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.     

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 of porous Mo2Ti2AlC3 ceramic and its basic application properties

Porous Mo₂Ti₂AlC₃ was synthesized by reactive synthesis of the mixed powder of molybdenum, aluminum, titanium hydride and graphite at 1500 °C. The effects of sintering temperature on the phase transition and pore structure parameters of porous Mo₂Ti₂AlC₃ were deeply discussed, and the pore forming mechanism in the sintering process was further deduced. The results showed that the pore formation of porous Mo₂Ti₂AlC₃ consists of the following aspects: (i) At 500 °C, stearic acid was completely pyrolyzed; (ii) At 700 °C, titanium hydride was completely decomposed into titanium and hydrogen via endothermic reaction; (iii) The partial diffusion effect of aluminum in the formation of TiAl and Mo₃Al intermetallic compounds; (iv) The solid-solid reaction of titanium and molybdenum with graphite generated TiC and Mo₂C; (v) TiC, Mo₂C, Ti₂AlC, Mo₃Al and graphite further reacted to form Mo₂Ti₂AlC₃,where pore formation was mainly controlled by (iii) and (v). In addition, the corrosion resistance and mechanical properties of porous Mo₂Ti₂AlC₃ were also explored.     

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.     

Reaction,densification, and mechanical properties of Ti2AlCx ceramics at low applied pressure and temperature

Ti₂AlC_x ceramic was produced by reactive hot pressing (RHP) of Ti:Al:C powder mixtures with a molar ratio of 2:1:1–.5 at 10–20 MPa, 1200–1300°C for 60 min. X-ray diffraction analysis confirmed the Ti₂AlC with TiC, Ti₃Al as minor phases in samples produced at 10–20 MPa, 1200°C. The samples RHPed at 10 MPa, 1300°C exhibited ≥95 vol.% Ti₂AlC with TiC as a minor phase. The density of samples increased from 3.69 to 4.04 g/cm³ at 10 MPa, 1200°C, whereas an increase of pressure to 20 MPa resulted from 3.84 to 4.07 g/cm³ (2:1:1 to 2:1:.5). The samples made at 10 MPa, 1300°C exhibited a density from 3.95 to 4.07 g/cm³. Reaction and densification were studied for 2Ti–Al–.67C composition at 10 MPa, 700–1300°C for 5 min showed the formation of Ti–Al intermetallic and TiC phases up to 900°C with Ti, Al, and carbon. The appearance of the Ti₂AlC phase was ≥1000°C; further, as the temperature increased, Ti₂AlC peak intensity was raised, and other phase intensities were reduced. The sample made at 700°C showed a density of 2.87 g/cm³, whereas at 1300°C it exhibited 3.98 g/cm³; further, soaking for 60 min resulted in a density of 4.07 g/cm³. Microhardness and flexural strength of Ti₂AlC_0.8 sample were 5.81 ± .21 GPa and 445 ± 35 MPa.     

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.     

HiPIMS induced high-purity Ti3AlC2 MAX phase coating at low-temperature of 700 °C

Ti₃AlC₂, one of Ti-Al-C MAX phases, has received extensive attention due to its unique nano-laminated structure and combined properties of metals and ceramics. However, ultra-high synthesis temperature exceeding 800 °C is a critical challenge for broad application of Ti₃AlC₂ coatings on temperature-sensitive substrates. In this study, Ti-Al-C coatings were deposited on Ti-6Al-4V substrates using high-power impulse magnetron sputtering (HiPIMS) and DC sputtering (DCMS) for comparison. Different from as-deposited amorphous Ti-Al-C coating by DCMS, nanocrystalline TiAl_x compound was achieved by HiPIMS deposition due to highly ionized plasma flux with high kinetic energy. Furthermore, HiPIMS promoted the generation of dense and smooth Ti₃AlC₂ phase coating after low-temperature annealing at 700 °C, while annealed DCMS coating only obtained Ti₂AlC. In-situ XRD demonstrated such Ti₃AlC₂ phase could be early involved in crystallization at 450 °C, lowest than synthesis temperature ever reported. The mechanical properties of Ti₃AlC₂ coating were also discussed in terms of structural evolution.     

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.     

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.     

An optimized method for synthesizing phase-pure Ti3AlC2 MAX-phase through spark plasma sintering

So far many attempts have been made to synthesize phase-pure Ti₃AlC₂ MAX-phase. But still the challenge posed by the presence of TiC and Ti-Al based intermetallic transient impurity phases in the final product is a persisting problem. Spark plasma sintering (SPS) technique has been the most successful method to decrease the impurity content of the final product. Even so, synthesis of phase-pure Ti₃AlC₂ MAX-phase, without any TiC and Ti-Al based intermetallic impurities, has not been achieved and reported in literature with substantial evidences. Further, high purity Ti₃AlC₂ MAX-phase synthesized using SPS technique has shown lack of phase and microstructural stability above 1350°C temperature. In this work, we have reported an optimized method for producing phase-pure Ti₃AlC₂ MAX-phase (having more than 99 % purity) using commercial grade Ti, Al and C elemental powders through SPS technique. The final product also showed very good high temperature stability up to 1500°C under flowing Argon inert atmosphere.     

