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.     

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.     

Structure of a new bulk Ti5Al2C3 MAX phase produced by the topotactic transformation of Ti2AlC

Upon annealing cold-pressed Ti₂AlC, ?325 mesh powders, at 1500 °C for 8 h in argon, the resulting partially sintered sample contained 43(±2) wt.% of the layered ternary carbide Ti₅Al₂C₃. Herein, the X-ray powder diffraction pattern of Ti₅Al₂C₃ is reported for the first time and its structure and stoichiometry are confirmed through high-resolution transmission electron microscopy. This phase has a trigonal structure (space group P3m1) with a unit cell consisting of 3 formula units and cell parameters of a = 3.064 Å, c = 48.23 Å. The lattice parameters determined through first principles calculations agree reasonably well with the experimentally determined values. At 147.1 GPa, the calculated bulk modulus falls between the bulk moduli of Ti₂AlC and Ti₃AlC₂. The transformation from Ti₂AlC to Ti₅Al₂C₃ is topotactic.     

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.     

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.     

Oxidation behavior of Ti3SiC2 powder synthesized by using biochar,Si and Ti

Biochar was proposed as a novel carbon source for synthesizing Ti₃SiC₂ powder with high purity by a simple pressureless sintering at 1673 K, and Ti₃SiC₂ grains exhibited the typical nanolayered structure. The oxidation behavior of Ti₃SiC₂ powder showed the parabolic law during isothermal oxidation from 1273 K to 1473 K. Dense and continuous oxidation layer consisting of mixed TiO₂ and SiO₂ was formed rapidly on the surface of Ti₃SiC₂ particles as a diffusion barrier, which effectively retarded the inward diffusion of oxygen, conferring good oxidation resistance of the powder.     

Low-temperature synthesis of high-purity Ti3AlC2 by MA-SPS technique

Ternary carbide Ti₃AlC₂ was synthesized by mechanical alloying (MA) and spark plasma sintering (SPS) from elemental powder mixtures of Ti, Al and C, and the effect of Al content on formation of Ti₃AlC₂ during both processes was investigated. The results showed that adding proper Al content in the staring materials significantly increased the phase purity of Ti₃AlC₂ in the synthesized samples. Dense and high-purity Ti₃AlC₂ with <1 wt.% TiC could be successfully obtained by spark plasma sintering of powders mechanically alloyed for 9.5 h from a starting powder mixtures of 3Ti/1.1Al/2C at a lower sintering temperature of 1050 °C for 10–20 min.     

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.     

A multicomponent porous MAX phase (Ti0.25Zr0.25Nb0.25Ta0.25)2AlC: Reaction process,microstructure and pore formation mechanism

A multicomponent porous MAX phase (Ti_0.25Zr_0.25Nb_0.25Ta_0.25)₂AlC has been successfully synthesized by using pressureless sintering of mixed elemental powders. The microstructure and phase composition of the samples sintered at various temperatures have been characterized by using SEM, XRD, EDS and other analyses, from which conclusions regarding the reaction and pore forming processes could be drawn. During the whole sintering process, the pores did mainly arise from the diffusion related reactions between Al and other elements at low temperatures (below 1200 °C), and the formation reaction of the MAX phase took place at higher temperatures (above 1200 °C). An exception is the clearance holes that were left from the pressing. The optimum sintering temperature for the final MAX phase (Ti_0.25Zr_0.25Nb_0.25Ta_0.25)₂AlC was 1600 °C. A too high sintering temperature (1700 °C) caused a serious loss of Al atoms and a decomposition of the synthesized MAX phase.     

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.     

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.     

Fabrication,Mechanical Properties,and Tribological Behaviors of Ti3Al0.8Sn0.4C2 Solid Solution by Two‐Time Hot‐Pressing Method

A nearly pure, dense Ti₃Al_0.8Sn_0.4C₂ solid solution bulk has been prepared by two‐time hot‐pressing sintering a purity Ti₃Al_0.8Sn_0.4C₂ powders at 1450°C with 30 MPa for 30 min in Ar atmosphere. The Ti₃Al_0.8Sn_0.4C₂ powders have been synthesized by sintering a mixture of Ti, Al, Sn, and TiC powders with a molar ratio of 1:0.8:0.4:1.85 at 1450°C for 5 min. The Vickers microhardness and flexural strength of the Ti₃Al_0.8Sn_0.4C₂ have been measured to be 3.51 GPa and 620 MPa, respectively. Buckling and kinking of the layered structure as well as grain delamination crack deflection have been extensively observed around indentations and on the fracture surfaces. The tribological behaviors have been investigated by dry sliding a low‐carbon steel disk. Ti₃Al_0.8Sn_0.4C₂ bulk has a friction coefficient of 0.3–0.48 and a very low wear rate of 0.4–4.3 × 10^?6 mm³/Nm for the test conditions. These tribological properties are attributed to the presence of a compact self‐generating film, which covers uniformly over the friction surface of Ti₃Al_0.8Sn_0.4C₂.     

