosion wear mechanism is evolved from forging extrusion to ploughing and micro-cutting.

The equal molar AlCoCrFeNi high entropy alloys with 5% (volume fraction) TiB₂ particles reinforcement were fabricated by spark plasma sintering (SPS). Effect of SPS temperature and pressure on microstructure evolution and mechanical properties of TiB₂/AlCoCrFeNi composites was studied using X-raydiffraction (XRD), density testing, scanning electron microscopy (SEM) and mechanical properites testing. The results show that increasing SPS temperature and pressure can improve hardness and compressive strength of TiB₂/AlCoCrFeNi composites. The relative density, compressive strength, yield strength and hardness of the TiB₂/AlCoCrFeNi composite after sintering at 1200℃ and 30MPa are 99.6%, 2416MPa, 1474MPa and 470HB, respectively. During spark plasma sintering, phase transformation occurs in the high entropy alloy matrix of the composite. The composite after sintering at 1200℃ and 30-45MPa is composed of phases BCC, B2, FCC, TiB₂ and σ.

Based on different high-entropy alloys (HEAs) systems, the latest research progress in additive manufactured high-entropy alloys was reviewed. The rapid solidification microstructure, segregation and precipitation behaviors of high-entropy alloys fabricated by additive manufacturing with different compositions were described. Especially, the analysis was focused on the mechanical properties, deformation and strengthening mechanisms. It was pointed out that the appropriate additive manufacturing process should be selected for different high-entropy alloy systems, and the influencing factors of forming quality need to be further studied. Finally, it was proposed that high-entropy alloys with both excellent strength and high plasticity can be developed and prepared by additive manufacturing technology.

In recent years, the sudden rise of high entropy alloys (HEAs) has become a hot research topic in the field of metal materials. The high entropy alloy is located in the central region of phase diagram, which has broad alloy composition space and possible formation of microstructure. The synergistic regulation of composition and preparation process can obtain richer structure. Unconventional chemical structure is expected to break through the performance limit of traditional anti-wear and lubricating alloys. In this work, the classification of wear-resistant HEAs was discussed. The effects of the addition of chemically active metals, soft metals and refractory metals on the wear resistance and lubrication properties of HEAs were analyzed. The effects of non-metallic elements and ceramic phases on the tribological properties of HEAs matrix composites were summarized. The effects of heat treatment and surface engineering technology on the surface microstructure and tribological behavior of HEAs were reviewed. The design method of HEAs with anti-wear lubrication under severe working conditions was discussed. The future research and application of HEAs in the field of friction and wear were prospected. High entropy alloys have great potential to solve the bottleneck problems of traditional alloys, such as to realize stable lubrication and anti-wear under extreme working conditions and to ensure anti-wear under specific functions.

The meaning and characteristics of refractory high entropy alloys were briefly described, and the preparation methods of various refractory high entropy alloys (bulk, film and coating) were summarized.The comprehensive properties of refractory high entropy alloys were emphatically expounded. It was suggested that the composition design should be optimized by constructing a special database of refractory high entropy alloys, and the manufacturability of different preparation methods should be focused on. In view of the shortcomings of high room temperature brittleness, high density and high cost of refractory high entropy alloys at present, different preparation methods could be selected according to the properties of refractory high entropy alloys for future industrial application.

AlCoCrCuFe high-entropy alloy was fabricated by melting-casting method. The phase structure, microstructure and friction and wear property for this alloy without and with CeO₂ doping were investigated by XRD, SEM, EDS, microhardness tester and friction-wear tester, respectively. The results show that AlCoCrCuFe alloy has BCC+FCC dual phase structure. 1%(mass fraction)CeO₂ addition improves the diffraction peak of AlCoCrCuFe alloy. The microstructure of two above alloys is typical dendrite structure. The interdendrite region is mainly Cu-rich and Ce-rich area, and the dendrite microstructure is layered grid structure of spinodal decomposition. After CeO₂ adding, the microhardness increases from 441.5HV to 475.3HV, and the friction coefficient and the mass loss rate decrease from 0.55 and 1.44% to 0.4 and 1.28%, respectively.

The AlCoCuFeMnNi high-entropy alloy cladding layer with CeO₂ was prepared by plasma cladding technology on 45 steel. The microstructure and phase composition of the cladding layer were investigated by XRD, SEM and EDS, and its microhardness and wear property were also tested. The results show that the phase structure of the cladding layer is mainly composed of BCC dendrites and FCC interdendrites. Thermodynamic calculation shows a small amount of AlCoNi phase exists in the cladding layer without CeO₂, and a large number of Fe-rich precipitated particles in the dendrites is observed, the hardness value exhibits gradient changes from 260HV_0.2 to 420HV_0.2, and their friction coefficient is between 0.16 and 0.57. After adding 1%(mass fraction) CeO₂, the Fe diffusion decreases into cladding layer, and a 32μm Fe-rich peritectic transition layer is formed on the bottom of the cladding layer. The average hardness value is about 400HV_0.2, and their friction coefficient is relatively stable (0.28-0.31). The mass loss of the layer with CeO₂ is 74.4% of that without CeO₂. The grain refinement strengthening is the main reason of the improvement of wear properties.

