SPS原位反应制备TiC/Ti2AlC/TixAly系复合材料的微观结构及其导电性能

以Al₄C₃、Ti和石墨混合粉体为原料, 采用放电等离子技术原位反应制备了TiC/Ti₂AlC两相复合材料和TiC/Ti₂AlC/Ti_xAl_y三相复合材料. 利用XRD、SEM、TEM研究了复合材料的相组成和微观结构, HRTEM的观察结果显示复合材料的相界面清洁干净, 无非晶相存在. 同时研究了TiC/Ti₂AlC/Ti_xAl_y三相复合材料的原位反应烧结过程, 并对复合材料的导电行为进行了表征. 在室温时TiC/Ti₂AlC材料的电导率大于TiC/Ti₂AlC/Ti_xAl_y三相复合材料,其中TiC/40vol%Ti₂AlC的电导率最高达到8.83×105S/m. TiC/Ti2AlC两相复合材料和TiC/Ti₂AlC/Ti_xAl_y三相复合材料的电导率均随温度的升高而下降, 呈现电导的金属性特征, 同时电导率随温度变化关系符合Arrhenius理论.     

Oxygen incorporation in Ti2AlC thin films studied by electron energy loss spectroscopy and ab initio calculations

Substitution of C with O in hexagonal inherently nanolaminated Ti₂AlC has been studied experimentally and theoretically. Ti₂Al(C_1?x O _x ) thin films with x ≤ 0.52 are synthesized by both cathodic arc deposition with the uptake of residual gas O, and solid-state reaction between understoichiometric TiC _y and Al₂O₃(0001) substrates. The compositional analysis is made by analytical transmission electron microscopy, including electron energy loss spectroscopy. Furthermore, predictive ab initio calculations are performed to evaluate the influence of substitutional O on the shear stress at different strains for slip on the (0001) basal plane in the [?1010] and [1?210] directions.     

Effect of TiO2 and Al2O3 doping on sintering behavior and microwave dielectric properties of Ba4(Sm0.5Nd0.5)28/3Ti18+xO54+2x ceramics

The effects of TiO₂ and Al₂O₃ doping on the phase formation, the microstructure and microwave dielectric properties of Ba_6?3x (Sm_1?y ,Nd _y )_8+2x Ti₁₈O₅₄ (x = 2/3 and y = 0.5; BSNT) ceramics were investigated. X-ray diffraction patterns showed that the main crystal phase of BSNT + xTiO₂ (x = 0–2) ceramics sintered at 1,280 and 1,300 °C for 5 h was Ba(Sm, Nd)₂Ti₄O₁₂, accompanied by a small number of second phases: Ba₂Ti₉O₂₀ and TiO₂ (x ≥ 1.0), while the new phase BaAl₂Ti₅O₁₄ appeared and the two phases Ba₂Ti₉O₂₀ and TiO₂ disappeared in BSNT ? 2TiO₂ ceramic doped with ≥2 wt% Al₂O₃ successively as identified by scanning electron microscopy and energy dispersive spectroscopy analysis. The TiO₂ and Al₂O₃ working as sintering aids conduced effectively to promote the densification and grain growth, and thus decreasing the sintering temperature, so when the amounts of TiO₂ was increased, Q × f and τ _f values increased continuously. The BSNT ? 2TiO₂ ceramics doped with y wt% Al₂O₃ decreased the density and dielectric constant, increased the Q × f value remarkably and the τ _f values was adjusted from 25.3 to ?7.3 ppm/ °C. When doped with 1.5 wt% Al₂O₃ sintered at 1,260 °C for 5 h, the ceramics obtained the excellent microwave dielectric properties: ε _r  = 74.3, Q × f = 11,928 GHz, and τ _f  = +5.39 ppm/ °C.     

