Characterization of local chemical ordering and deformation behavior in high entropy alloys by transmission electron microscopy

Short-range ordering(SRO)is one of the most important structural features of high entropy alloys(HEAs).However,the chemical and structural analyses of SROs are very difficult due to their small size,complexed compositions,and varied locations.Transmission electron microscopy(TEM)as well as its aberration correction techniques are powerful for characterizing SROs in these compositionally complex alloys.In this short communication,we summarized recent progresses regarding characterization of SROs using TEM in the field of HEAs.By using advanced TEM techniques,not only the existence of SROs was confirmed,but also the effect of SROs on the deformation mechanism was clarified.Moreover,the perspective related to application of TEM techniques in HEAs are also discussed.

A critical review on solid-state welding of high entropy alloys-processing,microstructural characteristics and mechanical properties of joints

The high entropy alloys(HEAs)are the newly developed high-performance materials that have gained significant importance in defence,nuclear and aerospace sector due to their superior mechanical properties,heat resistance,high temperature strength and corrosion resistance.These alloys are manufactured by the equal mixing or larger proportions of five or more alloying elements.HEAs exhibit superior mechanical performance compared to traditional engineering alloys because of the extensive alloying composition and higher entropy of mixing.Solid state welding(SSW)techniques such as friction stir welding(FSW),rotary friction welding(RFW),diffusion bonding(DB)and explosive welding(EW)have been efficiently deployed for improving the microstructural integrity and mechanical properties of welded HEA joints.The HEA interlayers revealed greater potential in supressing the formation of deleterious intermetallic phases and maximizing the mechanical properties of HEAs joints.The similar and dissimilar joining of HEAs has been manifested to be viable for HEA systems which further expands their industrial applications.Thus,the main objective of this review paper is to present a critical review of current state of research,challenges and opportunities and main directions in SSW of HEAs mainly CoCrFeNiMn and Al_xCoCrFeNi alloys.The state of the art of problems,progress and future outlook in SSW of HEAs are critically reviewed by considering the formation of phases,microstructural evolution and mechanical properties of HEAs joints.

Spark plasma sintering of tungsten-based WTaVCr refractory high entropy alloys for nuclear fusion applications

W-based WTaVCr refractory high entropy alloys (RHEA) may be novel and promising candidate materials for plasma facing components in the first wall and diverter in fusion reactors. This alloy has been developed by a powder metallurgy process combining mechanical alloying and spark plasma sintering (SPS). The SPSed samples contained two phases, in which the matrix is RHEA with a body-centered cubic structure, while the oxide phase was most likely Ta2VO6through a combined analysis of X-ray diffraction (XRD),energy-dispersive spectroscopy (EDS), and selected area electron diffraction (SAED). The higher oxygen affinity of Ta and V may explain the preferential formation of their oxide phases based on thermodynamic calculations. Electron backscatter diffraction (EBSD) revealed an average grain size of 6.2μm. WTaVCr RHEA showed a peak compressive strength of 2997 MPa at room temperature and much higher micro-and nano-hardness than W and other W-based RHEAs in the literature. Their high Rockwell hardness can be retained to at least 1000°C.

Research Progress of High Entropy Ceramic Materials

High-entropy materials(HEMs)have better mechanical,thermal,and electrical properties than traditional materials due to their special"high entropy effect".They can also adjust the performance of high entropy ceramics by adjusting the proportion of raw materials,and have broad application prospects in many fields.This article provides a review of the high entropy effect,preparation methods,and main applications of high entropy ceramic materials,especially exploring relevant research on high entropy perovskite ceramics.It is expected to provide reference for the promotion of scientific research and the development of further large-scale applications of high-entropy ceramic materials.

Research Progress of Non-oxide and High Entropy Ceramic Coatings

Ceramic coatings play a keyrole in extending the service life of materials in aerospaceandenergy fields byprotectingmaterials from high temperature,oxidation,corrosion and thermal stress.Non-oxide and high entropy ceramics are new emerging coating materials which have been researched and developed in recent years.Compared with traditional oxide ceramics,non-oxide ceramics have better high temperature stability,oxidation resistance and erosion resistance.These characteristics make non-oxide ceramics perform well in extreme environments.It is particularly noteworthy that the non-oxide high entropy ceramic is a uniform solid solution composed of at least four or fiveatoms.Their unique structure and outstanding propertiesshow great potential application in the field of coating.In this paper,the researches aboutregulating microstructure,preparation technology and properties of nitride and its high entropy system,carbide and its high entropy system and boride and its high entropy system in coating field are summarized,and their future development and prospects are prospected.

