Absstract

The microstructural characteristics of the Ti₃AlC₂/Al₂O₃ interface have been investigated by means of transmission electron microscopy (TEM). The orientation relationships (ORs) between the Ti₃AlC₂ and the Al₂O₃ oxide scale are shown to be

(0001)Ti₃AlC₂‖(0001)Al₂O₃ and ［11 20］ Ti₃AlC₂‖［12 10］ Al₂O₃ (OR-1)

(0001)Ti₃AlC‖(0001)Al₂O₃ and ［11 20 ］Ti₃AlC₂‖［1 100 ］Al₂O₃ (OR-2).

 A high-resolution TEM image of the interface is presented and two interfacial models are proposed based on the observed orientation relationships.

Absstract

Microstructural relationships between Ti₅Si₃ and TiC, and Ti₃SiC₂ and TiC are reported. Ti₅Si₃ and TiC were determined to share a coherent interface following a new set of orientation relationships, i.e., (11 2 4)Ti₅Si₃‖(1 1 0)TiC and [22 4 3]Ti₅Si₃‖[110]TiC.  Moreover, direct atomic resolution observation through Z-contrast scanning transmission electron microscopy showed that Ti₃SiC₂ nucleated within TiC. The close microstructural relationships between compounds in the Ti–Si–C system benefit the understanding of the reaction mechanism for the synthesis of Ti₃SiC₂.

Absstract

Ti₃Al_1-xSi_xC₂ solid solutions exhibited abnormal high coefficients of thermal expansion at temperatures of >940 ^oC during heating, which was ascribed to the precipitation of Si as Ti₅Si₃. Ti₅Si₃ phase usually located at the grain boundaries of the solid solutions.

Absstract

Ti₃AlC₂ suffers severe Na₂SO₄-induced corrosion attacks at temperatures higher than 800 °C in air. A convenient and efficient pre-oxidation method is proposed to enhance the corrosion resistance of Ti3AlC2. The corrosion weight-changes of the pre-oxidized samples were decreased by about four orders of magnitude compared with those of the untreated specimens. The mechanism on improvement of corrosion resistance was investigated by means of thermogravimetric analysis, X-ray diffraction and scanning electron microscopy/energy-dispersive spectroscopy. A continuous and adherent -Al₂O₃ scale was prepared by high-temperature pre-oxidation treatment in air. The preformed dense Al2O3 scale has good compatibility with the Ti₃AlC₂ substrate, and consequently, can act as an efficient barrier against corrosion. Long-time corrosion tests demonstrate that the Al₂O₃ scale conserves after corrosion attack and is capable of long-term stability.

Absstract

Polycrystalline Ti₃SiC₂ suffered from serious hot corrosion attack in the mixture of 75wt.%Na₂SO₄ + 25wt.%NaCl melts at 850 ^oC. In order to improve the hot corrosion resistance of this material, pre-oxidation treatment was conducted at 1200 ^oC in air for 2 h. A duplex oxide scale with an outer layer of TiO₂ and an inner layer of a mixture of TiO₂ and SiO₂ was formed during the pre-oxidation. Because the outer oxide layer of the pre-oxidation treated specimens could inhibit hot corrosion process, they exhibited good hot corrosion resistance in the mixture of 75wt.%Na₂SO₄ + 25wt.%NaCl melts at 850 ^oC for 50 h. However, during the hot corrosion the outer layer of TiO2 would degrade gradually. Once the outer layer damaged, the hot corrosion rate increased sharply, the corrosion behavior was similar to Ti₃SiC₂ corroded under the same conditions. The microstructure and phase compositions of the hot corrosion samples were investigated by SEM/EDS and XRD.

Absstract

The oxidation behavior of Ti₃Al_1−xSi_xC₂ (x≤0.25) solid solutions was investigated in flowing air at 1000–1400 ^◦C for up to 20 h. Similar to Ti₃AlC₂, Ti₃Al_1−xSi_xC₂ (x≤0.15) solid solutions display excellent oxidation resistance at all temperatures because of the formation of the continuous α-Al₂O₃ protective layers. However, Al₂(SiO₄)O formed during oxidation of Ti₃Al_1−xSi_xC₂ (x =0.2 and 0.25) solid solutions at and above 1100^◦C, which is believed to be responsible for the deterioration of oxidation resistance of Ti₃Al_0.75Si_0.25C₂ at 1300^◦C. Additionally, Ti₅Si₃ was also found in the oxidized samples. This implies that Ti₅Si₃ precipitated from Ti₃Al_1−xSi_xC₂ solid solutions during oxidation. But it has been proven that Ti₅Si₃ has little effect on the oxidation resistance of the material, which is attributed to an interstitial carbon effect.