TiC-CoCrFeNi composite was fabricated by mechanical alloying and consequently vacuum hot pressing sintering, and the effects of milling time on the microstructure and mechanical properties of the composite was investigated. The results show that a single-phase solid solution with fcc structure is obtained after milled for 10h of Co, Cr, Fe and Ni powders. TiC and Cr₇C₃ structured carbides are formed and dispersed in the CoCrFeNi solid solution after hot pressing sintered at 1200℃ for 1h. Milling time has a significant effect on the size and amount of TiC and Cr₇C₃ structured carbides, which can affect the mechanical properties of the composite. When the milling time reaches 10h, the hardness and yield strength of the composite reach the maximum values of 671HV and 1440MPa, respectively, which is probably attributed to the dramatically increasing of nano-sized TiC in sintered bodies.

High entropy alloys have been proposed in 2004, which are expected to be widely used in aerospace, petrochemical and other fields due to their excellent properties compared with the traditional alloys, and have become a hot spot in current metal material research. It has become one of the methods to improve the comprehensive properties of high entropy alloys by introducing suitable reinforcement phase into the high entropy alloy matrix, and to form high entropy alloy matrix composites (HEAMCs). In this review, according to the current research status in HEAMCs at home and abroad in the past few years, the reinforcement phase selection, preparation method, phase structure, microstructure and strengthening mechanism of HEAMCs were systematically introduced, and the evolution of properties of HEAMCs were summarized, including strength and plasticity, hardness, wear resistance and corrosion resistance. Finally, the challenges to HEAMCs were discussed and future research directions in HEAMCs were suggested.The wettability between the reinforcement phase and the matrix seriously affects the preparation and performance of large-scale composites, and finding an efficient and simple method to prepare large-scale composites is a problem that needs to be solved in high-entropy alloy matrix composites; reinforcing particles will lead to a decrease in plasticity, and the balance between strength and plasticity of metal matrix composites also needs to be studied.

The friction and wear of mechanical parts mainly occurs on the surface of the material, and about 80% of the failures of parts are caused by surface wear.Friction and wear increase the loss of material and energy, and reduce the reliability and safety.Using laser cladding technology to prepare a high entropy alloy coating on the surface of the substrate can achieve a good metallurgical combination between the coating and the substrate, so as to achieve the purpose of improving surface wear resistance.The main factors affecting the wear resistance of the high entropy alloy coating are the mechanical and physical properties of the coating material (such as hardness, plasticity and toughness), defects generated during the cladding process (such as surface roughness, pores and cracks), friction conditions (such as high temperature environment and corrosive environment).In this paper, the influencing factors and strengthening mechanism of laser cladding high entropy alloy coatings were reviewed and summarized.First of all, the influence of laser process parameters (such as laser power, laser scanning speed, spot diameter) and post-treatment processes (such as heat treatment and rolling) on the quality and performance of the coating were explained.Secondly, the influence of component element selection, high temperature environment and corrosive environment on the wear resistance of the coating was described.Finally, the problems existing in the preparation of high entropy alloy coatings by laser cladding technology were analyzed, and the future development trends were forecasted, such as developing new materials based on far-equilibrium material design theory, using electric field-magnetic field synergy or laser-ultrasonic vibration composite technology to improve the wear resistance of coatings, etc.

Due to excellent comprehensive properties such as high strength, high hardness, and excellent high-temperature oxidation resistance, the refractory high-entropy alloys have broad application prospects and research value in the fields of aerospace and nuclear energy. However, the refractory high-entropy alloys have very complex composition features, making it difficult to perform alloy design. It seriously restricts the development of high-performance refractory high-entropy alloys. In recent years, the machine learning technique has been gradually applied to various high-performance alloys with efficient and accurate modeling and prediction capability. In this review, there was a comprehensive summary of research achievements on machine learning-driven design of refractory high-entropy alloys. A detailed review on the applications and progress of machine learning technique in different aspects was given, including alloy phase structure design, mechanical property prediction, strengthening mechanism analysis and acceleration of atomic simulations. Finally, the currently existing problems in this direction were summarized. The prospect about promoting the design of high-performance refractory high-entropy alloys was presented, including development of high-quality database for refractory high-entropy alloys, establishment of quantitative relation of "composition-process-structure-property" and achievement of multi-objective optimization of high-performance refractory high-entropy alloys.

