Molten salt synthesis and formation mechanism of Ti3AlC2: A new path from Ti2AlC to Ti3AlC2

Fine, pure Ti₃AlC₂ powder is prepared in a very mild condition via Ti₃Al alloy and carbon black with the assistance of molten salts. X-ray diffraction, scanning electron microscopy, TG-DSC, and transmission electron microscopy (TEM) characterizations show that the high purity, nanosized Ti₃AlC₂ can be obtained at 900°C with the 1:1 salt-to-material ratio. The formation mechanism of Ti₃AlC₂ through this strategy of alloy raw material is fully studied under further TEM investigations, showing that the reaction process can basically be described as Ti₃Al and C → TiAl and TiC → Ti₂AlC and TiC → ψ and TiC → Ti₅Al₂C₃ and TiC → Ti₃AlC₂, where the key ψ, a modulated Ti₂AlC structure, is determined for the first time containing alternate-displacement Al layers along (0 0 0 2) of Ti₂AlC phase with a distinct selected area electron diffraction pattern. Such alternant displacement is considered a precondition of forming Ti₅Al₂C₃ through topotactic transition, followed by Ti₅Al₂C₃ converting into Ti₃AlC₂ by the diffusion of Ti, C atoms in the outside TiC. Several parallel orientations can be observed through the phase transition process: Ti₂AlC (0 0 0 2)//ψ (0 0 0 1), ψ (0 0 0 1)//Ti₅Al₂C₃ (0 0 0 3), Ti₅Al₂C₃ (0 0 0 3)//Ti₃AlC₂ (0 0 0 2). Such parallel orientations among these phases apply an ideal condition for the topotactic reaction. The distinct path of the phase transition brings a significant change of heat effect compared with the traditional method, leading to a fast reaction rate and a mild reaction condition.     

Investigation of Ti2AlC formation mechanism through carbon and TiAl diffusional reaction

Ti₂AlC formation mechanism with carbon and TiAl alloy as reactants at 1000–1200 °C have been investigated through Raman spectrometer and transmission electron microscope (TEM). Reaction products TiC_x and Ti₂AlC are confirmed. Full width at half height (FWHH) of Ti₂AlC Raman peak is used to characterize crystallinity, which indicates better perfection of Ti₂AlC grains near TiAl area and with higher reaction temperature. Variations of TiC_x microstructure, especially stacking faults(SF) and nanotwins, are observed by TEM along reaction layer. Ti₂AlC lattice uniformity of different reaction temperature is also confirmed by TEM, which is consistent with Raman result. TiC_x is believed first formed through reaction between TiAl and C. With help of carbon vacancy, Al-doped TiC_x SF and nanotwin can be formed under hot pressing, which becomes the basal frame of Ti₂AlC.     

Ti-Al-C三元系统中Ti3AlC2,Ti2AlC和TiC物相含量的测定

研究了Ti-Al-C三元体系中X射线衍射定量相分析方法,提出了Ti3AlC2,Ti2AlC和TiC这3种物相的特征衍射峰Ti3AlC2的(002)衍射峰,2θ=9.5°;Ti2AlC的(002)衍射峰,2θ=13.0°;TiC的(111)衍射峰,2θ=35.9°.采用混样无标样法推导了一套Ti3AlC2,Ti2AlC和TiC的定量计算公式,根据其特征衍射峰的强度就可计算这3个物相的相对含量.因为该公式推导过程本身就验证了其正确性(即自验证性),所以这个公式可以广泛用于Ti-Al-C三元体系中Ti3AlC2,Ti2AlC和TiC物相的定量测定.因此,这种方法非常简便、易行.     

