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The elastic stiffness and shear deformation mode of Al3BC3, a metal borocarbide containing linear C–B–C units, are studied based on the first-principles total energy calculations. The predominant effect of C–B–C units on mechanical properties is reported by leading to low shear modulus. The low shear-strain resistance originates from the deformation mode as follows: the rigid linear C–B–C units tilt with respect to the c direction easily, and the corner-sharing Al5C bipyramid slabs simultaneously slide along the basal plane with low resistance. The proposed deformation mode may be universal for the ternary metal borocarbides family containing short linear C–B–C units.

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A series of Ti₃Al_1-xSi_xC₂ (x≤0.25) solid solutions were synthesized using an in situ hot pressing/solid–liquid reaction method. It was revealed that the lattice parameter c decreased dramatically but a remained almost unchanged with an increase of Si in the Ti₃Al_1-xSi_xC₂ solid solutions. A relationship between lattice parameter c (in nm) and Si content x, c(x) = 1.8541 (8.5674×10 ^-2)x, was established. A significant strengthening effect was observed when x was greater than 0.15 in the Ti₃Al_1-xSi_xC₂ solid solutions: the Vickers hardness, flexural strength and compressive strength were enhanced by 26%, 12% and 29%, respectively, for Ti₃Al_0.75Si_0.25C₂ solid solution.  The electrical resistivities of Ti₃Al_1-xSi_xC₂ solid solutions were in the range 0.35–0.37μΩm and increased slightly with an increase of Si content. The addition of Si to Ti₃AlC₂ to form solid solutions had no deleterious effect on the oxidation resistance at 1100 ^oC due to the formation of a continuous Al₂O₃ layer.

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Microstructures of Ti2AlC were investigated by means of X-ray diffraction, transmission electron microscopy, and analytical electron microscopy. The as-synthesized Ti2AlC is predominantly single phase and free of amorphous grain-boundary phases. High-resolution imaging reveals that the stacking sequence of Ti and Al atoms along the [0001]Ti2AlC direction is ABABAB. Two intergrown structures, i.e., Ti3AlC2–Ti2AlC and Ti2AlC–TiC–Ti2AlC, were determined using high-resolution imaging and energy dispersive X-ray analysis. ?Ti3AlC2 and TiC share close crystallographic relationships with Ti2AlC, which opens up the possibility of tuning the properties of Ti–Al–C carbides by controlling the microstructures. Investigation of the microstructure of TiAl-containing Ti2AlC revealed that Ti2AlC preferentially forms at TiAl twins.

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The microstructures of bulk Zr2Al3C4 and Zr3Al3C5 ceramics have been investigated using transmission electron microscopy and scanning transmission electron microscopy. These two carbides were determined to have a point group 6/mmm and a space group P63/mmc using selected-area electron diffraction and convergent beam electron diffraction. The atomic-scale microstructures of Zr2Al3C4 and Zr3Al3C5 were investigated through high-resolution imaging and Z-contrast imaging. Furthermore, intergrowth between Zr2Al3C4 and Zr3Al3C5 was identified. Stacking faults in Zr3Al3C5 were found to result from the insertion of an additional Zr–C layer. Cubic ZrC was occasionally identified to be incorporated in elongated Zr3Al3C5 grains. In addition, Al may induce a twinned ZrC structure and lead to the formation of ternary zirconium aluminum carbides.

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In this paper, we predicted the possible mechanical properties and presented the electronic structure of Zr3Al3C5 by means of first-principles pseudopotential total energy method. The equation of state, elastic parameters including the full set of second order elastic coefficients, bulk and shear moduli, Young’s moduli, and Poisson’s ratio , and ideal tensile and shear strengths are reported and compared with those of the binary compound ZrC. Furthermore, the bond relaxation and bond breaking under tensile and shear deformation from elasticity to structural instability are illustrated. Because shear induced bond breaking occurs inside the NaCl type ZrCx slabs, the ternary carbide is expected to have high hardness and strength, which are related to structural instability under shear deformation, similar to the binary carbide. In addition, mechanical properties are interpreted by analyzing the electronic structure and chemical bonding characteristics accompanying deformation paths. Based on the present results, Zr3Al3C5 is predicted to be useful as a hard ceramic for high temperature applications.

