High-entropy Carbides Research Articles

New Route to Synthesize High-Entropy Carbide Powders by Mechanical Alloying

A new route for the synthesis of the (NbTa)0.67\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{0.67}$$\\end{document}(TiHfZr)0.33\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{0.33}$$\\end{document}C and (NbTa)0.67\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{0.67}$$\\end{document}(MoHfW)0.33\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{0.33}$$\\end{document}C high-entropy carbides powders by mechanical alloying is presented. Namely, pure metal powders were wet-milled in toluene using a planetary mill under an argon atmosphere. The synthesized materials crystallize with a rock-salt structure (space group Fm3¯m\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Fm{\\bar{3}}m$$\\end{document}). A significant reduction in structural defects with annealing of the samples was detected with X-ray diffraction and positron annihilation experiments. Simultaneously, only simple metallic properties without any trace of magnetic or superconducting phase transitions were observed.

Self-Propagating High-Temperature Synthesis of High-Entropy Carbides in the Gasless Thermal Explosion Mode

Self-Propagating High-Temperature Synthesis of High-Entropy Carbides in the Gasless Thermal Explosion Mode

The Characteristics of Light (TiCrAl0.5NbCu)CxNy High-Entropy Coatings Deposited Using a HiPIMS/DCMS Technique

Multi-component high-entropy (TiCrAl0.5NbCu)CxNy coatings targeting applications requiring medium-to-high friction and wear-resistant surfaces were fabricated through the co-sputtering of elemental targets in an Ar + CH4 + N2 reactive atmosphere using a hybrid HiPIMS/DCMS technique. Two sets of samples were fabricated: (a) (TiCrAl0.5NbCu)Cx high-entropy carbides (HEC) and (b) (TiCrAl0.5NbCu)CxN0.13 high-entropy carbonitrides (HECN), 0 ≤ x ≤ 0.48. The structural, mechanical, tribological, and corrosion resistance properties were thoroughly investigated. The metallic sample exhibits a single BCC structure that changes to FCC via an intermediary amorphous phase through the addition of C or N to the content of the films. The crystallinity of the FCC phases is enhanced and the density of the films decreases down to 5.5 g/cm3 through increasing the carbon fraction up to 48%. The highest hardness of about 16.9 GPa and the lowest wear rate of about 5.5 × 10−6 mm3/Nm are presented by the samples with the largest carbon content, x = 0.48. We found a very good agreement between the evolution of H/E and H3/E2 parameters with carbon content and the tribological behavior of the coatings. The best corrosion resistance was presented by the low-carbon carbonitride samples, showing a charge transfer resistivity of about 3 × 108 Ω∙cm, which is more than three times larger than that of the metallic HEA. The best tribological characteristics for envisioned application were presented by (TiCrAl0.5NbCu)C0.3N0.13, showing a coefficient of friction of 0.43 and a wear rate of about 7.7 × 10−6 mm3/Nm.

SELF-PROPAGATING HIGH-TEMPERATURE SYNTHESIS OF HIGH-ENTROPY CARBIDES IN THE REGIME OF A GASLESS THERMAL EXPLOSION

High-entropy carbides are a new class of inorganic compounds promising for a wide range of applications. The paper presents a new concept for the synthesis of powders of high-entropy carbides by the method of self-propagating high-temperature synthesis (SHS) in the gasless thermal explosion mode from previously mechanically synthesized and structured reaction mixtures. For the first time, high-entropy carbides TaTiNbVWC5 and TaNbVMoWC5 were obtained by this method, their crystal structure was determined, which was compared with similar compositions obtained by sintering.