High-voltage arc erosion behavior and mechanism of Ti3AlC2 under different ambient atmospheres

The arc erosion behavior of Ti₃AlC₂ in oxygen, air, nitrogen, carbon dioxide, argon, and sulfur hexafluoride atmospheres at 9 kV voltage was studied. The breakdown strength increased in the order of oxygen, air, carbon dioxide, nitrogen, argon, and sulfur hexafluoride, whereas the arc energy decreased. Cracks, pits, and bulges on the eroded surface were investigated using scanning electron microscopy and three-dimensional (3D) laser confocal scanning microscopy. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to determine the composition of the eroded surface. The arc energy and electromagnetic force lead to the formation of erosion characteristics. The mechanism of erosion under the different atmospheres is discussed systematically, and is called the “decomposition-oxidation” process in oxygen, air, and carbon dioxide and the “decomposition-rereaction” process in nitrogen and sulfur hexafluoride. This study provides a reference for the application of MAX phase materials in high-voltage electrical appliances.     

Preparation of Ti2AlC through in-situ selective laser forming and reaction sintering

This paper proposes the preparation of Ti₂AlC complex parts through the combination of in-situ selective laser forming (ISLF) and reaction sintering. By studying the impaction of scan speed and laser power on the phase composition and microstructure of the formed test specimen, it was found that the test specimen mainly consisted of Ti3Al, TiAl, TiC, and graphite reacted incompletely. When the laser power was fixed, the lower the scan speed, the higher the TiC content. The TiC phase developed from nanoparticles to coarse dendritic structures gradually. When the laser power is further increased, the dendritic structure does not increase significantly. The process optimization found that when the laser power was 40 W and the scan speed was 100 mm/s, the density of the formed test specimen was 80.308%, which was mainly because the difference between the diffusion coefficients of Ti and Al resulted in Kirkendall pores, causing the incomplete densification of the formed part.     

Investigation of Ti3AlC2 formation mechanism through diffusional reaction between carbon and Mo-modified Ti6Al4V

Carbon and Mo-coated Ti6Al4V alloy diffusion couple was used to investigate Mo-modified diffusion reaction between carbon and Ti6Al4V. Randomly dispersed carbide particles were observed in Ti6Al4V alloy after 900℃ exposure. Carbide particles were found evolving from defective TiC_x (x<1) to mixture of TiC_x and defective Ti₃AlC₂. Although Mo atoms were hardly detected in particles, their dilution effect along Ti alloy grain boundary (GB) is beneficial to carbon diffusion and carbide formation along GB. Based on high-resolution TEM (HRTEM) imaging, high density stacking faults (SFs) and nanotwins were observed in TiC_x particles which explain Raman activation of defects in TiC. Formation of SFs and nanotwins in TiC_x is attributed to carbon vacancies, which inversely promotes incoherent twin boundary (ITB) formation. Transformation from TiC_x to Ti₃AlC₂ is considered driven by Al indiffusion along ITBs. Intergrowth of TiC_x in defective Ti₃AlC₂ is the compromise to low Al concentration in Ti6Al4V.     

Influence of Ti3AlC2 content and load on the tribological behaviors of Ti3AlC2p/Al composites

In this study, Ti₃AlC₂ particles doped aluminum matrix composites were prepared by ultrasonic agitation casting method. Microstructure, mechanical properties, and tribological properties of pure aluminum and Ti₃AlC_2p/Al composites were characterized. Influence of different loads (10, 20, 30, and 40 N) and Ti₃AlC₂ contents (1.0, 2.0, 3.0, and 4.0 wt%) on the tribological behaviors of the composites were studied. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Energy dispersion spectroscopy (EDS), and 3D laser confocal were used to assist the analysis. The results indicated that fine and uniformly microstructure and the optimum comprehensive mechanical properties were exhibited on 2.0 wt%-Ti₃AlC_2p/Al composites. The abrasive grooves were widened and deepened with an increase in the load. The abrasion performance of composites improved distinctly with the addition of the Ti₃AlC₂ particles, which changed the wear mechanism from adhesive wear to abrasive wear. The 30 N load and the composites of 2.0 wt% Ti₃AlC₂ revealed the optimum tribological properties. The improvement of the tribological behavior of composites was attributed to the refinement of microstructure, the improvement mechanical properties and the three dimensional layered Ti₃AlC₂ phases with self-lubricating properties.     