Novel Cr2AlC MAX-phase/SiC fiber composites: Synthesis,processing and tribological response

A new class of ceramic matrix composites based on Cr₂AlC MAX-phase containing 5 and 10 wt.% of SiC fibers was developed in this investigation. The Cr₂AlC/SiC composites were performed through two consecutives steps: i) synthesis of the pure Cr₂AlC phase from its elemental constituents by reactive method under argon atmosphere at 1400 °C and particle size refinement, and ii) processing of Cr₂AlC powder and SiC fibers followed by densification using the field assisted sintering technology/spark plasma sintering. Cr₂AlC/SiC composites presented high density (98.6%) with an excellent dispersion of the fibers within the matrix and a strong matrix/fiber interfase. Tribological behavior of the developed composites was studied under dry conditions to reveal the role played by the SiC fibers. Incorporation of the SiC fibers within the Cr₂AlC matrix reduced the friction coefficient up to 20% for low testing loads, while the wear resistance increased up to 70–80% independently of the applied load.     

Formation of MAX solid solutions (Ti,V)2AlC and (Cr,V)2AlC with Al2O3 addition by SHS involving aluminothermic reduction

MAX solid solutions (Ti,V)₂AlC and (Cr,V)₂AlC with Al₂O₃ addition were produced by solid state combustion involving aluminothermic reduction in the mode of self-propagating high-temperature synthesis (SHS). Starting materials included TiO₂/V₂O₅/Al/Al₄C₃ and Cr₂O₃/V₂O₅/Al/Al₄C₃ powder mixtures. Attempts were made to attain (Ti_1−xV_x)₂AlC and (Cr_1−yV_y)₂AlC with the V content in terms of x and y from 0.1 to 0.7. Combustion exothermicity was increased by increasing V₂O₅ for the yield of a higher proportion of V at the substitution site, which not only increased the combustion temperature and reaction front velocity, but also facilitated the evolution of solid solutions. Due to insufficient reaction exothermicity, (Ti_1−xV_x)₂AlC/Al₂O₃ in situ composites were only produced under x≥0.4. On the other hand, the formation of (Cr_1−yV_y)₂AlC/Al₂O₃ was achieved with y from 0.1 to 0.7, because reduction of Cr₂O₃ is more energetic than that of TiO₂. The laminated microstructure characteristic of the MAX ternary carbide was observed for both Al₂O₃-added (Ti,V)₂AlC and (Cr,V)₂AlC composites synthesized in this study.     

Sintering and mechanical properties of titanium diboride with aluminum nitride as a sintering aid

Titanium diboride (TiB₂) was hot-pressed at 1800 °C with aluminum nitride (AlN) as a sintering aid. The presence AlN had a strong influence on the sinterability and mechanical properties of the TiB₂. When a small amount (⩽5 wt.%) of AlN was added to the TiB₂ the titania (TiO₂) present on the surface of the TiB₂ powder was eliminated by a reaction with AlN to form TiN and Al₂O₃. The elimination of TiO₂ markedly improved the sinterability and consequently the mechanical properties of TiB₂. However, when too much AlN was added (⩾10 wt.%), the sinterability and the mechanical properties decreased, apparently due to the remaining unreacted AlN.     

Microwave dielectric characteristics of Nb2O5-added 0.9Al2O3–0.1TiO2 ceramics

The sintering characteristics, phase composition, and microwave dielectric properties of Nb₂O₅-added 0.9Al₂O₃–0.1TiO₂ ceramics sintered at 1300–1500 °C have been investigated. Results show that Nb⁵⁺ and Al³⁺ can co-substitute for Ti⁴⁺ and form Ti_0.8Al_0.1Nb_0.1O₂, which can lower effectively the sintering temperature, and improve the quality factor of 0.9Al₂O₃–0.1TiO₂ ceramics.     

