An experimental approach to delineate the reaction mechanism of Ti3AlC2 MAX-Phase formation

A comprehensive reaction mechanism of Ti₃AlC₂ MAX-phase formation from its elemental powders while spark plasma sintering has been proposed. Microstructural evaluation revealed that Al-rich TiAl₃ intermetallic forms at around 660 °C once Al melts. Gradual transition from TiAl₃ to Ti-rich TiAl and Ti₃Al intermetallic phases occurs between 700 °C and 1200 °C through formation of layered structure due to diffusion of Al from periphery toward the centre of Ti particles. Formation of TiC and Ti₃AlC transient carbide phases were observed to occur through two different reactions beyond 1000 °C. Initially, TiC forms due to interaction of Ti and C, which further reacts with TiAl and Ti and gives rise to Ti₃AlC. Later, Ti₃AlC also forms due to diffusion of C into Ti₃Al above 1200 °C. Above 1300 °C, Ti₃AlC phase decomposes into Ti₂AlC MAX-phase and TiC in presence of unreacted C. Finally, Ti₂AlC and TiC reacts together to from Ti₃AlC₂ MAX-phase above 1350 °C and completes at 1500 °C.     

Reactive synthesis for porous TiVAlC ceramics by TiH2,V,Al and graphite powders

Using the mixed powder of TiH₂, graphite, aluminum and vanadium as starting materials, porous TiVAlC ceramics were fabricated by the reactive synthesis technology at 1300 °C. The chemical steadiness of porous TiVAlC along with the effects of sintering temperature on the viscous permeability coefficient, strength, porosity, pore size and volume expansion rate of the porous TiVAlC were explored, and the mechanism of pore formation was also revealed. The preparation process includes five steps as follows: (i) the complete decomposition of stearic acid at 500 °C; (ii) the pyrolysis of TiH₂ at 700 °C, converting TiH₂ into hydrogen and titanium (iii) The solid-liquid chemical reaction of solid vanadium, titanium and molten aluminum at 700 °C, converting the mixture into V–Al and Ti–Al compounds; (iv) At 900–1100 °C, Surplus V and Ti interact with graphite to synthesize carbides of TiVC₂, VC, and TiC; (v) Reactive synthesized carbides (TiVC₂, VC, and TiC), Ti₂AlC, V–Al and Ti–Al compounds that yield porous TiVAlC at 1300 °C.     

The synthesis and mechanical properties of high pure Ti2Al(Sn)C solid solution

High pure Ti₂Al_(1?x)Sn_xC (x = 0‐1) powders were synthesized using Ti, Al, Sn, and TiC powders as raw materials by pressureless sintering method. The influence of sintering temperature and raw material ratio on the purity of Ti₂AlC and Ti₂Al_0.8Sn_0.2C powders were investigated. The results show that pure Ti₂AlC and Ti₂Al_0.8Sn_0.2C powders were obtained from the mixed raw materials ratio of Ti:1.1Al:0.9TiC and Ti:0.9Al:0.2Sn:0.9TiC at 1450°C, respectively. Subsequently, fully dense Ti₂AlC and Ti₂Al_0.8Sn_0.2C bulks were prepared using mechanically alloying and hot pressed sintering method. The Vickers hardness of Ti₂AlC and Ti₂Al_0.8Sn_0.2C approaches approximately about 6 GPa and 4 GPa, the flexural strength was measured to be 650 ± 36 MPa and 521 ± 33 MPa, respectively. Microstructural analysis reveals that grain delamination, kink bands, and crack deflection occurred around the indentation area and at the fracture surface.     

The phase transformation and microstructure of TiAl/Ti2AlC composites caused by hot pressing

Two series of raw materials were adopted to form TiAl/Ti₂AlC composites: Ti/Al/TiC and Ti/Al/C. Differential thermal analysis (DTA) of starting powers and X-ray diffraction (XRD) of samples sintered at different temperatures from 600 °C to 1300 °C by hot pressing were utilized to analyze the phase transformation and the mechanism of synthesis. Scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS) was utilized to investigate the morphology characteristics of the products. The experimental results showed that Ti reacted with Al to form TiAl intermetallics below 900 °C; and above 900 °C, TiAl reacted with TiC to produce dense TiAl/Ti₂AlC composites. The products sintered at 1200 °C had fine crystals and dense fibres, and the distribution of Ti₂AlC particles in TiAl matrix was homogeneous.     

