Microwave sintering of titanium diboride

Using a 2.45 GHz, 6 kW microwave furnace adapted for inert gas sintering, titanium diboride (TiB₂) can be rapidly microwave-sintered to >90% of theoretical density with sintering temperatures of 1900 to 2100 °C and soak times of 30 min or less. Densification behaviour with low-level additives was evaluated; 3 wt% chromium diboride (CrB₂) was an excellent sintering aid-grain growth inhibitor. A special covering system was required to produce oxidefree TiB₂. Specimen surface and interior temperatures were determined with a hole experiment. Comparison with conventional sintering indicates that microwave sintering of TiB₂-3 wt% CrB₂ occurs at lower temperatures (i.e., 200 °C lower) and can yield material with improved hardness, grain size, and fracture toughness.A portion of the material in this article was presented at a Symposium on Microwave Processing of Ceramics, 91st Annual Meeting of the American Ceramic Society, Indianapolis, Indiana, April 1989.Operated for the U.S. Department of Energy by Martin Marietta Energy Systems, Inc., under contract DE-AC05-84OR21400.     

Preparation of titanium diboride nanopowder

We have studied the reaction between NaBH₄ and TiCl₄ at elevated temperatures in the range 570–1020 K and pressures of up to 10 MPa, with no solvent. The results indicate that nanoparticulate titanium diboride forms at temperatures above 820 K. According to electron microscopy data, the titanium diboride powder obtained at 1020 K consists of spherical particles 35–50 nm in diameter, in reasonable agreement with the equivalent particle diameter of ≃45 nm evaluated from the specific surface area of the TiB₂ and with the crystallite size D _hkl ≃ 30 nm evaluated from X-ray diffraction data.     

Improving the uniformity in combustion-synthesized titanium carbide

The effects of reactant dispersion and outgassing on the microstructure of combustion-synthesized TiC were investigated. Pressure filtration casting of Ti+C colloidal suspension was shown to be an effective reactant dispersion method to improve the homogeneity of the product. The relation between the nature of the outgassing atmosphere and the formation of titanium oxide and precombustion titanium carbide was investigated. The combination of the colloidal dispersion technique with outgassing was shown to provide TiC products with reduced defects and improved microstructure.     

High temperature deformation of titanium diboride

Specimens of high purity polycrystalline titanium diboride, TiB₂, were tested in uniaxial compression under vacuum at a strain rate of 5 × 1O^–4sec^–1 to determine the plastic yield behaviour. Generally, plastic deformation was detected only above 170O° C. The apparatus was able to apply u stress of 900 MPa at a maximum temperature of 2000° C. The yield stress data fit an Arrhenius function, with an apparent activation energy of O.8 eV atom^–1. Dislocation glide over the Peierls barrier is thought to be the deformation mechanism, The dependence of the yield stress on the grain size obeyed the Hall-Petch relation within the bounds of experimental error. A TiB₂ single crystal containing TiC precipitates was also compression - tested at 2000° C, and its yield stress was approximately four times the stress predicted by the Hall-Petch expression for e pure TiB₂ single crystal, suggesting that the precipitate raises the yield stress but that the intrinsic lattice resistance is still significant. Submicrometre-sized graphite inclusions were observed in the polycrystalline specimens, but are thought not to have a direct effect on the yield stress in the temperature regime of the present study.     

The wettability of titanium diboride by molten aluminum drops

Titanium diboride is widely accepted to be completely wet by liquid aluminum, yet few published wetting studies demonstrate this behavior, and reported contact angles vary widely. Sessile drop substrates from four different sources were selected and their microstructures and chemistries characterized. The results of sessile drop experiments using four techniques to modify oxide film behavior were compared. The Al-TiB₂ interfaces were examined in metallographic sections or after chemical removal of the Al drop. Al wets a material containing 5.5 wt% Ni in vacuum experiments before the hold temperature of 1025^° C is reached. The other TiB₂ substrates are completely wet by Al at 1025^° C, but only after prolonged holds under vacuum. Elimination of boron oxide from the TiB₂ surface leads to a spreading condition. The role of the substrate microstructure (porosity, grain size, roughness, and carbon content) in altering the wetting kinetics is discussed.     

