A Nitride-Reinforced NbMoTaWHfN Refractory High-Entropy Alloy with Potential Ultra-High-Temperature Engineering Applications

Refractory high-entropy alloys(RHEAs)have promising applications as the new generation of hightemperature alloys in hypersonic vehicles,aero-engines,gas turbines,and nuclear power plants.This study focuses on the microstructures and mechanical properties of the NbMoTaW(HfN)_(x)(x=0,0.3,0.7,and 1.0)RHEAs.The alloys consist of multiple phases of body-centered cubic(BCC),hafnium nitride(HfN),or multicomponent nitride(MN)phases.As the x contents increase,the grain size becomes smaller,and the strength gradually increases.The compressive yield strengths of the NbMoTaWHfN RHEA at ambient temperature,1000,1400,and 1800℃ were found to be 1682,1192,792,and 288 MPa,respectively.The high-temperature strength of this alloy is an inspiring result that exceeds the high temperature and strength of most known alloys,including high-entropy alloys,refractory metals,and superalloys.The HfN phase has a significant effect on strengthening due to its high structural stability and sluggish grain coarsening,even at ultra-high temperatures.Its superior properties endow the NbMoTaWHfN RHEA with potential for a wide range of engineering applications at ultra-high temperatures.This work offers a strategy for the design of high-temperature alloys and proposes an ultra-high-temperature alloy with potential for future engineering applications.

Microstructure and mechanical properties of a low activation cast WTaHfTiZr refractory high-entropy alloy

In the face of the requirement that nuclear fusion reactor materials exhibit more excellent thermal,mechanical and physical properties,a novel refractory highentropy alloy,WTaHfTiZr was proposed.The constituent elements were selected in consideration of low activation,high melting point and high thermostability.The alloys were prepared by arc melting.The as-cast alloy shows a dendrite microstructure with two disordered BCC phases,which caused by the preferential nucleation of W and Ta with much higher melting points during solidification.It exhibits a high compressive yield strength of 1,900 MPa and fracture strain of 8.1% at room temperature,and its yield strengths are up to 612 MPa at 700 ℃ and 203 MPa at 1,000 ℃,respectively.The high strengths are attributed mainly to solid solution strengthening and second phase strengthening.This alloy shows great promise as one of the next-generation nuclear fusion reactor materials.

Phase,microstructure and compressive properties of refractory high-entropy alloys CrHfNbTaTi and CrHfMoTaTi

New refractory high-entropy alloys,CrHfNbTaTi and CrHfMoTaTi,derived from the well-known HfNbTaTiZr alloy through principal element substitution were prepared using vacuum arc melting.The phase components,microstructures,and compressive properties of the alloys in the as-cast state were investigated.Results showed that both alloys were composed of BCC and cubic Laves phases.In terms of mechanical properties,the yield strength increased remarkably from 926 MPa for HfNbTaTiZr to 1258 MPa for CrHfNbTaTi,whereas a promising plastic strain of around 15.0%was retained in CrHfNbTaTi.The morphology and composition of the network-shaped interdendritic regions were closely related to the improved mechanical properties due to elemental substitution.Dendrites were surrounded by an incompact interdendritic shell after Mo incorporation,which deteriorated yield strength and accelerated brittleness.

Effect of Ti content on microstructure and properties of TixZrVNb refractory high-entropy alloys

This study aimed to investigate the microstructure and mechanical properties of TixZrVNb(x=1,1.5,2)refractory high-entropy alloys at room and elevated temperatures.The TiZrVNb alloy consisted of the body-centered cubic(bcc)matrix with a small amount of V2Zr phase.The Ti1.5ZrVNb and Ti2ZrVNb alloys exhibited a single-phase bcc structure.At room temperature,the tensile ductility of the as-cast alloys increased from 3.5%to 12.3%with the increase in the Ti content.The TixZrVNb alloys exhibited high yield strength at 600°C,and the ultimate yield strength was more than 900 MPa.Softening occurred at 800°C,but the ultimate yield strength could still exceed 200 MPa.Moreover,the TixZrVNb alloys displayed low densities but high specific yield strengths(SYSs).The lowest density of TixZrVNb alloys was only 6.12 g/cm^3,but the SYS could reach about 180 MPa·cm^3·g^−1,which is better than those of most reported high-entropy alloys(HEAs).

