First-principles study,fabrication, and characterization of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high-entropy ceramic

The formation possibility of (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C high-entropy ceramic (HHC-1) was first analyzed by the first-principles calculations, and then, it was successfully fabricated by hot-pressing sintering technique at 2073 K under a pressure of 30 MPa. The first-principles calculation results showed that the mixing enthalpy and mixing entropy of HHC-1 were −0.869 ± 0.290 kJ/mol and 0.805R, respectively. The experimental results showed that the as-prepared HHC-1 not only had an interesting single rock-salt crystal structure of metal carbides but also possessed high compositional uniformity from nanoscale to microscale. By taking advantage of these unique features, it exhibited extremely high nanohardness of 40.6 ± 0.6 GPa and elastic modulus in the range from 514 ± 10 to 522 ± 10 GPa and relatively high electrical resistivity of 91 ± 1.3 μΩ·cm, which could be due to the presence of solid solution effects.     

Synthesis,mechanical, and thermophysical properties of high-entropy (Zr,Ti,Nb,Ta,Hf)C0.8 ceramic

Two high-entropy carbides, including stoichiometric (Zr,Ti,Nb,Ta,Hf)C and nonstoichiometric (Zr,Ti,Nb,Ta,Hf)C_0.8, were prepared from monocarbides and ZrH₂. Their sinterability, microstructures, mechanical properties, thermophysical properties, and oxidation behaviors were systematically compared. With the introduction of carbon vacancy, the sintering temperature was lowered up to 300°C, Vickers hardness was almost unaffected, whereas the strength decreased significantly generally due to the decrease of covalent bonds. The thermal conductivity shows a 50% decrease for nonstoichiometry high-entropy carbide, which is a major consequence of the lower electrical conductivity. The oxidation resistance in high temperature water vapor was not sensitive to carbon stoichiometry.     

Predictions of structural,electronic, mechanical,and thermodynamic properties of TMBCs (TM = Ti,Zr, and Hf) ceramics

In the present work, we have investigated the structural, electronic, elastic, and thermodynamic properties of transition-metal boron-carbon compounds (TMBCs) (TM = Ti, Zr, Hf) using the first-principles calculations. The results showed that TMBCs are energetically and thermodynamically stable, and the sequence of phase stability is HfBC > ZrBC > TiBC. B-C bonds can be formed in TMBCs ceramics due to the strong hybridization between B-2p and C-2p states. The elastic anisotropies of TMBCs were illustrated by elastic anisotropy indexes, 3D surface constructions, and 2D projections, and the results indicated that the sequence of elastic anisotropy is ZrBC > TiBC > HfBC. Finally, the calculated minimum thermal conductivities, based on the Clarke's and Cahill's models, of all TMBCs are anisotropic with the sequence of ZrBC > TiBC > HfBC.     

(Ti,Mo,W,Ta,V,Nb)(C,N)多元陶瓷相的价电子结构

运用固体与分子经验电子理论(EET理论)计算了(Ti,Mo,W,Ta,V,Nb)(C,N)多元陶瓷相的价电子结构.结果表明,价电子结构参数(nA)随碳化物添加量的增加而增加.不同碳化物对价电子结构参数的影响不同,其中VC的影响最为显著.价电子结构参数(nA)可以用来评价金属陶瓷的力学性能,提出了相关的判据关系式.     

Phase stability,mechanical, and thermal properties of high-entropy transition and rare-earth metal diborides from first-principles calculations

The narrow composition design space of high-entropy transition metal diborides (HE TMB₂) limits their further development. In this study we designed six quaternary and quinary high-entropy transition metal and rare-earth diborides (HE TMREB₂) and investigated their phase stability using the energy distribution of the local mixing enthalpy of all possible configurations. The results show that both quaternary and quinary HE TMREB₂ have higher enthalpic driving forces, which facilitates the formation of single-phase AlB₂-type structures between TMB₂ and REB₂. Calculations of elastic constants show that the TMB₂ component has the greatest effect on the c₄₄ elastic constant and shear modulus G, while REB₂ significantly influences the bulk modulus B. Furthermore, LuB₂ and TmB₂ substantially affect the elastic modulus anisotropy of HE TMB₂. Rare-earth atoms in HE TMREB₂ can enhance the nonharmonic interactions between phonons, which results in a significant hindrance in the thermal transport of low-frequency phonons as well as an increase in the volume thermal expansion coefficients. Thus, the incorporation of REB₂ into HE TMB₂ has a significant impact on the phase stability and properties.     

