TiB2 Crystals Research Articles

In Situ Formation of Titanium Diboride/Magnesium Titanate Composites by Magnesiothermic-Based Combustion Synthesis

In situ formation of TiB2–Mg2TiO4 composites was investigated by combustion synthesis involving the solid-state reaction of Ti with boron and magnesiothermic reduction of B2O3. Certain amounts of MgO and TiO2 were added to the reactant mixtures of Ti/B/Mg/B2O3 to act as the moderator of highly exothermic combustion and a portion of the precursors to form Mg2TiO4. Two combustion systems were designed to ensure that synthesis reactions were sufficiently energetic to carry on self-sustainably, that is, in the mode of self-propagating high-temperature synthesis (SHS). Consistent with thermodynamic analyses, experimental results indicated that the increase in pre-added MgO and TiO2 decreased the combustion temperature and propagation velocity of the flame front. MgO was shown to have a stronger dilution effect on combustion exothermicity than TiO2, because the extent of magnesiothermic reduction of B2O3 was reduced in the MgO-added samples. In situ formation of the TiB2–Mg2TiO4 composite was achieved from both types of samples. It is believed that, in the course of the SHS progression, Mg2TiO4 was produced through a combination reaction between MgO and TiO2, both of which were entirely or partially generated from the metallothermic reduction of B2O3. The microstructure of the products exhibited fine TiB2 crystals in the shape of short rods and thin platelets that existed within the gaps of Mg2AlO4 grains. Both constituent phases were well distributed. A novel and efficient synthesis route, which is energy- and time-saving, for producing Mg2TiO4-containing composites was demonstrated.

Controlled Synthesis of TiB2-TiC Composite: Substantiation of the Homogenizing Joule Thermostatting Efficiency and Improvement of SHS-Compaction Technology in a Vacuum

This research aims to improve and substantiate the efficiency of homogenization heat-stabilizing Joule heating on ceramic-matrix composites of TiB2-TiC system with a 2:1 component ratio during its synthesis. For this purpose, an improved technological approach is proposed, which is based on the known method of SHS-compacting but differs by the possibility of controlled Joule influence on the synthesis products, which is achieved by the use of a special electrothermal vacuum press-mold functioning according to a particular control algorithm. The task of controlled Joule heating is a compensation of the temperature gradient formed in the synthesized workpiece, which is solved by passing in it a direct current directed in line with the vector of propagation of the combustion wave. An indicator of assessment of the degree of compensation of the noted temperature gradient is the Seebeck effect, excited between the upper and lower surface of the SHS workpiece, which should be brought to zero in the process of Joule thermostatting. It was experimentally revealed that compensation of the noted temperature gradient with heat released predominantly by electrically conductive and Joule-heated TiC grains leads to their softening, which contributes to more uniform compaction of the workpiece due to diffusion coalescence of these grains around prism-shaped hard TiB2 crystals. Such consolidation leads to a significant increase in the quality of structural packaging and a reduction in the number and volume of micropores, as a result of which the performance properties of the composite improve on average by 10–15 %.

Formation of TiB2-MgAl2O4 Composites by SHS Metallurgy.

TiB2-MgAl2O4 composites were fabricated by combustion synthesis involving metallothermic reduction reactions. Thermite reagents contained Al and Mg as dual reductants and TiO2 or B2O3 as the oxidant. The reactant mixtures also comprised elemental Ti and boron, as well as a small amount of Al2O3 or MgO to serve as the combustion moderator. Four reaction systems were conducted and all of them were exothermic enough to proceed in the mode of self-propagating high-temperature synthesis (SHS). The reaction based on B2O3/Al/Mg thermite and diluted with MgO was the most exothermic, while that containing TiO2/Al/Mg thermite and Al2O3 as the diluent was the least. Depending on different thermites and diluents, the combustion front temperatures in a range from 1320 to 1720 °C, and combustion wave velocity from 3.9 to 5.7 mm/s were measured. The XRD spectra confirmed in situ formation of TiB2 and MgAl2O4. It is believed that MgAl2O4 was synthesized through a combination reaction between Al2O3 and MgO, both of which can be totally or partially produced from the metallothermic reduction of B2O3 or TiO2. The microstructure of the TiB2-MgAl2O4 composite exhibited fine TiB2 crystals surrounded by large densified MgAl2O4 grains. This study demonstrated an energy-saving and efficient route for fabricating MgAl2O4-containing composites.