Laser metal deposition of refractory high-entropy alloys for high-throughput synthesis and structure-property characterization

Progress in materials development is often paced by the time required to produce and evaluate a large number of alloys with different chemical compositions.This applies especially to refractory high-entropy alloys(RHEAs),which are difficult to synthesize and process by conventional methods.To evaluate a possible way to accelerate the process,high-throughput laser metal deposition was used in this work to prepare a quinary RHEA,TiZrNbHfTa,as well as its quaternary and ternary subsystems by in-situ alloying of elemental powders.Compositionally graded variants of the quinary RHEA were also analyzed.Our results show that the influence of various parameters such as powder shape and purity,alloy composition,and especially the solidification range,on the processability,microstructure,porosity,and mechanical properties can be investigated rapidly.The strength of these alloys was mainly affected by the oxygen and nitrogen contents of the starting powders,while substitutional solid solution strengthening played a minor role.

Microstructure characteristics and mechanical properties of NbMoTiVWSix refractory high-entropy alloys

Refractory high-entropy alloys are considered as potential structural materials for elevated temperature applications.To obtain refractory high-entropy alloys with high strength,different amounts of Si were added into the NbMoTiVW refractory high-entropy alloys.The effects of Si on the phase constitution,microstructure characteristics and mechanical properties of NbMoTiVWSi_(x) alloys were investigated.Results show that when the addition of Si is 0,0.025 and 0.05(molar ratio),the alloys are consisted of primary BCC and secondary BCC in the intergranular area.When the addition of Si is increased to 0.075 and 0.1,eutectic structure including silicide phase and secondary BCC phase is formed.The primary BCC phase shows dendritic morphology,and is refined by adding Si.The volume fraction of intergranular area is increased from 12.22%to 18.13%when the addition of Si increases from 0 to 0.1.The ultimate compressive strength of the NbMoTiVW alloy is improved from 2,242 MPa to 2,532 MPa.Its yield strength is also improved by the addition of Si,and the yield strength of NbMoTiVWSi_(0.1) reaches maximum of 2,298 MPa.However,the fracture strain of the alloy is decreased from 15.31%to 12.02%.The fracture mechanism of the alloys is changed from mixed fracture of ductile and quasi-cleavage to cleavage fracture with increasing of Si.The strengthening of alloys is attributed to the refinement of primary BCC phase,volume fraction increment of secondary BCC phase,and formation of eutectic structure by addition of Si.

Effects of Nitrogen Doping on Microstructures and Irradiation Resistance of Ti-Zr-Nb-V-Mo Refractory High-Entropy Alloy

Interstitial strengthening with nitrogen(N)is one of the effective ways to improve the mechanical properties of HEAs,but the effects of N on the microstructures and mechanical properties of the irradiated HEAs have not been studied extensively.Here,the microstructures and mechanical properties of N-free and N-doped Ti_(2)ZrNbV_(0.5)Mo_(0.2)HEAs before and after He irradiation were investigated.The results showed that the solid solution strengthening caused by interstitial N improved the yield strength at room temperature and 1023 K without significantly reducing plasticity.N doping significantly promoted the growth,aggregation and wider spatial distribution of He bubbles by enhancing the mobility of He atoms/He-vacancy complexes,with the average size of He bubbles increasing from 10.4 nm in N-free HEA to 31.0 nm in N-doped HEA.In addition,N-doped HEA showed a much higher irradiation hardness increment and hardening fraction than N-free HEA.Contrary to conventional materials doped with N,the introduction of N into Ti_(2)ZrNbV_(0.5)Mo_(0.2)HEA had adverse effects on its resistance to He bubble growth and irradiation hardening.The results of this study indicated that N doping may not improve the irradiation resistance of HEAs.

