High-entropy Carbides Research Articles

Role of carbon vacancies in determining the structural, mechanical, and thermodynamic properties of (HfTaZrNb)C1-x high entropy carbides: a first-principles study

Role of carbon vacancies in determining the structural, mechanical, and thermodynamic properties of (HfTaZrNb)C1-x high entropy carbides: a first-principles study

Compositional optimization for enhanced oxidation resistance of high-entropy carbide ceramics

Excellent oxidation resistance is essential for the viability of high-entropy carbides (HECs) in high-temperature environments. However, the vast HEC compositional space presents a significant challenge in developing desirable formulations with improved oxidation tolerance. In this work, we establish a computational framework to guide the optimization of oxidation-tolerant HEC compositions and further validate the theoretical predictions through systematic oxidation experiments. Leveraging first-principles calculations, we initially determine the heterogeneous oxygen adsorption ability of different constituent elements in HECs, which is subsequently harnessed to regulate preferential oxidation and suppress the formation of detrimental oxides during the oxidation process. Incorporating this fundamental-level understanding, multi-objective optimization (MOO) was performed to identify a compositional region with potentially intrinsic oxidation immunity, achieving a balance between the compactness of the oxidation product and the thermodynamic stability of the HEC matrix. Based on our analysis, we propose a compositional design strategy involving the strategic reduction of elements with higher oxygen adsorption energies, such as Nb, Ta, and Ti in (NbTaZrHfTi)C, to effectively improve the oxidation performance of HECs. The theoretical findings are robustly corroborated by our comparative oxidation experiments, which demonstrate that the selected NbTa-depletion HEC exhibits superior antioxidant performance compared to the equiatomic counterpart. The integrated computational-experimental approach presented in this work serves as a powerful framework to accelerate the discovery and development of oxidation-resistant HECs, paving the way for their expanded applications in demanding high-temperature, oxygen-rich environments.

The effect of configurational entropy and refractory elements on the oxidation behaviour of high entropy carbides containing up to eight refractory elements

High entropy carbides (HECs) have attracted increasing attention due to their promising oxidation resistance properties. The oxidation behaviour of (Zr,Nb,Ta,Hf,Cr)C (HEC5-Cr), (Ti,V,Cr,Zr,Nb,Hf,Ta,W)C (HEC8-Cr), and (Ti,V,Zr,Nb,Mo,Hf,Ta,W)C (HEC8-Mo) powders was compared and studied from room temperature to 1200 °C. It was found that the oxidation onset temperatures (OOT) for the HEC5-Cr, HEC8-Cr, and HEC8-Mo powders were 790 °C, 688 °C, and 617 °C respectively. The high entropy effect benefits the structural stability of carbides and thus improves their oxidation resistance. However, there is no direct correlation between the configurational entropy value and the OOT of the studied HECs. Compared to the configurational entropy value, the refractory elements in HECs and the thermal stability of the produced oxides have a more significant impact on the oxidation resistance of HECs. Most of the group IV, V, and VI metals except W, Mo, and V have main oxides suitable for high-temperature applications, making them attractive candidates for oxidation-resistant HECs.

Effect of processing parameters and carbon to metal ratio on microstructure and mechanical properties of (MoTaTiVW)Cx high entropy carbide produced by reactive spark plasma sintering