Synthesis and formation mechanism of high-purity Ti3AlC2 powders by microwave sintering

High-purity titanium aluminum carbide (Ti₃AlC₂) powders were synthesized by a microwave sintering method using different titanium sources as raw materials. The prepared products were characterized by differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results indicated that the synthesized Ti₃AlC₂ powders have high purity (97.5%) and even distribution of the grain size when using a 3TiH₂/1.2Al/2C mixture as raw materials when the microwave sintering temperature and time were 1300°C and 30 minutes, respectively. The formation mechanism of the Ti₃AlC₂ is described as proceeding via four stages. The solid-phase reaction between titanium and aluminum occurs below the melting point of aluminum and the main product is a Ti₃Al phase, which is an observed intermediate compound for the formation of Ti₂AlC and Ti₃AlC₂. Thus, this study provides a beneficial approach to low-temperature synthesis of high-purity Ti₃AlC₂ materials.     

Spark plasma sintering of TiB2-based ceramics with Ti3AlC2

A TiB₂–Ti₃AlC₂ ceramic was manufactured by spark plasma sintering at 1900 °C temperature for 7 min soaking time under 30 MPa biaxial pressure. The role of Ti₃AlC₂ additive on the microstructure development, densification behavior, phase evolution, and hardness of the ceramic composite were studied. The phase characterization and microstructural investigations unveiled that the Ti₃AlC₂ MAX phase decomposes at the initial stages of the sintering. The in-situ formed phases, induced by the decomposition of Ti₃AlC₂ additive, were identified and scrutinized by XRD and FESEM/EDS techniques as well as thermodynamics principles. The sintered TiB₂–Ti₃AlC₂ ceramic approached a near full density of ~99% and a hardness of ~28 GPa. The densification mechanism and sintering phenomena were discussed and graphically illustrated.     

Oxidation and hot corrosion behaviors of MAX-phase Ti3SiC2, Ti2AlC,Cr2AlC

Oxidation and hot corrosion behaviours of Ti₃SiC₂, Ti₂AlC and Cr₂AlC at 750 °C were investigated in this work. Ti₃SiC₂ and Ti₂AlC showed a linear increase in mass gain and a relatively poor oxidation resistance. This might be attributed to the porous TiO₂ scale. A dense α-Al₂O₃ layer was formed during the oxidation test. Cr₂AlC exhibited the best oxidation resistance. This dense oxide scale can effectively isolate the substrate from contact with oxygen leading to excellent oxidation resistance. In contrast to the oxidation test, Ti₃SiC₂ and Ti₂AlC showed relatively better resistance to hot corrosion, while Cr₂AlC showed inferior resistance to NaCl introduced hot corrosion. The hot corrosion mechanism of the MAX phases was analyzed. Due to the formation of Na₂TiO₃, Ti containing MAX phases showed a continuous increase in the mass gain. The corrosion products of Cr₂AlC were Al₂O₃, Cr₂O₃ and Na₂CrO₄. However, due to the volatilization of Na₂CrO₄, Cr₂AlC showed a mass loss during the hot corrosion test. The chemical reaction process of the MAX phase was also analyzed.     

Molten salt shielded synthesis and formation mechanism of Ti2AlC in NaCl–KCl medium

Herein, a highly crystalline Ti₂AlC was synthesized via the improved molten salt synthesis method called molten salt shielded synthesis. To achieve this goal, the mixture of Ti, Al, and graphite and KCl–NaCl eutectic composition salt was heated at 1000, 1050, and 1100 °C for 0.5, 1, and 1.5 h. The X-ray diffraction (XRD) patterns showed that the optimum condition for obtaining the more crystalline Ti₂AlC was achieved at 1100 °C for 1.5 h. Such phase identification, and transmission electron microscopy (TEM) images, proved that applying a protective carbon layer on the surface of salt led to inhibiting the diffusion of oxygen into the surface of the green pellet. As a result, the crystallinity of Ti₂AlC improved, while the content of undesirable compounds such as Al₂O₃ and Ti_xO_y decreased drastically. In order to shed light on the Ti₂AlC synthesis mechanism, differential thermal analysis (DTA) was employed. The DTA curve revealed that the Ti₂AlC formation completed in three levels. First, the partial dissolution of Ti in KCl–NaCl salt followed by a reaction with liquid Al resulted in the TiAl formation. Next, Ti(II) reacted in-situ on the surface of graphite that resulted in the non-stoichiometric TiC (TiC_1-x) formation, and, at last in a reaction between TiAl and TiC_1-x, Ti₂AlC phase formation took place at 940 °C.     

