[en] The hot corrosion behavior of polycrystalline Ti₃SiC₂ under thin films of Na₂SO₄ was studied at 900 and 1000 deg. C in air. The microstructure and composition of the scales were investigated by scanning electron microscope/energy dispersive spectroscope and X-ray diffraction. The results demonstrated that Ti₃SiC₂ suffered from serious attack during hot corrosion at 900 and 1000 deg. C. The corroded scale had a duplex microstructure, the outer layer consisted of coarse grains with pores, the inner layer consisted of fine grains and was compact. The whole corroded layer consisted of a mixture of TiO₂ and SiO₂ after hot corrosion attack, which was different from the scale formed during the oxidation of Ti₃SiC₂ in air

[en] ZrB_2 based ultra-high temperature ceramics (UHTCs) exhibit a unique combination of excellent properties that makes them promising candidates for applications in extreme environments. Evaluating the correlation between microscopic defects and macroscopic performance of these materials is crucial for the design of UHTCs. The present work deals with a first-principles investigation on segregations of solute atoms (Y, Nb, Ta, Mo and W) in ZrB_2 grain boundaries and their influences on grain boundary strengths. Opposite segregation tendency between Y and Nb, Ta, Mo or W is obtained, where Y prefers sites with long M-B bonds, while Nb, Ta, Mo or W prefers sites with short M-B bonds. The short equilibrium M-B (M = Nb, Ta, Mo or W) bonds induce local contractions around grain boundaries, which in turn strengthens grain boundaries remarkably, thereby enhances the mechanical properties of ZrB_2 at evaluated temperatures. In contrast, segregation of Y poisons grain boundaries due to local expansions induced by long Y-B bonds, which will deteriorate the performance of ZrB_2 based UHTCs. The results provide useful guidelines for the design of ZrB_2 based UHTCs, since grain boundaries play a key role in determining high temperature mechanical properties.

[en] Superhard materials are usually found in high symmetry and high bond density compounds like diamond and cubic BN. Herein, using a combination of first-principles calculations and the microscopic and macroscopic models for hardness prediction, three new possible superhard boron-rich metal borides with low symmetry and open structure are predicted. The predicted hardness values are 37.9 GPa, 37.5 GPa and 41.3 GPa for YbAlB₁₄, LuAlB₁₄ and ScMgB₁₄, respectively, from the macroscopic model and 45.1 GPa, 50.8 GPa and 50.4 GPa, respectively, from the microscopic model. The reliability of this work is supported by comparing the measured hardness of AlMgB₁₄ and Y_0.62Al_0.71B₁₄ with the calculated Vickers hardness of AlMgB₁₄ and YAlB₁₄. The metal elements are found to influence the anisotropy of B−B bonds and elastic properties beside transferring valence electrons to the boron framework and stabilizing the crystal structure. From elastic property angle, it is the near-isotropic elastic modulus and high shear deformation resistance underpinned by the nearly isotropic B−B bonding that warrant these low symmetry and open structured ternary borides superhard.

[en] We investigated the stable (0001) surfaces of M₂AlC (M = Ti, V and Cr) using the first-principles plane-wave pseudopotential total energy method. Four possible (0001) terminations were considered by breaking the M-Al and M-C bonds. The corresponding surface energies were calculated and compared. The Al- and M(C)-terminated (0001) surfaces demonstrated better stability than the C- and M(Al)- terminated surfaces by their much lower surface energies. In addition, the stability of surfaces was predicted under various chemical environments as a function of chemical potentials. We further investigated the character of surface relaxations. The electronic structures of the stable Al- and M(C)-terminated surfaces were analyzed

[en] Highlights: • Detail electronic structure and chemical bonding of MAlB phases are disclosed. • The direction-dependence and temperature-dependence of mechanical properties of MAlB phases are investigated. • The effects of transition metals on the structure and elastic properties are discussed. Layered ternary borides, which also named as 'MAB' phases, are close structural analogs to the 'MAX' phases and believed to be more ductile and resistant to oxidation than binary borides. Although attentions have been drawn on their atomic arrangement and ground-state elastic properties, their thermal expansions and maintainability of mechanical properties at high temperatures have not been fully understood. Herein, with the combination of density functional theory (DFT) calculations and quasi-harmonic approximation (QHA), the mechanical and thermal properties of two MAB phases, MAlB (M = Mo, W), at ground-states and high temperatures were thoroughly investigated. The effects of transition metals on the structure, elastic anisotropy, thermal expansion, and temperature-dependent mechanical properties are discussed in detail. Possible improvement of high temperature mechanical properties of MoAlB by substitution of Mo by W is proposed.