Synthesis and reaction path of Ti-Al-C MAX phases by reaction with Ti-Al intermetallic compounds and TiC

In this study, it was verified that the synthesis of Ti-Al-C MAX phases has advantages when using intermetallic compounds rather than using only elemental powders. The formation behavior of the MAX phases was presented through diffusion experiments. In the case of using elemental powder, Ti₂AlC is produced at 1300°C, and Ti₃AlC₂ is produced at 1400°C. When intermetallic compounds are used, Ti₂AlC is produced at 1000°C, and Ti₃AlC₂ is produced at 1300°C. In the case of the elemental powder, it is verified that Ti₃AlC₂ content is decreased and Ti₂AlC is increased when heat treatment is performed at 1400°C for 3 h. Rather Ti₃AlC₂ content is increased when intermetallic compounds are used. When an intermetallic compound is used, synthesis occurs more actively at high temperatures, and the tendency to be thermally decomposed can be prevented. When TiAl and TiC are heat treated, Al of the intermetallic compound is diffused into TiC, and C of TiC is diffused into the intermetallic compound. Furthermore, there are many two-dimensional defects in TiAl, which act as a C diffusion channel. C diffuses into TiAl to produce TiC_X, and the MAX phases is generated by the short-range diffusion of Al. At the region of TiC, TiC transforms into TiC_X after C diffuses into TiAl, which consequently structure of TiC changes from cubic to hexagonal. This is the same crystal structure as the MAX phases, and it is confirmed that the (110) surface is maintained. A Ti-C layer structure of the (110) surface is maintained, and it was determined that Al is diffused during this time to generate the MAX phases.     

Molten salt synthesis of MAX phases in the Ti-Al-C system

The molten salt method was used to synthesise the MAX phase compounds Ti₂AlC and Ti₃AlC₂ from elemental powders. Between 900–1000?°C, Ti₂AlC was formed alongside ancillary phases TiC and TiAl, which decreased in abundance with increasing synthesis temperature. Changing the stoichiometry and increasing the synthesis temperature to 1300?°C resulted in formation of Ti₃AlC₂ alongside Ti₂AlC and TiC. The type of salt flux used had little effect on the product formation. The reaction pathway for Ti₂AlC was determined to be the initial formation of TiC_1-x templating on the graphite and titanium aluminides.     

Infiltration behavior of Cu and Ti fillers into Ti2AlC/Ti3AlC2 composites during tungsten inert gas (TIG)brazing

Herein we study the infiltration behavior of Ti and Cu fillers into a Ti₂AlC/Ti₃AlC₂MAX phase composites using a TIG-brazing process. The microstructures of the interfaces were investigated by scanning electron microscopy and energy dispersive spectrometry. When Ti₂AlC/Ti₃AlC₂ comes into contact with molten Ti, it starts decomposing into TiC_x, a Ti-richandTi₃AlC; when in contact with molten Cu, the resulting phases are Ti₂Al(Cu)C, Cu(Al), AlCu₂Ti and TiC. In the presence of Cu at approximately 1630 °C, a defective Ti₂Al(Cu)C phase was formed having a P63/mmc structure. Ti₃AlC₂ MAX phase was completely decomposed in presence of Cu or Ti filler-materials. The decomposition of Ti₂AlC to Ti₃AlC₂ was observed in the heat-affected zone of the composite. Notably, no cracks were observed during TIG-brazing of Ti₂AlC/Ti₃AlC₂ composite with Ti or Cu filler materials.     

Design and strengthening behaviour of Ti2AlC/TiAl composite by low-temperature hot-pressing process

In situ Ti₂AlC/TiAl composite was first fabricated by reactive hot-pressing technique at low temperature of 1150°C for 2?h using Ti₃AlC₂ and Ti–Al alloy powders. The composite with fine-grained structure consisted of TiAl, Ti₃Al and Ti₂AlC phases. The Vickers hardness, flexural strength and fracture toughness of the Ti₂AlC/TiAl composite reached 5.2?GPa, 937.7?MPa and 7.7?MPa?m^1/2, respectively. The action mechanism for the composite was mainly attributed to the grain refinement, the uniform distribution of the dispersed Ti₂AlC particles, transgranular cracking, crack deflection, crack bridging and pull-out of Ti₂AlC.     