Microstructure Recrystallization and Mechanical Properties of a Cold-Rolled TiNbZrTaHf Refractory High-Entropy Alloy

The equiatomic TiNbZrTaHf alloy was successfully rolled at room temperature with the reduction of ~ 85%. The microstructure and tensile properties were investigated after cold working and annealing. It was determined that the recrystallization temperature of the TiNbZrTaHf alloy between 850 and 900 ℃. Complete recrystallization and normal grain growth occurred, the high stability of single phase was maintained after annealing at 1000, 1200, and 1400 ℃. But the precipitated phase appeared after long term annealing, as seen after 500 h at 1000 ℃. After cold working, the tensile strength and the elongation of TiNbZrTaHf were 1137 MPa and 25.1%, respectively. The annealed alloy has a high tensile strength (σ_(b )= 863 MPa) and ductility (ε_(e )= 28.5%). Moreover, the oxidation of TiNbZrTaHf alloy at elevated temperatures has a significant impact on its mechanical properties. The poor oxidation resistance of TiNbZrTaHf can accelerate tensile failure by inducing fractures at grain boundaries.

Refractory high-entropy alloys:A focused review of preparation methods and properties

In recent years,high-entropy alloys(HEAs)have become prominent metallic materials due to their unique design strategies and excellent mechanical properties.The HEAs-inherent high-entropy,lattice-distortion,sluggish-diffusion,and cocktail effects make HEAs maintain high strength,oxidation resistance,corrosion resistance,wear resistance,and other excellent comprehensive properties,showing stronger competitiveness relative to traditional alloys.Refractory high-entropy alloys(RHEAs)are considered as a new kind of high-temperature materials with great application prospects due to their excellent mechanical properties and have the potential to replace nickel-based superalloy as the next generation of high-temperature materials.We reviewed the research status and preparation methods of RHEAs in recent years,including the metallurgical smelting,powder metallurgy,magnetron sputtering,and additive manufacturing technologies.The microstructure and phase-transformation process of RHEAs were analyzed.The mechan-ical properties and main strengthening and toughening mechanisms of RHEAs,such as solid-solution strengthening,precipitation strengthening,and the transformation-induced plasticity(TRIP),were discussed,and the deformation mechanism of RHEAs was revealed.The properties of RHEAs,including high strength,oxidation resistance,corrosion and wear resistance were reviewed.RHEAs will meet the huge market demand in the engineering materials field,but there are still many challenges,such as the tradeoff between high strength and high ductility,structural design,and performance optimization of RHEAs with brittle BCC structures.We believe that this combination of knowledge may shape the future of RHEAs and break through the mutually exclusive conundrum of high strength and high toughness for RHEAs.

Light-weight refractory high-entropy alloys: A comprehensive review

In recent years,high-entropy alloys(HEAs)have become a research hotspot in materials community,and great progress has been made in exploring various high-performance HEAs.As a special class,the light-weight refractory high-entropy alloys(RHEAs)own both excellent high-temperature comprehensive properties and low density and have accordingly attracted more and more attention.In this paper,we presented a comprehensive review of the recent progress and status in light-weight RHEAs.Based on an exhausting search of the literature reports,one strategy in terms of phase numbers after preparation was first proposed to classify the light-weight RHEAs into three categories.Then,the status on the fundamental thermodynamic and thermophysical data/databases,computational approaches for alloy designing,and preparation/fabrication techniques of light-weight RHEAs was introduced one after another.After that,the progress on mechanical properties and oxidation/corrosion/wear behaviors of light-weight RHEAs at room and high temperatures was summarized.Finally,the conclusions of this review were drawn.By pointing out the shortcomings of the current research,the follow-up development directions in the field of light-weight RHEAs were also given.

