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The Nanolaminate Ti₃SiC₂ ceramic exhibited unique mechanical properties, such as high modulus, low anisotropic hardness to modulus ratio and microscale ductility etc. The planar close-packed Si atoms were expected to play a dominant role in deducing these properties. By performing first-principles total energy calculations, we demonstrated that a reversible polymorphic phase transition occurred when shear strain energy was large enough to close an energy barrier. The phase transition path was described as the Si atoms sliding between 2b and 2d Wyckoff positions on the (112¯ 0) plane. The electronic band structure, lattice dynamics, and structure stability were discussed for the two polymorphs, respectively. We demonstrated that the a-Ti3SiC2 was more stable than b-Ti₃SiC₂ by comparing the ground-state total energy and ab initio Gibbs free energy. Raman and infrared active phonon modes were illustrated for feasibly identifying the two phases in experimental spectra. The results were used to assign peaks in the experimental Raman spectrum with distinct vibrational modes, and to clarify the origin of the uncertain peak. The calculated heat capacity and volume thermal expansion coefficient agreed with experimental values well. The elastic mechanical parameters of the polymorphs were presented and compared with respect to various strain modes. Based on electronic band structure discussions, we clarified the mechanism of anisotropic hardness of Ti₃SiC₂ , which attributed to different covalent bonding strengths involved in kink migration.

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An analytical relationship between the reduced modulus Er and hardness H for solid materials is established based on the conventional depth-sensing indentation method of Oliver and Pharr. It is found that the two properties are related through a material parameter that is defined as the recovery resistance Rs. This parameter is shown to represent the energy dissipation during indentation.  Based on indentation measurements with the use of a Berkovich indenter, the relationship is given as Er=0:6647 . Also presented is a simple set of procedures to determine the area of indent. The procedures require three measured quantities, i.e., the peak load and corresponding displacements as well as the depth of residual indentation, but do not require complicated curve fitting process and regression analysis which themselves involve the specimen material. Nano-indentation tests were conducted using a Berkovich indenter on five materials spanning a wide range of hardness and plasticity. Experimental results revealed two important features: (a) the reduced modulus predicted by the new Er–H relationship is the same as that obtained by the conventional method; (b) the elastic modulus and hardness values determined by the simple set of procedures are comparable to those obtained by using the conventional method.

Absstract

The oxidation behavior of the Ti₃Si_0:9Al_0:1C₂ solid solution in air was investigated at 1000–1350 ^oC. The parabolic rate constants of Ti₃Si_0:9Al_0:1C₂ were decreased by 2–4 orders of magnitude compared with those of Ti₃SiC₂ at 1000–1300 ^oC. At 1000–1100 ^oC, the oxide scales displayed a continuous a-Al₂O₃ inner layer and a discontinuous TiO₂ (rutile) outer layer. At 1200–1300 ^oC, the continuous inner layer was still α-Al₂O₃, but the outer layer was a mixture of TiO₂ (rutile) and Al₂TiO₅. The oxide layers were dense, adherent and resistant to thermal cycling. However, the oxidation resistance of Ti₃Si_0:9Al_0:1C₂ deteriorated at 1350 ^oC because of the depletion of a-Al₂O₃. This depletion was caused by the extensive reaction between TiO₂ (rutile) and a-Al₂O₃ to form Al₂TiO₅. The high activity and diffusion of Al and the low solubility of oxygen in the solid solution were the key factors for the formation of a continuous α-Al₂O₃ layer during high-temperature oxidation.

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A calculation of the electronic structure and formation energy for ZrV₂H_x (x ¼ 0:5, 1, 2, 3, 4, 6 and 7) is performed using a planewave pseudo-potential method. It is found that in ZrV₂H_x hydrogen forms stronger covalent bonds with vanadium than with zirconium if Zr atoms are in the neighborhood of V. A detailed analysis of how the densities of states change with the hydrogen count x in ZrV₂H_x shows the changes in the bonding and anti-bonding interactions of H with V and Zr. However, the covalent antibonding interactions between H and V seem to be mainly responsible for the variation in the formation energy of ZrV₂H_x with x. The value of projected density of states of V 3d at the Fermi level can be used as a rough comparative measure for these antibonding interactions and therefore allows us to predict the changes in stability of ZrV₂H_x with x.