AlCoCrFeNi_2.1 eutectic high-entropy alloy is characterized by a fine, homogeneous, and regular lamellar structure, as well as good organizational structure and mechanical properties with both strength and plasticity over a wide range of temperature (70-1000 K) and compositional deviation, thus making it the most widely studied eutectic high-entropy alloy at present. In this paper, regarding the additive manufacturing of AlCoCrFeNi_2.1 eutectic high-entropy alloy, the influence of different processes and process parameters on the microstructure and mechanical properties of the alloy was reviewed, and the phase distribution, microstructure, and strengthening mechanism of AlCoCrFeNi_2.1 eutectic high-entropy alloy prepared by the selective laser melting technology were highlighted. Finally, it points out the differences and deficiencies in phase formation mechanism and organization evolution process of the current additive manufacturing AlCoCrFeNi_2.1 eutectic high-entropy alloy and puts forward the development direction of material modification of AlCoCrFeNi_2.1 eutectic high-entropy alloy as the substrate of the material modification and the new technology of additive manufacturing high-entropy alloy, which will provide ideas for the promotion of the industrialized application of the alloy.

Soft magnetic materials have been widely applied in modern industries as energy materials. In recent years, with the increasingly high frequency and miniaturization of magnetic components, as well as the call of energy conservation and environmental protection, the development and research of high-performance soft magnetic material are of great important significance. The present work generalized the development history of soft magnetic alloys comprehensively, from the viewpoints of chemical compositions, microstructures, magnetic properties, application fields, and advantages and disadvantages of different soft magnetic alloys. The involved alloy systems include primarily traditional crystalline alloys, amorphous/nanocrystalline alloys, and high entropy alloys. It is found that the microstructure induced by alloy compositions plays a dominant role in soft magnetic property, especially the coercivity. Then the influence factors on the coercivity of alloys and the related micro-mechanisms were discussed, in which the grain size in traditional alloys or particle size in nano-crystalline alloys is crucial to achieve lower coercivity. Therefore, the development of the micro-mechanisms of coercivity in high entropy soft magnetic alloys was described. Finally, it was expected that high entropy soft magnetic alloys would be more beneficial to modulate alloy properties due to the diversification of microstructures induced by the mixing of multi-principal elements, which shows great potential to serve as a new generation of high temperature soft magnet materials.

High-entropy alloys have attracted great attention in various fields due to their high-entropy effect, severe lattice distortion, slow diffusion and special and excellent material performance due to the combination of various alloying elements in equal or near-equal molar proportions. Its high strength and hardness, fatigue resistance, excellent corrosion resistance, radiation resistance, near-zero thermal expansion coefficient, catalytic response, thermoelectric response and photoelectric conversion make high-entropy alloys have potential applications in many aspects. High-throughput computation and machine learning technology have rapidly become powerful tools to explore the huge composition space of high-entropy alloys and comprehensively predict material properties. The basic concepts of high-throughput computing and machine learning were introduced in this paper as well as the advantages of first-principles calculation, thermodynamic/kinetic calculation and machine learning in the research of high-entropy alloys. The application research status of high-entropy alloy composition screening, phase and microstructure calculations and performance prediction were summarized. In the final part, the existing problems, and the solutions and future prospects of this field were summarized, including developing tools for first-principles calculations and machine learning of high-entropy alloys, building high-quality databases for high-entropy alloys and integrating high-throughput computing with machine learning to globally optimize the mechanical property and service performance of high-entropy alloys.

Al_0.26CoCrFeNiMn high-entropy alloys were prepared by vacuum induction melting and casting method, and processed by homogenized annealing, rolling and recrystallized annealing. The microstructure evolution of the alloys during thermomechanical treatment was investigated based on ultrasonic method. The results show that the microstructure is coarse equiaxed grain after homogenized annealing, retaining deformed grain when recrystallized annealing temperature is 800℃, forming fine equiaxed grain when recrystallization is completed at 900℃, and then the grain grows at 1000℃.Besides, the grain size decreases with the increase of rolling ratio at the same recrystallized annealing temperature. At the aspect of ultrasonic characterization, the attenuation coefficient for probe testing with nominal frequency of 5MHz increases with the increase of grain size, and the attenuation coefficient with average grain size is corresponded in a cubic relationship, having a strong correlation. In order to further verify the feasibility of the method, the probe with nominal frequency of 7.5MHz was used to test, and the similar correlation was acquired. The attenuation coefficient can reveal the microstructure evolution of high-entropy alloys during thermomechanical treatment, especially for the relationship between grain size and thermomechanical treatment process.

Thermal barrier coating (TBC) materials are an important method to provide thermal protection and prolong service life for aero-engines and gas turbines. In recent years, various kinds of high-entropy (HE) rare earth oxides have emerged in the exploration of novel thermal barrier coating materials, in order to obtain thermal, mechanical, high temperature phase stability, sintering corrosion resistance and other properties better than single principal rare earth oxides through HE effect on the thermodynamics and kinetics of hysteresis diffusion effect, the structure of the lattice distortion effect and "cocktail" effect on the performance. The thermal, mechanical and other performances of HE rare-earth zirconates, cerates, hafnates, phosphates, tantalates, niobates, etc. were summarized and analyzed in comparison with the performance of the corresponding single phases to investigate the various factors affecting the performance. Finally, it was pointed out that in the future, it may be possible to combine experiments with first-principles calculations to select high-entropy ceramic thermal barrier coating materials with superior comprehensive performance; at the same time, extending high-entropy to complex components or medium-entropy ceramic thermal barrier coating materials is also an important development direction.