(MoTaTiVW)Cx high entropy ceramics with varying carbon to metal ratio in the range of 0.75-0.97 were synthesized by by reactive spark plasma sintering. The effect of ball milling time on particle size, phase evolution and carbon pick-up due to toluene decomposition were studied. Formation of single-phase high entropy carbide having face-centered cubic (FCC) crystal structure was confirmed by X-ray diffraction (XRD), electron backscattered diffraction (EBSD) and Selected area diffraction pattern (SAED) analysis of the sintered compacts. Transmission Kikuchi Diffraction images revealed presence of equiaxed nature of the grains. Transmission electron microscopy images revealed the presence of an intergranular phase at grain boundaries and triple junctions. 3D-Atom Probe Tomography studies showed uniform distribution of elements within the grains with some segregation of impurity elements Fe and Co to the grain boundaries indicating minor contribution of liquid phase sintering to densification. The grain size of single-phase sintered compacts ranged between 1.69 ± 0.60 μm to 4.6 ± 1.0 μm. A low sintering temperature and higher milling time resulted in finer grain size The microhardness was found to increase with milling time and C/M ratio. The microhardness, Young’s modulus and indentation fracture toughness lied between 2221 HV0.1 to 2732 HV0.1, 370 GPa to 473 GPa, 3.6 MPa.m1/2 to 4.4 MPa.m1/2, respectively. The highest microhardness and indentation fracture toughness was found to be 2732 ± 80 HV0.1 and 4.4 MPa.m1/2 on par with reported literature for other high entropy carbides.

Synthesis, microstructure, mechanical and tribological properties of high-entropy carbides (WZrNbTaTi)C-SiCw

In this work, composite high-entropy carbides (WZrNbTaTi)C-x SiCw (x = 0, 5 %, 10 %, 15 %, 20 %) are prepared using the two-step fast hot-pressing sintering (TSFHPS) method. The whiskers in the samples are evenly dispersed and well integrated with the ceramic matrix, forming high density materials. The mechanical properties and tribological properties at room temperature are studied, revealing that the addition of SiCw improves the performance of the materials. When the whisker content is 15 %, the hardness and fracture toughness of composite ceramics are the highest, which are 22.6 GPa and 6.8 MPa m1/2, respectively. The bending strength of the samples increases monotonically with the increase of whisker content, and reaches 639.2 MPa at 20 % content. At the same time, the addition of an appropriate amount of SiCw prevents the propagation of micro-cracks and the expansion of oxidative wear, thereby reducing the friction coefficient and wear rate, and improving the wear resistance of high-entropy carbides.

Recent Advances in High-Entropy Ceramics: Synthesis Methods, Properties, and Emerging Applications

High-entropy ceramics (HECs) represent an emerging class of materials composed of at least five different cations or anions in near-equiatomic proportions, garnering significant attention due to their extraordinary functional and structural properties. While multi-component ceramics have played a crucial role for many years, the concept of high-entropy materials was first introduced eighteen years ago with the synthesis of high-entropy alloys, and the first high-entropy nitride films were reported in 2014. These newly developed materials exhibit superior properties over traditional ceramics, such as enhanced thermal stability, hardness, and chemical resistance, making them suitable for a wide range of applications. High-entropy carbides, borides, oxides, oxi-carbides, oxi-borides, and other systems fall within the HEC category, typically occupying unique positions within phase diagrams that lead to novel properties. HECs are particularly well suited for high-temperature coatings, for tribological applications where low thermal conductivity and similar heat coefficients are critical, as well as for energy storage and dielectric uses. Computational tools like CALPHAD streamline the element selection process for designing HECs, while innovative, energy-efficient synthesis methods are being explored for producing dense specimens. This paper provides an in-depth analysis of the current state of the compositional design, the fabrication techniques, and the diverse applications of HECs, emphasizing their transformative potential in various industrial domains.

Novel synthesis of (Hf0.25Zr0.25Ti0.25Ta0.25)C-SiC biphasic ceramic micro-powders using single-source-precursor route

High-entropy carbides-SiC biphasic ceramics demonstrate outstanding mechanical and oxidation resistance properties. However, previous studies have predominantly focused on fabricating dense monolithic high-entropy carbides-SiC biphasic ceramics, with limited attention given to the production of high-purity, narrow particles size distribution high-entropy carbides-SiC biphasic ceramic powders. In this study, we successfully synthesized high-entropy carbides-SiC biphasic ceramic powders, specifically (Hf0.25Zr0.25Ta0.25Ti0.25)C-SiC, utilizing the polymer-derived-ceramic (PDC) route employing transition metal alcohol solution and methyltrimethoxysilane. The resultant powders exhibited an average particle size of ∼15 μm, with a narrow particle size distribution. Furthermore, variations in the heat treatment temperature demonstrated minimal impact on the particle size, while maintain compositional homogenous from nanoscale to microscale, and exhibiting low oxygen content less than 1.01 %. The introduction of the SiC phase also enhances the dynamic oxidation resistance of the high-entropy carbides. This pioneering work will not only provide a novel approach for obtaining other high-entropy carbides and SiC biphasic ceramic powder but also offers prospects for large-scale fabrication in industrial.