Dynamic compression behavior of TiZrNbV refractory high-entropy alloys upon ultrahigh strain rate loading

In this study,the dynamic compressive response behavior of a body-centered cubic(BCC)single-phase TiZrNbV refractory high-entropy alloy(RHEA)was investigated under impact at speeds of 313-1584 m s^(-1)using two-stage,gas-gun-driven,high-speed plate-impact experiments;recovery sample analysis;and theoretical calculations.The strain rate and pressure were approximately 10^(7) s^(−1) and 5.07-29.37 GPa,respectively.The results showed that the TiZrNbV RHEA had a Hugoniot elastic limit of 4.12-5.86 GPa and a spall strength of 1.84-2.03 GPa.The initial yield strength of the alloy showed a strong strain-rate dependence and could be described by the modified Zerilli-Armstrong model,while the phonon-damping effect was the main reason for its high strain-rate sensitivity.Microstructural analysis showed that the dynamic deformation of the TiZrNbV RHEA was controlled by the dislocation slip,dislocation proliferation,intersection of the deformation bands,and grain refinement.The analysis also showed that the intergranular,transgranular,and mixed-type cracks dominated the spall failure of the material.The dynamic Hall-Petch effect and pinning from the lattice distortion led to high dynamic yield strength.The critical strain rate for the phonon drag effect was positively related to the relative atomic mass and local strain field of the metals.Within the experimental loading range,the RHEA showed good structural stability,and simultaneously,the theoretical calculation method for the equation of state based on a cold-energy mixture could accurately predict its shock-response behavior.The valence-electron concentration(VEC)had a direct effect on the shock-compression properties of the HEAs;higher VEC implied more difficulty in compressing the HEAs.The findings of this study provide insights into understanding the mechanical response characteristics of RHEAs under extreme conditions such as high-speed impact and ultrahigh strain-rate loading.

Relationship between the unique microstructures and behaviors of high-entropy alloys

High-entropy alloys(HEAs),which were introduced as a pioneering concept in 2004,have captured the keen interest of nu-merous researchers.Entropy,in this context,can be perceived as representing disorder and randomness.By contrast,elemental composi-tions within alloy systems occupy specific structural sites in space,a concept referred to as structure.In accordance with Shannon entropy,structure is analogous to information.Generally,the arrangement of atoms within a material,termed its structure,plays a pivotal role in dictating its properties.In addition to expanding the array of options for alloy composites,HEAs afford ample opportunities for diverse structural designs.The profound influence of distinct structural features on the exceptional behaviors of alloys is underscored by numer-ous examples.These features include remarkably high fracture strength with excellent ductility,antiballistic capability,exceptional radi-ation resistance,and corrosion resistance.In this paper,we delve into various unique material structures and properties while elucidating the intricate relationship between structure and performance.

Effect of hafnium and molybdenum addition on inclusion characteristics in Co-based dual-phase high-entropy alloys

Specific grades of high-entropy alloys(HEAs)can provide opportunities for optimizing properties toward high-temperature applications.In this work,the Co-based HEA with a chemical composition of Co_(47.5)Cr_(30)Fe_(7.5)Mn_(7.5)Ni_(7.5)(at%)was chosen.The refractory metallic elements hafnium(Hf)and molybdenum(Mo)were added in small amounts(1.5at%)because of their well-known positive effects on high-temperature properties.Inclusion characteristics were comprehensively explored by using a two-dimensional cross-sectional method and extracted by using a three-dimensional electrolytic extraction method.The results revealed that the addition of Hf can reduce Al_(2)O_(3)inclusions and lead to the formation of more stable Hf-rich inclusions as the main phase.Mo addition cannot influence the inclusion type but could influence the inclusion characteristics by affecting the physical parameters of the HEA melt.The calculated coagulation coefficient and collision rate of Al_(2)O_(3)inclusions were higher than those of HfO_(2)inclusions,but the inclusion amount played a larger role in the agglomeration behavior of HfO_(2)and Al_(2)O_(3)inclusions.The impurity level and active elements in HEAs were the crucial factors affecting inclusion formation.