Macroscopic and microscopic mechanical responses, damage behavior and microstructure evolution of NiCoCrFe high entropy alloys (HEAs) during finite deformation under quasi-static loading were investigated by experiments and crystal plasticity finite element method. The microstructure of NiCoCrFe before and after tensile deformation was characterized by electron backscattering diffraction technique(EBSD). The internal state variables of dislocation density and continuum damage factors were introduced into the CPFEM model by modifying the strengthening model and the flow criterion, and the NiCoCrFe related model parameters were determined by combining the stress-strain curves of the tensile test. The results show that the CPFEM model considering the dislocation density and damage can effectively describe the macroscopic and microscopic mechanical responses of NiCoCrFe. CPFEM model can reasonably predict the deformation shape and size of NiCoCrFe necking region, among which, the length of the necking region obtained in the experiment is 7% smaller than the predicted result, and the width of the necking region predicted by CPFEM is 23% larger than the experimental result. The texture evolution predicted by CPFEM model after NiCoCrFe tensile deformation is in good agreement with the results that characterized by EBSD, showing weak (100)//RD and strong (111)//RD fiber texture. In the analysis of the 3D micro- structure damage, the damage predicted by the current CPFEM model appears as an inter-granular damage mechanism at the grain boundary where stress and dislocation density are concentrated, and the damage gradually expands to the grain interior with the increase of deformation.

The development of nuclear reactor structural materials with excellent comprehensive performance is the basis of nuclear energy development, and it is one of the difficulties that have long restricted the promotion of nuclear energy. Multiprincipal element alloys(MEAs) have been recognized as candidate materials for advanced reactor structural materials due to their good irradiation resistance and mechanical properties, which has expanded a broad space for the design of new radiation-resistant materials. In recent years, the research on the irradiation damage of multiprincipal element alloys has tried to reveal the influence of some factors and characteristics of multiprincipal element alloys on the formation and evolution of defects in the irradiation process. For example, the type, number and concentration of alloying elements, lattice distortion, chemical short range order, etc. Although some existing research results show that the above factors can improve the resistance of multiprincipal element alloys to irradiation damage, under different irradiation conditions, the influence mechanism of the above factors on the formation and evolution of defects in multiprincipal element alloys is quite different, and it is difficult to draw generalization conclusions. Focusing on the four aspects of irradiation swelling, helium bubble formation, irradiation-induced element segregation and phase transition, irradiation hardening of FCC and BCC systems.The research progress of multiprincipal element alloys in irradiation damage in recent years was reviewed, the mechanism of action of multiprincipal element alloys to improve radiation resistance was summarized.And based on this, the future research directions for multiprincipal element alloys used in nuclear power structures were prospected, including tuning short-range order, high-entropy ceramics, additive manufacturing technology, accelerating development of new materials by integrating high-throughput computing with machine learning, etc. Finally, it is pointed out that new radiation-resistance MEAs must be designed based on the actual environment of material service from the perspective of composition design.

FeCoCrNiAl_x (x=0, 0.5, 1.0, mole ratio, the same below) coatings were prepared on the surface of TC4 titanium alloy by electric explosion spraying technology. The effects of Al content on the phase structure, surface morphology, microhardness and wear resistance of high-entropy alloy coatings were studied by means of XRD, SEM, EDS, microhardness tester and friction and wear test. The results show that the grain size of the coatings is nano-scale, and simple FCC, BCC and FCC+BCC solid solutions are formed. With the increase of Al element, the phase structure is gradually changed from FCC phase to BCC phase. The surface of the coatings is smooth and dense, without obvious cracks, and the elements are evenly distributed on the surface of the coatings, and no obvious segregation of elements is found. The scratch test shows that the average critical load for the failure of the FeCoCrNiAl_1.0 coatings is 37.2 N. The coating is metallurgically bonded to the substrate. The hardness and wear resistance of the coatings are positively correlated with the Al content. When x is 1.0, the average microhardness reaches the maximum value of 531.8HV, which is about 1.62 times that of the substrate. The FeCoCrNiAl_1.0 coatings have the smallest amount of wear, and the wear resistance is about 3.9 times that of the substrate. The wear mechanism is mainly abrasive wear.

Refractory high entropy alloys NbMoTaWTi and NbMoTaWZr were prepared by vacuum arc melting.The microstructure and component distribution characteristics were analyzed, and the dynamic behavior during room temperature to 1500℃, as well as the isothermal oxidation behavior at 1200℃ were studied. Results reveal that NbMoTaWTi mainly consists of single body-centred cubic (BCC) phase, and NbMoTaWZr is composed of BCC and Zr-rich phases.These two alloys are both seriously oxidized above 700℃. Comparatively, NbMoTaWTi alloy is superior to NbMoTaWZr in antioxidation below 1300℃.For both two alloys, the oxygen diffusion inward mainly occurs during isothermal oxidation at 1200℃ and catastrophic oxidation takes place after 3 h. The Ti and Zr addition cannot cause selective oxidation. Although these two elements form a composite oxide layer with other refractory metal oxides, the density and the ability to prevent oxidation is not enough.