Synthesis of high-entropy carbides (TiTaNb)xHfyZrzC with strong thermal-oxidative resistant properties by mechanical alloying and spark plasma sintering

The paper presents the synthesis of single-phase high-entropy carbides (TiZrHfTaNb)C, (TiTaNb)0.45Hf0.275Zr0.275С and (TiTaNb)0.3Hf0.35Zr0.35С by mechanical alloying and spark plasma sintering. High-entropy carbides are promising as a material for jet engine parts. Mechanical alloying mode was obtained, in which the homogeneity of the powder and low technical grinding is achieved.The analysis of the microstructure, phase and chemical composition of the obtained samples was carried out. HEC with an FCC structure and a small content of zirconium and hafanium oxides is formed at a temperature of 1600 °C. Increasing the sintering temperature to 2000 °C promotes the dissolution of oxides and the formation of a single-phase HEC. The microhardness of the samples ranged from 1600 to 2000 HV. The compressive strength of the samples was from 600 to 800 MPa. According to the results of gas dynamic tests, the (TiTaNb)0.3Hf0.35Zr0.35С alloy showed excellent thermal-oxidative resistance up to a temperature of 2250 °C.

Influence of Alternative Hard and Binder Phase Compositions in Hardmetals on Thermophysical and Mechanical Properties

Due to the classification of Co as a CMR (carcinogenic, mutagenic, toxic to reproduction) as well as the classification of both Co and WC as CRM (critical raw materials) more and more research is being carried out to investigate possible substitutes for WC-Co hardmetals. To directly compare their microstructure as well as mechanical and thermophysical properties, five very different hardmetals were investigated. For this purpose, the compositions WC-Co, WC-FeNiMn, WC-HEA, NbC-Co and HEC-Co were selected in order to investigate alternative binders for cobalt as well as different alternative hard phases for WC. The results of the hardness measurements showed that for the hardmetals with alternative binders (WC-FeNiMn and WC-HEA) hardness values of 1327 HV10 and 1299 HV10 comparable to WC-Co with 1323 HV10 can be achieved. When WC is replaced by HEC as the hard phase, a significantly higher hardness of 1543 HV10 can be obtained, demonstrating the great potential of high-entropy carbides. Furthermore, the hot hardness measurements between RT and 900 °C showed significantly higher values (up to approx. 290 HV10) for the WC-HEA and HEC-Co hardmetals compared to those of WC-Co. However, the fracture toughness of the alternative hardmetals was lower compared to that of conventional WC-Co hardmetals. In terms of thermophysical properties, the results of the hardmetals with alternative binders were close to those of WC-Co. Thus, it can be shown that it is possible to produce alternative hardmetals with comparable properties to WC-Co and that with further optimization they show great potential to replace WC-Co in the near future.

Joining of C/C composite with high entropy alloy interlayers via spark plasma sintering and its mechanical strength at 1600 ℃

A novel method is developed to join C/C composites via spark plasma sintering (SPS) using ZrHfNbTa and TiZrHfTa high entropy alloys as interlayers. The joint microstructure, interface reaction and shear strength at 1600 ℃ were investigated. For both interlayers, the C/C joint consisted of a single high entropy carbide phase. However, the use of different high entropy alloy interlayers resulted in differences in the micro-homogeneity of the formed high entropy carbide as well as the thickness of the joint zone. Both formed (Zr-Hf-Nb-Ta)C and (Ti-Zr-Hf-Ta)C high entropy carbides, which were homogeneous on the atomic scale, and no impurities were found at the grain boundaries. The shear strengths of the C/C-ZrHfNbTa-C/C joint and C/C-TiZrHfTa-C/C joint at 1600 ℃ were comparable, with values of 23.5 ± 1.2 MPa and 24.6 ± 2.5 MPa, respectively, which are more than twice the shear strength obtained with pure Ti foil (11.6 ± 3.6 MPa).