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From crystallographic point of view, Al4SiC4 can be described as Al4C3-type and hexagonal SiC-type structural units alternatively stacked along 0001 direction. However, relationship between this layered crystal structure and mechanical properties is not fully established for Al4SiC4, except for the reported bulk modulus locating between those of Al4C3 and SiC. Based on the first-principles pseudopotential total energy method, we calculated the elastic stiffness of Al4SiC4, and reported on its ideal tensile and shear stress-strain relationships considering different structural deformation modes. Elastic properties of Al4SiC4 are dominated by the Al4C3-type structural units and exhibit similar results with those of Al4C3. Furthermore, the atomistic deformation modes of Al4SiC4 upon tensile and shear deformations are illustrated and compared with Al4C3 as well. Since the tension-induced bond breaking occurs inside the constitutive Al4C3-type unit, the ternary carbide has similar ideal tensile strength with Al4C3. On the other hand, despite the softening of strong coupling between Al4C3- and SiC-type structural units is involved in shear, the shear strength for Al4SiC4 is, however, lower than the tensile strength, since p-state involved Al-C bonds respond more readily to the shear deformation than to tension. In addition, based on the comparison of strain energies at the maximum stresses, i.e., ideal strengths, for both tension and shear, we suggest that structural failure occurs in tensile deformation firstly and, thus confirms an intrinsic brittleness of Al4SiC4. For crystal structure arranged in alternatively stacking configuration, such as Al4SiC4, mechanical properties can be traced back to the constituent units, and are also related to the coupling strengths between each constituent unit. The results might provide a computational method to predict ductile or brittle response of a solid to applied deformations.

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Deformation and failure modes were studied for Ti2AlC and Ti2AlN by deforming the materials from elasticity to structural instability using the first-principles density functional calculations. We found that the TiC0.5 /TiN0.5 slabs remain structurally stable under deformations, whereas the weak Ti-Al bonds accommodate deformation by softening and breaking at large strains. The structural stability of the ternary compound is determined by the strength of Ti-Al bond, which is demonstrated to be less resistive to shear deformation than to tension. The ideal stress-strain relationships of ternary compounds are presented and compared with those of the binary materials, TiC and TiN, respectively. For Ti2AlC and Ti2AlN, their ideal tensile strengths are comparable to those of the binary counterparts, while the ideal shear strengths yield much smaller values. Based on electronic structure analyses, the low shear deformation resistance is well interpreted by the response of weak Ti-Al bonds to shear deformations. We propose that the low shear strengths of Ti2AlC and Ti2AlN originate from low slip resistance of Al atomic planes along the basal plane, and furthermore suggest that this is the mechanism for low hardness, damage tolerance, and intrinsic toughness of ternary layered carbides and nitrides.

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Cr2AlC is a recently developed layered ternary carbide. In this work, the atomic scale microstructure is reported. The layer stacking sequence of Cr and Al atoms has been clearly resolved. The atomic scale characterizations were realized by means of high resolution transmission electron microscopy and Z-contrast scanning transmission electron microscopy. Furthermore, electron energy loss spectroscopic analysis revealed that the Cr–C bonds in Cr2AlC are characterized by a strong bonding.

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Deformation modes are studied for two basal-plane slip systems, [1ˉ210](0001) and [ˉ1010](0001), in ternary-layered Ti2AlC and Ti2AlN ceramics using the first-principles plane-wave pseudopotential total energy method. Based on the theoretical stress–strain curves, the [ˉ1010](0001) slip system leads to smaller ideal shear strength compared to the [1ˉ210](0001) mode, implying that the [ˉ1010](0001) slip system is predominant to the mechanical properties of these ternary-layered compounds. Bond-length relaxations are examined for materials strained from elasticity to structural instability. Interatomic bonds are demonstrated to respond to shear strain inhomogeneously because of different bonding strengths. The slips of atomic planes are determined by the failure of weak Ti–Al bonds. In addition, we predict a polymorphic phase transformation along the [1ˉ210](0001) shear deformation path. For the [ˉ1010](0001) slip system, in contrast, no polymorphic phase transformation is observed because TiC slabs do not hold the original NaCl-type structure and, in addition, Al layers change from a hexagonal to a cubic stacking in the shear deformed lattice. In other words, the structural units undergo different atomic configurations from those in the two polymorphs under applied shear strain.