Reaction synthesis of layered ternary Ti2AlC ceramic

Reactive sintering of 8Ti:Al₄C₃:C powder mixtures to form the ternary carbide Ti₂AlC is studied in the temperature range 570–1400 °C. After sintering at 1400 °C for 1 h, only the MAX phase Ti₂AlC and some TiC are produced. A series of intermediate phases, such as TiC, Ti₃Al, Ti₃AlC are detected during the reactive sintering process. From X-ray diffraction (XRD) and scanning electron microscopy (SEM) characterizations, a reaction path is proposed for the intermediate phases and Ti₂AlC formation. Results show that reaction kinetics may play an important role in the understanding of the reaction mechanisms.     

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.     

Synthesis,sintering, and effect of surface roughness on oxidation of submicron Ti2AlC ceramics

Submicron Ti₂AlC MAX phase powder was synthesized by molten salt shielded synthesis (MS³) using a Ti:Al:C molar ratio of 2:1:0.9 at a process temperature of 1000°C for 5  hours. The synthesized powder presented a mean particle size of ~0.9 µm and a purity of 91 wt. % Ti₂AlC, containing 6 wt. % Ti₃AlC₂. The Ti₂AlC powder was sintered by pressureless sintering, achieving a maximal relative density of 90%, hence field-assisted sintering technology/spark plasma sintering was used to enhance densification. The fine-grained microstructure was preserved, and phase purity of Ti₂AlC was unaltered in the latter case, with a relative density of 98.5%. Oxidation was performed at 1200°C for 50 hours in static air of dense monolithic Ti₂AlC with different surface finish, (polished, ground and sandblasted) which resulted in the formation of an approx. 8 µm thin aluminum oxide (Al₂O₃) layer decorated with titanium dioxide (rutile, TiO₂) colonies. Surface quality had no influence on Al₂O₃ scale thickness, but the amount and size of TiO₂ crystals increased with surface roughness. A phenomenon of rumpling of the thermally grown oxide (TGO) was observed and a model to estimate the extent of deformation is proposed.     

Synthesis and mechanism of ternary carbide Ti3AlC2 by in situ hot pressing process in TiC–Ti–Al system

Abstract

Ti₃AlC₂ is successfully synthesised by in situ hot pressing process from 2TiC/xAl/Ti (x?=?1, 1·2) raw powders. The phases and microstructure of the samples are identified by X-ray diffraction and scanning electron microscopy. It is found that aluminium content influences on the generating content of Ti₃AlC₂ significantly. High purity Ti₃AlC₂ can be obtained from a compacted cylinder composed of TiC–Ti–1·2Al at 1350°C for 2 h, and the purity of Ti₃AlC₂ is nearly 96·9 wt-%. The corresponding density and compressive strength are 3·93 g·cm^?3 and 377·34 MPa respectively. Ti₃AlC₂ grain exhibits typical plate-like structure. When aluminium melts, a mass of Al atoms diffuse to Ti grain rapidly, and Ti–Al intermetallic compounds generate. Then, Ti–Al intermetallic compounds react with TiC to form Ti₃AlC₂ directly. Using TiC powders as the raw materials provides Ti₆C octahedra directly. At elevated temperature, a part of aluminium will evaporate and lose. This will result in that every two layers of Ti₆C octahedra are linked by aluminium planes directly and Ti₃AlC₂ can be formed.     

Reactive synthesis for porous Ti3AlC2 ceramics through TiH2, Al and graphite powders

Porous Ti₃AlC₂ ceramics were fabricated by reactive synthesis. The process of fabrication involved five steps: (i) the pyrolysis of stearic acid at 450 °C; (ii) the decomposition of TiH₂ at 700 °C, which leads to the conversion of TiH₂ to Ti; (iii) the solid–liquid chemical reaction of solid Ti and molten Al at 800–1000 °C, which converts the mixture to Ti–Al compounds; (iv) the newly synthesized Ti–Al compounds that react with surplus Ti and graphite to form ternary carbides and TiC at 1100–1200 °C; and (v) reactive synthesized ternary carbides and TiC that yield porous Ti₃AlC₂ at 1300 °C.     