A simple route to synthesize high-entropy carbide (Hf0.2Zr0.2Ti0.2Ce0.2La0.2)C1-δ nanoparticles with large covalent radius difference

Synthesis of high-entropy carbides containing lanthanide elements for improved high-temperature oxidation resistance is still a challenge due to the large covalent radius difference. Nanoparticles of a novel high-entropy carbide ceramic, (Hf0.2Zr0.2Ti0.2Ce0.2La0.2)C1-δ were successfully synthesized by a combination of organic sol-gel route and carbothermal reduction. The (Hf0.2Zr0.2Ti0.2Ce0.2La0.2)Ox crystals with average size of ∼3 nm were synthesized at 220 °C in benzyl alcohol bath, dominated by the nucleation kinetics mechanism. Phase-pure (Hf0.2Zr0.2Ti0.2Ce0.2La0.2)C1-δ was achieved by using the (Hf0.2Zr0.2Ti0.2Ce0.2La0.2)Ox crystals and glucose as the carbon source in Ar atmosphere at 1500 °C. Average crystal size of the (Hf0.2Zr0.2Ti0.2Ce0.2La0.2)C1-δ nanoparticles was ∼30 nm estimated by XRD spectra and TEM, exhibited rock-salt phase and high elemental homogeneity. The synthesis of (Hf0.2Zr0.2Ti0.2Ce0.2La0.2)C1-δ nanoparticles at low temperature was attributed to the fast nucleation that depending on high atomic homogeneity of the ultra-fine (Hf0.2Zr0.2Ti0.2Ce0.2La0.2)Ox crystals.

Superhard bulk high-entropy carbides with enhanced toughness via metastable in-situ particles

Despite the extremely high hardness of recently proposed high-entropy carbides (HECs), the low fracture toughness limits their applications in harsh mechanical environment. Here, we introduce a metastability engineering strategy to achieve superhard HECs with enhanced toughness via in-situ metastable particles. This is realized by developing a (WTaNbZrTi)C HEC showing a solid solution matrix with uniformly dispersed in-situ tetragonal and monoclinic ZrO2 particles. Apart from a high hardness of 21.0 GPa, the HEC can obtain an enhanced fracture toughness of 5.89 MPa·m1/2, significantly exceeding the value predicted by rule of mixture and that of other reported HECs. The toughening effect is primarily attributed to the transformation of the metastable tetragonal ZrO2 particles under mechanical loading, which promotes crack tip shielding mechanisms including crack deflection, crack bridging and crack branching. The work demonstrates the concept of using in-situ metastable particles for toughening bulk high-entropy ceramics by taking advantage of their compositional flexibility.

Valence electron concentration as key parameter to control the fracture resistance of refractory high-entropy carbides.

Although high-entropy carbides (HECs) have hardness often superior to that of parent compounds, their brittleness-a problem shared with most ceramics-has severely limited their reliability. Refractory HECs in particular are attracting considerable interest due to their unique combination of mechanical and physical properties, tunable over a vast compositional space. Here, combining statistics of crack formation in bulk specimens subject to mild, moderate, and severe nanoindentation loading with ab initio molecular dynamics simulations of alloys under tension, we show that the resistance to fracture of cubic-B1 HECs correlates with their valence electron concentration (VEC). Electronic structure analyses show that VEC ≳ 9.4 electrons per formula unit enhances alloy fracture resistance due to a facile rehybridization of electronic metallic states, which activates transformation plasticity at the yield point. Our work demonstrates a reliable strategy for computationally guided and rule-based (i.e., VEC) engineering of deformation mechanisms in high entropy, solid solution, and doped ceramics.

A practical guideline for designing high-entropy carbides based on (Ti1/3Mo1/3W1/3)C with high valence electron concentration

Introductionof high entropy and high valence electron concentration (VEC) design criterion into TiC ceramic is a promising method for further improving its thermal stability and mechanical performance. Herein, structural, mechanical, electronic properties and stability of (M0.25Ti0.25Mo0.25W0.25)C (M=Cr, V, Nb, Ta, Zr, Hf) high-entropy carbides (HECs) were systematically investigated by combining density functional theory (DFT) and experimental method. All of the six HECs are stable. Among them, (Ta0.25Ti0.25Mo0.25W0.25)C owns the highest melting temperature, ductility and the best fracture toughness. (Zr0.25Ti0.25Mo0.25W0.25)C possesses the highest friction strength and excellent hardness. (Hf0.25Ti0.25Mo0.25W0.25)C exhibits the highest hardness and compressive strength as well as good fracture toughness. In addition, (Cr0.25Ti0.25Mo0.25W0.25)C was successfully fabricated to further prove the synthesizing feasibility of all samples in this study, and formation mechanism of the monophasic carbide solid solutions was probed. This research provides instructive information for further developing TiC ceramics and designing high-entropy carbides with high VEC with excellent mechanical performance.

