Optimization of the Ti3SiC2 MAX phase synthesis

The synthesis of Ti₃SiC₂ MAX phase by self-propagating high-temperature synthesis (SHS) and pressureless argon shielding synthesis has been investigated following different pathways pertaining to the reactant systems Ti/Si/C, Ti/SiC/C and Ti/TiC/Si. Silicon in excess ranging from 10 to 50 mol% was employed to obtain powders mainly constituted by Ti₃SiC₂.Optimizing the excess of silicon and the pressing technique, the resultant powders with Ti₃SiC₂ content near to 100% were obtained. Result was consequent to the use of pressureless argon shielding synthesis obtained with 30 mol% of silicon excess in the examined different systems. The Ti₃SiC₂ was also obtained by SHS, but with lower proportion (88% and 86% from 3Ti + 1.2SiC + 0.8C and 3Ti + 1.3Si + 2C respectively). These results driving from XRD patterns were confirmed by FESEM observations and the EDAX analyses.     

Spark plasma sintering of TiC particle-reinforced molybdenum composites

In order to improve the recrystallization resistance and the mechanical properties of molybdenum, TiC particle-reinforcement composites were sintered by SPS. Powders with TiC contents between 6 and 25 vol.% were prepared by high energy ball milling. All powders were sintered both at 1600 and 1800 °C, some of sintered composites were annealed in hydrogen for 10 h at 1100 up to 1500 °C. The powders and the composites were investigated by scanning electron microscopy and XRD. The microhardness and the density of composites were measured, and the densification behavior was investigated. It turns out that SPS produces Mo–TiC composites, with relative densities higher than 97%.The densification behavior and the microhardness of all bulk specimens depend on both the ball milling conditions of powder preparation and the TiC content. The highest microhardness was obtained in composites containing 25 vol.% TiC sintered from the strongest milled powders. The TiC particles prevent recrystallization and grain growth of molybdenum during sintering and also during annealing up to 10 h at 1300 °C. Interdiffusion between molybdenum and carbide particles leads to a solid solution transition zone consisting of (Ti_1 −x Mo_x)C_y carbide. This diffusion zone improves the bonding between molybdenum matrix and TiC particles. A new phase, the hexagonal Mo₂C carbide, was detected by XRD measurements after sintering. Obviously, this phase precipitates during cooling from sintering temperature, if (Ti_1 −x Mo_x)C_y or molybdenum, are supersaturated with carbon.     

Thermal stability of Ti3SiC2 thin films

The thermal stability of Ti₃SiC₂(0 0 0 1) thin films is studied by in situ X-ray diffraction analysis during vacuum furnace annealing in combination with X-ray photoelectron spectroscopy, transmission electron microscopy and scanning transmission electron microscopy with energy dispersive X-ray analysis. The films are found to be stable during annealing at temperatures up to ∼1000 °C for 25 h. Annealing at 1100–1200 °C results in the rapid decomposition of Ti₃SiC₂ by Si out-diffusion along the basal planes via domain boundaries to the free surface with subsequent evaporation. As a consequence, the material shrinks by the relaxation of the Ti₃C₂ slabs and, it is proposed, by an in-diffusion of O into the empty Si-mirror planes. The phase transformation process is followed by the detwinning of the as-relaxed Ti₃C₂ slabs into (1 1 1)-oriented TiC_0.67 layers, which begin recrystallizing at 1300 °C. Ab initio calculations are provided supporting the presented decomposition mechanisms.     

First-principles study of Ti3AC2 (A = Si,Al) (0 0 1) surfaces

We have systematically studied the mechanical properties and surface properties of (1 × 1) Ti₃AC₂ (A = Si, Al) (0 0 1) using the density functional theory (DFT). The calculated cleavage energy for each possible cleavage site shows that Ti–Si and Ti–Al are the weakest layers, while the Ti–C layer is the strongest one. It reveals that the main difference between Ti₃SiC₂ and Ti₃AlC₂ is that the Ti–Si bond is stronger than the Ti–Al bond. This shows the different mechanical and surface properties between them. The surface rumpling and surface energy of Ti₃SiC₂ and Ti₃AlC₂ are calculated. The study shows that the cleavage energy affects both the surface rumpling and the surface energy. The higher the cleavage energy, the larger the surface energy and surface rumpling. Furthermore, the most stable surface structures are predicted for different experimental conditions. The predicted surface structures agree with the available experimental results.     