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Nb2AsC showed superior mechanical properties to those of other layered ternary carbides (Kumar et al 2005 Appl. Phys. Lett. 86 111904). In the present density functional calculation, the underlying mechanism is interpreted by astonishing bonding features of Nb2AsC. The Nb d–As (px + py) and Nb d–As pz bonding states locate in the same energy range as those of Nb d–Cp bonding, which indicates that the Nb–As bond has similar bonding strength as the Nb–C bond does; and thereafter, Nb2AsC has improved mechanical properties compared to the others. The present reported bonding features are interestingly different from those experienced by T2AlC (T=Ti, V, Cr, and Nb), wherein the weak T–Al coupling was separated from T–C bonding states in the higher energy level by a pseudogap. This work proposes an effective method to strengthen the relative weaker T–A (A is the A-group element) bonding in layered ternary carbides.

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The isothermal oxidation behavior of bulk Ti3SiC2 at intermediate temperatures from 500 to 900 °C in flowing dry air was investigated. An anomalous oxidation with higher kinetics at lower temperatures was observed. This phenomenon resulted from the formation of microcracks in the oxide scales at low temperatures. The generation of these microcracks was caused by a phase change in the oxide products, i.e., the transformation of anatase TiO2 to rutile TiO2. This phase transformation resulted in tensile stress, which provided the driving force for the formation of the microcracks during oxidation. Despite the existence of microcracks, the intermediate-temperature oxidation of Ti3SiC2 generally obeyed the parabolic rate law and did not exhibit catastrophic destruction due to the fact that cracks occurring in the oxide layers were partially filled with amorphous SiO2. Therefore, further high oxidation kinetics was prevented.

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Bulk Ta4AlC3, a new layered-ternary carbide in the Ta–Al–C system, was synthesized and characterized. Transmission electron microscopy investigations on this new phase are reported here. Selected area electron diffraction and convergent beam electron diffraction analyses indicated that this ternary carbide crystallized with the space group P63/mmc. Atomic-scale microstructures of Ta4AlC3 were achieved by means of high-resolution transmission electron microscopy and Z-contrast scanning transmission electron microscopy. The experimental crystal structural parameters agreed well with the theoretical values obtained using density-functional calculations.

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In this article, a novel pressureless solid-liquid reaction method is presented for preparation of yttrium disilicate ( γ-Y2Si2O7). Single-phase γ-Y2Si2O7 powder was synthesized by calcination of SiO2 and Y2O3 powders with the addition of LiYO2 at 1400 °C for 4 h. The addition of LiYO2 significantly decreased the synthesis temperature, shortened the calcination time, and enhanced the stability of γ-Y2Si2O7. The sintering of these powders in air and O2 was studied by means of thermal mechanical analyzer. It is shown that the γ-Y2Si2O7 sintered in oxygen had a faster densification rate and a higher density than that sintered in air. Furthermore, single-phase γ-Y2Si2O7 with a density of 4.0 g/cm3 (99% of the theoretical density) was obtained by pressureless sintering at 1400 °C for 2 h in oxygen. Microstructures of the sintered samples are studied by scanning electron microscope.

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Finite element analyses were carried out to simulate the loading, unloading, and reloading processes of indentation tests. It was found that the validity of applying the elastic contact theory to the indentation unloading process is strongly related to the strain hardening and residual stress in impression. It is the combination of strain hardening and residual stress that causes the unloading or reloading curves to show elastic loading in the range from zero to the maximum load whereas the reloading curve on the impression without strain hardening and residual stress shows elastic–plastic loading in the same range. These computations indicate that applying the elastic contact theory to the unloading or reloading processes, the fundamental prerequisite of the instrumented indentation technique, is valid because of the existence of strain hardening and residual stress. The mechanism of this hardening effect is discussed through energy analysis.