Synthesis, microstructure and electromagnetic wave absorption properties of high-entropy carbide powders

With the continuous development of microwave technology, the electromagnetic (EM) environment we face is becoming increasingly complex. EM wave absorbing materials, which convert EM wave energy into other forms of energy such as thermal energy, have attracted much attention in EM shielding technology for reducing and isolating EM interference. Finding absorbing materials with both strong and wideband absorption performance in 2–18 GHz is particularly important. In this paper, (Zr0.2Ti0.2Hf0.2Ta0.2Cr0.2)C, (Zr0.2Ti0.2Hf0.2Ta0.2Ni0.2)C, (Zr0.2Ti0.2Hf0.2Nb0.2Cr0.2)C and (Zr0.2Ti0.2Hf0.2Nb0.2Ni0.2)C high-entropy carbide powders were prepared through calculation and design, and their properties were compared with TaC and NbC. According to the analysis of phase composition, microstructure, and EM wave absorption performance, the prepared high-entropy carbides have formed single-phase solid solutions, and their absorption performance has been improved. It can be noted that (Zr0.2Ti0.2Hf0.2Nb0.2Ni0.2)C has the minimum reflection loss (RL) of − 42.61 dB, and it has the maximum effective absorption bandwidth (EAB) of 3.60 GHz at the thickness of 1.00 mm. Through the analysis of EM parameters, it was found that high-entropy carbides with different compositions changed the dielectric loss, magnetic loss, and their coupling effect. This provides important insights for designing absorbing materials with ultra-thin and good conductivity.

Formation ability descriptors for high-entropy carbides established through high-throughput methods and machine learning

Formation ability descriptors for high-entropy carbides established through high-throughput methods and machine learning

PRODUCING METHOD OF HIGH ENTROPY CARBIDE IN AN IONIC MELT

Refractory metal carbides TiC, ZrC, HfC, NbC and TaC have excellent physical, chemical and mechanical properties as materials for ultra-high temperature ceramics. Of these, the most refractory are TaC and HfC, whose melting points approach 4000°C. It should be noted the high hardness, strength and wear resistance of refractory carbides. Hence, there is a natural interest in high-entropy carbides based on them, which are becoming an important class of new ceramic materials, since they potentially have more advanced applied properties. However, obtaining such materials by classical metallurgical methods is a difficult task. In modern research, samples of high-entropy carbides are most often synthesized using expensive special equipment (methods of plasma-spark sintering, high-energy planetary mills, etc.) and a relatively long preparation of precursors for sample production. This paper describes a new approach to the synthesis of multicomponent carbide (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C using an electrochemical process at a temperature not exceeding 1173 K. The method is based on the phenomenon of currentless metal transfer in molten salts. After the step-by-step transfer of metals, the sample was washed from the electrolyte, then sintered in a vacuum furnace. According to X-ray phase analysis, the resulting high-entropy carbide is a single-phase solid solution with an FCC structure. The diffraction pattern of the synthesized sample is in good agreement with the calculated diffraction pattern obtained by the Debye formula for a supercell of 64 000 atoms. A compact sample of high-entropy carbide was produced by pressing a tablet 10 mm in diameter with the addition of cobalt as a matrix metal. After vacuum sintering, the sample was ground to prepare for examination on a scanning electron microscope. Elemental mapping of the sample surface was performed, which showed a satisfactory distribution of metals that make up the high-entropy carbide. The measured microhardness of the sample turned out to be less than the values found in the publications of other authors, which may be due to some residual sample porosity.