Phase and morphology evolution of TiC in the Ti–Si–C system

TiC was synthesized by reactive pyrolysis of a mixture of poly(dimethylsilaacetylide), [-MeSi(H)C≡C-]_n, Ti, and TiSi₂ particles under Ar atmosphere between 1100 °C and 1400 °C. The pyrolysis products were studied by means of X-ray diffraction (XRD), scanning electron microsphere (SEM), and Energy-Disperse Spectrometer (EDS). The results showed that TiC crystals appeared at 1200 °C accompanied by Ti₅Si₃. At 1400 °C, single crystalline phase TiC was obtained. By changing the ratio of raw materials, TiC crystals with different morphologies, including octahedron, truncated-octahedron, and polyhedral were prepared.     

Synthesis of high purity titanium silicon carbide from elemental powders using arc melting method

The objective of this study is to investigate the formation of Ti₃SiC₂ from Ti/Si/C powders using the arc melting method. The results show that the sample sintered at 80 s produced a near single-phase of Ti₃SiC₂ (99.2 wt.%) with a relative density of 88.9%. These results were confirmed by phase determination using XRD analysis and were supported with micrographs from FESEM/EDX analyses. The relative density and porosity of all samples were dependent on the formation of macropores in bulk samples and micropores in TiC_x grains. The proposed reaction mechanisms for the synthesis of Ti₃SiC₂ by arc melting is that Ti₃SiC₂ might be formed from TiC_x + Si, Ti₅Si₃C_x + C, and Ti₅Si₃C_x + TiC_x at early arcing time (≤ 10 s), while TiC_x + TiSi₂ take place at 15 s to 80 s. After 80 s, decomposition of Ti₃SiC₂ into TiC_x, TiSi₂ and C was observed.     

Structure stability of Ti3AlC2 in Cu and microstructure evolution of Cu–Ti3AlC2 composites

The structural stability of Ti₃AlC₂ in Cu and the microstructure evolution of Cu–Ti₃AlC₂ composites prepared at different temperatures were investigated by high-resolution transmission electron microscopy and X-ray diffraction. A mild reaction between Ti₃AlC₂ and Cu occurred at 850–950 °C, and strong reactions occurred above 950 °C. The reaction was identified as diffusion of Al from Ti₃AlC₂ into Cu to form Cu(Al) solid solution. Ti₃AlC₂ retained its structure under the partial loss of Al. Further depletion of Al resulted in highly defective Ti₃AlC₂ accompanied by the inner diffusion of Cu into Ti₃AlC₂ along the passway left by the Al vacancies. When Al was removed, Ti₃AlC₂ decomposed and transformed into cubic TiC_x. In addition, TiC twins formed by the aggregation of C vacancies at twin boundaries. With the help of first-principles calculation and image simulation, an ordered hexagonal TiC_x was identified as a transition phase linking Ti₃AlC₂ and c-TiC_x. The effect of the reaction and phase transformation on the microstructure and properties of Cu–Ti₃AlC₂ composites was also discussed.     

Oxidation behavior of micro-sized Al2O3–TiC–Co composites prepared from cobalt-coated powders

The oxidation behavior of hot-pressed Al₂O₃–TiC–Co composites prepared from cobalt-coated powders has been studied in air in the temperature range from 200 °C to 1000 °C for 25 h. The oxidation resistance of Al₂O₃–TiC–Co composites increases with the increase of sintering temperature at 800 °C and 1000 °C. The oxidation surfaces were studied by XRD and SEM. The oxidation kinetics of Al₂O₃–TiC–Co composites follows a rate that is faster than the parabolic-rate law at 800 °C and 1000 °C. The mechanism of oxidation has been analyzed using thermodynamic and kinetic considerations.     