Theoretical insights into the behaviors of sodium and aluminum on the cathode titanium diboride surfaces

Density functional theory calculations were adopted to systematically investigate the adsorption and diffusion behaviors of sodium and aluminum over TiB2 surfaces or in TiB2 crystal to characterize the interaction mechanism between sodium and TiB2 cathode in aluminum reduction cells. Results suggest that Na and Al will stably adsorbe on the low-index TiB2 (0 0 0 1) surface, and the presence of vacant defects can significantly strengthen this adsorption. The migration of Na and Al over pristine TiB2 is anisotropic, with the largest energy barrier of 0.024/0.32 eV for Na and 0.28/1.57 eV for Al over Ti/B-terminated surfaces. The Ti vacancy in Ti-terminated surface is more effective to hinder Na and Al migration with the large diffusion barriers of 0.36 eV for Na and 2.07 eV for Al. Specially, for the B-terminated surface, B vacancy will promote the Na and Al diffusion with the lower barriers. Additionally, it is difficult for Na and Al to form interstitial defects and diffuse in covalent TiB2 crystal. Given these results, compared to graphite cathode, the sodium prefers to deposit on TiB2 surface, and the strong interaction between sodium and TiB2 promotes the early process of sodium penetration. On the other hand, the smoother landscape for Na diffusion on the TiB2 surface suggests the decreased stability of aluminum liquid, so that the current efficiency of aluminum reduction cell will decrease.

Thermal conductivity and thermal shock resistance of TiB2-basedUHTCs enhanced by graphite platelets

TiB2-SiC-C and TiB2-TiC-C heteromodulus ceramics were sintered by reactive hot pressing of TiC-B4C-Si and TiC-B4C precursors at 1830 °C and 30 MPa. Sintering time at the maximum temperature was between 2 and 8 min. The reactions in TiC-B4C-Si green body create submicron (100–400 nm) TiB2 crystals inside silicon carbide grains as well as BxSiyCz nanofibers, which nucleate near SiC surfaces. Low hardness graphite platelets are formed within the hard matrix during the in-situ exothermic reaction. “Soft-hard” grain combination produces substantially improved fracture toughness (up to 9 MPa m1/2). The materials also demonstrate high thermal conductivity (up to 120 W/m∙K) and high thermal shock resistance. The thermal shock crack growth is arrested by the graphite platelets.

Solidification orientation relationships between Al3Ti and TiB2

Orientation relationships (ORs) can form during solidification by a variety of mechanisms that are often difficult to distinguish after solidification. Here we study three ORs formed by the nucleation of Al3Ti on TiB2, and by the pushing and engulfment of TiB2 by growing Al3Ti facets in hyperperitectic Al-rich melts. The nucleation OR is identified by growing a relatively large TiB2 crystal, solidifying multiple small Al3Ti crystals on one (0001) facet of TiB2, and measuring the resulting OR by electron backscatter diffraction (EBSD). Pushing and engulfment ORs are investigated by statistical analysis of EBSD measurements, density functional theory (DFT) calculations of interface energies, and imaging of cross-sections of TiB2 particles being pushed and engulfed by Al3Ti facets. It is shown that the lowest energy OR is formed by nucleation as well as by pushing/engulfment. The higher energy ORs, formed by pushing and engulfment, correspond to local interfacial energy minima and can be explained by rotation of TiB2 particles on Al3Ti facets during pushing.

Synthesis and characterization of oxidation-resistant TiB2 coating on molybdenum substrate by chemical vapor deposition

Practical applications of molybdenum (Mo) and Mo-based alloy have been limited for their weak oxidation resistance properties under oxidizing environments. Anti-oxidation coating technology is a major solution for the improvement of oxidation resistance of Mo. In this study, titanium diboride (TiB2) was deposited as an anti-oxidation coating on pure molybdenum substrate from a gas mixture of TiCl4, BCl3 and H2 at 1000 °C by chemical vapor deposition (CVD). The result shows that good quality TiB2 thin coating has been grown on pure molybdenum and the thickness of the coating deposited at 1000 °C for 2 h reached 13 μm. The TiB2 coating was found to have a typically columnar particle structure and the length of the columnar particle is 1–2 μm. Meanwhile, TiB2 crystal has a preferential orientation growth in the (0 0 1) direction. The results of oxidation test reveal TiB2 coating formed on Mo substrate exhibits better oxidation resistance in air at 900 °C.

