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Raman active phonon frequencies and corresponding vibrational eigenmodes are reported for two M2AlC ceramics, Ti2AlC and Cr2AlC, by means of first-principles calculations. The theoretical modes are approved by the experimental Raman spectrum for Ti2AlC. Compared with that of the Ti3SiC2 counterpart, the number of Raman active modes is definitely determined by the chemical formula of transition-metal carbide layers; for example, TiC0.67 in Ti3SiC2 and TiC0.5 in M2AlC. All active modes originate from stretching and bending of Al–M–C covalent bond chains, whereas the vibration of M–C bond localized inside the M6C octahedra is not Raman active. The differences in vibration characteristics between Ti2AlC and Cr2AlC are discussed in terms of atomic force constants. By including the phonon and electron contributions, trends in heat capacities as a function of temperature were illustrated and compared.

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In this letter, the full set of elastic coefficients of LaPO₄ monazite is presented based on the first-principles plane-wave pseudopotential total energy method. Mechanical parameters bulk modulus, shear modulus, Young’s moduli, and Poisson’s ratio are also presented and compared with experimental results for polycrystalline monazite. The responses of electronic structure and chemical bonds to a series of ｛010 ｝〈001 〉shear strains are examined in order to study the mechanism of low shear strain resistance. The results show that small shear moduli originate from the inhomogeneous strengths of atomic bonds. For example, the weak La–O bonds accommodate the shear strain locally, while the PO₄ tetrahedra are almost rigid. The theoretical elastic stiffness may be useful to understand the deformation mechanisms of LaPO₄ monazite.

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An analytical relationship between the ratio of hardness to reduced modulus H/Er and the geometry of a residual indent is established based on the theories of depth-sensing indentation. Various material parameters, including elastic parameters, recovery deformation and energy-dissipation capacity, are uniquely determined by the value of H/Er, so that they can be estimated from a residual indent trail. Thus, we are able to know what has happened in the material simply by analyzing or comparing residual trails on the
material. The validity of this method has been confirmed by analyzing residual Vickers indents on quasi-plastic ceramic and brittle glass. In addition, it is demonstrated that the geometric constant e in traditional nanoindentation theories is related to the proportional factor g through the exponent of unloading curve m, i.e., e = m Æ g, where g is defined by hs = g Æ (hm hf).

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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.

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The corrosion behavior of polycrystalline Ti₃SiC₂ was studied in the mixture of Na₂SO₄–NaCl melts with various mass ratios at 850 ^◦C. The results demonstrated that Ti₃SiC₂ suffered from serious hot corrosion attack in the mixture of Na₂SO₄–NaCl melts when the concentration of Na₂SO₄ was higher than 35 wt.%. A large amount of corrosion products spalled from specimens during the tests and obvious mass loss was observed. Hot corrosion of Ti3SiC2 would become severe because NaCl had lower melting-point and caused Na₂SO₄–NaCl mixture melted below 850 ^◦C. However, when the concentration of Na₂SO₄ was lower than 25 wt.% in the mixture, a protective oxide layer (SiO₂ + TiO₂) formed on the substrate, the corrosion rate of Ti₃SiC₂ became quite slow and slight mass gain was observed, the corrosion products did not spall from substrate at 850 ^◦C. The microstructure and phase composition of the corroded samples were investigated by SEM/EDS and XRD.

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The retained strength and hardness of Ti3AlC2 bars quenched from 200 to 1300 ^oC in air, water and silicon oil were investigated to probe the thermal shock resistance of Ti3AlC2 in various media. The measured retained strength displayed different trends for the samples quenched in various media. For the samples quenched in air, the retained strength showed somewhat enhancement with increase of the temperature difference. For the samples quenched in water, the retained strength exhibited a complex evolution and could be divided into four zones, i.e., (i) no damage zone (20–300 ^oC), (ii) strength degradation zone (300–500 ^oC), (iii) stable strength zone (500–1000 ^oC), and (iv) strength enhancement zone (1000–1300 ^oC). Therefore, the minimum retained strength in the third zone, which is higher than 60% of the initial strength, provided a prediction that the strength loss by thermal shock for Ti3AlC2 should be less than 40%. SEM analysis revealed that an oxide scale of -Al2O3 was formed at high temperature for which the residual stress was calculated. The strength degradation in the second temperature zone was imputed to the weakening of grain boundaries caused by water infiltration, whereas the strength enhancements for the air-quenched samples in the fourth zone was attributed to the formation of oxide scale and the residual compressive stresses in the oxide layer. The damage caused by quenching in oil for this ceramic was demonstrated between that of air and water quenching. Finite element method (FEM) was used to simulate the failure in bending, and the results indicated that the strength of the sample with an oxide scale was about 5–10% higher than that of homogeneous sample.

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Highly oriented copper nanowires were electrochemically synthesized in a porous alumina membrane template using a new type of weak-acid electrolyte. The Cu nanowires that were deposited have (110) preferred orientation, which is different from most electrochemically deposited Cu nanowires, and they can grow homogeneously.  Transmission electron microscopy was used to investigate the room-temperature oxidation behavior, and it was observed that sample treatment methods greatly influence the oxidation rate of the wires. Cu nanowires with different diameters have different resistance to oxidation. The orientation relationship between oxide layer and small-diameter Cu nanowire was determined to be (001) Cu2O // (11¯ 1) Cu, (110) Cu2O // (110) Cu, and [1¯10] Cu2O // [1¯ 12] Cu. The possible oxidation process is also discussed.

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Ti₂AlC is strengthened by substituting Ti with V to form (Ti,V)₂AlC solid solutions. The Vickers hardness, flexural strength, shear strength and compressive strength are enhanced by 29%, 36% and 45% for (Ti_0.8,V_0.2)₂AlC solid solution, respectively. The strengthening mechanism is discussed.