Progress in weldability research of high entropy alloys

High entropy alloys usually show good weldability.The weldability problems of high entropy alloys are segregation,cracks,and hardening or softening of weld,etc.When an Al_(x)CoCrFeNi alloy is welded,Al and Ni will segregate to the interdendritic region in the weld,but the degree of segregation is less than that of the base metal.When an Al_(x)CoCrCu_(x)FeNi alloy or a CoCrCu_(x)FeNi alloy is welded,Cu tends to segregate to the interdendritic region in the weld.Increasing the cooling rate of the welding process,such as with laser welding,is conducive to reducing the segregation in the weld.The segregation in the weld and the heat affected zone,especially the segregation of Cu,will lead to the generation of hot cracks.Hot cracking is the main form of cracking in high entropy alloys joints.Welding will lead to changes in the hardness of the weld.The main factors affecting the hardness change are the grain sizes and the precipitations.With laser welding,if the base metal is cold rolled,the hardness of the weld will decrease.If the base metal is hot rolled and annealed or cast,the hardness of the weld will increase.With TIG welding,the hardness of the weld is usually lower than that of the base metal,unless the grain of the base metal is particularly coarse before welding.With friction stir welding,recrystallization and grain refinement occur in the stir zone,and the hardness of the stir zone will be significantly improved no matter the original base metal is cold rolled or cast.

FeCoNiCrMo high entropy alloy nanosheets catalyzed magnesium hydride for solid-state hydrogen storage

The catalytic effect of FeCoNiCrMo high entropy alloy nanosheets on the hydrogen storage performance of magnesium hydride(MgH_(2))was investigated for the first time in this paper.Experimental results demonstrated that 9wt%FeCoNiCrMo doped MgH_(2)started to dehydrogenate at 200℃and discharged up to 5.89wt%hydrogen within 60 min at 325℃.The fully dehydrogenated composite could absorb3.23wt%hydrogen in 50 min at a temperature as low as 100℃.The calculated de/hydrogenation activation energy values decreased by44.21%/55.22%compared with MgH_(2),respectively.Moreover,the composite’s hydrogen capacity dropped only 0.28wt%after 20 cycles,demonstrating remarkable cycling stability.The microstructure analysis verified that the five elements,Fe,Co,Ni,Cr,and Mo,remained stable in the form of high entropy alloy during the cycling process,and synergistically serving as a catalytic union to boost the de/hydrogenation reactions of MgH_(2).Besides,the FeCoNiCrMo nanosheets had close contact with MgH_(2),providing numerous non-homogeneous activation sites and diffusion channels for the rapid transfer of hydrogen,thus obtaining a superior catalytic effect.

Influences of Milling Time and NbC on Microstructure of AlCoCrFeNi_(2.1)High Entropy Alloy by Mechanical Alloying

AlCoCrFeNi_(2.1)eutectic high entropy alloy(EHEA)and AlCoCrFeNi_(2.1)-x NbC(x=2.5wt%,5.0wt%,7.5wt%,and 10wt%)high entropy alloy(HEAs)were prepared by mechanical alloying(MA).The effects of milling time and NbC content on the alloying behavior and grain size of the AlCoCrFeNi_(2.1)EHEA were investigated.The experimental results show that the AlCoCrFeNi_(2.1)EHEA primarily consists of order BCC(B2)and face-centered-cubic(FCC)phases,while the AlCoCrFeNi_(2.1)-x NbC(x=2.5wt%,5.0wt%,7.5wt%,and 10wt%)HEAs are composed of B2,FCC,and NbC phases.With the increase of milling time,the powder goes through three stages,irregularity,cold welding fracture and spheroidization.The particle size of AlCoCrFeNi_(2.1)EHEA powder shows a trend of first increasing and then decreasing.Therein,the particle size presents a normal distribution during 0-50 h alloying.With the addition of NbC,the AlCoCrFeNi_(2.1)-x NbC HEAs powders are significantly refined.And the degree of grain refinement gradually increases with the increase of NbC content.

