TiC Grains Research Articles

Microstructure and mechanical properties of in situ synthesized (TiB2 + TiC)/Ti3SiC2 composites

Based on thermodynamic analysis, highly dense (TiB 2 + TiC)/Ti 3SiC 2 composite ceramics with different TiB 2 volume contents were in situ fabricated in situ by hot-pressing at 1500 °C. Laminar Ti 3SiC 2 grains, columnar TiB 2 grains and equiaxed TiC grains were clearly identified from microstructural observation; grain boundaries were clean. The increase of TiB 2 volume content significantly restrains the grain growth of the Ti 3SiC 2 matrix. As the content of TiB 2 increases from 5 vol.% to 20 vol.%, the bending strength and fracture toughness of the composites both increase and then decrease, whereas the Vickers hardness increases linearly from 6.13 GPa to 11.5 GPa. The composite with 10 vol.% TiB 2 shows the optimized microstructure and optimal mechanical properties: 700 MPa for bending strength; 9.55 MPa m 1/2 for fracture toughness. These are attributed to the synergistic action of strengthening and toughening mechanisms such as particulate reinforcement, crack deflection, grain's pull-out and fine-grain toughening, caused by the columnar TiB 2 grains and equiaxed TiC grains.

Micromechanical and Tribological Properties of Nanocomposite nc-TiC/a-C Coatings

The nanocomposite coatings composed of nanocrystalline TiC grains embedded in hydrogen free amorphous carbon a-C matrix (nc-TiC/a-C) were deposited by magnetron sputtering on the two substrates, oxygen hardened Ti-6Al-4V alloy and heat treated VANADIS 23 steel. The Ti-6Al-4V alloy was oxygen hardened by plasma glow discharge. Micro-mechanical and tribological properties as well as coating adhesion to the substrates were investigated. Micro/nanostructure of the coatings and the substrates were examined using scanning- and transmission electron microscopy methods as well as X-ray diffractometry. Nano-, microhardness tests performed for the coated materials showed average hardness 13.4-14.7 GPa and modulus of elasticity 160 GPa. Scratch test revealed good adhesion of coatings to the both substrates. The nanocomposite coatings significantly improved tribological properties of the titanium alloy and steel, increased wear resistance and decreased friction coefficient.

Synthesis of Mesoporous Carbon‐Bonded TiC/SiC Composites by Direct Carbothermal Reduction of Sol–Gel Derived Monolithic Precursor

Mesosporous carbon‐bonded titanium‐carbide/silicon‐carbide (C‐TiC/SiC) ceramics with a high specific surface area (221 m2/g), ultra fine grains (10–50 nm), and high crystallinity were synthesized from direct carbothermal reduction (1100°–1450°C) of monolithic Ti–Si–O–C precursors made from controlled sol–gel process. The nano‐sized carbide grains are bonded into bulk materials by poly(furfuryl alcohol) (PFA) derived nanocrystalline carbon framework by “bridge,” “entanglement,” and “adhesive” effects. Graphenes were found to grow on {111} planes of TiC grains but this was not observed for SiC. The bonding of graphitic carbon layers on carbide grains support the nanostructure and also result in the desired combination of functional and mechanical properties.

Toughening mechanisms and microstructure of Al 2O 3–TiC–Co composites

Cobalt-coated Al 2O 3–TiC composites were prepared by hot-pressing at 1650 °C. Higher fracture toughness (7.81 MPa m 1/2) and bending strength (782 MPa) were obtained for composites containing both intragranular and intergranular TiC grains. TiC grains were dispersed homogeneously with an incoherent interface in the Al 2O 3 matrix and most cobalt particles resided at triple grain junctions. Grain/subgrain refinement, dislocation activities, and dispersion strengthening mainly account for the toughening mechanisms.