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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 t2g and eg 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 M_d-Al_p bonding states.

Absstract

We have investigated the elastic stiffness and electronic band structure of nanolaminate (M_xM _2−x )AlC solid solutions, where M and M = Ti,V and Cr, by means of the ab initio pseudopotential total energy method. The second-order elastic constants, bulk moduli and anisotropic Young’s moduli are computed for the solid solutions, in which x is changed from 0 to 2 in steps of 0.5. The bulk moduli of (M_xM _2−x )AlC is found to be approximately the average of the two end M₂AlC and M₂AlC phases as the substitution content x, as well as
the valence electron concentration (VEC), varies in the compounds. On the other hand, the shear modulus c₄₄, which by itself represents a pure shear shape change and has a direct relationship with hardness, saturates to a maximum as VEC is in the range 8.4–8.6. It implies that solid solution hardening may be operative for alloys having VEC values in this range. Furthermore, trends in the elastic stiffness are interpreted in terms of the electronic band structure. We show that monotonically incrementing the bulk moduli is attributed to the occupying states involving transition-metal d–Al p covalent bonding and metal-to-metal dd bonding. The maximum in c₄₄, on the other hand, originates from completely filling the shear resistive transition-metal d– Al p bonding states. Most importantly, we predict a method to optimize the desired elastic stiffness by properly tuning the valence electron concentration of (M_xM _2−x )AlC ceramics.

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Hot corrosion of layered machinable ceramic Ti₃AlC₂ coated with 3.8 ± 0.2 mg/cm² Na₂SO₄ in air at 700-1000°C has been investigated by means of thermogravimetric analysis, X-ray diffraction, Raman spectroscopy, and scanning electron microscopy/ energy-dispersive spectroscopy. The hot corrosion of Ti₃AlC₂ was slight at low temperatures of 700 and 800°C, while Ti₃AlC₂ was severely attacked by fused sodium sulfate at 900 and 1000°C. No protective scales were observed on Ti₃AlC₂ , and sulfur-rich layers were present at the Ti₃AlC₂ substrate/scale interface. The linear hot corrosion kinetics at 800 and 900°C demonstrated that electrochemical reactions of Ti₃AlC₂ with sodium sulfate at the substrate/scale interface dominated, while the parabolic kinetics at 1000°C implied that the rate-limiting step involved in the hot corrosion was the diffusion of hot corrosion medium through the scale formed. The hot corrosion behaviors were explained by a mechanism of electrochemistry coupled with basic dissolution-precipitation.

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The influence of Ce implantation into preformed Cr₂O₃ scales with a dose of 1×10¹⁷ ions/cm² on the subsequent oxidation behavior of Ni–20Cr alloy at 1050◦C in air has been investigated. The pre-oxidation was carried out at 1050^◦C in air for 0.5 and 1 hr respectively Cr₂O₃ and NiCr₂O₄ formed on Ni–20Cr alloy. The oxidation rate was decreased remarkably due to Ce implantation regardless of whether it was implanted
into the alloy or into the pre-formed oxide scales, and the beneficial effect decreased with increasing pre-oxidation time, the alloy implanted directly with Ce had the lowest oxidation rate constant. During cyclic oxidation (350 cycles) Ce implantation played a similar benefical effect on the oxide-spallation resistance for blank and pretreated alloys. The result indicates that Ce incorporated into the oxide scale affected the diffusion of the reaction species and also the spallation resistance of the oxide scales.  The change of the oxidation process is attributed to the segregation of Ce at the oxide grain boundaries.