Synthesis of Ti3AlC2 ceramic by high-energy ball milling of elemental powders of Ti,Al and C

Synthesis of the ternary carbide Ti₃AlC₂ by high-energy ball milling of elemental Ti, Al and C powders with a stoichiometric composition was tentatively investigated. The results show that high content Ti₃AlC₂ was successfully obtained after ball milling of powder mixture only for 3 h. The milled products consist of powder and a coarse granule with 8 mm in diameter, and both are mainly composed of Ti₃AlC₂ with TiC as impurity based on X-ray diffractometer (XRD) and energy-dispersive spectroscopy (EDS) characterization. It is believed that a mechanically induced self-propagating reaction (MSR) was triggered to form Ti₃AlC₂ and TiC during high-energy ball milling process.     

Effect of silicidation pretreatment with gaseous SiO on sinterability of TiC powders

Silicidation pretreatment with gaseous SiO at 1350 °C for 30 min is employed for chemically modifying commercially available TiC powder. Phase composition and microstructural features of the pretreated powder are discussed. Densification behavior of the pretreated TiC powder during hot pressing is studied in comparison with that of non-pretreated one. Significantly improved densification behavior and sinterability of TiC powder after silicidation pretreatment are explained by the effect of Ti₃SiC₂ acting as a solid lubricant. Nearly fully dense TiC-based ceramics having flexural strength of 370 MPa, fracture toughness of 5.6 MPa m^½, and microhardness of 24 GPa is obtained by hot pressing under conditions as mild as 1600 °C and 20 MPa.     

Formation of TiAl–Ti2AlC in situ composites by combustion synthesis

Preparation of TiAl–Ti₂AlC in situ composites with a broad range of composition was conducted by self-propagating high-temperature synthesis (SHS) with compressed samples from the mixture of elemental powders. When compared with SHS formation of monolithic TiAl, the addition of carbon particles to the Ti–Al powder mixture enhances the sustainability of the reaction. It was found that no prior heating was required for the samples prepared to produce the composites containing more than 20 mol% Ti₂AlC, in contrast to the need of preheating at 200 °C for single-phase TiAl formation. This is attributed to the fact that formation of Ti₂AlC is more exothermic than that of TiAl. As a result, the combustion temperature and combustion wave velocity increase with the content of Ti₂AlC formed in the TiAl–Ti₂AlC composite, and approach the values associated with formation of single-phase Ti₂AlC when considerable amounts of Ti₂AlC are yielded. The XRD analysis of the end products confirms formation of TiAl–Ti₂AlC in situ composites. Moreover, simultaneous formation of Ti₂AlC promotes the phase evolution of the aluminide compounds. That is, the secondary aluminide phase, Ti₃Al, was no longer detected in the TiAl–matrix composites containing Ti₂AlC of 30 mol% or above.     

Effect of temperature and Ar flow rate on Ti3SiC2 formation from TiSiO4 powders coated with pyrolytic carbon

In this study, equilibrium thermodynamic analysis was initially carried out for TiO₂:SiO₂:C molar ratio of 1:1:4 at 1600 K, 1700 K and 1800 K as a function of Ar/solid reactant ratio. It was predicted that single phase Ti₃SiC₂ is formed when a critical Ar/solid reactant ratio is exceeded. This behavior is ascribed to the reduction of partial pressures of gaseous reaction products of SiO and CO. Subsequently, formation of Ti₃SiC₂ phase from carbon coated TiSiO₄ powders by carbothermal reduction was investigated as a function temperature, isothermal holding time and Ar flow rate. Carbothermal reduction experiments at 1800 K and at a Ar flow rate of 250 cm³/min for 60 min showed that the optimal C content was determined to be 27.47 wt.%. The ternary carbide compound was not detected within 120 min at 1600 K and 1700 K, but a major TiOC phase along with a minor SiC phase. Whereas at 1800 K, the ternary carbide phase was observed and its amount increased from 6.80 wt.% at 0 min to 38.91 wt.% at 75 min above which it gradually decomposed into the binary carbides. The experiments carried out for various Ar flow rate at 1800 K for 75 min revealed that the highest ternary carbide content (47.84 wt.%) was obtained at a Ar flow rate of 425 cm³/min. The thermodynamic and experimental results indicate that Ti₃SiC₂ formation takes place via the reaction of pre-formed TiC and SiC phases with the remaining SiO₂.     