Probing the oxidation behavior of Ti2AlC MAX phase powders between 200 and 1000 °C

Oxidation of commercial Ti₂AlC MAX phase powders at 200–1000 °C has been investigated by XRD, XPS, SEM, STA and TGA coupled with FTIR. These powders are a mixture of Ti₂AlC, Ti₃AlC₂, TiC and Ti_1.2Al_0.8. Oxidation at 400 °C led to disappearance of carbide phases from Ti 2p, Al 2p and C 1s XPS spectra. At 600 °C, powders changed from dark grey to light grey with a significant volume increase due to crack formation. Powders were severely oxidized by detecting rutile with minor anatase TiO₂. At 800 °C, α-Al₂O₃ was detected while anatase transformed into rutile TiO₂. The cracks were healed and disappeared. At 1000 °C, the Ti₂AlC powders were fully oxidized into rutile TiO₂ and α-Al₂O₃ with a change of powder color from light grey to yellow. FTIR detected the release of C as CO₂ from 200 °C onwards but with additional CO above 800 °C.     

Influence of Mo doping on the tribological behavior of Ti3AlC2 ceramic at different temperatures

(Ti_0·8Mo_0.2)₃AlC₂ solid solutions were successfully synthesized from Ti, Al, TiC, and Mo powders using the in situ hot-pressing sintering method. The tribological properties of (Ti_0·8Mo_0.2)₃AlC₂ and the reference Ti₃AlC₂ in the temperature range 25–800 °C were evaluated in ambient air with the counterpart of Al₂O₃ balls. The results show that (Ti_0·8Mo_0.2)₃AlC₂ has improved lubricating properties and wear resistance above 400 °C compared with Ti₃AlC₂. This can be contributed to the formation of tribo-oxidation films containing MoO₃ and MoO_3-x. Structural characterization of the tribo-oxidation films was conducted using SEM, EDS, Raman spectroscopy, and XPS to evaluate the effect of Mo doping on the wear mechanisms of Ti₃AlC₂ in detail.     

Pressureless Sintering and Reaction Mechanisms of Ti3SiC2 Ceramics

Easy sinterable Ti₃SiC₂ powder was synthesized from a powder mixture with a molar ratio of 1.0 Ti, 0.3 Al, 1.2 Si, and 2.0 TiC by heating at 1200°C in the flowing Ar. Here, the Al powder acts as a deoxidation agent and provides a liquid phase for the reaction. The powder compacts subjected to pressureless sintering at 1300°C in Ar had a relative density up to 99%. The results of chemical analysis and the measured lattice constant suggest that the Al–Si liquid phase was formed at approximately 1200°C and that liquid‐phase sintering was promoted by the 0.1 molar ratio of Al and the 0.2 molar ratio of Si remaining in excess. The three‐point bending strength, fracture toughness, and electrical resistivity of the sintered samples were 380 MPa, 4.1 MPa m^1/2, and 0.34μΩm, respectively.     

Solid-state joining of SiC using a thin Ti3AlC2, TiC,or Ti filler

SiC monoliths containing 5 wt.% Al₂O₃-Y₂O₃ additive were joined using a thin Ti₃AlC_2, TiC, or Ti filler. After joining at 1900 °C for 5 h under 3.5 MPa, the joint properties were compared in terms of the microstructure, phase evolution, joining strength, and possible elimination of the joining layer. Although all samples showed a sound joint, the microstructure differed according to the filler. SiC joined with Ti₃AlC₂ filler showed an indistinguishable joining interface due to the filler decomposition followed by solid-state diffusion into the SiC base, whereas TiC filler remained at the interface without showing decomposition or diffusion. In contrast, the Ti filler showed a possible elimination of the joining layer because of the diffusion of Ti and the formation of TiC. The mean joining strengths for the Ti₃AlC₂, TiC, and Ti fillers were 300, 234, and 248 MPa, respectively, which were comparable to that of the base SiC material (250 MPa).     