FeCoNiAlCr_x(x=0, 0.2, 0.4, 0.6, 0.8, atomic ratio) high-entropy alloy ingots were prepared by vacuum arc melting method, and the effect of Cr content on the microstructure and mechanical properties of the alloy was investigated. The phase structure, microstructure and the composition of the alloy were analyzed and characterized by X-ray diffractometer (XRD), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS).The compression properties of the alloy were tested by universal testing machine. The results show that with the increase of Cr content, the microstructure of the alloy changes from a single-phase BCC structure to a BCC+FCC mixed structure; the microstructure of the alloy gradually changes from an equiaxed structure to a dendrite structure, and the grain size of the alloy is obviously refined. The five alloys prepared in this experiment have relatively good mechanical properties, and the compressive strength of the alloy increases greatly with the increase of Cr content. When x=0, the compressive strength and plastic strain of the alloy are the lowest, which are 1500 MPa and 13.56%, respectively; when x=0.8, the compressive strength and plastic strain of the alloy reach the maximum, which are 2460 MPa and 30.09%, respectively; the compressive strength of the alloy increases by 64%. It indicates that Cr addition plays an important role in the microstructure refinement, the improvement of compressive strength and ductility of FeCoNiAlCr_x high-entropy alloys.

High-entropy alloy coatings show great potential for improving the wear resistance of the stainless steel substrate. To investigate the effects of Cu/Si doping on the microstructure and high temperature tribological properties of FeCoCrNi high-entropy alloy coating, FeCoCrNiCu_x and FeCoCrNiSi_x series of high-entropy alloy coatings were prepared on the 304 stainless steel by laser cladding.The microstructure and phase distribution of the coatings were characterized by XRD, SEM and EDS, and the high temperature tribological properties of the coatings were tested by a high temperature friction and wear tester. The results show that both FeCoCrNiCu_x and FeCoCrNiSi_x high entropy alloy coatings form a single FCC-type solid solution with good metallurgical bonding to the substrate under suitable laser cladding parameters.The addition of Cu reduces the surface hardness of FeCoCrNi coatings, but improves the metallurgical bonding due to the increase of thermal conductivity of the coating; the addition of Si promotes grain refinement and improves the surface hardness of the coating. At 600 ℃, the addition of Cu/Si elements significantly improves the tribological properties of the coating, with the coefficients of friction of 0.24 and 0.19 for FeCoCrNiCu and FeCoCrNiSi coatings, respectively, and the wear rates are 1.58×10^-4 mm³·N^-1·m^-1 and 6.77×10^-5 mm³·N^-1·m^-1, respectively, which are 56.1% and 81.9% lower than FeCoCrNi coating.The main wear mechanisms of FeCoCrNiCu coating are oxidation wear, fatigue wear and slightly abrasive wear, while FeCoCrNiSi coating is oxidation wear.

Machine learning(ML) assisted high-entropy alloys(HEA) design is dedicated to solving the problem of long period and high cost of designing by traditional trial and error experimental methods. The classic AlCoCrCuFeNi HEA was taken as the research object. The phase structure prediction model and hardness prediction model were established respectively. The support vector machine(SVM) models have the best training performance in both tasks. The best phase classification accuracy is 0.944, and the root mean square error(RMSE) of the hardness regression model is 56.065HV. The two ML models are further connected in series. Based on the upper and lower limits of the data set, the high-throughput prediction and selection of phases and hardness of AlCoCrCuFeNi HEA are carried out simultaneously, thus realizing the efficient composition design of the new alloy. The experimental results show that the five new alloys are in accord with the predicted results, and the RMSE is 12.58HV. It shows that the ML models can predict the phase and hardness of HEA efficiently and accurately.

High entropy alloys (HEAs) show better wear resistance and corrosion resistance than traditional alloys, which has gradually become a research hotspot in the field of metal materials. CoCrFeNiMnAlW_x (x=0.12, 0.15, 0.19)high entropy alloys with different W content were prepared by metal thermal reduction. The effects of W addition on phase structure, microstructure and performance of CoCrFeNiMnAlW_x high entropy alloy were investigated. The phase structure, microstructure and element distribution of the alloy were characterized by XRD, SEM and EDS. Surface performance tester and electrochemical workstation were adopted to detect corrosion resistance and wear resistance performance of CoCrFeNiMnAlW_x high entropy alloy. Results show that the high entropy alloys with different W contents are both composed of BCC phases with two different lattice contents. There is no obvious change in the micro-tissue of the dendrites with the increase content of W. However, microstructure between dendrites changes significantly with the change of W content. The wear resistance and corrosion resistance have certain degree of improvement, the friction coefficient and wear rate of CoCrFeNiMnAlW_0.19 alloy are 0.684 and 1.06×10^-5 mm³/(N·m) respectively. The wear mechanism is converted from adhesive wear to the combination of adhesion wear and abrasive particle wear, and finally is transformed to friction wear. The wear resistance performance of CoCrFeNiMnAlW_x high entropy alloy in 3.5% NaCl solution is increased with the increase of W content. Corrosion current density is decreased from 6.08×10^-6 A/cm² to 1.72×10^-6 A/cm², and the corrosion rate is gradually reduced.