Absstract

The corrosion behavior of polycrystalline Ti₃SiC₂ was studied in the presence of Na₂SO₄ deposit and water vapor at 900 C and 1000 ^oC. The mass gain per unit area of the samples superficially coated with Na₂SO₄ exposed to water vapor was slightly lower than that of the samples corroded without water vapor. The microstructure and composition of the scales were investigated by SEM/EDS and XRD. Pores were observed in the corroded sample surfaces.  The main corrosion phases on the sample surface were identified by XRD as TiO₂, Na₂Si₂O₅ and Na₂TiO₃. After Ti₃SiC₂ corroded in the presence of the Na₂SO₄ deposit and water vapor, the scale had a three-layer microstructure, which was different from the duplex corrosion scale formed on Ti₃SiC₂ beneath the Na₂SO₄ film without water vapor. Because water vapor penetrated the corrosion layer and then reacted with SiO₂ to form volatile Si(OH)₄, an intermediate porous and TiO₂-enriched layer formed in the corrosion layer.

Absstract

In this paper, we investigated the reaction path to synthesize Ti₃SiC₂ by the in situ hot pressing/solid–liquid reaction method. The effect of different content of Al addition on this process was also examined. Ti3SiC2 mainly formed from the reaction between Ti₅Si₃C_x, TiC_x, TiSi₂ and graphite at 1400–1500 ^oC. As an inescapable impurity in Ti3SiC2, TiC_x was removed by addition of small amounts of Al. This was owing to the fact that the addition of a minor quantity of Al increased the amount of “effective TiC_x” and relatively decreased that of “invalid TiC_x”. Further increasing Al content, however, resulted in the presence of TiC_x again in the final product. This was due to the fact when significant amounts of Al was added, the stoichiometric ratio of silicon and graphite has been deviated from that for Ti3SiC2. More Si and less graphite were needed to prepare a monolithic Ti3Si(Al)C2 solid solution with high Al content.

Absstract

The hot corrosion behavior of Na₂SO₄-coated Ti₂AlC was investigated by means of thermogravimetric analysis, X-ray diffraction, and scanning electron microscopy/energy dispersive spectroscopy. This carbide displays good hot corrosion resistance below the melting point of Na₂SO₄ while the corrosion attacks become virulent when the salt is molten. A protectively continuous Al2O3 layer forms and imparts good corrosion resistance, and consequently, the corrosion kinetics is generally parabolic at 850 ^oC. However, porous oxide scales fail to protect the Ti2AlC substrate at 900 and 1000 ^oC. The segregation of sulfur at the corrosion scale/substrate interface accelerates the corrosion of Ti2AlC. Furthermore, a convenient and efficient pre-oxidation method is proposed to improve the high-temperature hot corrosion resistance of Ti₂AlC. An Al₂O₃ scale formed during pre-oxidation treatment can remarkably restrain the infiltration of the molten salt into the substrate and prevent the substrate from severe corrosion attacks.

Absstract

Al₃BC₃, an isostructural phase to Mg₃BN₃, experienced no pressure-induced phase transformation that occurred in the latter material (J. Solid State Chem. 154 (2000) 254–256). The discrepancy is not clear yet. Using the first-principles density functional calculations, we predict that Al₃BC₃ undergoes a hexagonal-to-tetragonal structural transformation at 24 GPa. The predicted phase equilibrium pressure is much higher than the previously reported pressure range, i.e., 2.5–5.3 GPa, conducted on phase stability of Al₃BC₃. A homogeneous orthorhombic shear strain transformation path is proposed for the phase transformation. The transformation enthalpy barrier is estimated to yield a low value, i.e., 0.129 eV/atom, which ensures that the transformation can readily take place at the predicted pressure.

Absstract

Al₂O₃ thin films were deposited on a Ti₃Al based alloy (Ti–24Al–14Nb–3V–0.5Mo–0.3Si) by sol–gel processing. Isothermal oxidation at temperatures of 900–1000 ^◦C and cyclic oxidation at 800–900 ^◦C were performed to test their effect on the oxidation behavior of the alloy. Results of the oxidation tests show that the oxidation parabolic rate constants of the alloy were reduced due to the applied thin film. This beneficial effect
became weaker after longer oxidation time at 1000 ^◦C. TiO₂ and Al₂O₃ were the main phases formed on the alloy. The thin film could promote the growth of Al₂O₃, causing an increase of the Al₂O₃ content in the composite oxides, sequentially decreased the oxidation rate. Nb/Al enriched as a layer in the alloy adjacent to the oxide/alloy interface in both the coated and uncoated alloy. The coated thin film decreased the thickness of the Nb/Al enrichment layer by reducing the scale growth rate.