Radiation tolerance of Mn+1AXn phases,Ti3AlC2 and Ti3SiC2

During investigations of novel material types with uses in future nuclear technologies (ITER/DEMO and GenIV fission reactors), ternary carbides with compositions Ti₃AlC₂ and Ti₃SiC₂ have been irradiated with high Xe fluences, 6.25 × 10¹⁵ ions cm^?2 (～25–30 dpa), using the IVEM-TANDEM facility at Argonne National Laboratory. Both compositions show high tolerance to damage, and give indications that they are likely to remain crystalline to much higher fluences. There is a visible difference in tolerance between Ti₃AlC₂ and Ti₃SiC₂ that can be related to the changes in bonding within each material. These initial findings provide evidence for a novel class of materials (+200 compounds) with high radiation resistance, while, significantly, both of these materials are composed of low-Z elements and hence exhibit no long-term activation.     

Effect of sintering process on the microstructures and properties of in situ TiB2–TiC reinforced steel matrix composites produced by spark plasma sintering

Spark plasma sintering (SPS) is a new technique to rapidly produce metal matrix composites (MMCs), but there is little work on the production of TiB₂–TiC reinforced steel matrix composites by SPS. In this work, in situ TiB₂–TiC particulates reinforced steel matrix composites have been successfully produced using cheap ferrotitanium and boron carbide powders by SPS technique. The effect of sintering process on the densification, hardness and phase evolution of the composite is investigated. The results show that when the composite is sintered at 1050 °C for 5 min, the maximum densification and hardness are 99.2% and 83.8 HRA, respectively. The phase evolution of the composite during sintering indicates that the in situ TiB₂–TiC reinforcements are formed by a hybrid formation mechanism containing solid–solid diffusion reaction and solid–liquid solution-precipitation reaction. The microstructure investigation reveals that fine TiB₂–TiC particulates with a size of ～2 μm are homogeneously distributed in the steel matrix. The TiB₂–TiC/Fe composites possess excellent wear resistance under the condition of dry sliding with heavy loads.     

Characterization of hot pressed SiC whisker reinforced TiB2 based composites

TiB₂–SiC ceramic composites, with different contents of SiC whiskers (SiC_w), as a ceramic sinter-additive, were prepared by the hot pressing process at 1850 °C for 2 h under a pressure of 20 MPa. For comparison, a monolithic TiB₂ ceramic was also fabricated under the identical temperature, pressure, atmosphere, and holding time by the hot pressing process. The effects of fabrication process and SiC whiskers on microstructural features, phase evolution and mechanical properties were investigated. Hardness measurements revealed an initial increase in hardness for TiB₂–SiC compared to TiB₂. Also the improvement of the fracture toughness was attributed to the toughening and strengthening effects of SiC whiskers such as crack deflection. The results showed that promoted densification of TiB₂–SiC ceramic composites is due to addition of SiC whiskers and reduction of oxide impurities by reacting with SiC whiskers and removing them from the surface layer of TiB₂ particles. The reaction between TiB₂ particles and SiC whiskers led to in-situ formation of TiC phase in the matrix as well. In general, it is concluded that the sinterability of TiB₂-based composites was remarkably improved by introducing SiC whiskers compared to the single phase TiB₂ ceramic.     

Microstructure and properties of intermetallic composite coatings fabricated by selective laser melting of Ti–SiC powder mixtures

Transition metal silicides and carbides are attractive advanced materials possessing unique combinations of physical and mechanical properties. However, conventional synthesis of bulk intermetallics is a challenging task because of their high melting point. In the present research, titanium carbides and silicides composites were fabricated on the titanium substrate by a selective laser melting (SLM) of Ti–(20,30,40 wt.%)SiC powder mixtures by an Ytterbium fiber laser with 1.075 μm wavelength, operating at 50 W power, with the laser scanning speed of 120 mm/s. Phase analysis of the fabricated coatings showed that the initial powders remelted and new multiphase structures containing TiC_x, Ti₅Si₃C_x, TiSi₂ and SiC phases in situ formed. Investigation of the microstructure revealed two main types of inhomogeneities in the composites, (i) SiC particles at the interlayer interfaces and, (ii) chemical segregation of the elements in the central areas of the tracks. It was suggested and experimentally proven that an increase in laser power to 80 W was an efficient way to improve the laser penetration depth and the mass transport in the liquid phase, and therefore, to fabricate more homogeneous composite. The SLM Ti–(20,30,40 wt.%)SiC composites demonstrated high hardness (11–17 GPa) and high abrasive wear resistance (3.99 × 10⁻⁷–9.51 × 10⁻⁷ g/Nm) properties, promising for the applications involving abrasive wear.     