Comprehensive insights into recent innovations:Magnesium-inclusive high-entropy alloys

This review focuses on thermodynamic and physical parameters,synthesis methods,and reported phases of Magnesium(Mg)containing high-entropy alloys(HEAs).Statistical data of publications concerning Mg-containing HEAs were collected and analyzed.Data on the chemical elements included in Mg-containing HEAs,their theoretical end experimental densities,thermodynamic parameters,physical parameters,fabricated techniques and reported phases were also collected and discussed.On the basis of this information,a new classification for HEAs was proposed.It is also shown that the existing thermodynamic parameters cannot accurately predict the formation of a single phase solid solution for Mg-containing HEAs.The physical parameters of Mg-containing HEAs are within a wide range,and most of the synthesized Mg-containing HEAs have a complex multiphase structure.

Evolution of helium bubbles in FeCoNiCr-based high-entropy alloys containing γ′ nanoprecipitates

A series of high-entropy alloys(HEAs) containing nanoprecipitates of varying sizes is successfully prepared by a non-consuming vacuum arc melting method.In order to study the irradiation evolution of helium bubbles in the FeCoNiCrbased HE As with γ' precipitates,these samples are irradiated by 100-keV helium ions with a fluence of 5 × 10~(20) ions/m~2 at 293 K and 673 K,respectively.And the samples irradiated at room temperature are annealed at different temperatures to examine the diffusion behavior of helium bubbles.Transmission electron microscope(TEM) is employed to characterize the structural morphology of precipitated nanoparticles and the evolution of helium bubbles.Experimental results reveal that nanosized,spherical,dispersed,coherent,and ordered L1_2-type Ni_3Ti γ' precipitations are introduced into FeCoNiCr(Ni_3Ti)_(0.1) HEAs by means of ageing treatments at temperatures between 1073 K and 1123 K.Under the ageing treatment conditions adopted in this work,γ' nanoparticles are precipitated in FeCoNiCr(Ni_3Ti)_(0.1) HE As,with average diameters of 15.80 nm,37.09 nm,and 62.50 nm,respectively.The average sizes of helium bubbles observed in samples after 673-K irradiation are 1.46 nm,1.65 nm,and 1.58 nm,respectively.The improvement in the irradiation resistance of FeCoNiCr(Ni_3Ti)_(0.1) HEAs is evidenced by the diminution in bubbles size.Furthermore,the FeCoNiCr(Ni_3Ti)_(0.1) HEAs containing γ' precipitates of 15.8 nm exhibits the minimum size and density of helium bubbles,which can be ascribed to the considerable helium trapping effects of heterogeneous coherent phase boundaries.Subsequently,annealing experiments conducted after 293-K irradiation indicate that HEAs containing precipitated phases exhibits smaller apparent activation energy(E_a) for helium bubbles,resulting in larger helium bubble size.This study provides guidance for improving the irradiation resistance of L1_2-strengthened high-entropy alloy.

Effect of Mn content on microstructure and properties of AlCrCuFeMnx high-entropy alloy