In situ synthesis and characterization of a hierarchically structured Al2O3/Al3Ti composite

A hierarchically structured α-Al₂O₃/Al₃Ti composite was fabricated by an in situ process called exothermic dispersive synthesis from a powder blend of Al and TiO₂. The microstructure of the composite was investigated by X-ray diffraction, scanning electron microscopy, and X-ray energy dispersive spectroscopy. Three transitional phases, specifically TiO, Ti₂O₃, and γ-Al₂O₃, were found to form during the reactive process. Using differential scanning calorimetry, it was found that the reaction between the Al and TiO₂ occurred through three intermediate steps and their corresponding activation energies were 390, 205, and 197 kJ/mol, respectively. Moreover, the reaction rate of the third step was found to be much higher than that of the second step, and the time taken by each reaction step decreased with the increase of the heating rate. The findings are critical to understanding the microstructural development in the synthesis of strong and tough Al₂O₃/Al₃Ti composites.     

Synthesis of Ti3AlC2/Al2O3 nanopowders by mechano-chemical reaction

Ti₃AlC₂/Al₂O₃ nanopowders were synthesized by the combination of mechanically-induce self-propagating reaction (MSR) of Ti, C, Al and TiO₂ powder mixtures and subsequently heat treatment. Effects of high energy milling and heat treatment temperatures on the phase transformation were investigated in detail. X-ray diffraction (XRD) was used to characterize the powders of milled and annealed, respectively. The morphology and microstructure of as fabricated products were also studied by scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS). Results show that TiC, Ti_xAl_y and Al₂O₃ transitional phases were formed when the initial powder mixtures were milled for 24 h. The desired Ti₃AlC₂/Al₂O₃ nanopowders with high purity were obtained when annealed the as-milled powders at 1100 °C. SEM image confirmed that the as fabricated Ti₃AlC₂/Al₂O₃ particles has nanocrystalline layered structural matrix of Ti₃AlC₂, and the second phase of nanosized Al₂O₃ disperses uniformly in the Ti₃AlC₂ matrix.     

Study of interfacial reactions in ionized metal plasma (IMP) deposited Al-0.5%wtCu/Ti/SiO2/Si structure

Interfacial reactions in Al-0.5%wtCu/Ti/SiO₂/Si structure have been investigated up to the annealing temperature of 600°C for 30 min in Argon ambient. Annealing temperature at above 500°C, Al alloy and Ti start to react and produce Al₃Ti, which was already reported. Annealing at higher temperatures (550°C, and 600°C) made Al₃Ti transformed into Al₅Ti₂, which is thermodynamically more stable than Al₃Ti. The unreacted 52 nm thick Ti which existed underneath of Al₅Ti₂ might lead to retardation of the reaction between Al₅Ti₂ and the underlying SiO₂. Hence, the formation of ternary compound (Al _x Ti _y Si _z ) which is believed to be detrimental to the contact metallization layers was protected.     

Combustion reaction of Ti–Al–C–N system

The combustion reaction of Ti–Al–C–N system was investigated by using Ti powders and one CNx precursor powder as reactant powder blends. The reactant powder blends ratio was adjusted to obtain different materials. The phase composition of the samples was investigated by X-ray diffraction (XRD). The microstructure of the samples was observed by scanning electron microscopy (SEM). The result showed that Ti₂Al(C, N)–TiAlx, AlN–Ti(C, N) and Ti₃Al(C, N)₂–TiC composites can be fabricated by changing the reactant powder blends ratio.     

Interaction of ZrFe<Subscript>2</Subscript> doped with Ti and Al with hydrogen

The influence of doping with Ti and Al on the structure and hydrogen sorption properties of ZrFe₂ was studied by XRD, XRSMA, and measurement of hydrogen absorption and desorption isotherms at pressure up to 300 MPa. The hydrogen capacity and equilibrium desorption pressures of hydrides decrease with increasing Al content at a constant ratio of Ti and Zr. The increase in the Ti content at a constant content of Al in alloys also leads to a decrease in hydrogen capacity; however, the equilibrium desorption pressures of hydrides increase considerably. Zr_1−x Ti _x (Fe_1−y Al _y )₂ (x= 0.2–0.8; y = 0.05–0.4) alloys were investigated.     