Homogeneous boron doping in a TiO2 shell supported on a TiB2 core for enhanced photocatalytic water oxidation

Photocatalytic water oxidation for O2 evolution is known as a bottle neck in water splitting. Various strategies have been conducted to keep the energetics of photogenerated holes or to create more holes in the bulk to reach the surface for efficient photocatalytic water oxidation. Our previous study demonstrated the effectiveness of interstitial boron doping in improving photocatalytic water oxidation by lowering the valence band maximum of TiO2 with a concentration gradient of boron. In this study, homogeneous doping of interstitial boron was realized in a TiO2 shell with mixed anatase/rutile phases that was produced by the gaseous hydrolysis of the surface layer of TiB2 crystals in a moist argon atmosphere. Consequently, the homogeneous doping and lowered valence band maximum improved the energetics of holes for efficient photocatalytic water oxidation.

Self-Propagating High Temperature Synthesis of TiB2–MgAl2O4 Composites

Metal borides are widely used as heat-insulating materials, however, the range of their application in high-temperature conditions with oxidative medium is significantly restricted. To improve the thermal stability of structural materials based on titanium boride, and to prevent the growth of TiB2 crystals, additives based on alumina-magnesia spinel with chemical resistant and refractory properties have been used. The aim of this work is to study the structure of TiB2 with alumina-magnesia spinel additives obtained by self-propagating high-temperature synthesis (SHS). TiB2 structure with uniform fine-grained distribution was obtained in an MgAl2O4 matrix. The material composition was confirmed by X-ray diffraction analysis (DRON-3M, filtered Со kα-emission), FTIR spectroscopy (Thermo Electron Nicolet 5700, within the range of 1300–400 cm−1), and scanning electron microscopy (Philips SEM 515). The obtained material represents a composite, where the particles of TiB2 with a size of 5 µm are uniformly distributed in the alloy of alumina-magnesia spinel.

In situ synthesis of polycrystalline cubic boron nitride with high mechanical properties using rod-shaped TiB2 crystals as the binder

ABSTRACTPolycrystalline cubic boron nitride (PcBN) was sintered in the temperature range of 1050–1650°C under an ultra-high pressure of 5.0 GPa for 20 min using an in situ synthesis method. X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and energy-dispersive spectrometry were used to study the compositional and morphological changes during the synthetic process. The results showed that rod-shaped TiB2 crystals could be detected at 1150°C, and their quantity increased with sintering temperatures. The TiB2 crystals distributed homogeneously and grew randomly among the cBN grain boundaries and interstitial spaces, ultimately forming an interlocking structure. The PcBN prepared at 1650°C exhibited optimal comprehensive mechanical properties, with a micro-hardness of 44.42 GPa and a flexural strength of 862.5 MPa. Moreover, based on the experimental results, the chemical reaction mechanism and the strengthening mechanisms occurred during the sintering process were also proposed to properly interpret the synthesis of PcBN.

Interfacial reactions between titanium and boron nitride nanotubes

Titanium-coated boron nitride nanotubes prepared by sputtering method have been heated under different conditions to investigate reaction phases at their interface. It is found that TiB2 nanocrystals firstly nucleate on the surface of nanotubes at 800°C for 30min. By the time 1h, these nucleated TiB2 crystals grow larger by consuming inner walls of nanotubes and tend to transform into TiB phase. With an exposure to higher temperatures and longer periods, needle-like TiB whiskers and plate-like TiB2 remain in the composite.