Synthesis and characterization of (Zr1/3Nb1/3Ti1/3)C metal carbide solid-solution ceramic

(Zr_1/3Nb_1/3Ti_1/3)C metal carbide solid-solution ceramic has been successfully fabricated by hot pressing sintering at 2473 K using ZrC, NbC, and TiC powders as raw materials. The results show that the as-prepared solid-solution ceramic possesses a single rock-salt crystal structure of metal carbides and simultaneously exhibits high compositional uniformity from nanoscale to microscale. By taking advantage of these unique features, it shows relatively high hardness of 38.8 ± 4.4 Gpa and elastic modulus of 481.8 ± 31.0 Gpa and relatively low thermal conductivity of 17.1 ± 0.3 W/(m·K) and thermal diffusivity of 6.1 ± 0.1 mm²/s, which may attribute primarily to the presence of solid solution effects.     

Synthesis,microstructure, physical and mechanical properties of phase-pure entropy-enhanced (Nb0.8Ti0.05Ta0.05V0.05M0.05)4AlC3 (M = Hf,Zr) ceramics

The first 413-phase entropy-enhanced (Nb_0.8Ti_0.05Ta_0.05V_0.05M_0.05)₄AlC₃ (M = Hf, Zr) (EEMAX_Hf and EEMAX_Zr) ceramics were successfully consolidated by spark plasma sintering (SPS) using Nb, Ti, Ta, V, Zr, Hf, Al and graphite as initial materials. The formation of solid solution with five transition metals at the M sites of hexagonal M₄AlC₃ unit cell was confirmed by elemental analyses. Compared with pure Nb₄AlC₃, both the electrical and thermal conductivities of the entropy-enhanced ceramics showed a slight decrease, which is attributed to the lattice distortion and the increasing lattice defects that prevents the transfer of electrons and phonons. On the other hand, the mechanical properties of entropy-enhanced ceramics were greatly enhanced compared to pure Nb₄AlC₃. The measured fracture toughness of EEMAX_Hf and EEMAX_Zr ceramics were 8.2 MPa·m^1/2 and 10.0 MPa·m^1/2, respectively, which were increased by 18.8% and 44.9% compared to Nb₄AlC₃. The compressive strength of EEMAX_Hf and EEMAX_Zr ceramics were 987 MPa and 1187 MPa, respectively, being 92.0% and 130.9% higher than that of Nb₄AlC₃, respectively. EEMAX_Hf and EEMAX_Zr ceramics also possessed the higher Vickers hardness of 6.8 GPa and 7.4 GPa, respectively.     

Effects of (Ti,Ta,Nb,W)(C,N) on the microstructure,mechanical properties and corrosion behaviors of WC-Co cemented carbides

Homogeneous solid-solution (Ti, Ta, Nb,W)(C,N) powders were synthesized through carbothermal reduction-nitridation method. The effects of (Ti, Ta, Nb,W)(C,N) powders on the microstructure, mechanical properties and corrosion resistance of WC-10Co cemented carbides were investigated using XRD, SEM, electrochemical test and mechanical properties tests. The results showed that cemented carbides with pre-alloyed powder addition had a similar microstructure appearance: weak core/rim structure consisting of solid-solution phase embedded in the WC-Co system. The black core and gray rim, both of which contained similar elements, were identified as (Ti, Ta, Nb,W)(C,N), but the latter contained higher amount of heavy elements.With the addition of (Ti, Ta, Nb,W)(C,N) powders, the density, transverse rupture strength and fracture toughness of samples decreased monotonously. However, the hardness rose sharply at first, reached a peak at 15 wt% solid-solution addition, then slightly decreased, and finally increased again. Results also revealed that increasing (Ti, Ta, Nb,W)(C,N) made the open circuit potential (OCP) in 1 M sulphuric acid solution more negative than that of WC-Co, and all specimens exhibited pseudo-passivation phenomenon in the test solution. In addition, increasing pre-alloyed powders led to decreasing corrosion current density, which implies that (Ti, Ta, Nb,W)(C,N) could remarkably improve the corrosion resistance of WC-Co cemented carbides.     