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The isothermal-oxidation behavior of polycrystalline Ti₂SnC at 500–800^◦C in air has been investigated. The growth of the oxide scale on Ti₂SnC from 500^◦C to 700^◦C obeyed a parabolic law, whereas at 800^◦C it was a two-step parabolic process. Microstructure and composition analysis on the surface and sectioned samples demonstrated that the oxidation of Ti₂SnC was controlled by outward diffusion of titanium and carbon, and inward diffusion of oxygen. As the oxidation continued the oxygen potential in the inner layer was low, and metallic Sn was stable as an interfacial layer between the oxide scale and the Ti₂SnC matrix. This work confirmed the presence of metallic Sn in the oxidized Ti₂SnC and explained why metallic Sn was stable in the oxidized Ti₂SnC sample.

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Shear-induced failures in uniaxial compression and Hertzian contact damage of Ti₃AlC₂ fabricated by a solid–liquid reaction synthesis were investigated, and shear strength was evaluated using double notched samples and punch hole tests respectively. Compressive strength of 749 MPa and the slip angle of 38 between the slip plane and the loading direction were measured when the applied load was parallel to the hot-pressing direction of the material; whereas the compressive strength of 841 MPa and the angle of 26 were obtained when the applied load was perpendicular to the hot-pressing direction. The shear strength obtained using double-notched sample was 96 MPa and that from punch-shear test was 138 MPa, respectively. SEM fractographs of specimens failed in shear and compression tests indicated a combination of intergranular and transgranular fracture and also an evidence of friction in the slip plane during compression failure process. Punch-shear test was developed and confirmed to be a simple and feasible method for determining the shear strength of layered machinable ceramics.

Absstract

Ti₃SiC₂ combines many of the merits of both metals and ceramics and is potential in diverse high temperature applications. However, TiC always exists as an impurity phase, which is deleterious to the high-temperature oxidation resistance of Ti₃SiC₂. Although many attempts have been made, it is difficult to eliminate TiC from Ti3SiC2 due to the close structural relations between the two compounds. In this paper we describe our innovative work to prepare TiC free Ti₃SiC₂, by the substitution of a small amount of Si with Al. The substitution of Si with Al resulted in the formation of Ti₃Si_1-xAl_xC₂ solid solution. The mechanical properties of the Al doped Ti₃SiC₂ were close to those of Ti₃SiC₂, but the oxidation resistance was significantly improved due to the formation of protective Al₂O₃ layer during high-temperature oxidation.

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Ti₃AlC₂/Al₂O₃ composites were fabricated by the in-situ hot pressing/solid–liquid reaction process. The hardness, strength and toughness are enhanced by the incorporation of Al2O3. The strengthening and toughening mechanisms are discussed.

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Chemical reactions and stability of Ti₃SiC₂ in Cu during processing of Cu/Ti₃SiC₂ composites in the temperature range of 900-1070 ^oC were investigated using X-ray diffraction and scanning electron microscopy. The results indicated that Cu reacted with Ti3SiC2 above 900 ^oC, and the reaction products were closely related to the reaction temperature and the relative ratio of Cu and Ti₃SiC₂. At low Ti₃SiC₂ content or temperatures below 1000 ^oC, Cu(Si) solid solution and TiC_x were formed, whereas at high reaction temperature and high Ti₃SiC₂ content, Cu-Si intermetallic compounds like Cu₅Si, Cu₁₅S_i4 and (Cu, Si) eta' as well as TiC_x were observed. The transporting process for the reaction was investigated and described. It was found that de-intercalation of Si from Ti₃SiC₂ dominated the reaction, which dissolved in copper to form Cu(Si) solid solution or Cu-Si intermetallic compounds such as Cu₅Si, Cu₁₅Si₄ and (Cu, Si) eta'. TiC_x was always present as the decomposition product of Ti₃SiC₂.