Investigating the influence of epitaxial modulation on the evolution of superhardness of the VN/TiB2 multilayers

A series of the VN/TiB2 nanomultilayers with different modulation ratios (tVN:tTiB2) and different modulation periods were synthesized via a magnetron sputtering system. The cross-sectional transmission electron microscopy (TEM) and x-ray diffraction (XRD) examinations indicated that in the alternately deposited monolayers of the VN and TiB2, due to the influence of the crystal (111)VN texture, TiB2 layer presented epitaxial growth on the surface of the VN layer when its tVN:tTiB2 was 5:1. Moreover, the formation of the TiB2 crystal promoted the growth of (200)VN and significantly improved the preferential growth of nanomultilayers. With decreasing tVN:tTiB2 to 1:7, the thin VN layer was crystallized under the introduction of crystalline TiB2 layers. A type of double epitaxial growth was observed to be a main reason for the coherent growth of the VN/TiB2 nanomultilayers within a certain thickness. Consequently, the multilayers appeared to have a corresponding superhard effect, which presented a corresponding anomalous enhancement of hardness and elastic modulus. The highest hardness and elastic modulus reached 41.8 and 492.4GPa, respectively, at a 1:7 ratio of tVN:tTiB2. Residual stresses were also released due to the coherent growth in the interfaces. Meanwhile, the coherent growth model of the multilayer was used to explain the growth mechanism of the VN/TiB2 nanomultilayers in this study.

Effect of Ti content and stirring time on microstructure and mechanical behavior of Al-B4C composites

Ti content and stirring time are two important parameters of stir casting technique for Al-B4C composites. In this study, their effect on microstructure and mechanical behavior of Al-B4C composites were investigated. A simple way to fabricate tri-modal Al-B4C-Al3Ti composites was proposed and efforts have been made to correlate microstructure evolution with mechanical response of the composites. It was found that, higher Ti levels resulted in higher volume fraction and particle size of Al3Ti, denser TiB2 interface and even more uniform spatial distribution of B4C. A prolonged stirring time, led to copious coarse TiB2 crystals at the expense of Al3Ti particles and generally more homogeneous spatial arrangement of particles. As a result, their tensile strength showed an inclined leaf-shape curve. These observations were interpreted in terms of synergy of Al3Ti particles, uniformity of particle spatial distribution, coarse TiB2 crystals and Al-B4C interface.

Grain refinement by trace TiB2 addition in conventional cast TiAl-based alloy

Effects of trace TiB2 addition on the solidification microstructure of Ti–48Al–2Cr–2Nb+(0.18, 0.54, 0.9) wt.% TiB2 alloys were investigated by arc melting and electromagnetic levitation melting in this paper. Results showed that the matrix alloy generated a large-grained (~800μm) fully lamellar microstructure, and the grain size decreased with increasing TiB2 additions. With 0.18wt.% TiB2 addition, the grains remained fully lamellar, at about 700μm in diameter. For more TiB2 addition (0.54wt.%), grain size decreased to an average value of 200μm. The grain refinement could be explained by constitutional undercooling and α phase nucleation on the β laths of secondary TiB2/β borides. With 0.9wt.% TiB2, the alloy melt produced primary TiB2 crystals during solidification to nucleate β grains. The β grains in turn nucleated and were transformed to α grains with the generation of secondary TiB2 flakes. This resulted in equiaxed, refined (~200μm) lamellar grains without element segregation. The elimination of element segregation could be attributed to the fine equiaxed dendrites, corresponding to the result of electromagnetic levitation melting quenching experiments. With equivalent B additions, the grain refinement effect was equal, except for the cavity defects.

The free growth criterion for grain initiation in TiB 2 inoculated γ-titanium aluminide based alloys

γ-titanium aluminide (γ-TiAl) based alloys enable for the design of light-weight and high-temperature resistant engine components. This work centers on a numerical study of the condition for grain initiation during solidification of TiB2 inoculated γ-TiAl based alloys. Grain initiation is treated according to the so-called free growth criterion. This means that the free growth barrier for grain initiation is determined by the maximum interfacial mean curvature between a nucleus and the melt. The strategy presented in this paper relies on iteratively increasing the volume of a nucleus, which partially wets a hexagonal TiB2 crystal, minimizing the interfacial energy and calculating the corresponding interfacial curvature. The hereby obtained maximum curvature yields a scaling relation between the size of TiB2 crystals and the free growth barrier. Comparison to a prototypical TiB2 crystal in an as cast γ-TiAl based alloy allowed then to predict the free growth barrier prevailing under experimental conditions. The validity of the free growth criterion is discussed by an interfacial energy criterion.