Self-healing capacity of Mullite-Yb2SiO5 environmental barrier coating material with embedded Ti2AlC MAX phase particles

Repetitive heating and cooling cycles inevitably cause crack damage of hot gas components of gas turbine engines, such as blades and vanes. In this study the self-healing capacity is investigated of mullite + ytterbium monosilicate (Yb₂SiO₅) as EBC material with Ti₂AlC MAX phase particles embedded as a crack-healing agent. The effect of Ti₂AlC in the EBC was compared with the self-healing ability of the mullite + Yb₂SiO₅ material. After introducing cracks by Vickers indentation on the surface of each sample, crack healing was realized by controlling the temperature and time during the post-heat-treatment process. For the mullite + Yb₂SiO₅ composite with Ti₂AlC particles, crack healing occurred at 1000 °C, while in the case of the mullite + Yb₂SiO₅ composite without Ti₂AlC, a sustained temperature of 1300 °C or higher was required. Compared with the healing of the mullite + Yb₂SiO₅ composite by the formation of a eutectic phase, the addition of Ti₂AlC promoted healing via the oxidation of Ti and Al. Notably, the surface formation of a ternary oxide of Ti–Yb–O was confirmed, which completely covered the damage area. Consequently, the addition of a Ti₂AlC MAX phase to the EBC composite resulted in a complete strength recovery, while the mullite + Yb₂SiO₅ composite without Ti₂AlC showed a strength recovery of about 80%. Furthermore, by analyzing the indentation load–displacement curve to indicate the role of Ti₂AlC, the addition of Ti₂AlC improved both the hardness and stiffness of the composite.     

Wetting and interfacial behavior of Al–Ti/4H–SiC system:A combined study of experiment and DFT simulation

Wetting and interfacial behavior of molten Al-(10, 20, 30, 40) at.%Ti alloys on C-terminated 4H–SiC at 1500 and 1550 °C were investigated experimentally, and theoretical bonding strength, structure stability and electronic structure of interfacial reaction products/C-terminated 4H–SiC interfaces were evaluated by first-principle calculations. The wetting experiments show that the Al–Ti/SiC systems present excellent wettability with contact angle of less than 15° except the Al–40Ti/SiC system performed at 1500 °C × 30 min. The SEM-EDS and TEM analyses demonstrate that the reaction products are mainly composed of Al₄C₃, TiC, Ti₃SiC₂, Ti₅Si₃C_X and τ phase, and their formation and evolution can be mainly affected by the Ti concentration in the Al–Ti alloys and wetting temperature. Moreover, the calculated results show that the SiC/C-terminated TiC interface presents the highest work of separation and its electronic property reveals that the localization of electrons and formation of covalent bond between interfacial C atoms lead to the excellent bonding strength of SiC/TiC interface.     

Sintering,mechanical, electrical and oxidation properties of ceramic intermetallic TiC–Ti3Al composites obtained from nano-TiC particles

The paper discusses the development of a new material system for interconnect application in Solid Oxide Fuel Cells (SOFC) based on TiC–Ti₃Al. Nano-sized TiC powders utilized in this research were synthesized using carbon coated TiO₂ precursors from a patented process. The pressureless sintering of TiC–Ti₃Al in a vacuum was applied at temperatures between 1100 °C and 1500 °C and content of Ti₃Al was varied in the range of 10–40 wt%. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used for phase evaluation and sintering behavior. Relative density increased markedly with increasing sintering temperature because of grain growth and formation of the Ti₃AlC₂ secondary phase. Dense products (>95% TD) were prepared from nanosized TiC powders with 10 and 20 wt% Ti₃Al, but with about 8 to 10% porosity for 30 and 40 wt% Ti₃Al. The mechanical properties were determined from Vickers hardness and fracture toughness calculations. Vickers hardness decreased and fracture toughness increased with increasing Ti₃Al content. The electrical conductivity and oxidation behavior of TiC–Ti₃Al composites were investigated to evaluate the feasibility for SOFC interconnect application. The electrical conductivity measurements in the air at 800 °C for 100 h were made using the Kelvin 4-wire method.