Self-propagating high temperature synthesis of intragranular TiC-Ti3AlC2 composites

Abstract

Abstract

TiC-Ti₃AlC₂ composites with a novel intragranular structure were fabricated by self-propagating high temperature synthesis technique using Ti, Al and C elemental powders. X-ray diffraction results indicated that the as synthesised composites were composed of TiC and Ti₃AlC₂, where the amounts of TiC were measured to be 5·4, 9·2 and 16·7?wt-%. Transmission electron microscopy observation showed that the globular TiC was located within the plate-like Ti₃AlC₂ grain, which confirms the ubiquitous existence of intragranular structure in the composites.     

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.     

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.     

Oxidation and creep behavior of textured Ti2AlC and Ti3AlC2

The oxidation and creep behaviors of textured Ti₂AlC and Ti₃AlC₂ ceramics were characterized. The oxidation behavior of the two materials, which was studied in air at temperatures ranging from1000 to 1300 °C, was observed to be anisotropic and the materials exhibited a better oxidation resistance along a direction transverse to the c-axis. The correlation between the overall parabolic rate constant and oxidation temperature of both textured materials was characterized, providing new insights into the oxidation kinetics. The results indicate that the texturing has a negligible influence on the creep behavior in the assessed temperature range of 1000?1200 °C in air, for the applied stresses ranging from 40 to 80 MPa. In this stress regime, the creep behavior of textured Ti₂AlC and Ti₃AlC₂ appears to be controlled by grain boundary sliding. This behavior can be rationalized based on a model for superplastic deformation, indicating pure-shear motion under stationary conditions accommodated by lattice or grain-boundary diffusion.     

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.     

Ti-Al-C MAX phases by aluminothermic reduction process

A new approach to synthesis of Ti₂AlC-Ti₃AlC₂/Al₂O₃ compounds is developed based on thermite reaction in the TiO₂-Al-C system. The effect of Al excess is also discussed. XRD analysis has proved that this parameter can be used to improve the product purity, i.e., the amount of TiC in the final product. It has also been shown that, with increasing Al excess, the composition of a major MAX phase undergoes a change from Ti₂AlC to Ti₃AlC₂.      

Preparation and tribological behaviour of laminated Ti3AlC2 crystals as additive in base oil

Abstract

Laminated ternary compound Ti₃AlC₂ crystals were synthesised by pressureless sintering the mixture powders of 3Ti/1·1Al/1·8C, 3Ti/1Al/1·8C/0·2Sn, 1Ti/1·8TiC/1Al and 1Ti1·8TiC1Al0·1Sn at 1400°C with preliminary liquid magnetic stirring mixing. The X-ray diffraction results indicate that Ti₃AlC₂ prepared from 3Ti/1Al/1·8C/0·2Sn has the highest purity, and the addition of appropriate Sn favours the synthesis of high purity Ti₃AlC₂. Scanning electron microscopy images show that Ti₃AlC₂ samples exhibit lamellar-like microstructure with thickness of ～100 nm. The tribological properties of Ti₃AlC₂ as an additive in 100SN base oil were evaluated with a ball on disc tester. The results show that the Ti₃AlC₂ additives exhibited good friction reduction and wear resistance at 5 wt-% concentration. Under determinate conditions, the base oil containing 5 wt-% Ti₃AlC₂ samples presented good tribology performance under the load of 15 N. The improved tribological properties of the Ti₃AlC₂ samples could be attributed to the formation of tribofilm in friction process.     

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.     

Temperature-driven deintercalation and structure evolution of Ag/Ti3AlC2 composites

Ag/Ti₃AlC₂ composites are promising sliding contact material. Here, Ag/Ti₃AlC₂ composites were obtained via the hot pressing technique and their structure evolutions upon sintering temperature were investigated. Sintering temperature controlled the deintercalation of Al layers from Ti₃AlC₂, thus controlling the interfacial structure of Ag/Ti₃AlC₂ composites. Amorphous interface was found after sintering at 750?°C. TiC_x particles with a size of around 10?nm were found in the interfacial region after sintering at 800?°C. Increasing sintering temperature to 850?°C, stripy structures composed of alternately arranged silver-rich phase and TiC_x phase appeared around the edges of Ti₃AlC₂ particles. The over-saturated Al precipitates found in 850?°C sintered composites are cubic μ-Ag₃Al inter-metallic compounds, which have the coherent relationship with Ag matrix.     

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.