Absstract

The layered ternary carbide Ti₂SnC has outstanding mechanical and electrical properties. Therefore, the Cu matrix composite reinforced by Ti2SnC expects combined merits of both materials. In this article, we synthesized the copper matrix composites reinforced by Ti₂SnC ceramic particles using hot-pressing method. The tribological behaviors of pure copper and composites reinforced with Ti₂SnC were studied on a blockon-ring tester. The blocks were slid against 52100 steel rings under dry ambient conditions. It showed that the friction coefficient and wear rate of Cu matrix composites decreased significantly by incorporation of Ti2SnC particles. The wear rate of pure copper was about eight times that of the Cu–10 vol.% Ti2SnC composite under a sliding velocity of 1 and a normal load of 30 N. The satisfied low friction coefficient and wear resistance prove that Ti2SnC is a promising reinforcement material for Cu matrix composite. In addition, the effects of sliding velocity and normal load on wear behavior were investigated for pure copper and Cu–Ti2SnC composites.

Absstract

The reaction route, microstructure, and properties of Ti3Si(Al)C2/SiC composites with 5–30 vol.% SiC content prepared by in situ hot pressing/solid–liquid reaction synthesis process are investigated. In contrast to monolithic Ti3Si(Al)C2, the SiC particle-reinforced composites exhibit higher elastic modulus, Vickers hardness, fracture toughness, improved wear, and oxidation resistance, but have a slight loss in flexural strength. The improvement in the properties is mainly ascribed to the contribution of SiC particles, and the strength degradation is due to the residual tensile stresses in the matrix.

Absstract

The high-temperature mechanical properties of both Ti3AlC2 and Ti3AlC2/10 vol. %Al2O3 composites, including compressive strength, bending strength, and fracture behavior, were investigated. The strengthening effect caused by Al2O3 strongly depended on the mode of failure: significant enhancement in strength was only observed under the brittle mode of failure; whereas under the ductile mode of failure the strengthening effect was impaired due to Ti3AlC2 matrix softening and stress relaxation.

Absstract

Ti₃SiC₂ has many salient properties including low density, high strength and modulus, damage tolerance at room temperature, good machinablity, and being resistant to thermal shock and oxidation below 1100 ^oC. However, the low hardness and poor oxidation resistance above 11001C limit the application of this material. The poor oxidation resistance at temperatures above 1100 ^oC was because of the absence of protective layer in the scale and the presence of TiC impurity phase. TiC impurity could be eliminated by adding a small amount of Al to form Ti₃Si(Al)C₂ solid solutions. Although the high-temperature oxidation resistance was significantly improved for the Ti₃Si(Al)C₂ solid solutions, the strength at high temperatures was lost. One important way to enhance the high-temperature strength is to incorporate hard ceramic particles like SiC. In this article, we describe the in situ synthesis and simultaneous densification of Ti₃Si(Al)C₂/SiC composites using Ti, Si, Al, and graphite powders as the initial materials. The effect of SiC content on high-temperature mechanical properties and oxidation resistance were investigated. The mechanisms for the improved high-temperature properties are discussed.

Absstract

In order to meet increasing requirement for the evaluation and design of damage tolerance of brittle materials, a simple expression was proposed to estimate the damage tolerance quantitatively using basic material parameters, based on the features of damage tolerance and fracture mechanics. The calculated results from this equation indicated that the damage tolerance of nanolayer grained ceramics was over 10 times higher than that of some brittle ceramics, while glass has the lowest damage tolerance. Therefore, the damage tolerance of various ceramics can be estimated directly using their basic properties data without the necessity of additional tests.  SEM examination and finite element simulation showed that the high damage tolerance of the layer grained ceramics was attributed to delamination in grains, interlocking microstructures and crack deflection.

Absstract

Ti₃AlC₂/TiC-Al₂O₃ composite was synthesized by a combustion reaction in TiO₂-Al-C system. The effect of the compositions in raw materials on the products was investigated. Ti₃AlC₂/TiC-Al₂O₃ composite was obtained at the molar ratio of TiO₂:Al:C=3.0:5.0~5.1:1.8~2.0. The reaction path for the 3TiO₂-5Al-2C system was proposed. Al₃Ti, Ti₂O₃, TiO, and δ-Al₂O₃ are found to be transitional phases. Finally, Ti₃AlC₂/TiC-Al₂O₃ composite forms at ~900 ^oC of furnace temperature. The measured Vickers hardness, fracture toughness, and flexural strength of the nearly dense sample from 3TiO₂-5Al-2C are 13.3±1.1 GPa, 5.8±0.3 MPa·m^1/2, and 466±39 MPa, respectively.