Effective grain pinning revealed by nanoscale electron tomography

The grain pinning behavior of TiC particles in a rapidly solidified MRE-Fe-B (MRE = Nd + Y + Dy) nanocrystalline hard magnet was studied using electron tomography (ET). The 3D reconstruction overcomes the inherent 2D nature of conventional transmission electronmicroscopy (TEM) to reveal how this grain boundary phase controls the nanoscale structure in the rapidly solidified alloy. The 3D reconstruction was performed on the optimally annealed alloy (750 °C/15 min) with hard magnetic properties of Mr = 8.1 kGs, Hc = 6.2 kOe, (BH)max = 11.2 MGOe measured at 300 k. The sampled volume, 425 × 425 × 92.5 nm3, contains more than 20 grains of the RE2-14-1 phase and more than 70 TiC nanoparticles. The TiC grains’ shapes depend on their sizes and locations along the grain boundary. Most of the TiC particles are oval or short rod like shapes and range from 5 nm to 10 nm. TiC particles less than 10 nm formed between adjacent 2-14-1 grains, while the largest ones formed at triple junctions. There are ∼1.7 × 108 TiC particles within a 1 mm3 volume in the alloy. This accounts for the strong grain boundary pinning effect, which limits grain growth during annealing.

Layer formation by resputtering in Ti–Si–C hard coatings during large scale cathodic arc deposition

This paper presents the physical mechanism behind the phenomenon of self-layering in thin films made by industrial scale cathodic arc deposition systems using compound Ti–Si–C cathodes and rotating substrate fixture. For the as-deposited films, electron microscopy and energy dispersive X-ray spectrometry reveals a trapezoid modulation in Si content in the substrate normal direction, with a period of 4 to 23 nm dependent on cathode configuration. This is caused by preferential resputtering of Si by the energetic deposition flux incident at high incidence angles, when the substrates are facing away from the cathodes. The Ti-rich sub-layers exhibit TiC grains with sizes up to 5 nm, while layers with high Si-content are less crystalline. The nanoindentation hardness of the films increases with decreasing layer thickness.

A comparative study of TiN and TiC: Oxidation resistance and retention of xenon at high temperature and under degraded vacuum

Dense TiN and TiC samples were prepared by hot pressing using micrometric powders. Xenon species (simulating rare gas fission products) were then implanted into the ceramics. The samples were annealed for 1 h at 1500 °C under several degraded vacuums with PO2 varying from 10−6 to 2×10−4 mbars. The oxidation resistance of the samples and their retention properties with respect to preimplanted xenon species were analyzed using scanning electron microscopy, grazing incidence x-ray diffraction, Rutherford backscattering spectrometry, and nuclear backscattering spectrometry. Results indicate that TiC is resistant to oxidation and does not release xenon for PO2≤6×10−6 mbars. When PO2 increases, geometric oxide crystallites appear at the surface depending on the orientation and size of TiC grains. These oxide phases are Ti2O3, Ti3O5, and TiO2. Apparition of oxide crystallites is associated with the beginning of xenon release. TiC surface is completely covered by the oxide phases at PO2=2×10−4 mbars up to a depth of 3 μm and the xenon is then completely released. For TiN samples, the results show a progressive apparition of oxide crystallites (Ti3O5 mainly) at the surface when PO2 increases. The presence of the oxide crystallites is also directly correlated with xenon release, the more oxide crystallites are growing the more xenon is released. TiN surface is completely covered by an oxide layer at PO2=2×10−4 mbars up to 1 μm. A correlation between the initial fine microstructure of TiN and the properties of the growing layer is suggested.

<I>In-situ</I> Synthesis and Mechanical Properties of (TiB<SUB>2</SUB>+TiC)/Ti<SUB>3</SUB>SiC<SUB>2</SUB> Composites

Based on thermodynamic analysis,(TiB2+TiC)/Ti3SiC2 composite ceramics were in-situ prepared by hot-pressing with TiH2,Si,graphite and B4C powders as raw materials.The phase composition and microstructure of the materials were characterized by XRD,SEM and TEM.Effects of sintering temperature on phase composition,sintering performance,microstructure and mechanical properties were investigated.The results indicate that fully densified(TiB2+TiC)/Ti3SiC2 composite ceramics could be obtained after sintered at 1400-1600℃.With the increase of sintering temperature,the bending strength,fracture toughness and Vickers hardness of the composite increase gradually.The materials sintered at 1500-1600℃ show optimized microstructure and consequently excellent mechanical properties,i.e.,bending strength of above 700MPa,fracture toughness of above 9MPa·m1/2 and Vickers hardness of 7.33-8.31GPa.As reinforcing agents,columnar TiB2 grains and equiaxed TiC grains,were incorporated by in-situ reaction during hot-pressing process to increase the strength and toughness of Ti3SiC2 matrix dramatically by the synergistic action of the mechanisms such as particulate reinforcement,crack deflection,grain's pull-out and fine-grain toughening.