AlCrCuFeMnx(x=0,0.5,1,1.5,and 2)high-entropy alloys were prepared using the vacuum arc melting technology.The microstructure and mechanical properties of AlCrCuFeMnxwere analyzed and tested by XRD,SEM,TEM,nanoindentation,and electronic universal testing.The results indicate that the AlCrCuFeMnxhigh-entropy alloy exhibits a dendritic structure,consisting of dendrites with a BCC structure,interdendrite regions with an FCC structure,and precipitates with an ordered BCC structure that form within the dendrite.Manganese(Mn)has a strong affinity for dendritic,interdendritic,and precipitate structures,allowing it to easily enter these areas.With an increase in Mn content,the size of the precipitated nanoparticles in the dendritic region initially increases and then decreases.Similarly,the area fraction initially decreases and then increases.Additionally,the alloy’s strength and wear resistance decrease,while its plasticity increases.The Al Cr Cu Fe Mn1.5alloy boasts excellent mechanical properties,including a hardness of 360 HV and a wear rate of 2.4×10^(-5)mm^(3)·N^(-1)·mm^(-1).It also exhibits impressive yield strength,compressive strength,and deformation rates of 960 MPa,1,700 MPa,and 27.5%,respectively.

Atomistic evaluation of tension–compression asymmetry in nanoscale body-centered-cubic AlCrFeCoNi high-entropy alloy

The tension and compression of face-centered-cubic high-entropy alloy(HEA) nanowires are significantly asymmetric, but the tension–compression asymmetry in nanoscale body-centered-cubic(BCC) HEAs is still unclear. In this study,the tension–compression asymmetry of the BCC Al Cr Fe Co Ni HEA nanowire is investigated using molecular dynamics simulations. The results show a significant asymmetry in both the yield and flow stresses, with BCC HEA nanowire stronger under compression than under tension. The strength asymmetry originates from the completely different deformation mechanisms in tension and compression. In compression, atomic amorphization dominates plastic deformation and contributes to the strengthening, while in tension, deformation twinning prevails and weakens the HEA nanowire.The tension–compression asymmetry exhibits a clear trend of increasing with the increasing nanowire cross-sectional edge length and decreasing temperature. In particular, the compressive strengths along the [001] and [111] crystallographic orientations are stronger than the tensile counterparts, while the [110] crystallographic orientation shows the exactly opposite trend. The dependences of tension–compression asymmetry on the cross-sectional edge length, crystallographic orientation,and temperature are explained in terms of the deformation behavior of HEA nanowire as well as its variations caused by the change in these influential factors. These findings may deepen our understanding of the tension–compression asymmetry of the BCC HEA nanowires.

High-entropy alloys in thermoelectric application:A selective review

Since the superior mechanical,chemical and physical properties of high-entropy alloys(HEAs)were discovered,they have gradually become new emerging candidates for renewable energy applications.This review presents the novel applications of HEAs in thermoelectric energy conversion.Firstly,the basic concepts and structural properties of HEAs are introduced.Then,we discuss a number of promising thermoelectric materials based on HEAs.Finally,the conclusion and outlook are presented.This article presents an advanced understanding of the thermoelectric properties of HEAs,which provides new opportunities for promoting their applications in renewable energy.

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.

Local chemical inhomogeneities in TiZrNb-based refractory high-entropy alloys

Multi-principal element solid solutions are prone to develop local chemical inhomogeneities,i.e.,chemi-cal order/clustering and/or compositional undulation.However,these structural details from short-range(first couple of nearest-neighbor atomic shells)to nanometer length scale are very challenging to re-solve in both experimental characterization and computer simulations.For instance,Monte Carlo model-ing based on density-functional-theory calculations is severely limited by the sample size and the sim-ulation steps practical in the simulations.Adopting the cluster expansion approach,here we systemati-cally reveal the local chemical inhomogeneity,including chemical order and compositional fluctuation,in three representative equiatomic TiZrNb-based body-centered cubic refractory high-entropy alloys(HEAs):TiZrNb,TiZrHfNb and TiZrHfNbTa.Ti-Zr pairs are found to exhibit the highest degree of chemical pref-erence among all atomic pairs.Such chemical short-range order(CSRO)induces an accompanying com-positional undulation,both extending to characteristic dimensions of the order of one nanometer.The chemical inhomogeneity trend uncovered for this series of TiZrNb-based HEAs is expected to impact their mechanical properties;e.g.,incorporating the CSRO effects in a current model significantly improves its agreement with experimental measured yield strength.