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The compound Ti₃SiC₂ is a promising reinforcement for copper, but at a high volume content of Ti₃SiC₂, densification of Cu/Ti₃SiC₂ composites becomes difficult. To improve the densification behaviour and microstructure of Cu/Ti₃SiC₂ composites, Ti3SiC2 particulates were coated with a layer of copper by an electroless plating method before being incorporated into the copper matrix. Results demonstrated that, compared with uncoated Ti₃SiC₂ reinforced copper, copper coated Ti₃SiC₂ reinforced composites exhibit both high density and a homogeneous distribution of Ti₃SiC₂ within the copper matrix. The precoated layer of copper prevents direct contact and agglomeration of Ti₃SiC₂ particulates. Owing to the improved density and microstructure, the mechanical properties of Cu/Ti₃SiC₂ composites are also enhanced. Mechanical property investigation revealed that a significant strengthening effect is observed for Cu/Ti₃SiC₂ composites at both room and high temperatures.

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Large quantities of ZnO nanorods have been synthesized by the seed-mediated method in the presence of
polyethylene glycol at 90 ^oC. The products are characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and high-resolution transmission electron microscopy. The as-grown ZnO nanorods are uniform with a diameter of 40–70nm and length about 2 mm. The nanorods grew along the [0 0 1] direction. Possible roles of ZnO seeds and polymer in the growth of ZnO nanorods are also discussed.

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Various sample thicknesses and punch diameters were used in punch-shear tests for investigating the size effects in evaluating the shear strength of machinable ceramics. The ratio of the sample thickness to the punch diameter was defined as the relative thickness that strongly affects the measured shear strength in a certain range. It was found that there was a threshold value in the relative thicknesses (~ 0.1 for Ti₃AlC₂ and ~ 0.13 for Ti₃SiC₂) above which the measured shear strength increased linearly with increasing thickness. The threshold provided a thickness-requirement for valid punch-shear tests, and stable shear strength was obtained under this requirement using 3 mm and 1.8 mm punch diameters respectively. Experiments using various loading rates suggested that the shear strength measured by the punch shear test was almost immune from the loading rates. The mechanism of the size effects was analyzed through optical and SEM observations.

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The microstructural characteristics of the bulk Ti₃AlC₂ ceramic, synthesized by a solid–liquid reaction and simultaneous in-situ hot pressing, have been studied by electron diffraction and high-resolution transmission electron microscopy.  It was found that the as-synthesized ceramic is fully dense and free from amorphous phase at the grain boundaries. The ceramic is predominantly a single phase of hexagonal Ti₃AlC₂ with a minor volume of tetragonal Al₃Ti and cubic TiC.  The Al₃Ti grains are crystallographically independent of the Ti₃AlC₂, whereas TiC is intergrown with Ti₃AlC₂, forming thin platelets and maintaining a close orientational relationship with the Ti₃AlC₂ matrix.

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Shear strength and shear-induced Hertzian contact damage in Ti₃SiC₂ were investigated using double-notched-beam specimen and steel spherical indenter, respectively. The shear strength of 40 MPa that was only about 10% of bending strength was obtained for this novel ceramic. The SEM fractograph of specimens failed in shear test indicated a combination of intergranular and transgranular fracture. Under a contact load, plastic indent without cone crack could be formed on the surface of Ti3SiC2 sample. Optical observation on side view showed half-circle cracks around the damage zone below the indent, and the crack shape was consistent with the contrail of the principal shearing stress. The low shear strength and the shearing-activated intergranular sliding were confirmed being the key factors for failure in Ti₃SiC₂.

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The average growth stress of NiO scale formed on pure Ni at 1000 ^oC in air was investigated by a modified deflection technique. The growth stress in NiO scale was tensile with a magnitude of about 10 MPa during the 10-h oxidation period. The growth stress level decreased with increasing oxidation time or oxide thickness. It varied from 37 MPa for a scale thickness of 4.8 mm to 13 MPa for a thickness of 13.8 mm. At the later stage of oxidation, the growth stress did not change noticeably. The planar stress state in the substrate was both compressive and tensile during the first hour of the deflection test. After that, only compressive stress existed in the substrate alloy.