Sol-gel method preparation and high-rate energy storage of high-entropy ceramic (FeCoCrMnNi)C porous powder

Among the existing electrochemical capacitive energy storage electrode materials, the (TiNbTaZrHf)C, (VCrNbMoZr)2N, (CoCrFeMnNi)3O4, (FeCoCrMnMg)3O4 and (FeCoCrMnCuZn)3O4 all have excellent capacitive performance, but the energy density is limited due to the narrow potential window. In order to solve the problem of low energy density of electrochemical supercapacitor, a kind of high-entropy carbides (HEMCs) with broad potential window is designed from generalized analysis of electrochemical window of active elements. This high-entropy carbide contains five active elements of Fe, Co, Cr, Mn and Ni. The prepared HEMC-3 has abundant pores and a specific surface area of 417.8 m2 g−1. The large specific surface area makes the HEMCs have high-rate energy storage performance. Electrochemical tests show that the HEMC-3 presents a potential window of [-1.0, 0.6] versus Hg/HgO, and can obtain a specific capacitance of 169.7 F g−1 at a current density of 0.5 A g−1. Due to the designed wide potential window of the prepared HEMCs, the energy density has been greatly improved, which shows a new strategy for developing of electrochemical supercapacitor electrode materials.

Mechanism and kinetics of high-temperature oxidation of medium- and high-entropy carbides in air

Medium- and high-entropy carbides (MECs and HECs) have a wide range of advanced applications and, in many cases, outperform monocarbides. However, the behavior, oxidation mechanisms, and factors defining oxidation resistance of some popular HECs at high temperatures have not been studied in detail yet. In this study, the oxidation behavior of MEC and HECs including (Ta,Ti,Nb,Zr)C, (Ta,Ti,Nb,Zr,Mo)C, (Ta,Ti,Nb,Zr,Hf)C, and (Ta,Ti,Nb,Zr,W)C was investigated under both non-isothermal and isothermal conditions at temperatures up to 1200 °C, and a possible oxidation path was described. It was found that under non-isothermal heating conditions, the oxidation of HECs occurred in three stages, each governed by first-order chemical reactions, and was limited by the formation of ZrO2, Me2Me6O17, and MeMe2O7 oxides, respectively. In the case of isothermal heating, the oxidation of (Ta,Ti,Nb,Zr)C, (Ta,Ti,Nb,Zr,Mo)C, and (Ta,Ti,Nb,Zr,Hf)C follows a logarithmic law and was limited by the formation of a protective Me2Me6O17 layer. In contrast, isothermal oxidation of (Ta,Ti,Nb,Zr,W)C followed a linear law due to the formation of volatile WO3. The results of the study have a high potential for practical application for the production of HEC ceramics for various high-temperature purposes and could be used for further understanding of functional characteristics and development of the new ceramic compositions.

Ablation behavior of high-entropy carbides ceramics (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C upon exposition to an oxyacetylene torch at 2000 °C

The ablation performance of a high-entropy ceramic carbide, (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C, was performed by oxyacetylene ablation flame, simulating the extreme service environment at 2000 ºC. Phase and microstructure characterization at multi-length scales was carried out. During ablation, a compositionally and microstructurally complex oxidation layer formed on the ablation surface, which consisted of a combination of (ZrxHf1-x)6(NbyTa1-y)2O17, Ti(NbxTa1-x)2O7, and Tix(ZraHfbNbcTa1-a-b-c)1-xO2. Based on the microstructure information, the ablation mechanisms were proposed considering the oxidation thermodynamics and kinetics. Comparable rates of O inward diffusion and Ti outward diffusion are suggested, and a particular innermost dense layer composed of isolated (ZrxHf1-x)6(NbyTa1-y)2O17 grains embedded in a continuous Ti(NbxTa1-x)2O7 matrix is considered to be beneficial for a better ablation resistance.