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As a typical damage-tolerant ceramic, Ti₃SiC₂ should have stable mechanical properties, but the great scatter in reported values of fracture toughness (K_IC), from 4.5 to 16 MPa·m_1/2, has formed a paradox bewildering material scientists. This uncertainty results in many questions concerning the fracture toughness
of Ti₃SiC₂. For example, what is the credible K_IC value? How do the temperature and the notch-width effect the measured toughness value? Are the conventional testing methods and sample-size requirements that are widely used for brittle ceramics suitable for Ti₃SiC₂ that possesses many metallic properties? It is these questions that intrigue us and led us to explore the various effects on the measured toughness of Ti₃SiC₂, including the effects of sample thickness, notch-width, sample width, testing temperature, and testing method. The aim of this work is to reveal the intrinsic fracture resistance of Ti₃SiC₂ and the variation of the measured K_IC with changed sample size and testing conditions.

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Brittle-ductile alternate laminates with strong interfaces, Al₂O₃-Ti₃SiC₂ multilayer composites, are fabricated by hot-pressing thin Al₂O₃ tapes coated with a mixture of titanium, silicon, and graphite powders at 1550^oC.  The damage evolution and subcritical growth of macrocracks in these novel composites are investigated by three-point bending tests at high temperatures.  By comparing the mechanical response in different loading directions, the anisotropic mechanical properties of the laminated beams are revealed.  The experimental results show that, with the stress redistribution and the neutral axis shift toward the compressive surface during the creep damage, increased cracks occur in the Al₂O₃ layers and slowly extend the neutral axis without brittle fracture.  It is demonstrated that the creep damage of the  Al₂O₃-Ti₃SiC₂ -laminated composites in bending is governed by a combination of stress relaxation in Ti₃SiC₂ larers and subcritical crack growth in Al₂O₃ layers.  This result implies that contrallable subcritical crack growth and crack arrest in brittle ceramics could be accomplished by means of a ductile-brittle joined structure.  This prediction is confirmed by using a bending test on an aluminum beam with a glass coating bonded on the side surface.

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Single-phase bulk Cr₂AlC, a non-Ti-containing carbide with superior expected properties, was successfully fabricated by in-situ hot pressing/solid-liquid reaction process. The reaction process was investigated using differential scanning calorimetry. The products were characterized using X-ray diffraction and scanning electron microscopy. Four distinguished stages of the reactions from Cr, Al, and C elemental powders were discussed. The obtained X-ray diffraction data are in good agreement with calculated ones and those from JCPDS card No. 29-0017. A new set of Xray diffraction data. comprising reflections, 2 theta, and intensities of Cr2AlC is presented. Lattice parameters obtained by Rietveld XRD refinement of Cr₂AlC are a = 2.858 angstrom and c = 12.818 angstrom, respectively. The measured Vickers hardness of the as-synthesized Cr₂AlC is 5.5 ± 0.4 GPa, which is twice that of Ti₂AlC. CrAlC displays excellent oxidation resistance. The parabolic rate constant of Cr₂AlC was decreased by 3 - 4 orders of magnitude compared with those of Ti₃SiC₂ at 1200 ^oC.

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Ti₂SnC dispersion-strengthened (DS) copper matrix composites were prepared by the hot-pressing method. The change of microstructure, mechanical properties, and electrical resistivity of the composites as a function of Ti₂SnC volume fraction were studied. The results demonstrated that the grain size of Cu decreased pronouncedly by incorporating Ti₂SnC, and the strengthening effect was significant. Improvements in yield strength of up to four times that of pure copper were found in Cu-1 vol.% Ti₂SnC, however, the conductivity of the composite was still 85.6 % of pure copper. The high strength and low electrical resistivity indicated that Ti₂SnC is a promising reinforcement for copper.

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Interfacial reaction between Cu and Ti₂SnC during the processing of Cu-Ti₂SnC composites was investigated by using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy.  It is shown that the reaction is closed related to the reaction temperature, reaction time, and relative ratio of Cu and Ti₂SnC.  The reaction products are Cu(Sn) solid solution and TiC_X, and the transporting process during interfacial reaction is described.  The interfacial reaction products are further confirmed by measuring the hardness across the interface using nanoidentation test.  It is shown that the reaction is controlled by the-intercalation of Sn from Ti₂SnC to form Cu(Sn) solid solution and TiC_X, and crystallographic relation of   TiC_x //  Ti₂SnC和  TiC_x //  Ti₂SnC were observed at the interface.  The effect of interfacial reaction on the mechanical and electrical properties of the composite is also discussed.　

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Cu₂O nanowires were successfully synthesized by an electrochemical method using an alumina membrane as template through precise control of the pH value of the electrolyte.  The deposition process was monitored by the time–current curve. Characterization was performed by means of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The growth directions of the Cu₂O nanowires were determined and the possible growth mechanism is discussed.

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Creep and stress relaxation of Ti₃AlC₂ were investigated using three-point bending tests at 800-1200 ^oC under various load levels. The results show that the creep rate significantly increases with increasing temperature in the rang of 1000-1200 ^oC. Subcritical crack growth during the creep process was found to be the main failure mechanism, i.e., the stress intensity factor increases with the creep-induced crack growth and results in the ultimate fracture. The lower limit of stress relaxation was considered as the threshold value of zero-creep stresses, and the ratio of the threshold stress to the applied stress was defined to be a parameter of creep resistance for estimating deformation behavior at high temperature. SEM examination confirmed that the creep failure in Ti₃AlC₂ was governed by such a damage evolution: cavitation --> crack initiation --> crack extension --> fracture.