Sintering mechanisms of Yttria with different additives

The sintering behaviour of conventional yttria powder was investigated, with emphasis on the effect of sintering additives such as B₂O₃, YF₃, Al₂O₃, ZrO₂, and TiO₂, etc. at sintering temperatures from 1000 °C to 1600 °C. Powder shrinkage behaviour was analysed using a dilatometer. The powder sintering mechanisms were identified at different temperatures using powder isothermal shrinkage curves. This analysis showed that the sintering additives B₂O₃ and YF₃ could improve yttria sintering by changing the diffusion/sintering mechanisms at certain temperatures, while sintering additives TiO₂, Al₂O₃ and ZrO₂ appeared to retard the powder densification at temperatures around 1000 °C and are more suitable when used at temperatures in excess of 1300 °C. The powder with La₂O₃ added had the slowest densification rate throughout the test temperatures in this experiment and was also found to be more suitable when used at temperatures higher than 1550 °C.     

Healing performance of Ti2AlC ceramic studied with in situ microcantilever bending

In situ microcantilever bending tests were carried out to evaluate the healing efficiency of pre-notched Ti₂AlC ceramic after annealing at 1200 °C for 1.5 h. Microcantilevers of different orientations were fabricated with focused ion beam method at different locations, i.e. in a singular Ti₂AlC grain, at a grain boundary or at the Ti₂AlC–Al₂O₃ interface after healing. Ti₂AlC microcantilever shows an anisotropic bending strength (ranging between 9.6 GPa and 4.6 GPa depending on the precise crystallographic orientations) that is closely related to the different atomic bonds in the layered structure. After healing, the Ti₂AlC–Al₂O₃ microcantilevers exhibit almost the same strength of about 5.2 GPa, i.e. slightly higher than the cleavage strength (4.6 GPa) of the initial Ti₂AlC microcantilevers. It suggests that the orientation of the matrix grain has no significant influence on the strength of healed microcantilevers. Furthermore, it turns out that the strength of the microcantilever with a healed grain boundary is at least twice the strength of the initial Ti₂AlC cantilever with a grain boundary. It is concluded that the oxidation dominated self-healing mechanism of Ti₂AlC ceramics can result in a perfect recovery of mechanical performance. The paper shows that the in situ microcantilever bending test provides a quantitative method for the evaluation of the strength of self-healing ceramics.     

Ultra-high temperature ablation behavior of Ti2AlC ceramics under an oxyacetylene flame

The linear and mass ablation rates of Ti₂AlC ceramics under an oxyacetylene flame at a temperature up to 3000 °C were examined by measuring the dimensions and weight change of the ablated samples. The linear ablation rate was decreased from 0.14 μm s⁻¹ for the first 30 s of the ablation to 0.08 μm s⁻¹ after 180 s. Ti₂AlC ceramics gained small amounts of weight upon ablation, which is attributed to the formation of oxidation products on the ablated surface. The ablation surface exhibits a two-layer structure: an oxide outer layer, consisting mainly of α-Al₂O₃ and TiO₂ and some Al₂TiO₅, and a porous sub-surface layer containing Ti₂Al_1−xC and TiC_xO_y. With increasing ablation time, the content of TiO₂ and Al₂TiO₅ in the outer layer increased, and more pores developed in the sub-surface layer. The thermal oxidation of Ti₂AlC under the flame and scouring of the viscous oxidation products by high-speed flow of gas torch are the main ablation mechanisms.     

Green synthesis,formation mechanism and oxidation of Ti3SiC2 powder from bamboo charcoal,Ti and Si

Using low-cost and highly reactive bamboo charcoal, Ti and Si elemental powders as starting materials, Ti₃SiC₂ powder was synthesized via a simple and cost-efficient pressureless sintering technique in argon atmosphere. The influences of synthesis temperature, holding time and Si content on the Ti₃SiC₂ content of the synthesized products were investigated, and the analysis indicated that the relative content of Ti₃SiC₂ reached 98.9 wt% with a molar ratio of 3Ti/1.2Si/2.2C at 1400 °C for 1.5 h. The Ti₃SiC₂ with good crystallinity and homogeneous nanolayered structure was synthesized at lower temperatures due to the high reactivity and high specific surface area of bamboo charcoal. The non-isothermal oxidation behavior showed that Ti₃SiC₂ powder was stable in air below 540 °C. With the temperature increasing up to 1300 °C, continuous and dense TiO₂ and SiO₂ oxidation layers were formed on the surface of Ti₃SiC₂ particles, which conferred good oxidation resistance to Ti₃SiC₂ powder.