Preparation of titanium diboride TiB2 by spark plasma sintering at slow heating rate

A systematic study of the spark plasma sintering of TiB₂ starting from mixture of elemental powders was performed to investigate the temperature distribution between pressing tool and sinter body. The reaction mechanism and time-dependant evolution of the sintering behavior are established. The simultaneous application of pulsed high dc current and load leads to a microstructure with needle-shaped grains. Electron backscattering diffraction analyses show the preferred orientation of small crystallites parallel to the pressing direction.     

Mechanical properties of titanium diboride based cermets

Mechanical properties of titanium diboride (TiB₂) cermets critically depend on the composition of the binder phase. Both, fracture toughness and hardness are substantially increased by avoiding the formation of extremely brittle secondary borides which form during sintering by chemical reactions between TiB₂ and the metallic additives. Fractographic observations of TiB₂ cermets without secondary borides show the presence of ductile ligaments of the binder phase bridging the advancing crack tip. The powder metallurgy processing route applied to these materials allows modification of the binder phase structure from the ferritic iron-aluminium phase to Fe-Ni-Al austenite by changing the aluminium content of the powder mixtures. The highest toughness values have been obtained for the TiB₂ cermets with an austenitic binder phase. X-ray diffraction analyses of the fracture surfaces of such samples show that the binder phase is metastable exhibiting stress induced martensitic transformation during fracture. This new family of materials presents an outstanding combination of hardness and toughness, comparable to those obtained with commercial grades of tungsten carbide (WC) hardmetals.     

Explosive shock-compression processing of titanium aluminide/titanium diboride composites

Explosive shock-compression processing is used to fabricate Ti₃Al and TiAl composites reinforced with TiB₂. The reinforcement ceramic phase is either added as TiB₂ particulates or as an elemental mixture of Ti + B or both TiB₂ + Ti + B. In the case of fine TiB₂ particulates added to TiAl and Ti₃Al powders, the shock energy is localized at the fine particles, which undergo extensive plastic deformation thereby assisting in bonding the coarse aluminide powders. With the addition of elemental titanium and boron powder mixtures, the passage of the shock wave triggers an exothermic combustion reaction between titanium and boron. The resulting ceramic-based reaction product provides a chemically compatible binder phase, and the heat generated assists in the consolidation process. In these composites the reinforcement phase has a microhardness value significantly greater than that of the intermetallic matrix. Furthermore, no obvious interface reaction is observed between the intermetallic matrix and the ceramic reinforcement.     

Electrical resistivity of plasma-sprayed titanium diboride coatings

Plasma-sprayed TiB₂ coatings (50–600 m thick) on alumina substrates have been developed and characterized. X-ray diffraction studies, thermal analysis and oxygen analysis of the coatings show that there is appreciable oxidation of TiB₂ during the spray process. Partial oxidation of the boride during spraying strongly influences the electrical conductivity of the coatings, which is found to be 100–1000 times less than that of pure TiB₂. Although use of argon as shield gas during the spray process brings down the resisitivity substantially, partial oxidation of TiB₂ could not be totally avoided.     

Influence of reactant characteristics on the microstructures of combustion-synthesized titanium carbide

The influence of reactant characteristics on morphological development through the stages of combustion synthesis was investigated using a titanium-carbon system. The effect of the characteristics of a variety of carbons (carbon blacks, graphites, and cokes) and a variety of titanium powders on the density and microstructure of combusted and uncombusted sample compacts was studied. The size of the titanium particles had a relatively small influence on the density of the final (TiC) product but had a significant effect on its microstructure. The structure of the carbon blacks (as judged by the n-dibutyl phthalate absorption number, DBP) had a direct influence on the density of the uncombusted and combusted samples: low-structure carbon blacks resulted in higher densities for both cases. Products made with natural graphites had higher densities than those made with synthetic graphites. The surface area of carbon and graphite reactant powders had less influence on the density of the product than on its network morphology. Cored structures in TiC products made from certain carbon and graphite powders were observed and are explained in terms of their ash (oxide) content.     

Preparation of titanium diboride powders from titanium alkoxide and boron carbide powder

Titanium diboride powders were prepared through a sol-gel and boron carbide reduction route by using TTIP and B₄C as titanium and boron sources. The influence of TTIP concentration, reaction temperature and molar ratio of precursors on the synthesis of titanium diboride was investigated. Three different concentrations of TTIP solution, 0.033/0.05/0.1, were prepared and the molar ratio of B₄C to TTIP varied from 1.3 to 2.5. The results indicated that as the TTIP concentration had an important role in gel formation, the reaction temperature and B₄C to TTIP molar ratio showed obvious effects on the formation of TiB₂. Pure TiB₂ was prepared using molar composition of Ti: B₄C = 1: 2.3 and the optimum synthesis temperature was 1200°C.     