Mechanically activated combustion synthesis of Ti3AlC2/Al2O3 nanocomposite from TiO2/Al/C powder mixtures

Ti₃AlC₂/Al₂O₃ nanocomposite powder was synthesized by mechanical-activation-assisted combustion synthesis of TiO₂, Al and C powder mixtures. The effect of mechanical activation time of 3TiO₂-5Al-2C powder mixtures, via high energy planetary milling (up to 20?h), on the phase transformation after combustion synthesis was experimentally investigated. X-ray diffraction (XRD) was used to characterize as-milled and thermally treated powder mixtures. The morphology and microstructure of as-fabricated products were also studied by scanning electron microscopy (SEM) and field-emission gun electron microscopy (FESEM). The experimental results showed that mechanical activation via ball-milling increased the initial extra energy of TiO₂-Al-C powder mixtures, which is needed to enhance the reactivity of powder mixture and make it possible to ignite and sustain the combustion reaction to form Ti₃AlC₂/Al₂O₃ nanocomposite. TiC, AlTi and Al₂O₃ intermediate phases were formed when the initial 10?h milled powder mixtures were thermally treated. The desired Ti₃AlC₂/Al₂O₃ nanocomposite was synthesized after thermal treatment of 20?h milled powder and consequent combustion synthesis and FESEM result confirmed that produced powder had nanocrystalline structure.     

Ordering of Cubic Titanium Monoxide into Monoclinic Ti5O5

The structures of ordered and disordered cubic Ti _x O _z = TiO _y (y= z/x) phases containing both Ti and O vacancies were studied by x-ray diffraction. The results demonstrate that annealing TiO _y with y= 0.9–1.1 below 1300 K leads to the formation of a monoclinic ordered Ti₅O₅phase (sp. gr. C2/m(A12m/1)). Symmetry analysis of the monoclinic Ti₅O₅superstructure indicates that the TiO _y –Ti₅O₅ordering transition is defined by the {k ₁₀} Lifshits and {k ₄} and {k ₁} non-Lifshits wavevector manifolds. The ordering is shown to be a first-order transition accompanied by a decrease in the unit-cell volume of the parent lattice. The Ti and O distribution functions in titanium monoxide are calculated, and the Ti- and O-site near-neighbor environments in the monoclinic Ti₅O₅superstructure are identified. The region of possible long-range-order parameters is outlined.     

Property-microstructure correlation in in situ formed Al2O3, TiB2 and Al3Ti mixture-reinforced aluminium composites

The in situ formed Al₂O₃, TiB₂ and Al₃Ti mixture-reinforced aluminium composites were successfully fabricated by the reaction sintering of the TiO₂-B-Al system in a vacuum. With increasing boron content in the TiO₂-B-Al system, the amount of generated TiB₂ in the composites increased and Al₃Ti content decreased. At the same time the distribution uniformity of the in situ formed Al₂O₃ and TiB₂ particulates was obviously improved, and the size of the Al₃Ti particles was reduced. The in situ Al₂O₃ and TiB₂ particulates had sizes from 0.096–1.88 m. The interface between the in situ formed particulates and the aluminium matrix was clean, and no consistent crystallographic orientation relationship was found. The strength and elastic modulus of the composites was significantly improved by lowering the Al₃Ti content. When the boron content in the TiO₂-B-Al system rose, the morphology of the tensile fracture surface of the composites was changed from large fractured Al₃Ti blocks and fine dimples, to fine dimples and pulled-out particulates. The strengthening and fracture of the composites have been modelled.     

Microstructure and oxidation behaviour of TiAl(Nb)/Ti<Subscript>2</Subscript>AlC composites fabricated by mechanical alloying and hot pressing