The effects of refractory elements on the properties of quaternary high entropy carbides—A first-principles and experiment study

In this work, the effects of refractory transition metal elements on the properties of quaternary equimolar ratio high entropy carbides composed of ⅣB, ⅤB and ⅥB transition metals and C element were studied by first-principles calculations and experiments.The calculation results show that all 75 kinds of high entropy carbides were thermodynamically and mechanically stable and can be formed spontaneously. Among them, Hf can promote the synthesis of high entropy carbides, while Cr has the opposite effect. In addition, single phase solid solution structures can be formed for all 73 HECs except (TiZrHfCr)C and (ZrHfVCr)C. Thereafter, the mechanical properties of these 75 quaternary HECs were compared. The calculation results show that W, Mo, and Ta have a positive effect on improving the elastic modulus, fracture toughness, hardness and melting point of high entropy carbides, while Cr has the opposite effect. The calculation results of phonon spectra show that the six high entropy carbides represented by (TiHfNbTa)C, (TiVNbTa)C, (TiZrNbTa)C, (TiZrNbCr)C, (TiZrNbMo)C and (TiZrNbW)C have lattice dynamic stability. (TiHfNbTa)C, (TiVNbTa)C, (TiZrNbTa)C, (TiZrNbMo)C and (TiZrNbW)C face-centered cubic high entropy carbides were prepared by spark plasma sintering and the microstructure and properties were tested. The experimental results are basically consistent with the calculated results.

Composition optimization of (Hf, Ta, Zr, Cr)C high‐entropy carbides for good oxidation resistance

AbstractOxidation resistance is crucial to the potential applications of high‐entropy carbides (HECs) at elevated temperatures. Here, we realize the exploration of (Hf, Ta, Zr, Cr)C high‐entropy carbides (HEC‐TM, TM = Hf, Zr, Ta, and Cr) with good oxidation resistance by optimizing their compositions. To be specific, 21 kinds of HEC‐xTM (x = 0–25 mol%) samples are fabricated by a high‐throughput ultrafast high‐temperature sintering technique, followed by oxidation testing at 1673 K for 30 min. Among all the HEC samples, the as‐fabricated HEC‐0Zr samples are proved to possess the best oxidation resistance with an oxidation depth of only 53 µm. Further study on isothermal oxidation kinetics demonstrates that the as‐fabricated HEC‐0Zr samples follow a linear oxidation law. The good oxidation resistance of the as‐fabricated HEC‐0Zr samples is believed to result from the (Ta, Me)2O5 phase with a low melting point, which can promote the densification of the oxide layer. This research opens up a new way for efficiently discovering new HECs for extreme applications.

Mechanical properties of high-entropy carbides prepared by high-pressure sintering: Role of carbon stoichiometry

The scarcity of carbon in high-entropy carbides is widely recognized to significantly affect their fabrication and performance. However, achieving both high hardness and plasticity in these carbides remains a formidable challenge. This study investigates the impact of carbon vacancies on the phase structure and mechanical properties of high-entropy carbides, particularly (Hf0.25Zr0.25Nb0.25Ta0.25)Cx (x = 1, 0.8, 0.6, or 0.5), utilizing density functional theory calculations and experimental analysis. Theoretical computations indicate a rock-salt structure for the carbides, with reduced cell parameters as carbon vacancies increase. Notably, carbides with lower carbon content exhibit softer characteristics, as evidenced by Pugh's ratio. Experimental fabrication and testing reveal a decrease in Vickers hardness with decreasing carbon content, attributed to changes in bonding types. Certain compositions, particularly (Hf0.25Zr0.25Nb0.25Ta0.25)C0.6 and (Hf0.25Zr0.25Nb0.25Ta0.25)C0.5, display plastic behavior alongside high hardness, offering promising avenues for future research and design of high-entropy ceramics.