Theoretical investigation of anisotropic mechanical and thermal properties of ABO3 (A=Sr,Ba; B=Ti,Zr, Hf) perovskites

As promising TBC (thermal barrier coating) candidates, perovskite oxides own designable properties for their various options of cations and structural diversity, but limited comprehensions of structure‐property relationship delay their engineering applications. In this work, mechanical/thermal properties of ABO₃ (A=Sr, Ba; B=Ti, Zr, Hf) perovskites and their anisotropic nature are predicted employing density functional theory. Their theoretical minimum thermal conductivities range from 1.09 to 1.74 W·m^?1·K^?1, being lower than Y₂O₃ partially stabilized ZrO₂. Reduced thermal conductivities up to 16% along particular directions are reached after considering thermal conductivity anisotropy. All compounds own high hardness while SrZrO₃, SrHfO₃, and BaHfO₃ possess well damage tolerance. We found that small electronegativity discrepancy leads to big anisotropy of chemical bond, Young's/shear moduli and thermal conductivities, together with good damage tolerance. These results suggest that the next generation TBCs with extra low thermal conductivity should be achieved through combining material design and orientation‐growth tailoring.     

The mechanical properties and electronic structures of V,Cr, Mn,Fe, Ni atoms doped MoCoB by first-principles calculation

MoCoB-based cermets have been regarded as the potential substitution of WC cermets with high hardness, high melting point and high oxidation resistance. Ternary borides-based cermets are widely used in extreme environment, such as high-pressure environment. Therefore, it is significant to explore the mechanical properties and electronic structures of transition elements X (X = V, Mn, Fe, Ni) atoms doped MoCoB under high pressure, which are performed by first-principles calculations to provide guidance for industry applications. The analysis of cohesive energy and formation enthalpy indicates high pressure leads to unstable states with lower lattice constants and crystal volumes. The deviation of cohesive energy and formation enthalpy indicate Mo₄Co₃FeB₄ and Mo₄Co₃NiB₄ have similar stability. The shear modulus, Young's modulus and bulk modulus increase under high pressure, which consists with the increasing of covalence. The variation of ductility and anisotropy indicate similar upward trend, which is verified by Poisson's ratio, B/G ratio and anisotropy index A^U. The analysis of overlap population indicates high pressure leads to the increasing of covalence of B-Co covalent bonds and the decreasing of the covalence of B-Mo covalent bonds. The analysis of electronic structures indicates the high pressure leads to higher hybridization and lower density of states of metallic bonds. The analysis of charge density difference consists with the variation of mechanical properties, implying shorter bond length and higher bonds strength under high pressure.     

Temperature-dependent elastic and thermodynamic properties of ZrC,HfC, and their solid solutions (Zr0.5Hf0.5)C

Zirconium carbide (ZrC) and hafnium carbide (HfC) have been identified as ultrahigh temperature ceramics with excellent thermal conductivity performance. The temperature profiles of ZrC and HfC have been studied; however, the temperature-dependent of solid solution of (Zr_0.5Hf_0.5)C is still lacking. Herein, we report the temperature-dependent elastic and thermodynamic properties of (Zr_0.5Hf_0.5)C using first-principles calculations. The covalent characters of ZrC, HfC, and (Zr_0.5Hf_0.5)C are weakened at high temperatures by analyzing their respective electronic structures. In addition, the equilibrium volumes at different temperatures can be determined from the energy–volume (E–V) curves under the quasi-harmonic approximation. Throughout the temperature ranges studied, the HfC material shows the highest bulk modulus and lowest thermal expansion. When T > 1000 K, (Zr_0.5Hf_0.5)C exhibits better shear and Young's modulus performance close to HfC and shows the highest anisotropy. The lattice thermal conductivity decreased as temperature increased for ZrC, HfC, and (Zr_0.5Hf_0.5)C, and (Zr_0.5Hf_0.5)C has the smallest lattice thermal conductivity. These results provide fundamental and useful information for the practical application of ZrC, HfC, and (Zr_0.5Hf_0.5)C.     

Ordered structure and mechanical properties of ternary Sc0.5TM0.5B2 (TM = Ti,V, Zr) alloys under high pressure

The structural and mechanical properties of ScB₂ and Sc_0.5TM_0.5B₂ (TM = Ti, V, Zr) alloys are investigated in the pressure range of 0–150 GPa based on density functional theory. The ground state structures of ScB₂ are screened out by structural substitution, and the P6/mmm is determined as the initial structure of alloying research according to structural stability. The structures of alloy generation and recognition (SAGAR) code combined with first-principles calculations selected the stable structures of Sc_0.5TM_0.5B₂ (TM = Ti, V) and Sc_0.5Zr_0.5B₂ alloys as ordered structure types Ⅰ and Ⅱ, respectively. The formation enthalpy, phonon dispersion and elastic constants demonstrate that Sc_0.5TM_0.5B₂ (TM = Ti, V, Zr) alloys are thermodynamically, dynamically and mechanically stable. In the whole pressure range, the elastic moduli of Sc_0.5TM_0.5B₂ (TM = Ti, V, Zr) increased significantly compared with ScB₂. This indicates that the introduction of TM improves the mechanical properties of ScB₂. The Vickers hardness (H_V) of the ScB₂ ground state is 42.4 GPa, and the H_V of the Sc_0.5TM_0.5B₂ (TM = Ti, V, Zr) alloys are increased to 47.8, 50.3 and 44.8 GPa, respectively. The electronic structures and chemical bonding reveal that the Sc_0.5TM_0.5B₂ (TM = Ti, V, Zr) alloys have stronger B–B, B–TM covalent bonds, charge interaction, and higher valence electron density, which significantly improves the hardness. The results show that ScB₂ and Sc_0.5TM_0.5B₂ (TM = Ti, V, Zr) alloys are potential superhard multifunctional materials.     