Tensile properties of Ti3SiC2 in the 25–1300°C temperature range

Although significant progress has been achieved in understanding the mechanical behavior of bulk, polycrystalline Ti₃SiC₂ in compression and flexure, as far as we are aware there are no reports in the literature dealing with its mechanical response under tension. In this paper, we report on the functional dependence of the tensile response of fine-grained (3–5 μm) Ti₃SiC₂ samples on strain rates in the 25–1300°C temperature range. The tensile response of Ti₃SiC₂ is a strong function of strain rate and temperature. Increases in testing temperatures, and decreases in testing strain rates lead to large (≈25%) tensile plastic deformations. Strain-rate jump/drop tests and stress-jump creep tests confirm the high values for the strain-rate sensitivity coefficients (0.42–0.56) obtained from the tensile tests. These values are equal to, or greater than, the strain-rate sensitivity of most superplastic ceramics. The large strains to failure result primarily from a high degree of damage, not from a microstructure that remains self-similar throughout deformation (as in superplasticity). Another important distinction between superplasticity in ceramics and the deformation of Ti₃SiC₂ is that in the former the grains are typically about an order of magnitude smaller than the ones tested here.     

Influence of SPS parameters on the density and mechanical properties of sintered Ti3SiC2 powders

The densification of Ti₃SiC₂ MAX phase was performed by the Spark Plasma Sintering (SPS) technique. The SPS parameters, such as sintering temperature, pressure and soaking time, were optimized to obtain fully densified samples which were characterized to obtain the best mechanical properties. The sintering temperature was varied from 1070 to 1300 °C, the soaking time from 1 to 10 min and the applied pressure from 60 to 180 MPa. The best full densified samples were sintered at 1300 °C applying 60 MPa for 7 min. Ti_xC_y and TiSi₂ secondary phases were found in samples densified at 1200, 1250 and 1300 °C, due to decomposition of Ti₃SiC₂. These secondary phases, detected by XRD patterns, were confirmed by microhardness testing, FESEM observations and EDAX analyses.     

Microstructure and some properties of TiAl-Ti2AlC composites produced by reactive processing

The TiAl–Ti₂AlC composites with and without impurities, Ni, Cl and P, were prepared by combustion reaction from the elemental powders and cast after arc melting. The resulting composites had about 18 vol% Ti₂AlC in the lamellar matrix consisting of γ-TiAl and Ti₃Al (α₂). In the homogenized specimens, the α₂ phase decomposed to γ-TiAl and Ti₂AlC. The composite material had a high strength both at ambient and elevated (1173 K) temperatures; about 800 and 400 MPa, respectively, with an ambient temperature ductility of 0.7% at bending test. The fracture toughness test also proved that the homogenized composite has higher toughness than the as cast one. The toughness value reached to 17.8 MPa m^1/2. The zigzag cracks propagated in the homogenized composite and the reinforcement Ti₂AlC particles and the finely precipitated Ti₂AlC particles were main obstacles to the crack propagation. The composite with impurities showed a marginal improvement in the oxidation resistance over the composites without impurities.     

Investigation of formation mechanism of Ti3SiC2 by self-propagating high-temperature synthesis

In this study, 3Ti/Si/2C powders were used as raw materials to synthesize Ti₃SiC₂ by self-propagating high-temperature synthesis (SHS). The phase compositions and morphological characteristics of Ti₃SiC₂, as well as the effects of grain granularity on Ti₃SiC₂ synthesis, were investigated. The reaction paths and dynamic behaviors of the synthesized Ti₃SiC₂ were studied using combustion front quenching. The results showed that the liquid phase of Ti–Si was formed during SHS. The mechanism for TiC formation exhibited a great effect on Ti₃SiC₂ synthesis. The proposed reaction mechanism for the synthesis of Ti₃SiC₂ by SHS suggested that Ti₃SiC₂ might be formed from the liquid phase of Ti–Si and the solid phase of TiC.