High-temperature oxidation behavior of high entropy carbide (TaNbTiV)C in atmospheres with different oxygen contents

A high entropy carbide (TaNbTiV)C was fabricated via spark plasma sintering of high entropy powders that were prepared using a molten salt method. The as-fabricated high entropy carbide was then investigated for its high-temperature oxidation behavior at 850 °C, under atmospheres with three different oxygen contents (20 vol%, 40 vol%, and 60 vol%). After the high-temperature oxidation process, the main phases observed in all the specimens were incompletely oxidized phase (NbxTi1-x)CyO2-y and oxides (Nb0.5Ta0.5)2O5, TiO2, and Nb2O5. Importantly, the thickness of the oxide layer initially increased and then decreased as the oxygen content increased. When the oxygen content reached 40 vol%, the ceramic displayed severe oxidation behavior with observed thicker of the oxide layer, as well as numerous pores and cracks in the oxide layer. This work provides new insights into the oxidation behavior of high entropy carbides.

Low-frequency and broadband electromagnetic wave absorption mechanism of (Hf0.2Zr0.2Ti0.2Ce0.2La0.2)C1-δ/PyC microspheres with hierarchy pores from 4 GHz to 17 GHz

Achieving low-frequency broadband electromagnetic wave absorption in extreme environments over 2000 °C is challenging. Biomimetic hierarchy materials of high entropy lanthanide carbides were designed to optimize electromagnetic wave absorption. Composite microspheres of (Hf0.2Zr0.2Ti0.2Ce0.2La0.2)C1-δ and SiC embedded in pyrolytic carbon (MC/PyC) were prepared by using the electro-spray method followed by heat treatment. The specimens of MC/PyC microspheres with a thickness of 2.5 mm exhibited a minimum reflection loss value of − 48.9 dB, and electromagnetic wave absorption from 4.0 GHz to 17 GHz. The enhanced absorption capability is attributed to the synergistic effect of dielectric losses from conduction and polarization, along with magnetic losses due to eddy currents and natural resonance of the high-entropy carbides within the multiscale pores. The composition and featured microstructure MC/PyC microspheres were noteworthily stable under the 2200 °C oxyacetylene ablation, ensuring consistent high-performance electromagnetic wave absorption even in harsh conditions.

Unraveling Lattice-Distortion Hardening Mechanisms in High-Entropy Carbides.

Uncovering the hardening mechanisms is of great importance to accelerate the design of superhard high-entropy carbides (HECs). Herein, the hardening mechanisms of HECs by a combination of experiments and first-principles calculations are systematically explored. The equiatomic single-phase 4- to 8-cation HECs (4-8HECs) are successfully fabricated by the two-step approach involving ultrafast high-temperature synthesis and hot-press sintering techniques. The as-fabricated 4-8HEC samples possess fully dense microstructures (relative densities of up to ≈99%), similar grain sizes, clean grain boundaries, and uniform compositions. With the elimination of these morphological properties, the monotonic enhancement of Vickers hardness and nanohardness of the as-fabricated 4-8HEC samples is found to be driven by the aggravation of lattice distortion. Further studies show no evident association between the enhanced hardness of the as-fabricated 4-8HEC samples and other potential indicators, including bond strength, valence electron concentration, electronegativity mismatch, and metallic states. The work unveils the underlying hardening mechanisms of HECs and offers an effective strategy for designing superhard HECs.