High entropy fluorides as conversion cathodes with tailorable electrochemical performance

With the recent development of high entropy materials, an alternative approach to develop advanced functional materials with distinctive properties that show improved values compared to conventional materials has been provided. The high entropy concept was later successfully transferred to metal fluorides and high entropy fluorides(HEFs) were successfully synthesized. Owing to their high theoretical specific capacities in energy storage applications, HEFs were utilized as cathode materials for lithiumion batteries(LIBs) and their underlying storage mechanisms were investigated. Instead of a step-bystep reduction of each individual metal cation, the HEFs seem to exhibit a single-step reduction process,indicating a solid solution compound instead of merely a mixture of different metal fluorides. It was also observed that the electrochemical behavior of the HEFs depends on each individual incorporated element. Therefore, by altering the elemental composition, new materials that exhibit improved electrochemical properties can be designed. Remarkably, HEFs with seven incorporated metal elements exhibited a better cycling stability as well as a lower hysteresis compared to binary metal fluorides.These findings offer new guidelines for material design and tailoring towards high performance LIBs.

Processing effects on the magnetic and mechanical properties of FeCoNiAl_(0.2)Si_(0.2) high entropy alloy

High entropy alloys with the composition of FeCoNiA10.2Si0.2 were prepared by arc melting and induction melting, denoted by A1 and A2, respectively. The samples prepared by these two techniques have a face-centered cubic （FCC） phase structure and a typical dendrite morphology. The tensile yield strength and maximum strength of A2 samples are about 280 and 632 MPa, respectively. Moreover~ the elongation can reach 41.7%. These two alloys prepared by the different methods possess the similar magnetic properties. The saturation magnetization and coercivity can reach 1.151 T and 1400 A/m for A1 samples and 1.015 T and 1431 A/m for A2 samples, respectively. Phases in A2 samples do not change, which are heat treated at different temperatures, then quenched in water. Only the sample, which is heat treated at 600~C for 3 h and then furnace cooled, has a new phase precipitated. Besides, the coercivity decreases obviously at this temperature. Cold rolling and the subsequent heat treatment cannot improve the magnetic properties effectively. However, cold rolling plays an important role in improving the strength.

Effects of Annealing on Microstructure, Mechanical and Electrical Properties of AlCrCuFeMnTi High Entropy Alloy

The multi-component A1CrCuFeMnTi high entropy alloy was prepared using a vacuum arc melting process. Serial annealing processes were subsequently performed at 590 ℃, 750 ℃, 955 ℃ and 1 100 ℃ respectively with a holding time of 4 h at each temperature. The effects of annealing on microstructure, mechanical and electrical properties of as-cast alloy were investigated by using differential thermal analysis （DTA）, X-ray diffraction （XRD）, scanning electron microscopy （SEM） and transmission electron microscopy （TEM）. The experimental results show that two C14 hexagonal structures remain unchanged after annealing the as-cast A1CrCuFeMnTi alloy specimens being heated to 1 100℃. Both annealed and as-cast microstructures show typical cast-dendrite morphology and similar elemental segregation. The hardness of alloys declines as the annealing temperature increases while the strength of as-cast alloy improves obviously by the annealing treatment. The electrical conductivities of annealed and as-cast alloys are influenced by the distribution of interdendrite re^ions which is rich in Cu element.

Superior corrosion resistance-dependent laser energy density in(CoCrFeNi)95Nb5 high entropy alloy coating fabricated by laser cladding

(CoCrFeNi)95Nb5 high entropy alloy(HEA)coatings were successfully fabricated on a substrate of Q235 steel by laser cladding technology.These(CoCrFeNi)95Nb5 HEA coatings possess excellent properties,particularly corrosion resistance,which is clearly superior to that of some typical bulk HEA and common engineering alloys.In order to obtain appropriate laser cladding preparation process parameters,the effects of laser energy density on the microstructure,microhardness,and corrosion resistance of(CoCrFeNi)95Nb5 HEA coating were closely studied.Results showed that as the laser energy density increases,precipitation of the Laves phase in(CoCrFeNi)95Nb5 HEA coating gradually decreases,and diffusion of the Fe element in the substrate intensifies,affecting the integrity of the(CoCrFeNi)95Nb5 HEA.This decreases the microhardness of(CoCrFeNi)95Nb5 HEA coatings.Moreover,the relative content of Cr2O3,Cr(OH)3,and Nb2O5 in the surface passive film of the coating decreases with increasing energy density,causing corrosion resistance to decrease.This study demonstrates the controllability of a high-performance HEA coating using laser cladding technology,which has significance for the laser cladding preparation of other CoCrFeNi-system HEA coatings.