In order to study the effect of Al content on the microstructure properties of FeCoCrNi alloy, Al_xCoCrFeNi high-entropy alloy (0≤x≤0.9) was prepared by multi-channel laser cladding. The phase composition, microstructure, chemical composition and hardness of the alloy were test by X-ray diffractometry, metallography microscope, scanning electron microscope, electron probe and microhardness tester. The results show that with the increase of Al content, Al_xCoCrFeNi high-entropy alloy changes from single FCC phase (x≤0.35) to FCC+BCC biphase structure (0.35 < x < 0.85), and finally to BCC structure (x≥0.85). The microstructure of the high entropy alloy consists of epitaxial columnar dendrites and uniform equiaxed dendrites. When Al content reaches to x=0.5, characteristic structure of spinodal decomposition with alternating light and dark contrast starts to appear between the dendrites, which consists of disordered phase A2 and ordered phase B2.The microhardness test results show that the microhardness of Al_xCoCrFeNi alloys almost increases with the increasing Al content and a total of 146% improvement has been achieved when the Al content increases from x=0 to x=0.9. It should be noted that cracks begin to appear in the alloy when the Al content increases to a certain value (x≥0.6).The size and density of cracks increase with the increase of Al content.There are two main reasons. Firstly, the hot-cracking increases since the solidification range widen and their viscosity values near the solidification temperatures increase with the increasing Al content.Besides, cold cracking also increases as the brittle BCC and σ phases increase with the increasing Al content.

High-entropy alloys exhibit excellent properties such as high strength and toughness, good wear resistance, superb corrosion resistance and superior high-temperature oxidation resistance, which have good potential applications in terms of energy chemical industry, aerospace and national defense. The mechanical behavior of high-entropy alloys under the condition of dynamic loading is different from that under the quasi-static loading, presenting higher strength, more twins and adiabatic shear bands and so on. And different phase structures have a significant impact on the dynamic properties and deformation mechanism of high-entropy alloys. Moreover, the high-entropy alloys have a certain research value in the field of energetic structural materials due to their good energy release characteristics under the condition of dynamic loading. Usually, the stability of dynamic experiment is unacceptable and the test is also difficult to achieve. In contrast, the dynamic mechanical properties of high-entropy alloys can be well predicted based on the constitutive models with experimental verification. As above-mentioned analysis, the dynamic mechanical behavior of high-entropy alloys with different phase structures, energy release characteristics and constitutive models were reviewed. Meanwhile, the comprehensive properties and their constitutive models as well as the simulation calculations were prospected. Finally, it is pointed out that the dynamic mechanical properties of high-entropy alloys can be improved by adjusting the type and proportion of elements, phase structure and concentration distribution. At the same time, the influence mechanism of temperature and strain rate on the dynamic mechanical behavior of high-entropy alloy needs further study. The model calculation also needs to play a greater role in revealing its deformation mechanism and predicting its performance at high strain rate.

The equiatomic CoCrFeNiMn high entropy alloys (HEAs) has been successfully manufactured using direct laser deposition (DLD) technique. The size and number of porosities, the microstructures along the height of samples and the tensile properties of DLDed HEAs prepared under room (293 K) and cryogenic temperatures (77 K and 200 K) were investigated. The results show that DLDed HEAs exhibit directional solidification, forming dendritic columnar grains with long pores at the grain boundary in bottom regions and transiting to equiaxed grains close to top regions. And in the top regions, the pores are round and the numbers are greatly reduced. Compared with tensile properties of DLDed HEAs, the 77 K tensile samples cut from the top region have better performance, but the elongation of 293 K tensile samples in the middle region and 200 K tensile samples in the bottom region are similar, owing to the difference of porosity and microstructure in the two regions.

In order to improve the mechanical properties and corrosion resistance of Al-Cr-Fe-Co-Ni system high-entropy alloys (HEAs), the effects of Mo on the microstructure, mechanical properties and corrosion behavior of the Al_0.3CrFeCoNiMo_x (x=0.2, 0.4, 0.6, 0.8, molar ratio, the same below) HEAs were studied. The results show that the microstructures of the HEAs evolve from the FCC phase (x=0.2) to the FCC+σ phases (x=0.4-0.8) with the increase of Mo content. The compressive yield strength and hardness of the HEAs are enhanced from 304 MPa and 214HV (x=0.2) to 1192 MPa and 513HV (x=0.8), respectively, while the plastic strain decreases from > 50% to 5.2%, mainly due to the solution strengthening and the increase in σ phase of the alloys.Among the present alloys system, the Al_0.3CrFeCoNiMo_0.4 and Al_0.3CrFeCoNiMo_0.6 alloys show relatively high yield strength (571-776 MPa) combined with good plasticity (plastic strain of 10.3%-23.8%).In 3.5%(mass fraction) NaCl solution, the Al_0.3CrFeCoNiMo_x HEAs are spontaneously passivated and exhibited low corrosion rates of 3.6×10^－4-5.9×10^－4 mm/a, and the addition of Mo effectively suppresses pitting corrosion. The corrosion resistance of the Al_0.3CrFeCoNiMo_x HEAs could be improved with the increase of Mo content, which is related to the increment in the electrochemical impedance and the thicknesses of the passive films on the alloys. Appropriate addition of Mo to the Al-Cr-Fe-Co-Ni system HEAs can lead to a combination of good mechanical properties and corrosion resistance.