Fermi energy engineering of enhanced plasticity in high-entropy carbides

Mechanical properties of a range of high-entropy rocksalt carbides are investigated via ab-initio modeling and experimental verification. It is found that elastic constants, hardness, and fracture resistance depend on the electronic structure of the system, which is parameterized through two descriptors: the valence electron concentration (VEC) and the integrated density of states from the pseudo-gap energy to the Fermi energy (iDOS(Epg,EF)). Compositions incorporating more electrons (increasing VEC) shift EF further above Epg, filling more deformable metal d-derived t2g bonding orbitals. MoNbTiVWC5, MoNbTaVWC5, CrMoNbVWC5, and CrMoTaVWC5, – stabilizing as a single-phase rocksalt solid solution – each have a VEC ≥ 9.0, an iDOS(Epg, EF) > 5.75 states/cell, and a higher electron abundance than any of the rocksalt binary and ternary carbides. The materials approach the ductility range, achieving an attractive combination of fracture resistance (i.e. enhanced plasticity) and hardness rarely found in ceramic materials. Entropy, allowing high VEC values to be reached within cubic structures, enables a new path for tailoring mechanical properties.

High-throughput assessment of stability and mechanical properties of medium- and high-entropy carbides: Bridging empirical criteria and ab-initio calculations

This study assesses the effectiveness of empirical stability descriptors — the normalized geometric packing parameter, lattice size difference, electronegativities mismatch, and the convex hull analysis based on information on mutually dissolving components, and ab-initio calculated entropy forming ability (EFA) for high-throughput materials discovery in high-entropy ceramics. A sample containing 53,130 medium and high-entropy carbide combinations from the Materials Project database has been analyzed, comparing solid solutions selected based on empirical descriptors and EFA calculations. The suitability of various electronegativity scales is evaluated, while Automatic FLOW (AFLOW) partial occupation (AFLOW-POCC) and special quasirandom structures (SQS) calculations provide insights into enthalpies, bulk moduli, and lattice parameters. It is shown that local distortions play a role in determining the stability of high-entropy ceramics and estimated lattice parameters, Young’s modulus, fracture toughness, and Vickers hardness by computational and experimental approaches. Our analysis demonstrates the efficacy of density functional theory (DFT) approaches for high-throughput screening and highlights the need for the further exploration of the cocktail effect on high-entropy ceramics’ mechanical properties.

Theoretical design and experimental verification of high-entropy carbide ablative resistant coating

Composition design of high-entropy carbides is a topic of great scientific interest for the hot-end parts in the aerospace field. A novel theoretical method through an inverse composition design route, i.e. initially ensuring the oxide scale with excellent anti-ablation stability, is proposed to improve the ablation resistance of the high-entropy carbide coatings. In this work, the (Hf0.36Zr0.24Ti0.1Sc0.1Y0.1La0.1)C1-δ (HEC) coatings were prepared by the inverse design concept and verified by the ablation resistance experiment. The linear ablation rate of the HEC coatings is −1.45 ​μm/s, only 4.78 % of the pristine HfC coatings after the oxyacetylene ablation at 4.18 ​MW/m2. The HEC possesses higher toughness with a higher Pugh's ratio of 1.55 in comparison with HfC (1.30). The in-situ formed dense (Hf0.36Zr0.24Ti0.1Sc0.1Y0.1La0.1)O2-δ oxide scale during ablation benefits to improve the anti-ablation performance attributed to its high structural adaptability with a lattice constant change not exceeding 0.19 % at 2000–2300 ​°C. The current investigation demonstrates the effectiveness of the inverse theoretical design, providing a novel optimization approach for ablation protection of high-entropy carbide coatings.