[en] High temperature mechanical and thermodynamic properties of TiB₂ are important to its applications as ultrahigh temperature ceramic, which were not well understood. In this study, the thermodynamic and mechanical properties of TiB₂ were investigated by the combination of first principle and phonon dispersion calculations. The thermal expansion of TiB₂ was anisotropic, α_c/α_a is nearly constant (1.46) from 300 K to 1500 K, theoretically. The origination of this anisotropy is the anisotropic compressibility. The heat capacity at constant pressure was estimated from the theoretical entropy and fitted the experimental result quite well when higher-order anharmonic effects were considered. Theoretical isentropic elastic constants and mechanical properties were calculated and their temperature dependence agreed with the existed experiments. From room temperature to 1500 K, the theoretical slope is −0.0211 GPa·K⁻¹, −0.0155 GPa·K⁻¹, and −0.0384 GPa·K⁻¹ for B, G, and E, respectively. Our theoretical results highlight the suitability of this method in predicting temperature dependent properties of ultrahigh temperature ceramics and show ability in selecting and designing of novel ultrahigh temperature ceramics

[en] The influence of deposition temperature on phase formation of V₂AlC is studied by magnetron sputtering from elemental targets. At substrate temperatures below 750 ⁰C, we observed the formation of Al_xV_y and V₂C using x-ray diffraction (XRD) analysis. At 750 ⁰C, a phase pure polycrystalline V₂AlC film on a ∼12 nm thick transition layer has been observed using XRD and transmission electron microscopy. Selected area electron diffraction indicates that the film grown on the transition layer consists of phase pure V₂AlC. As the substrate temperature is increased to 850 ⁰C, the formation of V₂C in addition to the V₂AlC phase is observed. This may be due to desorption of aluminium causing the decomposition of V₂AlC into vanadium carbides and aluminium. The V₂AlC film is fully dense and polycrystalline and the elastic modulus based on nanoindentation is within the expected error margin consistent with previously reported theoretical calculations and the diamond anvil cell measurement of bulk V₂AlC samples.

[en] We investigate the elastic stiffness and electronic band structure of nanolaminate M₂AlC (M=Ti,V,Nb, and Cr) ceramics by using the ab initio pseudopotential total energy method. The relationship between elastic stiffness and valence electron concentration (VEC) is discussed. The results show that the bulk and shear moduli enhance monotonously as VEC increases in M₂AlC. The shear modulus c₄₄, which by itself represents a pure shear shape change and is directly related to hardness, reaches its maximum when the VEC is in the range of 8.4-8.6. This implies that the bulk modulus, shear modulus, and hardness vary in different trends when the VEC changes in M₂AlC. Furthermore, trends in the elastic stiffness are well explained in terms of electronic band structure analysis, e.g., occupation of valence electrons in states near the Fermi level of M₂AlC. We show that increments of bulk and shear moduli originate from additional valence electrons filling states involving Md-Alp covalent bonding and metal-to-metal t_2g and e_g orbitals. For the case of c₄₄, strengthening the M-Al pd covalent bonds effectively enhances the shear resistance and excessive occupation of dd orbitals gives rise to a negative contribution. The maximum of c₄₄ is attributed to the complete filling of the Md-Alp bonding states

[en] The crystal structures of all layered ternary carbides called '312' phases including Ti₃AlC₂, Ti₃SiC₂ and Ti₃GeC₂ have been fully optimized by means of ab initio total-energy calculations. The equilibrium lattice parameters, the atomic positions in the unit cell and interatomic distances have been determined. The differences between the calculated and the measured lattice constants are generally less than 1%. It is also shown that c/a of the hexagonal lattices decreases from Ti₃AlC₂ to Ti₃GeC₂. The calculated bulk moduli are 190 GPa for Ti₃AlC₂, 202 GPa for Ti₃SiC₂ and 198 GPa for Ti₃GeC₂, respectively, which are comparable to that of TiC. The electronic structures reveal that the Ti(1, 2) and C atoms form a strong Ti(2)-C-Ti(1)-C-Ti(2) covalent bond chain, while the bonding between Ti(2) and M (M=Al, Si, Ge) is relatively weak. The strong Ti(2)-C-Ti(1)-C-Ti(2) covalent bond chain corresponds to the high strength and modulus, while the metallic bond corresponds to the metallic conductivity of these ternaries. (author)

[en] We have investigated the electronic structure, chemical bonding, and equations of state of Zr₂Al₃C₅ by means of the ab initio pseudopotential total energy method. The chemical bonding displays layered characteristics and is similar to that of nanolaminate ternary aluminum carbides Ti₂AlC and Ti₃AlC₂. Zr₂Al₃C₅ could be fundamentally described as strong covalent bonding among Al-C-Zr-C-Zr-C-Al atomic chains being interleaved and mirrored by AlC₂ blocks. The interplanar cohesion between covalent atomic chains and AlC₂ blocks is very weak based on first-principles cohesion energy calculations. Inspired by the structure-property relationship of Ti₂AlC and Ti₃AlC₂, it is expected that Zr₂Al₃C₅ will have easy machinability, damage tolerance, and oxidation resistance besides the merits of refractory ZrC. Zr₂Al₃C₅ has a theoretical bulk modulus of 160 GPa and illustrates elastic anisotropy under pressure below 20 GPa