Electrochemical Synthesis and Magnetic Studies of Ni-Fe-Co-Mn-Bi-Tm High Entropy Alloy Film

Amorphous Ni-Fe-Co-Mn-Bi-Tm high entropy alloy films were firstly prepared by potentiostatic electro- deposition. The results indicate that the six elements can be co-deposited in an organic system of DMF （N,N-dimethylformamide）-CH3CN. The surface of the film is composed of compact and uniform particles with triangular cone shape and a particle size of 100-200 nm. A simple face-centered-cubic structure is identified by XRD patterns after the films were annealed under Ar atmosphere. The annealed alloy film shows soft magnetic properties and the magnetization decreases with Fe content decreasing.

Research Progress of Refractory High Entropy Alloys:A Review

Owing to superior comprehensive performance than conventional superalloys at high temperature,refractory high entropy alloy(RHEA)is becoming a promising candidate for the next generation high-temperature material.Herein,contemporary aspects of corresponding development of RHEAs are reviewed to discuss various factors affecting the organization structure and service performance.It mainly covers alloying system and strengthening mechanism,the preparation method,plastic deformation and the related mechanism,as well as microstructure control by heat treatment.Firstly,the alloy systems and strengthening mechanism are introduced.This is followed by different preparation methods and the comparison of strengths and shortcomings based on different RHEAs.Then,hot deformation behavior and plastic deformation under different loadings are analyzed.Based on this,the influence of heat treatment on microstructures prior to and after the deformation is further summarized.Finally,some important research areas to be carried out in future are pointed out.This review will give a deep understanding of the effects of different factors on the service performance and provide scientific guide in designing RHEAs with improved performance.

Microstructural evolution and mechanical properties of FeCoCrNiCu high entropy alloys:a microstructure-based constitutive model and a molecular dynamics simulation study

High entropy alloys(HEAs)attract remarkable attention due to the excellent mechanical performance.However,the origins of their high strength and toughness compared with those of the traditional alloys are still hardly revealed.Here,using a microstructure-based constitutive model and molecular dynamics(MD)simulation,we investigate the unique mechanical behavior and microstructure evolution of FeCoCrNiCu HEAs during the indentation.Due to the interaction between the dislocation and solution,the high dislocation density in FeCoCrNiCu leads to strong work hardening.Plentiful slip systems are stimulated,leading to the good plasticity of FeCoCrNiCu.The plastic deformation of FeCoCrNiCu is basically affected by the motion of dislocation loops.The prismatic dislocation loops inside FeCoCrNiCu are formed by the dislocations with the Burgers vectors of a/6[112]and a/6[112],which interact with each other,and then emit along the<111>slip direction.In addition,the mechanical properties of FeCoCrNiCu HEA can be predicted by constructing the microstructure-based constitutive model,which is identified according to the evolution of the dislocation density and the stress-strain curve.Strong dislocation strengthening and remarkable lattice distortion strengthening occur in the deformation process of FeCoCrNiCu,and improve the strength.Therefore,the origins of high strength and high toughness in FeCoCrNiCu HEAs come from lattice distortion strengthening and the more activable slip systems compared with Cu.These results accelerate the discovery of HEAs with excellent mechanical properties,and provide a valuable reference for the industrial application of HEAs.

Progress in Light-weight High Entropy Alloys

High entropy alloys(HEAs)possess good mechanical properties and a wide range of industrial applications.In this paper,phase formation prediction theory,microstructure,properties and preparation methods of light-weight HEAs(LWHEAs)were reviewed.The problems and challenges faced by LWHEAs development were analyzed.The results showed that many aspects are still weak and require investigation for future advanced alloys,such as clarification of the role of entropy in phase formation and properties of HEAs,improved definition and different generations division of HEAs,close-packed hexagonal(HCP)phase structure prediction and corresponding alloy design and fabrication.Finally,some suggestions were presented in this paper including in-depth research on formation mechanism of multi-component alloy phase and strengthening of large-scale HEA preparation methods via technology compounding and 3D printing technology.Also,there is a need for more research on the in-situ preparation of HEA coatings and films,as well as developing LWHEAs with superior strength and elevated temperature resistance or ultra-low temperature resistance to meet the requirements of future engineering applications.