Solid state reactions in highly dispersed Ce1−xYbxO2−y-Al2O3 system

This paper reports the results of studies on the interaction of Ce_1−xYb_xO_2−y mixed oxide nanocrystals (x=0.3 and 0.5) with gamma Al₂O₃. Nano-sized (3–4 nm) crystalline Ce-Yb mixed oxides, prepared by a water-in-oil (W/O) microemulsion technique, or simple co-impregnation by aqueous solutions of lanthanide ions, were supported on high surface area gamma alumina. Solid state reactions occurring in these systems upon heat treatment in air or hydrogen at 900–1100 °C were studied by TEM, SAED and XRD techniques. As a result of solid state reactions between pure or mixed lanthanide oxides and high surface area Al₂O₃, various aluminates were formed, depending on the heating temperature and atmosphere. Lattice parameters measured for the Yb₄Al₂O₉ (monoclinic) and CeAlO₃ (tetragonal) aluminates formed in Ce_0.5Yb_0.5O_1.75-Al₂O₃ and CeO₂+Yb₂O₃-Al₂O₃ systems suggest that they may contain other Ln-ions in the structure, and thus should be described as Ce_1−aYb_aAlO₃ and (Yb_1−bCe_b)₄Al₂O₉. Similar products were formed from solid state reactions in Ce-rich (CeO₂-Al₂O₃ and Ce_0.7Yb_0.3O_1.85-Al₂O₃) or Yb-rich (Yb₂O₃-Al₂O₃ and Ce_0.5Yb_0.5O_1.75-Al₂O₃) systems. The author postulates that, different solid state reaction routes were taken, depending on the Yb-ions concentration in the oxide structure, which was connected with the microstructure of lanthanide oxides (F or F# for Ce-rich systems and C or C# for Yb-rich systems).     

Experimental investigation and mathematical modelling of water vapour corrosion of Ti3SiC2 and Ti2AlC ceramics and their mechanical behaviour

Commercially available Ti₃SiC₂ and Ti₂AlC ceramics were used in this study to investigate their wet corrosion and mechanical behaviour as they were under investigation for years for their applications in the field of nuclear as cladding materials and aerospace. The test coupons of dimension 3 × 4 × 40 mm³ and 3 × 4 × 20 mm³ were machined out from commercially available samples for the 3-pt bend test and wet corrosion test, respectively. The water vapour corrosion studies of these samples were carried out at 800 ℃, 1000 ℃, 1200 ℃ for 10, 20 and 100 h in gas flow condition containing 50 % steam + 50 % air. Phase analysis of the as-received Ti₃SiC₂ and Ti₂AlC ceramics revealed the presence of other impurity phases such as TiC and TiSi₂. The XRD patterns of the oxidised samples show the formation of rutile as the major phase in both materials. The oxidation layer formed on Ti₃SiC₂ sample was measured to be 280 μm after exposing the sample in steam for 100 h at 1200 °C. The water vapour corrosion studies reveal that Ti₂AlC has high oxidation resistance compared with the Ti₃SiC₂ due to the formation of protective layers of TiO₂ and Al₂O₃ which resulted in reduced weight gain and oxidation layer thickness. Three-point bend tests were conducted at room temperature for the samples after the water vapour corrosion test at 1000 °C/100 h. The TAC samples showed no degradation in the bending strength (244 MPa) whereas the TSC samples showed reduced strength of 320 MPa. The tensile strength of the samples was measured at room temperature and hydrothermal condition (250 °C and 250 bars pressure) and it was observed that Ti₃SiC₂ had high tensile strength (190 MPa) in hydrothermal conditions. The tensile strength results were validated using Finite element analysis (FEA) using ANSYS and the FEA results showed a negligible variance of 7 % compared with experimental method. Mathematical modelling based on one dimensional solution of diffusion equation combined with Deal-Grove model was employed to study and compare the oxidation thickness for the linear and parabolic models for the ceramics. The model was effective in validating the oxidation thickness of Ti₃SiC₂ showing that the experimental thickness was closer to that of mathematical model.     