Fabrication and characterization of sintered TiC–SiC composites

Some TiC–SiC composites with different SiC volume contents (0, 10, 25 and 50%) are prepared by spark plasma sintering (SPS). The relationship between density, grain growth and temperature is studied in order to fabricate dense and nano-sized TiC–SiC composites. A sintering by SPS at 1800 °C during 5 min allowed to form TiC–SiC composites with relative density above 95% and an homogeneous distribution of TiC (grain size from 270 to 900 μm) and nano-sized SiC. With the increasing of SiC volume contents, Vickers hardness and fracture toughness are improved; thermal conductivity at room temperature is increased whereas at high temperature it is reduced. In future studies, those materials will be irradiated to characterize monolithic TiC and TiC–SiC composites behaviour under irradiation.

Synthesis of TiC from porous carbon coating on Si–C–O (Nicalon) fibres by reactive chemical vapour deposition in pressure-pulsed mode or at atmospheric pressure

TiC coatings were synthesised on NL202 Nicalon fibres through the treatment of a porous carbon coating at the surface of the fibres by thermal reactive chemical vapour deposition (RCVD) either in pressure-pulsed (P-RCVD) mode at low pressure or at atmospheric pressure (AP-RCVD) with a H2/TiCl4 gaseous mixture. The conversion rate of C into TiC was studied as a function of the H2/TiCl4 pulse number for the P-RCVD method and as a function of the treatment time for the AP-RCVD method. Chemical and micro-structural characterisations of the coatings were carried out by X-ray photoelectron spectroscopy, Auger electron spectroscopy and scanning and transmission electron microscopies. Mechanical assessment was achieved by tensile tests at room temperature. For the P-RCVD method, the conversion rate increased when the number of H2/TiCl4 pulses increased from 20 to 100. Once 20 H2/TiCl4 pulses were performed, an interlayer made of TiC was observed at the carbon coating/fibre interface indicating that the H2/TiCl4 gas has diffused inside the porous carbon and has reacted in depth and more particularly at the coating/fibre interface. With 100 pulses, the carbon coating was totally converted. For the AP-RCVD method, the conversion rate increased when the treatment time increased from 30 to 60 min. The reaction of conversion began preferentially at the outermost surface of the carbon coating but several isolated TiC grains were also observed at the coating/fibre interface. At atmospheric pressure, the gaseous mixture was more reactive than at low pressure whereas the diffusion of the gaseous species was limited inside the carbon coating. The mechanical properties of the fibre decreased when the conversion rate of the carbon into TiC increased both in P-RCVD and AP-RCVD.

Condensation and mixing in supernova ejecta

Low-density graphite spherules from the Murchison carbonaceous chondrite contain TiC grains and possess excess 28Si and 44Ca (from decay of short-lived 44Ti). These and other isotopic anomalies indicate that such grains formed by condensation from mixtures of ejecta from the interior of a core-collapse supernova with those from the exterior. Using homogenized chemical and isotopic model compositions of the eight main burning zones as end-members, Travaglio et al. (1999) attempted to find mixtures whose isotopic compositions match those observed in the graphite spherules, subject to the condition that the atomic C/O ratio = 1. They were partially successful, but this chemical condition does not guarantee condensation of TiC at a higher temperature than graphite, which is indicated by the spherule textures. In the present work, model compositions of relatively thin layers of ejecta within the main burning zones computed by Rauscher et al. (2002) for Type II supernovae of 15, 21 and 25 M ʘ are used to construct mixtures whose chemical compositions cause equilibrium condensation of TiC at a higher temperature than graphite in an attempt to match the textures and isotopic compositions of the spherules simultaneously. The variation of pressure with temperature and the change in elemental abundances with time due to radioactive decay were taken into account in the condensation calculations. Layers were found within the main Ni, O/Ne, He/C and He/N zones that, when mixed together, simultaneously match the carbon, nitrogen and oxygen isotopic compositions, 44Ti/ 48Ti ratios and inferred initial 26Al/ 27Al ratios of the low-density graphite spherules, even at subsolar 12C/ 13C ratios. Due to the relatively large proportion of material from the Ni zone and the relative amounts of the two layers of the Ni zone required to meet these conditions, predicted 28Si excesses are larger than observed in the low-density graphite spherules, and large negative δ 46Ti/ 48Ti, δ 47Ti/ 48Ti, δ 49Ti/ 48Ti and δ 50Ti/ 48Ti are produced, in contrast to the observed normal δ 46Ti/ 48Ti and δ 47Ti/ 48Ti, large positive δ 49Ti/ 48Ti and smaller positive δ 50Ti/ 48Ti. Although better matches to the observed δ 46Ti/ 48Ti, δ 47Ti/ 48Ti and 28Si excesses can be found using much smaller amounts of Ni zone material and some Si/S zone material, it is very difficult to match simultaneously the Ti and Si isotopic compositions in any mixtures of material from these deep layers with He/C and He/N zone material, regardless of the condensation sequence. The occurrence of Fe-rich, Si-poor metal grains inside the graphite spherules does not have a satisfactory explanation.