Microstructures and Mechanical Properties of AlCrFeNiMo0.5Tix High Entropy Alloys

Effects of Ti addition on the microstructures and mechanical properties of AICrFeNiMo0.5 Tix （x = 0, 0.25, 0.4, 0.5, 0.6, 0.75） high entropy alloys （HEAs） are investigated. All these HEAs of various Ti contents possess dual BCC structures, indicating that Ti addition does not induce the formation of any new phase in these alloys. As Ti addition x varies from 0 to 0.75, the Vickers hardness （HV） of the alloy system increases from 623.7HV to 766.2HV, whereas the compressive yield stress firstly increases and then decreases with increasing x above 0.5. Meanwhile, the compressive ductility of the alloy system decreases with Ti addition. The AlCrFeNiMo0.5 Ti0.6 and AlCrFeNiMo0.5Ti0.75 HEAs become brittle and fracture with very limited plasticity. In the AlCrFeNiMo0.5Tix HEAs, the AlCrFeNiMoo.5 HEA possesses the highest compressive fracture strength of 4027 MPa and the largest compressive plastic strain of 27.9%, while the AlCrFeNiMo0.5 Ti0.5 HEA has the highest compressive yield strength of 2229 MPa and a compressive plastic strain of 10.1%. The combination of high strength and large plasticity of the AICrFeNiMoo.sTix （x = 0, 0.25, 0.4, 0.5） HEAs demonstrates that this alloy system is very promising for engineering applications.

A review on microstructures and properties of high entropy alloys manufactured by selective laser melting

High entropy alloys(HEAs)with multi-component solid solution microstructures have the potential for large-scale industrial applications due to their excellent mechanical and functional properties.However,the mechanical properties of HEAs limit the selection of processing technologies.Additive manufacturing technology possesses strong processing adaptability,making itthe best candidate method to overcome this issue.This comprehensive review examines the current state of selective laser melting(SLM)of HEAs.Introducing SLM to HEAs processing is motivated by its high quality for dimensional accuracy,geometric complexity,surface roughness,and microstructure.This review focuses on analyzing the current developments and challenges in SLM of HEAs,including defects,microstructures,and properties,as well as strengthing prediction models of fabricated HEAs.This review also offers directions for future studies to address existing challenges and promote technological advancement.

Extended damage range of(Al0.3Cr0.2Fe0.2Ni0.3)3O4 high entropy oxide films induced by surface irradiation

Irradiation makes structural materials of nuclear reactors degraded and failed.However,the damage process of materials induced by irradiation is not fully elucidated,mostly because the charged particles only bombarded the surface of the materials(within a few microns).In this work,we investigated the effects of surface irradiation on the indirect irradiation region of the(Al0.3Cr0.2Fe0.2Ni0.3)3O4 high entropy oxide(HEO)films in detail by plasma surface interaction.The results show that the damage induced by surface irradiation significantly extends to the indirect irradiation region of HEO film where the helium bubbles,dislocations,phase transformation,and the nickel oxide segregation were observed.

Parametric study of spark plasma sintering of Al_(20)Cr_(20)Fe_(25)Ni_(25)Mn_(10) high entropy alloy with improved microhardness and corrosion

Multicomponent Al_(20)Cr_(20)Fe_(25)Ni_(25)Mn_(10) alloys were synthesized using spark plasma sintering at different temperatures(800,900,and 1000℃)and holding times(4,8,and 12 min)to develop a high entropy alloy(HEA).The characteristics of spark plasma-synthesized(SPSed)alloys were experimentally explored through investigation of microstructures,microhardness,and corrosion using scanning electron microscopy coupled with energy dispersive spectroscopy(EDS),Vickers microhardness tester,and potentiodynamic polarization,respectively.Xray diffraction(XRD)characterization was employed to identify the phases formed on the developed alloys.The EDS results revealed that the alloys consisted of elements selected in this work irrespective of varying sintering parameters.The XRD,EDS,and scanning electron microscopy collectively provided evidence that the fabricated alloys were characterized by globular microstructures exhibiting face-centered cubic phase,which was formed on a basis of solid solution mechanism.This finding implies that the SPSed alloy showed the features of HEAs.The alloy produced at 1000℃ and holding time of 12 min portrayed an optimal microhardness of HV 447.97,but the value decreased to HV 329.47 after heat treatment.The same alloy showed an outstanding corrosion resistance performance.The increase in temperature resulted in an Al_(20)Cr_(20)Fe_(25)Ni_(25)Mn_(10) alloy with superior density,microhardness,and corrosion resistance over the other alloys developed at different parameters.