Comparison of corrosion behavior of Ti3SiC2 and Ti3AlC2 in NaCl solutions with Ti

Corrosion behavior of self-sintered, ternary-layered titanium silicon carbide (Ti₃SiC₂) and titanium aluminum carbide (Ti₃AlC₂) fabricated by an in-situ solid-liquid reaction/hot pressing process was investigated by potentiodynamic polarization, potentiostatic polarization and electrochemical impedance spectroscopy in a 3.5% NaCl solution. Commercially pure titanium (Ti) was selected for comparison through XRD, XPS, SEM and EDS examinations for elucidating both the passivation behavior and corrosion mechanism of the alloys. Both Ti₃SiC₂ and Ti₃AlC₂ exhibited significantly superior passivation characteristics compared to Ti; Ti₃SiC₂ also showed better corrosion resistance. The silicon/aluminum site is prone to attack, and the difference in the diffusion rate between the A-site atoms and titanium decreases the passivation ability of the MAX phase. CP titanium exhibited a lower passivation current density and did not undergo breakdown in the test potential region while two MAX phases are destroyed. Nevertheless, the corrosion resistances of Ti₃SiC₂ and Ti₃AlC₂ are comparable to that of CP titanium.     

Thermodynamic description of the ZrO2-TiO2-Al2O3 system based on experimental data

Phase relations in the system ZrO₂-TiO₂-Al₂O₃ have been investigated in the temperature range between 1557 and 1897 K using equilibration method. X-ray powder diffraction (XRD) and scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM/EDX) have been used for sample characterization. Melting relations and sub-solidus reactions have been investigated using differential thermal analysis (DTA). Two invariant reactions are found at the subsolidus temperatures of 1593 and 1648 K. Three eutectic reactions have been established in the system based on microstructure investigation. Temperatures of these reactions were in the range between 1909 and 1978 K. Based on the obtained experimental results thermodynamic parameters of the system have been assessed. Isothermal sections and liquidus surface of the ZrO₂-TiO₂-Al₂O₃ system have been calculated.     

Optimized mechanical properties and oxidation resistance of low carbon Al2O3-C refractories through Ti3AlC2 addition

The mechanical properties and oxidation resistance of the Al₂O₃-C refractories are of critical importance for iron and steel making processes. However, the evaporation of antioxidants related phases such as Al(g), Si(g), and SiO(g) would deteriorate these properties, especially during high-temperature treatment/application. Therefore, in the present work, a small amount of Ti₃AlC₂ compared with Al was introduced to overcome these problems. The phase compositions, microstructures, mechanical properties, and oxidation resistance of Ti₃AlC₂ containing refractories were investigated. The partial oxidation of Ti₃AlC₂ led to inherited lamellar structures such as Ti₃Al_1-xC₂, TiC, and granular Al₂TiO₅ phases. The controlled oxidation of Ti₃AlC₂ and its volume expansion contributed to the compact-structure, thereby limiting the escape of Si and SiO vapors at high temperatures. Consequently, the mechanical properties and oxidation resistance of Ti₃AlC₂ containing Al₂O₃-C refractories treated at 1600 ℃ were improved.     

Structural characteristics and optical properties of lead-free Bi(Zn1/2Ti1/2)O3-BaTiO3 ceramics

The xBi(Zn_1/2Ti_1/2)O₃-BaTiO₃ (xBZT-BT) ceramics, where x (mol) =0, 0.03, 0.06, 0.09, and 0.12, have been prepared by a solid-state reaction method. The phase transition, microstructure and optical properties were investigated. X-ray diffraction patterns indicate that the as-prepared samples have a polycrystalline perovskite structure. For x<0.06, the xBZT-BT ceramics exhibit clear tetragonal symmetry, and transform to rhombohedral phase as 0.06< x≤0.12. Coexistence of both tetragonal phase and rhombohedral phase is observed for x=0.06. The lattice strain is estimated by the Williamson-Hall analysis model, which suggests that the incorporation of substitution ions into the host lattice produces the inner stress field gives rise to structure distortions. The Raman scattering spectra corroborate the decrease in tetragonality with increasing the x, where the characteristic variation of phonon modes indirectly reveal the incorporation of Bi(Zn_1/2Ti_1/2)O₃. Furthermore, the optical band gaps of xBZT-BT ceramics show a non-linear change, which can be explained by the crystal field theory and phase structure effect.     