Hardness and toughness improvement of SiC-based ceramics with the addition of (Hf0.2Mo0.2Ta0.2Nb0.2Ti0.2)B2

The influences of different contents ranging 0–15 wt% of high-entropy boride (HEB) (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ on the mechanical properties of SiC-based ceramics using Al₂O₃-Y₂O₃ sintering additives sintered by spark plasma sintering process were investigated in this study. The results showed that the introduction of 5 and 10 wt% (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ could facilitate the densification and the grain growth of SiC-based ceramics via the mechanism of liquid phase sintering. However, the grain growth of SiC-based ceramics was inhibited by the grain boundary pinning effect with the addition of 15 wt% (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂. The SiC-based ceramics with 15 wt% (Hf_0.2Mo_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ showed the enhanced hardness (21.9±0.7 GPa) and high toughness (4.88±0.88 MPa·m^1/2) as compared with high-entropy phase-free SiC-based ceramics, which exhibited a hardness of 16.6 GPa and toughness of 3.10 MPa·m^1/2. The enhancement in mechanical properties was attributed to the addition of higher hardness of HEB phase, crack deflection toughening mechanism, and presence of residual stress due to the mismatch of coefficient of thermal expansion.     

First-principles investigation of new structure,mechanical and electronic properties of Mo-based silicides

The adjustment of strength and ductility of high-temperature ceramics is still a big challenge. Although Mo-based silicides are promising high-temperature materials, the influence of Mo concentration on the mechanical and electronic properties of Mo-based silicides is unclear. In addition, it is necessary to explore the novel Mo-based silicides. In this paper, we present results of novel phases, mechanical and electronic properties of the stable Mo-based silicides within various stoichiometries. Two new Mo-based silicides: MoSi (Cmcm and Pnma) and Mo₂Si (I4/mcm) are predicted. The calculated results show that the volume deformation resistance of Mo-based silicides increases with increasing Mo concentration. MoSi₂ shows the strongest elastic stiffness and shear deformation resistance due to the strong Mo-Si bonds. The calculated intrinsic hardness of MoSi₂ (37.7 GPa) is much larger than that of other Mo-based silicides. In particular, MoSi₂ and MoSi show brittle behavior. However, other silicides exhibit ductility. We further find that high concentration of Mo can improve the electronic properties of Mo-based silicides because of the formation of Mo-Mo metallic bond. Finally, our works indicate that the adjustment of the Mo stoichiometric ratio to improve the mechanical and electronic properties of Mo-based silicides.     

Change in mechanical properties,composition, and structure of hydrated polyurethaneurea

Relative to the extensive study on the relationship between the structure and properties of polyurethane (PU) and polyurethaneurea (PUU), basic information available on hydrated PUU is limited. In this study, PUU films were immersed in a saline solution (0.9 wt % NaCl, 37°C) for periods of up to 60 days and evaluated by the change in static and dynamic mechanical properties, composition, and hydrogen‐bonding structure. It was found that immersion in a saline solution could greatly increase the average molecular weight and degree of phase separation, which enhanced the mechanical strength and reduced the flexibility. The increase in the average molecular weight is believed to come from the chain extensions or crosslink reactions between intramolecules and/or between intermolecules and from the leaching out of poly(tetramethylene oxide) (PTMO) oligomers. FTIR analysis indicated that H₂O molecules did not obviously alter the hydrogen‐bonding structure of PUU. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 252–260, 2001     

Pressureless sintering and properties of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high-entropy ceramics: The effect of pyrolytic carbon

A novel (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C high-entropy ceramic was successfully prepared by pressureless sintering at 2200 °C. With increasing content of resin-derived-carbon, the density, and mechanical and thermal properties increased up to a maximum content of 2～4 wt% resin addition, after which further addition was detrimental. All specimens showed high strength (≥347±36 MPa), with the highest value achieving 450±64 MPa, and fracture toughness significantly higher (>20 %) than those of the corresponding monocarbides and Ta_0.5Hf_0.5C, (Ta_1/3Zr_1/3Nb_1/3)C. The thermal conductivity was approximately equivalent to the lowest value of the corresponding mono-carbides, which was assumed to be due to the lattice distortion effect.     