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Diffusion bonding of Ti3SiC2 and nickel has been conducted at temperatures of 800 °C–1100 °C for 10–90 min under 6–20 MPa in a vacuum. The phase composition and microstructure of the joints were investigated by x-ray diffraction, scanning electron microscopy, and electron probe microanalysis. The total diffusion path of the joining is determined to be Ni/Ni31Si12 + Ni16Ti6Si7 + TiCx/Ti3SiC2 + Ti2Ni + TiCx/ Ti3SiC2. The growth of the reaction layer follows parabolic law, and the temperature dependence of the reaction constant, k, can be expressed as k =1.68 × 10?4 exp(?118 ± 12 kJ/RT) m/s1/2. The diffusion of nickel through the reaction zone toward Ti3SiC2 is the main controlling step in the bonding process. Joint strengths were determined through shear tests. The maximum shear strength of 121 ± 7 MPa, which is close to the shear strength of Ti3SiC2, has been obtained under the condition of 1000 °C for 10 min under 20 MPa.

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Many layered ternary ceramics displayed unusual thermal shock behavior, i.e., the retained strengths of as-quenched samples increased with increasing quench temperature above a critical quench temperature. However, the causation was not clear even though this phenomenon has been observed for 10 years. In this study, the thermal shock behavior of Ti3SiC2 and Ti3AlC2, two representative members of layered ternary ceramics, was investigated. The results indicated that the formation of surface oxides was responsible for this abnormal phenomenon. These results might contribute to the understanding of this unusual behavior of other layered ternary ceramics.

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Tribological properties of Ti3SiC2 and Al2O3-reinforced Ti3SiC2 composites (10 and 20 vol% Al2O3) were investigated by using an AISI-52100 bearing steel ball dryly sliding on a linear reciprocating athletic specimen. The friction coefficients were found varying only in a range of 0.1 under the applied loads (2.5, 5, and 10 N), and the wear rates of the composites decreased with increasing Al2O3 content. The enhanced wear resistance is mainly attributed to the hard Al2O3 particles nail the surrounding soft matrix and decentrale the shear stresses under the sliding ball to reduce the wear losses.

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Direct atomic resolution observations of the layered stacking characteristics of TaCx slabs and Al atomic planes in ternary Ta–Al–C carbides were achieved. Layered ternary Ta–Al–C compounds have diverse structures. A previously unknown Ta6AlC5 carbide, as well as intergrown Ta2AlC–Ta4AlC3 and Ta4AlC3–Ta6AlC5 structures were identified. Theoretical lattice parameters and bulk modulus of Ta2AlC, Ta3AlC2, Ta4AlC3, and Ta6AlC5 are presented. Furthermore, the Ta–C bonds are much stronger than the Ta–Al bonds in ternary Ta–Al–C carbides, which accounts for the enhancement of bulk modulus with increasing Ta–C layers.

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A simple and versatile precracking method using a triangular notch as a crack starter in limited bending was developed, which is suitable for both brittle ceramics and quasi-plastic materials that are difficult to precrack by the conventional bridge-indentation technique. Slow growth of large crack in brittle or quasi-brittle ceramics was controlled and observed in situ in this way. The precracking tests performed on various ceramics exhibited high reliability and feasibility. The precracked specimens were subsequently used to measure the fracture toughness, and the resultant data showed that the fracture toughness determined by using the precracked specimens reflected the minimum value of the toughness measured in single edge-notched beam (SENB) tests.

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Microstructures of oxidized Ti₃AlC₂ and Ti₂AlC were investigated using transmission electron microscopy. The presence of Ti-rich precipitates in the Al₂O₃ oxide scale and the enrichment of Ti in the Al₂O₃ grain boundaries were identified. High diffusivity of Al in the carbides contributes to the selective oxidation of Al. Moreover, the adhesive strength between the oxide scales and the substrates exceeds 85 MPa. Strong adhesive strength contributes to the thermal-cyclic stability of the oxide scales.