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.
Fe40Cr25Ni25Al5Ti5 (atom fraction/%) medium entropy alloys (MEAs) were prepared by a vacuum arc-melting furnace with a water-cooled copper mold, and the microstructure, mechanical properties and strengthening deformation mechanism of the alloy in solid solution and annealed states were studied by scanning electron microscopy (SEM), energy spectrometry(EDS), X-ray diffractometer (XRD), transmission electron microscopy(TEM) and tensile testing machine.The results show that the Fe40Cr25Ni25Al5Ti5 medium entropy alloy has FCC+BCC1+BCC2 triple-phase organization of solid solution state, with yield strength of 520 MPa, fracture strength of 852 MPa and elongation of 13%. After annealing at 600 ℃ for 2 h, the phase composition of the alloy has no change, the size of granular BCC2 phase increases, and the volume fraction of FCC zone and BCC zone has no change significantly. The yield strength is 668 MPa, the fracture strength is 1029 MPa, and the elongation is reduced to 9%. The strength of Fe40Cr25Ni25Al5Ti5 alloy originates from the synergistic effect of coherent strengthening, solid solution strengthening and phase boundary strengthening. Dislocation slips are the main deformation mechanism of alloy.
The high temperature oxidation behavior of NbMoTaWV refractory high entropy alloy(RHEA) prepared by arc melting was studied by static oxidation experiment, XRD and FSEM. The results reveal that the NbMoTaWV RHEA is not protective due to severe cracks in the oxide layer at 1000 ℃ and 1200 ℃. The mass gain follows a linear oxidation law. Molten oxides formed at 1400 ℃, which release the growth stress of the oxide layer and fill the holes left by the volatilization of Mo and V oxides.In the oxidation process of NbMoTaWV, oxygen diffuses into the matrix and first oxidizes with Nb and Ta in the diffusion layer to generate needle-rod like oxidation products, and then oxidizes with other alloy elements. In the subsequent oxidation process, the oxides of W are dissolved in Nb and Ta oxides, while the granular mixed oxides of Mo and V volatilize at high temperatures.
To obtain Al-Co-Cr-Fe-Ni high entropy alloys (HEAs) with high strength and high ductility, Al1.2CoxCrFeNi(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 Al1.2CoxCrFeNi 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 Al1.2CoxCrFeNi 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 Al1.2CoxCrFeNi 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.
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.
AlFeNiCrCoTi0.5 high entropy alloy powder was prepared by mechanical alloying, and (AlFeNiCrCoTi0.5)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 (AlFeNiCrCoTi0.5)p/6061Al composites with different volume fractions were analyzed. The results show that the alloying time of AlFeNiCrCoTi0.5 metal powder increases with the increase of melting point of the elements. The higher the melting point of the elements, the earlier the alloying. AlFeNiCrCoTi0.5 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.
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, FeCoCrNiCux and FeCoCrNiSix 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 FeCoCrNiCux and FeCoCrNiSix 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.
FeCoNiAlCrx(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 FeCoNiAlCrx high-entropy alloys.
The effect of Cr content on the microstructure and corrosion resistance of CrxMnFeNi(x=0.8, 1.0, 1.2, 1.5) high entropy alloys was studied by using XRD, SEM/EDS, EBSD and electrochemical testing. The results show that Cr0.8 MnFeNi high entropy alloy exhibits single FCC phase structure, while CrxMnFeNi(x=1.0, 1.2, 1.5) high entropy alloys show FCC+BCC dual phase structure, and the BCC phase ratio of dual phase alloys increases with the increase of Cr content.The corrosion resistance of high entropy alloys gradually increases with the decrease of Cr content.Among them, Cr0.8MnFeNi high entropy alloy has the best corrosion resistance, because the composition of the alloy is more uniform.In addition, CrxMnFeNi high entropy alloys have extensive passive region and obvious pseudo passive region in 0.5 mol/L H2SO4 solution, which indicates the alloys have greater research value and development potential in corrosion resistance.
The recycled copper alloy powders prepared by physical method were further alloyed, and therefore four medium entropy alloys (MEAs), (Fe40Ni40Mn20)50Cu50, (Fe38Ni38Mn19Al5)50Cu50, (Fe36Ni36Mn18Al10)50Cu50 and (Fe32Ni32Mn16Al20)50Cu50were successfully prepared via mechanical alloying (MA) and spark plasma sintering (SPS). The influence of Al content on the microstructure and mechanical properties of the MEAs were systematically studied. Following 60 h of MA, the mechanical alloyed powders of the four MEAs consist of a primary supersaturated FCC solid solution along with a trace amount of WC contaminants. Following SPS, the (Fe40Ni40Mn20)50Cu50, (Fe38Ni38Mn19Al5)50Cu50 and (Fe36Ni36Mn18Al10)50Cu50 show a dual-phase structure consisting of a Cu-rich phase (FCC1) and a Fe-Ni-rich phase (FCC2), displaying a multiscale grain structure of ultrafine grains and micron grains. However, the (Fe32Ni32Mn16Al20)50Cu50 alloy shows a primary Cu-rich phase (FCC1) with a small amount of Fe-Mn rich phase (FCC2) and Ni-Al rich B2 phase. The plasticity of the four MEAs is gradually decreased, while the strength and hardness are gradually increased with the increase of Al content. The compressive yield strength, compressive strength and Vickers hardness of (Fe40Ni40Mn20)50Cu50 MEAs are 878 MPa, 1257 MPa and 248.5HV, respectively. Compared with (Fe40Ni40Mn20)50Cu50, the compressive yield strength and hardness of (Fe32Ni32Mn16Al20)50Cu50 are increased by 50.1% and 50.4%, respectively, whereas the fracture strain is decreased from 19.55% to 8.31%.
AlxCoCrFeNi(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 AlxCoCrFeNi 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 AlxCoCrFeNi was discussed.The results show that Al0.3CoCrFeNi and Al0.5CoCrFeNi are FCC structure, while Al0.7CoCrFeNi and Al1.0CoCrFeNi are BCC/B2 structure. The microstructure of Al0.3CoCrFeNi and Al0.5CoCrFeNi are mainly composed of equiaxed crystals, while Al0.7CoCrFeNi and Al1.0CoCrFeNi are mainly composed of columnar crystals. It indicates that the content of Al has great influence on the microstructure of AlxCoCrFeNi 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 Al0.3CoCrFeNi and Al0.5CoCrFeNi. 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 Al0.3CoCrFeNi is about 273 GPa, and Al0.5CoCrFeNi is about 233 GPa,while Al0.7CoCrFeNi and Al1.0CoCrFeNi 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 AlxCoCrFeNi includes only two stages, which are instantaneous creep and steady-state creep. The creep mechanism is mainly dislocation creep. Among the samples, Al0.7CoCrFeNi has the best creep resistance. Al0.3CoCrFeNi has the best print formability, with the yield strength of 702 MPa, and the elongation is 27.5%.
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 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.
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.
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.
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. CoCrFeNiMnAlWx (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 CoCrFeNiMnAlWx 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 CoCrFeNiMnAlWx 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 CoCrFeNiMnAlW0.19 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 CoCrFeNiMnAlWx 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.
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.