Predicting properties of high entropy carbides from their respective binaries

Using Density Functional Theory calculations, this paper addresses two questions: (1) to what degree can the properties of high entropy carbides created by equi-molar combinations of five of the set of eight refractory metals Hf, Nb, Mo, Ta, Ti, V, W, and Zr be predicted from their respective binary compounds, and (2) can empirical relationships from properties of the binary compounds be used to predict phase stability for these materials. For the former question, it is found that lattice constant, binding energy and bulk modulus are well approximated by binary carbide averages, but carbon vacancy formation energies are less predictable. To address the second question, correlations are explored between binary properties and the entropy forming ability (EFA) of all 56 possible five-element combinations of the eight refractory elements. Significant correlations are not found between cohesive energies or lattice constants of the binary constituents of each composition and that composition’s EFA, but there is a correlation between EFA and the standard deviation of the distribution of bulk moduli of the constituent binaries.

Single-Phase High-Entropy Carbides Synthesized from a Mixture of Pre-Mechanically Alloyed TiZrHfTaNb HEA Powders and Carbon

In this paper, single-phase chemically homogeneous high-entropy ceramics (HEC) were synthesized from a mixture of pre-mechanically alloyed TiZrHfTaNb high-entropy alloy (HEA) powders and carbon. Mechanical alloying (MA) of a TiZrHfTaNb alloy with different process times made it possible to obtain a powder with uniform distribution of chemical elements and with the main phase in the form of a body-centered cubic (BCC) solid solution. HEC with the chemical formula MeC and space group Fm-3m begins to form at temperatures of about 1600 °C in the process of sintering a mixture of pre-mechanically alloyed HEA powders and carbon. A detailed study of the diffraction patterns revealed peaks of mixed zirconium-hafnium oxide (ZrHf)O2, which is also confirmed by the microstructure analysis results and distribution elements. Increasing the process temperature to 2000 °C leads to the formation of a single-phase and chemically homogeneous (TiZrHfTaNb)C HEC.

Reduced He ion irradiation damage in ZrC-based high-entropy ceramics

Excellent irradiation resistance is the basic property of nuclear materials to keep nuclear safety. The high-entropy design has great potential to improve the irradiation resistance of the nuclear materials, which has been proven in alloys. However, whether or not high entropy can also improve the irradiation resistance of ceramics, especially the mechanism therein still needs to be uncovered. In this work, the irradiation and helium (He) behaviors of zirconium carbide (ZrC)-based high-entropy ceramics (HECs), i.e., (Zr_0.2Ti_0.2Nb_0.2Ta_0.2W_0.2)C, were investigated and compared with those of ZrC under 540 keV He ion irradiation with a dose of 1×10¹⁷ cm⁻² at room temperature and subsequent annealing. Both ZrC and (Zr_0.2Ti_0.2Nb_0.2Ta_0.2W_0.2)C maintain lattice integrity after irradiation, while the irradiation-induced lattice expansion is smaller in (Zr_0.2Ti_0.2Nb_0.2Ta_0.2W_0.2)C (0.78%) with highly thermodynamic stability than that in ZrC (0.91%). After annealing at 800 ℃, ZrC exhibits the residual 0.20% lattice expansion, while (Zr_0.2Ti_0.2Nb_0.2Ta_0.2W_0.2)C shows only 0.10%. Full recovery of the lattice parameter (<i>a</i>) is achieved for both ceramics after annealing at 1500 ℃. In addition, the high entropy in the meantime brings about the favorable structural evolution phenomena including smaller He bubbles that are evenly distributed without abnormal coarsening or aggregation, segregation, and shorter and sparser dislocation. The excellent irradiation resistance is related to the high-entropy-induced phase stability, sluggish diffusion of defects, and stress dispersion along with the production of vacancies by valence compensation. The present study indicates a high potential of high-entropy carbides in irradiation resistance applications.