Plasma Arc Welding of TiB2–20 vol% TiC

Ceramics consisting of titanium diboride with titanium carbide additions were fusion welded to produce continuous joins. A welding current of 135 A, welding speed of 8 cm/min, and plasma flow rate of 0.75 L/min were combined with a preheat temperature of ~1450°C to fusion weld coupons of TiB2 containing 20 vol% TiC with a thickness of 4 mm. The resulting fusion zone (FZ) was 3.9 mm deep at the center of the joint and 10.4 mm wide. During cooling of the melt pool, four distinct regions of crystal growth and nucleation were observed due to thermal gradients. Regions at the top and bottom of the FZ exhibited smaller TiB2 crystals due to higher nucleation rates whereas regions in the middle of the FZ showed higher growth rates, with TiB2 crystals up to 1.2 mm in length. Thermal gradients also affected cooling of the eutectic phase, causing a cellular structure to appear in the cooled eutectic. Plasma arc welding was a viable method for joining diboride‐based ceramics.

Crystal Growth, Fracture Behavior and Toughening of Rapidly-Solidified TiC-TiB<sub>2</sub> Prepared by Combustion Synthesis in High-Gravity Field

Micro-nanocrystalline microstructures characterized by TiB2 platelets of the average thickness close to or smaller than 1 μm, were achieved in full-density solidified TiC-TiB2 composites by combustion synthesis in high-gravity field. XRD, FESEM and EDS results showed that a large number of fine TiB2 platelets were uniformly embedded in irregular TiC grains, a few Cr-Al metallic phases or in between those phases. The achievement of micro-nanocrystalline microstructures results from low-velocity faceting growth of TiB2 crystal, high-velocity non-faceting growth of TiC solid and high diffusion rate of C relative to B in liquid TiC-TiB2. The relative density, Vickers hardness, flexural strength and fracture toughness of TiC-TiB2 composites measured 99.3%, 21.5 ± 1.5 GPa, 845 ± 35 GPa and 16.8 ± 1.5 MP • m0.5, respectively. High flexural strength of the materials benefits mainly from the achievement of micro-nanocrystalline microstructure in the full-density solidified ceramic and high fracture toughness contributed from intensive coupled toughening mechanisms by a large number of fine TiB2 platelets and a few Cr-Al metallic phases.

Deformation Behavior and Damage Evaluation in a New Titanium Diboride (TiB2) Steel‐Based Composite

AbstractDeformation behavior and damage evaluation of a new composite steel has been investigated by means of in situ three‐point bend tests in the scanning electron microscope. The titanium diboride (TiB2)‐reinforced steel composite is produced by in situ precipitation of the TiB2 particles during eutectic solidification. This production process developed by ArcelorMittal leads to a steel composite with a significant increase in specific stiffness (>20%), and good strength/ductility compromise. The microstructures obtained consist of primary TiB2 crystals surrounded by a eutectic mixture of ferrite and TiB2 particles. The primary mode of damage is particle fracture and inhomogeneous plastic deformation in the matrix. In contrast with other production process, particle fracture was more common than interfacial debonding indicating that interfacial strength is not the limiting factor in damage accumulation and fracture in this composite. Crack growth occurred by particle fracture ahead of the crack tip, producing large microvoids, which then link up to the growing crack by ductile failure of the remaining matrix ligaments. The results suggest also that the cracks tended to avoid direct particle interactions.

TiB2 and TiC single crystals by plasma-arc remelting of SHS products

The growth of TiB2 and TiC single crystals by plasma-arc remelting of SHS products was explored. The process was markedly improved by using special stabilization systems. The synthesized crystals exhibited a cellular structure and a strong dependence of the longitudinal distribution of impurity elements on a growth rate.

First principle study on the elastic and thermodynamic properties of TiB2 crystal under high temperature

This paper predicts the elastic and thermodynamic characteristics of TiB2 crystal through the method of density functional theory within the generalized gradient approximation (GGA). The five independent elastic constants (Cij), the bulk modulus (B0), the dependence of bulk modulus (B0) on temperature T and pressure P and the coefficient of thermal expansion (αL) at various temperatures have been evaluated and discussed. According to calculation, the bulk modulus will increase with increasing pressure while decrease with the increasing temperature. The coefficient of thermal expansion is consistent with the famous Grüneisen's law when the temperature is not too high. The obtained results agree well with the experimental and other theoretical results.