TiAl-based intermetallic matrix composites with dispersed Ti₂AlC particles and different amounts of Nb were successfully synthesized by mechanical alloying and hot pressing. The phase evolution of Ti–48 at%. Al elemental powder mixture milled for different times with hexane as a process control agent was investigated. It was found that after milling for 25 h, a Ti(Al) solid solution was formed; also with increase in the milling time to 50 h, an amorphous phase was detected. Formation of a supersaturated Ti(Al) solid solution after 75 h milling was achieved by crystallization of amorphous phase. Addition of Nb to system also exhibited a supersaturated Ti(Al,Nb) solid solution after milling for 75 h, implying that the Al and Nb elements were dissolved in the Ti lattice in a non-equilibrium state. Annealing of 75 h milled powders resulted in the formation of equilibrium TiAl intermetallic with Ti₂AlC phases that showed the carbon that originates from hexane, participated in the reaction to form Ti₂AlC during heating. Consolidation of milled powder with different amounts of Nb was performed by hot pressing at 1000°C for 1 h. Only the presence of γ-TiAl and Ti₂AlC was detected and no secondary phases were observed on the base of Nb. Displacement of γ-TiAl peaks with Nb addition implied that the Nb element was dissolved into TiAl matrix in the form of solid solution, causing the lattice tetragonality of TiAl to increase slightly. The values for density and porosity of samples indicated that condition of hot pressing process with temperature and pressure was adequate to consolidate almost fully densified samples. The isothermal oxidation test was carried out at 1000°C in air to assess the effect of Nb addition on the oxidation behaviour of TiAl/Ti₂AlC composites. The oxidation resistance of composites was improved with the increase in the Nb content due to the suppression of TiO₂ growth, the formation and stabilization of nitride in the oxide scale and better scale spallation resistance.     

Sol–gel preparation and characterization of black titanium oxides Ti2O3 and Ti3O5

Sub-micron black titanium oxides powders Ti₂O₃ and Ti₃O₅ were synthesized via a facile and economical sol–gel method in this paper. Relatively pure Ti₃O₅ and Ti₂O₃ can be obtained by annealing the as-prepared PEG600-based gel with a heating rate of 10 °C min^?1 and maintaining the temperature at 1,000 and 1,200 °C, respectively, under high pure (99.999 %) argon atmosphere for 4 h. The FESEM images reveal that the morphologies of the as-prepared Ti₃O₅ and Ti₂O₃ are sphere-like and plate-like mixed type structures. The particle size of Ti₃O₅ sample is in the range of 50–200 nm. However, the appearance of Ti₂O₃ is 200–500 nm irregular flake structures covered with 20–50 nm spherical particles. This PEG600-based sol–gel approach has a low reaction temperature of 1,000 °C herein for the preparation of Ti₃O₅, which is ascribed to that, the molecular PEG-600 was distributed well in the homogeneous gel through secondary cross-linking of the organic molecules, and with the increasing of heating temperature, molecular PEG-600 was carbonized. These nanoscaled and homogeneous mixtures of carbon and TiO₂ made the carbon thermal reduction reaction occur subsequently at 1,000 °C. The Raman vibrational wavenumbers of as-prepared Ti₃O₅ and Ti₂O₃ are perfectly coincident with those of calculated results of pure Ti₃O₅ and Ti₂O₃. Besides, the Fourier transform infrared spectra of Ti₃O₅ and Ti₂O₃ were also investigated in this article. Finally, the powder electrical resistivity of Ti₃O₅ and Ti₂O₃ is 4.7 × 10^?3 and 2.5 × 10^?3 Ω m, respectively.     

Fabrication of Ti2AIC by hot pressing of Ti, TiC, Al and active carbon powder mixtures

Polycrystalline Ti₂AlC samples were synthesized by hot pressing of Ti, Al, TiC and active carbon powder mixtures. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used for phase identification and microstructure evaluation. No other phase except Ti₂AlC was detected in samples synthesized by hot pressing of the 0.5TiC/1.5Ti/1.0Al/0.5C powder mixtures at 1400°C for 1 and 3 h under a pressure of 30 MPa. The densities of these two samples were 96.1 and 98% of the theoretical value of pure Ti₂AlC, respectively. The reason that the densities of these two samples were lower than the theoretical density of pure Ti₂AlC is that pore existed in these two samples. At lower temperature of 1300°C, the speed of the reaction forming Ti₂AlC was slow. While at higher temperature of 1500°C, Ti₂AlC transformed to Ti₃AlC₂. So these two temperatures are not suitable for the fabrication of Ti₂AlC.     

Studies of Ta,Al, and Carbon Sources on Combustion Synthesis of Alumina–Tantalum Carbide Composites

Alumina–tantalum carbide (Al₂O₃–TaC and Al₂O₃–Ta₂C) composites were synthesized by incorporating aluminothermic reduction into a self-propagating combustion process. The test specimens adopted were composed of Ta₂O₅, Al, Al₄C₃, and carbon powders. Experimental evidence showed that the use of Al₄C₃ to provide Al and carbon decreased combustion exothermicity and hindered Ta₂O₅ from being fully reduced. In contrast, for the formation of Al₂O₃–TaC and Al₂O₃–Ta₂C composites, Al₄C₃-free samples with molar ratios of Ta₂O₅:Al:C = 3:10:6 and 3:10:3, respectively, exhibited the highest combustion temperature and reaction rate in the self-propagating high-temperature synthesis process and yielded products with very few minor phases.     