Microstructure of high velocity oxy-fuel sprayed Ti2AlC coatings

The microstructure formation and phase transformations in Ti2AlC-rich coatings deposited by High Velocity Oxy-fuel spraying of Maxthal 211(A (R)) powders is presented. High resolution electron microscopy analysis, using both scanning and transmission electron microscopy with energy dispersive spectrometry and energy filtering, combined with X-ray diffraction reveals that the coatings consist of Ti2AlC grains surrounded by regions of very small TiC grains embedded in Ti (x) Al (y) . The composition of the Ti (x) Al (y) depends on its surrounding and varies with size and distribution of the adjacent TiC grains. Impact of spray parameters on coating microstructure is also discussed. Two spray parameters were varied; powder size distribution and flame power. They were found to greatly affect the coating microstructure. Increasing powder size and decreasing flame power increase the amount of Ti2AlC, but produces thinner coatings with lower cohesion. Larger powder size will also decrease oxygen incorporation.

In situ fabrication of TiC/Ti3Al/Ti2AlC composite by spark plasma sintering technology

A powder mixture of Ti, Al 4 C 3 and graphite was adopted to synthesize a new TiC/Ti 3 Al/Ti 2 AlC composite by spark plasma sintering. The reaction route was proposed from XRD analysis. TiC, Ti 3 Al and Ti 2 AlC formed from 800°C with TiC x and Ti 3 Al y acting as the intermediate phases until the reaction process completed at 1200°C. It was found from TEM observation that Ti 3 Al underwent an intergrowth relationship with TiC grains inside the Ti 2 AlC matrix due to the in situ reaction sintering. The average hardness, fracture toughness, Young's modulus and bending strength of the composite were 10.2 ± 0.6 GPa, 6.5 ± 0.2 MPa·m 1/2 , 436 ± 11 MPa and 227 ± 13 GPa, respectively.

Growth morphologies and mechanisms of TiC grains during Selective Laser Melting of Ti–Al–C composite powder

Selective Laser Melting (SLM) was used to consolidate the high-energy ball milled Ti–Al–C composite powder. The morphologies of the in-situ formed TiC grains under different laser processing parameters were investigated. It showed that with increasing the applied laser powers, the TiC grain morphologies experienced successive changes from a laminated shape to an octahedron shape, and then to a truncated near-octahedron shape, and finally to a near-spherical shape. It was believed that the continuously elevated SLM operating temperatures induced by the increasing laser powers played an important role in determining the TiC grain morphologies. Reasonable metallurgical mechanisms for grain growth behind the microstructural developments were proposed.

Hollow Spherical Ti-Al-C Clusters Prepared by Combustion Synthesis

Hollow spherical Ti–Al–C clusters have been prepared by combustion synthesis in air with the assistance of mechanical activation. The clusters have a two-layered shell structure, where the outer layer consists of Ti–Al–C ternary phases and the inner layer is composed of TiC grains. The reaction procedure and microstructure evolution during the combustion reaction is discussed. It is proposed that the presence of a Ti–Al melt is important for the formation of the hollow spherical clusters.