High-pressure elasticity of novel (VNbTaTi)C high-entropy carbides

Acoustic velocities and elasticity of high P-T synthesized new (VNbTaTi)C high-entropy carbides (HECs) are reported at high pressure using large volume press (LVP)-based ultrasonic interferometry technique. Using the finite-strain equation approach, the bulk modulus and shear rigidity, and their pressure dependences, are derived from the current experimental data, yielding B0 = 261.6(2) GPa, G0 = 191.4(3) GPa, ∂B/∂P = 4.34(18), and ∂G/∂P = 1.83(6). Interestingly, the (VNbTaTi)C HECs is more incompressible than the superhard γ-B (∼213.9 GPa), and almost as incompressible as superhard PcBN (∼254 GPa). Meanwhile, its shear rigidity (∼191.4 GPa) is comparable to the superhard B6O-B4C composite (G = 202 GPa), and as stiff as the (TiZrNbTa)C (193 ∼ 195 GPa) HECs. More strikingly, the fracture toughness is enhanced up to ∼4.6(3) MPa·m1/2, and the Vickers hardness of (VNbTaTi)C HECs is ∼22.6(2) GPa, which are significantly stronger than other high-entropy carbides. These new findings and results may be very important for its applications in extreme environments.

Tailoring crystal structure of high-entropy carbides in Si-based ceramic nanocomposites through precursor engineering

High-entropy carbides with tunable crystallization and growth have been demonstrated using single-source precursor derived ceramic route. The in situ nanocrystallization of high-entropy carbide phases, (Ta0.2W0.2V0.2Mo0.2Nb0.2)SiδC and (Ta0.167W0.167V0.167Mo0.167Nb0.167Si0.167)C in amorphous Si-based ceramic matrices was achieved by using polysiloxanes and polycarbosilanes as polymer precursors respectively. The results exemplify a prominent role of the architectures of the polymeric precursors in controlling the structural features of these ceramics at various length scales. In particular, it was observed that high-entropy carbides with rock salt and zinc blende crystal structures were formed when polysiloxanes and polycarbosilanes with different backbone structure were used as polymeric precursors respectively. This is attributed to the thermodynamics of nucleation of the carbidic phases in these nanocomposites. Furthermore, the precursor architecture that dictates free carbon content, influenced nanostructural features and porosity in the material. Therefore, engineering such compositionally complex phases is feasible by selecting suitable polymeric precursors.

Prediction of grain boundary strength and mechanical responses of high-entropy carbides reinforced by hybrid graphene and SiC nanowire

The excellent performance of high entropy ceramics (HECs) has attracted extreme attention in the most advanced functional applications and engineering structures, yet the poor toughness limits the application of HECs in many fields. As a consequence, advanced technology and new doping phases are urgently demanded to enhance the comprehensive mechanical properties of HEC composites. The microstructure model of 0.2 wt% graphene (G) and SiC nanowires (SiCnw) doped in (HfNbTaTiZr)C is established in Abaqus with Python language. The effects of HEC-HEC interface strength and HEC-G interface strength on crack propagation and mechanical properties of HEC composites were investigated. The improvement of mechanical properties of (HfNbTaTiZr)C with different content of SiCnw was predicted respectively. The composites obtain the optimal combination of mechanical properties at the content of 1.5 wt% SiCnw. The accuracy of the simulation is verified by the experimental data of mechanical properties changing trend and toughening mechanism, providing theoretical support for toughening HEC composites.

A first-principles investigation of light weight high mechanical performance high-entropy carbides: The research of bottom-up compositional designing

Designing high-entropy carbides with light weight and high mechanical performance is a promising strategy for upgrading titanium carbides applied in fields of aerospace materials. Herein, a bottom-up compositional designing method and the density functional theory calculations are combined, and properties of the six high-entropy carbides are investigated developed from the light-weight (Ti1/3Zr1/3V1/3)C medium-entropy carbide. The five of them are predicted to be preparable except for (W0·25Ti0·25Zr0·25V0.25)C. Among them, (Nb0·25Ti0·25Zr0·25V0.25)C exhibits the both low density and excellent mechanical performance: for density 5.471g/cm3, Young's modulus 456.4GPa, Vicker's hardness 23.76GPa, fractural toughness 3.371MPa∙m1/2, low-temperature and high-temperature friction resistance indicators 0.052 and 0.064GPa, and melting point 3974±300K. The ab-initio simulations also reveal its good high-temperature strength and toughness, showing the predictable potential for upgrading traditional titanium carbide based materials.