Prediction of solid solution characteristics of MC (M = Zr,Nb, and Ta) in TiC lattice using phase stability diagrams

Phase stability diagrams of Ti-M-O-C (M = Zr, Nb, and Ta, molar ratio of Ti and M = 1:1) systems at 1800 K were drawn as a function of the carbon activity, oxygen partial pressure, and solution formation characteristics. The solid solution characteristics were varied with the kind of M. The solid solution carbide, (Ti_0.5Zr_0.5)C, was less stable than the TiC-ZrC mixture while other solid solution carbides (Ti_0.5Nb_0.5)C and (Ti_0.5Ta_0.5)C were more stable than the mixtures of monocarbides. Thus, the (Ti_0.5Zr_0.5)C phase was not included in the phase stability diagram of the Ti-Zr-O-C system unlike the other solid solution carbides. The validity of the drawn stability diagrams was proved by experimental results. Thus, the conditions for synthesis of solid solution carbides by carbothermal reduction, or fabrication of TiC-based composites with solid solution phases, can be deduced using the phase stability diagrams.     

Insight into the structures,melting points,and mechanical properties of NbSi2 from first-principles calculations

Studies are carried out on the equilibrium structural, mechanical properties, and melting points of NbSi₂ with four ground-state crystal structures (C40, C11_b, C54, and C49) using first-principles approach. By means of the calculated formation enthalpies and phonon dispersion, it is found that these NbSi₂ phases are thermodynamically and dynamically stable. C54-NbSi₂ is uncovered to possess the lowest energy and formation enthalpy, implying that it is expected to be the most favorite structure for NbSi₂. The results of the calculated elastic constants reveal that four NbSi₂ phases are mechanically stable. We further find that the mechanical properties of C54-NbSi₂ are superior to those of the other NbSi₂ phases. The melting points of these NbSi₂ phases are calculated to examine their thermal stability. The elastic anisotropy is calculated and discussed using three patterns. The results prove that C54- and C40-NbSi₂ have good elastic isotropy, as confirmed by the given three-dimensional plots of elastic moduli. Analyzing the difference charge density and Mulliken overlap population provides the explanation about the relationship between bonding characteristics and mechanical properties.     

Mechanical properties,thermal expansion performance and intrinsic lattice thermal conductivity of AlMO4 (M=Ta,Nb) ceramics for high-temperature applications

The vital thermo-mechanical properties of thermal and environment barrier coatings (TBCs/EBCs) include high hardness, low Young's modulus, matching thermal expansion coefficients (TECs) with substrate and low thermal conductivity. The effect of distortion degree of crystal structure on thermo-mechanical properties of AlMO₄ (M=Ta, Nb) ceramics are assessed in this work. AlMO₄ ceramics display modest TECs and no phase transformation is detected from room temperature to 1200?℃. The experiment thermal conductivity can be dropped further as the theoretical minimum thermal conductivity of AlTaO₄ and AlNbO₄ is 1.48?W?m^?1 K^?1 and 1.05?W?m^?1 K^?1, respectively. The temperature dependent phonon thermal diffusivity of AlMO₄ ceramics has been confirmed; the intrinsic lattice thermal conductivity is determined. The extraordinary thermo-mechanical properties make it clear that AlMO₄ ceramics are suitable for high-temperature applications.     

Effect of ZrO2 whiskers on the microstructure and mechanical properties of a Ti(C,N)-based cermet cutting tool material

ZrO₂-whisker-reinforced Ti(C,N)-based cermets were designed and prepared via a vacuum hot-pressing process. The ZrO₂ whisker characteristics and the phase, microstructure and mechanical properties of the cermets were investigated, and the relevant toughening mechanisms were examined. The results indicate that the ZrO₂ whiskers retain their morphology and significantly improve the mechanical properties of the cermets. The relative density and hardness decreased slightly with increasing ZrO₂ whisker content. At a whisker content of 7.5 wt.%, the fracture toughness and flexural strength reached their maximum values of 7.825 MPa·m^1/2 and 1094 MPa, respectively. This enhancement in the mechanical properties is due to stronger bonding between the interfaces through dimples, tearing ridges and whisker pull-out, in addition to the inhibition of crack propagation via crack deflection, crack branching and crack bridging.