Reaction of TiC with SiCl 4 vapor with formation of SiC–TiC composites

The reaction of TiC grains with SiCl 4 vapor was carried out in Ar at 1000–1600 °C to produce TiC–SiC composite powders. The compositions of these powders were controlled by controlling the conversion to SiC, which depends on the reaction temperature. The phases formed by the solid–gas reaction were identified by X-ray analysis (XRD) and the molar ratio of SiC to TiC in the powder was estimated from the area ratios of the respective XRD peaks. The reaction begins at about 1100 °C and its rate increases with temperature to 1600 °C, at which 92% SiC is formed. The morphology of the TiC–SiC composites was observed by scanning electron microscopy and transmission electron microscopy and a reaction mechanism is deduced from the above results and thermodynamic considerations. TiC–SiC composites were densified by spark plasma sintering at 1750 °C and 40 MPa to the relative density 92–96% and a Vickers hardness of 1700–1900 Hv.

Grain refinement of AZ31 magnesium alloy by Al-Ti-C-Y master alloy

Al-Ti-C-Y master alloy was prepared by combining SHS technique and melting-casting method. The microstructure of master alloy and its grain-refining effect on AZ31 alloy were investigated by means of OM, XRD, SEM and EDS. Experimental results indicated that the prepared master alloy consisted of α-Al, TiAl 3, TiC and Al 3Y phases, and exhibited good grain-refining performance of AZ31 alloy. Morphology of α-Mg changed from coarse dendritic to fine equiaxed and the average grain size of α-Mg matrix reduced from the original 580 to 170 μm after adding 1.0 wt.% master alloy. The grain refining efficiency of Al-Ti-C-Y master alloy on AZ31 alloy was mainly attributed to heterogeneous nucleation of TiC particles and grain growth restriction of Al-Y compound or TiC at grain boundaries.

Fabrication and properties of TiC-based cermet with intra/intergranular microstructure

An intra/intergranular-type TiC-based cermet acquired by spark plasma sintering (SPS) was reported. Composition and microstructure were studied by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). Results revealed that some added TiN nanoparticles less than 80 nm could be entrapped within TiC grains, others ranged from 50 nm to100 nm distributed at the ceramic grain boundaries and formed an intra/inter-type microstructure finally. Properties testing indicated that the material had higher fracture toughness ( K IC) and transverse rupture strength (TRS) at room temperature. The strengthening and toughening mechanisms were also further discussed, respectively.

WC p/Ti–6Al–4V graded metal matrix composites layer produced by laser melt injection

The laser melt injection (LMI) process was explored to produce WC particles (WC p) reinforced Ti–6Al–4V metal matrix composites (MMC). In particular monocrystalline WC powder was used as injection particles to avoid the intercrystalline cracking often observed in granular or cast WC p reinforced MMC. WC p were injected into the extended part of the melt pool just behind the laser beam. The process allowed for the minimization of the WC p dissolution caused by the direct irradiation of the laser beam, and the decomposition reaction between WC p and Ti melt. Different parameters were applied, and a processing window of LMI was obtained. WC p exhibit a graded distribution along the depth direction of the MMC layer. New phases such as TiC and W 2C are observed in the MMC layer, in which TiC is the predominant phase. TiC grains present a continuous decrease in both amount and size with the distance from the surface to the bottom of the MMC layer. Two types of reaction layers around WC p can be distinguished, namely an irregular reaction layer and a cellular reaction layer. The growth and final morphology of reaction layers are most likely being dominated by the composition of the neighbouring melt pool. A gradual hardness distribution in the depth direction of the composites layer is observed. Moreover, the transition from the MMC layer to the substrate also exhibits a gradual change in the hardness.

Effects of nano TiN addition on the microstructure and mechanical properties of TiC based steel bonded carbides

TiC based steel bonded carbides with the addition of nano TiN were prepared by vacuum sintering techniques. The microstructure was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the mechanical properties, such as bending strength, impact toughness, hardness, and density, were measured. The results indicate that the grain size becomes small and there is uniformity in the steel bonded carbide with nano addition; several smaller carbide particles are also found to be inlaid in the rim of the larger carbide grains and prevent the coalescence of TiC grains. The smaller and larger carbide grains joint firmly, and then the reduction of the average size of the grains leads to the increase in the mechanical properties of the steel bonded carbides with nano addition. But the mechanical properties do not increase monotonously with an increase in nano addition. When the nano TiN addition accounts for 6–8 wt.% of the amount of steel bonded carbides, the mechanical properties reach the maximum values and then decrease with further increase in nano TiN addition.