High entropy alloys (HEAs) were first reported in the early 2000s. High mixing entropy of the HEAs makes it present good thermal stability. Meanwhile, the large lattice distortion in the HEAs leads to significant solution strengthening. Additionally, cluster structures are formed within grains due to the large negative enthalpy. Consequently, the movement of dislocation is effectively hindered, and the strength of the HEAs is remarkably improved. Given to these unique characteristics, the HEAs is expected to have excellent physical and chemical properties at low and high temperatures. As a result, the HEAs have become a hot area with lots of published research papers. Based on existing physical and mechanical properties of the HEAs with BCC and FCC structure, relation among electron concentration, lattice constant, atomic mismatch, mixing enthalpy, hardness, elasticity modulus and normalized hardness were analyzed to develop a formula calculating elasticity modulus and hardness of the HEAs. On this basis, the composition design method of the HEAs with BCC and FCC structures is established by considering density, ductility and working environment. Finally, it is pointed out that the persistent strength of HEAs, the uniformity of composition and properties of large-sized ingots, and the preparation of large-sized alloy ingots are key issues that need to be addressed in the engineering application of HEAs.

The refractory high entropy alloys (HEAs) based on refractory elements are developed for potential applications in high temperature areas, since these alloys always have melting temperature higher than 1800 ℃, high temperature structural stability and high resistance to heat softening. However, large density induced lower specific strength and room temperature brittleness hinder their application.In this study, the light-weight non-equimolar (Ti₃₅Zr₄₀Nb₂₅)_100-xAl_x (x=0, 5, 10, 15, 20) HEAs were designed and fabricated, then the effect of Al content on the phases, microstructure and mechanical properties were investigated. X-ray diffraction results indicate that the phase changes from the disorder BCC to ordered B2 of other alloys with the increase of Al content. Five alloys have similar phase morphology. Lots of long and slender dendrites grow along the cooling direction at the edge of the ingots, while equiaxed dendrites form at the center of the samples. Energy dispersive X-ray analysis imply the enrichment of Nb in dendritic regions, while Al and Zr segregate in the interdendritic regions.This can be attributed to the highest melting temperature of Nb and stronger bonding between Al and Zr. Room temperature tests reveal that the increase of Al content leads to the increase of both the yield stress and compression stress, but has less influence on the room temperature ductility, the fracture strain of all alloys exceeds 50%.

Al_xCoCrFeNi(x=0.3,0.5,0.7,1.0)high-entropy alloy was fabricated by selective laser melting (SLM),and pre-alloyed powder was prepared by gas atomization.The phase composition, microstructure, hardness, Young’s modulus and creep curve of Al_xCoCrFeNi were comprehensively analyzed through X-ray diffractometer, scanning electron microscope and nanoindentation experiments, respectively. The influence of Al content on the microstructure and nanoindentation of Al_xCoCrFeNi was discussed.The results show that Al_0.3CoCrFeNi and Al_0.5CoCrFeNi are FCC structure, while Al_0.7CoCrFeNi and Al_1.0CoCrFeNi are BCC/B2 structure. The microstructure of Al_0.3CoCrFeNi and Al_0.5CoCrFeNi are mainly composed of equiaxed crystals, while Al_0.7CoCrFeNi and Al_1.0CoCrFeNi are mainly composed of columnar crystals. It indicates that the content of Al has great influence on the microstructure of Al_xCoCrFeNi high-entropy alloy.With the increase of Al content, defects such as pores and cracks in the specimens increase. There is no obvious molten pool morphology observed in Al_0.3CoCrFeNi and Al_0.5CoCrFeNi. The residual stress increases with the increase of Al content. The hardness and Young’s modulus of the samples were measured. It was found that with the increase of Al content,the hardness of the sample increases from 447HV to 567HV.The Young’s modulus of Al_0.3CoCrFeNi is about 273 GPa, and Al_0.5CoCrFeNi is about 233 GPa,while Al_0.7CoCrFeNi and Al_1.0CoCrFeNi are about 240 GPa and 242 GPa,respectively. The changes in hardness and Young’s modulus are mainly related to the microstructure and phases of specimens. Different from the traditional creep curve, the creep curve of Al_xCoCrFeNi includes only two stages, which are instantaneous creep and steady-state creep. The creep mechanism is mainly dislocation creep. Among the samples, Al_0.7CoCrFeNi has the best creep resistance. Al_0.3CoCrFeNi has the best print formability, with the yield strength of 702 MPa, and the elongation is 27.5%.