Synthesis,microstructure and properties of (Ti1−x,Mox)2AlC phases

The reaction path of the (Ti_0.9, Mo_0.1)₂AlC phase from Ti, Al, TiC and Mo powder mixtures was investigated in detail ranging from 500 to 1450°C, and the results show that the reaction between Ti and Al produced Ti–Al intermetallics, and the reaction between Al and Mo formed Mo–Al intermetallics. And then the (Ti_0.9, Mo_0.1)₂AlC phase was formed by the reaction of Ti–Al, Mo–Al intermetallics, and TiC. At 1350°C for 2?h, a dense (Ti_0.9, Mo_0.1)₂AlC phase with purity was successfully fabricated. The Vickers hardness, flexural strength and fracture toughness were 5.48?GPa, 363.60?MPa and 5.78?MPa m^1/2, which were improved by 44, 34 and 136% for (Ti_0.80, Mo_0.20)₂AlC, respectively, compared with the single-phase Ti₂AlC.     

A study of the stabilization of aluminium titanate

A tialite ceramics (Al₂TiO₅) was synthesized at a temperature of 1500 °C, incorporating CaF₂, La₂O₃, SiO₂ or kaolin and MgO additives. Its thermal stability was investigated by thermocycling in a reducing medium. The batches containing SiO₂ or kaolin additives underwent a decomposition to rutile and corundum. A stabilized ceramic with added MgO was produced. X-ray and electron-probe microanalysis established the presence of Mg/Al, Ti_z/O₄, Al_2–x-y Ti_1+x Mg _y O₅ and Ca_1–x La _x /Al_12–y-z Mg _y Ti _z /O₁₉ solid solutions, which retained their chemical composition after thermocycling in a reducing medium.     

Interface compounds formed during the diffusion bonding of Al2O3 to Ti

The interfacial reaction products of Ti/Al₂O₃ joints obtained in the context of real diffusion bonding technology were investigated by means of X-ray diffraction analysis, X-ray photoelectron spectroscopy, and transmission electron microscopy. Some Ti reacted with Al₂O₃ giving titanium oxides, but the main mass transport occurred into the bulk Ti due to Al₂O₃ dissolution. The formation of a Ti[Al, O] solid solution followed by a order/disorder reaction yielded Ti₃Al. Further Al enrichment at the interface could lead to the formation of TiAl, which was not observed in the present work due to either the short residence time at the bonding temperatures or to its lower oxygen solubility. For joints obtained at 800°C and shear test fractured it was ascertained that the crack always propagated within the Ti₃Al layer.     

On the influence of alloy composition on the oxidation performance and oxygen-induced phase transformations in Ti–(0–8) wt%Al alloys

Adopting a high-throughput combinatorial approach, a compositionally graded Ti–xAl (0 ≤ x ≤ 8 wt%) specimen was prepared to conduct a rapid systematic investigation of the influence of composition and exposure time on the oxidation performance of the titanium-rich section of the binary Ti–Al system. The compositionally graded specimen was solution heat treated and subjected to oxidation tests at 650 °C for different exposure times. The morphology, structure, and composition of the oxide scale as well as the microstructural changes in the base material were studied across the entire composition range, using a suite of characterization techniques. The observations revealed the presence of Al₂O₃ in the topmost layer of the oxide scale in addition to TiO₂, indicating its early formation during oxidation. An increase in Al concentration improves the scaling rate of Ti; however, this is observed only for extended exposure times (i.e., 50 and 100 h), and a parabolic oxidation law is obeyed in the composition-time domain. The formation of the α₂ phase (Ti₃Al) also takes place for relatively higher Al contents (i.e., 8 wt%).     

Influence of sol–gel derived Al2O3 film on the oxidation behavior of a Ti3Al based alloy

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.