Lattice stability,mechanical and thermal properties of a new class of multicomponent (Fe,Mo, W)6C η carbides with different atomic site configurations

Transition metal carbides are candidates for high-temperature structural ceramics because of their high melting point, high hardness, and high strength. However, one challenge is overcoming their high intrinsic brittleness. In this study, we investigated a new class of (Fe, Mo, W)₆C carbides, which have three Wyckoff positions for metallic atoms (16d, 32e, and 48f) and one Wyckoff position for carbon (16c). These different Wyckoff positions provide a great opportunity to optimize the mechanical properties by the partial replacement of atoms at each Wyckoff position to obtain high-entropy carbides. The current results show that the phonon spectra have no imaginary frequency when Fe occupies the 16d or 32e positions, but a soft mode is observed when Fe occupies 48f. (Fe, Mo, W)₆C η carbides have a higher fracture toughness compared with those of M₃C and MC carbides owing to their low carbon content (14.3 at.%). The mechanical anisotropy of (Fe, Mo, W)₆C is weak, which is beneficial for increasing the damage tolerance. The thermal expansion coefficients of the (Fe, Mo, W)₆C η carbides are predicted to be approximately (8.5–9.5) × 10^?6 K^?1 at 1400 K.     

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.     

Pressure dependent mechanical properties of calcium carbides

The mechanical properties of newly synthesized Ca₂C₃ and Ca₂C under pressure have been studied by using the first-principles calculations with generalized gradient approximation. The equilibrium geometry, elastic stiffness constants, various moduli, and Pugh's ratio of the C2/m phase of Ca₂C₃ and the C2/m and Pnma phases of CaC₂ are systematically studied. The elastic stiffness constants of C2/m-Ca₂C₃ under 0–30GPa, C2/m-Ca₂C under 0–7.5 GPa, and Pnma-Ca₂C under 7.5–30 GPa satisfy the Born?Huang mechanical criteria. The three phases of calcium carbides exhibit ductile characteristics. The surface constructions of bulk and Young's moduli illustrate the mechanical anisotropy of Ca₂C₃ and Ca₂C. Our results are consistent with previously obtained experimental and theoretical data and have significant implications for the application of calcium carbides.     

Structure,mechanical, electronic and thermodynamic properties of Mo5Si3 from first-principles calculations

Transition metal silicides are promising advanced functional materials. However, the structure and relevant properties of Mo₅Si₃ are not well understood. In this work, we investigate the crystal structure, elastic properties, Vickers hardness, elastic anisotropy, electronic and thermodynamic properties of Mo₅Si₃ by using the first-principles calculations. Three structures: tetragonal, hexagonal and orthorhombic structures are considered. The calculated results show that those structures are thermodynamically stable. In particular, we firstly predict that Mo₅Si₃ with hexagonal (P63/mcm) structure is a stable phase. The calculated electronic structure shows that Mo₅Si₃ exhibits better electronic properties because of the charge overlap between Mo-4d state and Si-3p state near the Fermi level. Importantly, Mo₅Si₃ shows the strong deformation resistance and high elastic stiffness in comparison to other TM₅Si₃. Mo₅Si₃ with tetragonal structure has the smaller percentage anisotropy in compressibility and high percentage anisotropy in shear. We further find that the Debye temperature and heat capacity of tetragonal structure are larger than that of hexagonal structure. The high-temperature thermodynamic properties of Mo₅Si₃ are attributed to the vibration of Si atom.     

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.     

Investigation on composition-dependent properties of Mg5xAl23−5xO27+5xN5−5x (0 ≤ x ≤ 1): Part II. Mechanical properties via first-principles calculations combined with bond valence models

Due to the application in transparent windows, armors and domes, it is essential to have a deep insight into the mechanical properties of spinel-type MgAlON solid solution. Mg_5xAl_23?5xO_27+5xN_5?5x were chosen for the investigation of composition-dependent mechanical properties via combining first-principles calculations with bond valence models. The calculated mechanical properties showed a consistence with experimental values, supporting the validity of the investigation. The simulated elastic constants were used to check the mechanical stability and analyze the shear modulus of Mg_5xAl_23?5xO_27+5xN_5?5x. The mechanical properties of Mg_5xAl_23?5xO_27+5xN_5?5x weakened with increasing x. The composition-dependent mechanical properties were explored from the perspective of chemical bonds. The results demonstrated that decline trend of the mechanical properties, including hardness, bulk modulus and shear modulus of Mg_5xAl_23?5xO_27+5xN_5?5x is mainly impacted by the bonds in tetrahedra, since the linear density of bond valence and bond force constant in tetrahedra decreased dramatically, while those in octahedra were almost unchanged.     

Balance between strength and ductility of dilute Fe2B by high-throughput first-principles calculations

In boride ceramics, Fe₂B typically functions as the primary wear-resistant phase owing to its high hardness, but the elastic and ductile-brittle properties of Fe₂B greatly influence its wear resistance and industrial application. First-principles calculations using density functional theory (DFT) were utilized to systematically investigate the effects of Al, Si, and transition elements (including 3d, 4d, and 5d transition elements) on the mechanical properties and electronic structure of Fe₃₁MB₁₆ with a dilute solution model. The bulk modulus (B), shear modulus (G), and Young's modulus (E) were calculated through the stress-strain method. The Vickers hardness (H_V) and fracture toughness (K_IC) were calculated by semi-empirical models. The electronic structures and chemical bonding were analyzed using the charge density difference, electron localization function, and Mulliken bond population. Overall, Fe₃₁YB₁₆ had the largest moduli (B, G, and E) among all the Fe₃₁MB₁₆ compounds that were investigated, while Fe₃₁CrB₁₆ had the highest hardness. Compared with pure Fe₂B, the ductility of Fe₃₁MB₁₆ improved to varying degrees according to the B/G and Poisson's ratio (σ) criteria, respectively. However, adding alloying elements in this concentration did not change the intrinsic brittleness of Fe₂B. Moreover, the trends in the calculated K_IC of Fe₃₁MB₁₆ (M = Cr, Mn, Mo, and W respectively) were consistent with those of experiments. The electronic structure and Mulliken population analysis showed that the differences in the mechanical properties of the Fe₃₁MB₁₆ compounds were primarily determined by the M-B and B–B bonds because the alloying element M replaced Fe. These results provide guidance for improving the ductility of Fe₂B and its industrial application.