Al_xCoCrFeNi (0.5≤x≤0.8) high-entropy alloys were prepared by arc-melting method and the effect of 1100 ℃ heat treatment on the microstructure and mechanical properties of the alloys was investigated. The results show that the as-cast Al_xCoCrFeNi (0.5≤x≤0.8) high-entropy alloys present FCC dendrite (x=0.5 and 0.6), "eutectic-like" structure (x=0.7) and BCC/B2 dendrite (x=0.8) morphologies successively as Al content increases. Correspondingly, the yield strength and tensile strength of the alloys increase from 291 MPa and 733 MPa (x=0.5) to 1004 MPa and 1423 MPa (x=0.7), respectively, and the elongation decreases from 39.7% (x=0.5) to 6.8% (x=0.7). After heat treatment at 1100 ℃, a large number of rod-like B2 phases are precipitated from the FCC dendrite region, which can improve the strength of heat-treated alloys, while the BCC/B2 spinodal structure transforms into FCC and B2 dual-phase structure, which can enhance the plasticity of heat-treated alloys. Therefore, the yield strength and tensile strength of heat-treated Al_0.5CoCrFeNi alloy with FCC dendrite morphology increase to 370 MPa and 866 MPa, respectively, and the elongation decreases to 30.1%. However, both phase transition behaviors have great influence on the microstructure and property of Al_0.6CoCrFeNi alloy due to the increasing fraction of spinodal structure. Therefore, the mechanical properties of heat-treated Al_0.6CoCrFeNi alloy are basically unchanged in comparison to that of the as-cast alloy. The as-cast Al_0.7CoCrFeNi and Al_0.8CoCrFeNi alloys contain higher fractions of spinodal structure, while the heat-treated alloys present typical FCC and B2 dual-phase structure, resulting in an increase of plasticity but a decrease of strength. Correspondingly, the elongation of heat-treated Al_0.7CoCrFeNi alloy increases to 14.2%, and the yield strength and tensile strength decrease to 586 MPa and 1092 MPa, respectively.

To obtain Al-Co-Cr-Fe-Ni high entropy alloys (HEAs) with high strength and high ductility, Al_1.2Co_xCrFeNi(x=1, 1.6, 2.2, 2.8) HEAs were successfully prepared by arc melting method and its microstructure and mechanical properties were systematically studied. The results show that in Al_1.2Co_xCrFeNi alloy, Co element can induce the transformation from BCC to FCC phase. With the increase of the atomic ratio of Co from 1 to 2.8, the volume fraction of FCC phase increases from 0% to 59%, and the volume fraction of BCC phase decreases from 100% to 41%. The results of compression experiment show that the addition of Co plays an important role in improving the plasticity of Al_1.2Co_xCrFeNi high entropy alloys but has no obvious effect on the strength of the high entropy alloys. With the increase of Co content, the fracture strain of Al_1.2Co_xCrFeNi HEAs increases from 16.9% to 30%. The ultimate compressive strength decreases from 2128 MPa to 1913 MPa, and the maximum compressive strength is 2361 MPa, and the average hardness decreases from 513.7HV to 323.4HV. The increase of Co content decreases the atomic size difference of the alloys, which weakens the lattice distortion effect and solid solution strengthening effect caused by the large atomic radius of Al element. At the same time, the increase of Co content also increases the valence electron concentration (VEC) of the alloys. The changes of the above two parameters are the main factors for the increase of FCC phase volume fraction in the alloy. The increase of the volume fraction of FCC phase is the main reason for the improvement of the plasticity of the alloy.

AlFeNiCrCoTi_0.5 high entropy alloy powder was prepared by mechanical alloying, and (AlFeNiCrCoTi_0.5)_p/6061Al composites were prepared by cold isostatic pressing combined with equal-channel angular pressing. The alloying behavior between elemental metals and effects of milling time on powder morphology of high entropy alloy were investigated. The microstructure and properties of (AlFeNiCrCoTi_0.5)_p/6061Al composites with different volume fractions were analyzed. The results show that the alloying time of AlFeNiCrCoTi_0.5 metal powder increases with the increase of melting point of the elements. The higher the melting point of the elements, the earlier the alloying. AlFeNiCrCoTi_0.5 metal powder is fully alloyed and forms a FCC+BCC two-phase solid solution structure after 70 h ball milling time. A transition layer of element infiltration of elements is formed between Al matrix and the reinforcement. With the increase of the volume fraction of reinforcement, the agglomeration of reinforcement is intensified, the tensile strength increases and the plasticity decreases. When the volume fraction is 10%, the composites have good comprehensive properties. Compared with 6061 aluminum matrix, the tensile strength increases by 21.8% and the elongation decreases by 7.4%. For the composites after T6 treatment, the tensile strength and the elongation are 284.05 MPa and 11.51%, respectively.