Effect of target peak power density on the phase formation,microstructure evolution,and mechanical properties of Cr2AlC MAX-phase coatings

The applicability of Cr₂AlC MAX-phases as protective coatings in energy conversion or aerospace applications requires a dense, single-phase structure. Therefore, we study the effect of target power density and substrate bias on phase formation, microstructure evolution, and mechanical properties of Cr₂AlC coatings utilizing direct current (DCMS) and high-power pulsed magnetron sputtering (HPPMS). Generally, HPPMS results in coatings with superior density and hence larger elastic moduli compared to DCMS, indicating that ion bombardment by ionized film-forming species is beneficial. However, decreasing the substrate bias to ?200 V for DCMS and ?100 V for HPPMS favors the ion bombardment induced formation of the disordered (Cr,Al)₂C_x solid solution. It is evident that there is an optimum moderate ion energy for the formation of dense Cr₂AlC coatings. Too low energy results in the formation of under-dense coatings. Too high energy yields the formation of (Cr,Al)₂C_x in addition to Cr₂AlC.     

Phase equilibria and microwave dielectric properties in the ternary La2O3–TiO2–Nb2O5 system

High-temperature phase relations in the La₂O₃–TiO₂–Nb₂O₅ ternary system were investigated at 1290 °C using SEM, EDS and XRD, and a subsolidus phase equilibrium diagram was constructed. It was demonstrated that the addition of the La_1/3NbO₃ to the La₂O₃:3TiO₂ mixture completely stabilizes a perovskite-La_2/3TiO₃ compound, which is otherwise not stable at stoichiometric composition. On the other hand, a mixture La₂O₃:TiO₂ =1:3 dissolves in La_1/3NbO₃ up to 15 mol% TiO₂. Moreover, a newly defined composition of compound La₃₀Ti₁₆Nb₄O₈₇ formed in the La₂O₃-rich part of the system and a solid solution La₃Ti₅Nb₁₀O_39.5–La₂Ti₂Nb₈O₂₇ were proposed. In the binary subsystem LaTiNbO₆-La_0.462Ti_0.386Nb_0.614O₃, the components exhibit opposite temperature dependence of resonant frequency in the microwave frequency range. Thus, the dielectric properties of the prepared ceramics can be tailored by varying the component concentration and thermally stable dielectric properties can be achieved for a composition with a LaTiNbO₆/La_0.462Ti_0.386Nb_0.614O₃ molar ratio of 0.27.     

Mechanical properties and toughening mechanisms of highly textured Ti3AlC2 composite material

Composite material consisting of Al₂O₃ and TiC in a matrix of highly textured Ti₃AlC₂ was fabricated in a two-step fabrication process. The Lotgering orientation factor for {00 l} planes of Ti₃AlC₂ in the textured top surface plane reached 0.71. Texture analysis showed an orientation relationship among Ti₃AlC₂, Al₂O₃ and TiC grains of [110] Ti₃AlC₂ // [110] TiC, (001) Ti₃AlC₂ // (111) TiC, and [110] Ti₃AlC₂ // [120] Al₂O₃, (001) Ti₃AlC₂ // (001) Al₂O₃. The texture grained material exhibited excellent mechanical properties, with compressive and flexural strengths of more than 2.5 times those of conventional coarse grained Ti₃AlC₂, and fracture toughness and hardness were 50% higher than those of conventional coarse grained Ti₃AlC₂. The microstructures of textured Ti₃AlC₂ and reported textured Ti₂AlC were investigated and compared to interpret the differences in mechanical behavior of the two textured MAX phases.