Preparation of titanium diboride nanopowders of different particle sizes

Reactions between titanium and microcrystalline boron powders in a Na₂B₄O₇ ionic melt at temperatures from 700 to 850°C and those between TiCl₄ and NaBH₄ at temperatures from 300 to 750°C and hydrogen pressures of up to 10 MPa, with no solvent, have been studied by X-ray diffraction, scanning electron microscopy, thermogravimetry, and elemental analysis. The results demonstrate that TiB₂ formation occurs at t 〉 730°C and 550°C, respectively. According to scanning electron microscopy data, the TiB₂ powder consists of particles 70–75 and 35–50 nm size, and the crystallite size evaluated from X-ray diffraction data is 55 and 30 nm, respectively, in agreement with the equivalent particle diameters obtained from the specific surface area of the TiB₂ powders: 60 and 45 nm, respectively.     

Laser-induced vapour-phase syntheses of boron and titanium diboride powders

Small diameter boron and titanium diboride powders were synthesized from vapour phase reactants heated with infrared radiation from a CO₂ laser. Boron powders were synthesized from BCI₃ + H₂ gas mixtures undfrom B₂H₆. TiCl₄ + B₂H₆ gas mixtures yielded TiB₂ powder. BCI₃ + H₂ + TiCl₄ gas mixtures yielded TiCl₂ powder but no TiB₂. Novel equipment designed to vapourize TiCI₄ liquid is described, Detailed characterizations of the product powders are presented.     

The effect of processing parameters in the carbothermal synthesis of titanium diboride powder

The mechanism of the carbothermal method for synthesizing titanium diboride (TiB₂) powder has been studied. Mixtures of TiO₂, H₃BO₃ and carbon were heated in an argon atmosphere at 1000–1600 °C. The effect of the molar ratio and holding time on the phase evolution was studied by X-ray diffraction. The products were also characterized by scanning electron microscopy observations and particle size measurements.For a composition with a molar ratio of TiO₂:H₃BO₃:C = 1:2.4:5 heated for 1 h, the simultaneous presence of TiC and TiB₂ phases at 1100 °C and the transformation of TiO₂ to Ti₂O₃ at 1200 °C and higher confirms that TiB₂ synthesis is based on a TiC formation mechanism, in which TiC may be formed from a reaction between TiO₂ or Ti₂O₃ and carbon. Then TiC may react with liquid B₂O₃ and/or gaseous B₂O₂ to form the TiB₂ phase. The reaction is completed at 1500 °C. Also by increasing the molar ratio of boric acid to 3, the impurities decreased considerably and pressing of the material had an obvious effect on decreasing the impurities, due to an increase of the surface contact of particles, which causes an effective inhibition of boron escape from the reaction chamber. Under these experimental conditions, a relatively narrow size distribution of TiB₂ particles was produced. When the reaction time increased to 1.5–2 h, grain growth of particles occurred. Therefore, a wider distribution of particle size was obtained.     

Self-propagating high temperature synthesis and dynamic compaction of titanium diboride/titanium carbide composites

Self-propagating High-temperature Synthesis (SHS) of titanium and boron carbide (B₄C) combined with explosively driven Dynamic Compaction (DC) was employed for the fabrication of composite TiB₂/TiC compacts. A 2³ factorially designed experiment set was used to examine the effects of the TiB₂/TiC ratio, delay time and C/M ratio on the consolidation and properties of the compacts. The delay time is the time between completion of the SHS reaction and compaction. The C/M ratio, the ratio of the explosive mass to that of the flyer plate, influences the pressure applied to the samples during compaction. Composites with molar TiB₂/TiC ratios of 2:1 or 1:2 were prepared using Ti and B₄C or Ti, C and B₄C, respectively, as reactants. The SHS/DC of Ti and B₄C resulted in high quality, near fully dense TiB₂/TiC composite compacts. Under best conditions, the densities were greater than 98% of the theoretical maximum. While the microhardness and densities of the compacts with TiB₂/TiC ratio of 2:1 were comparable to those of monolithic TiB₂ and TiC, compacts with TiB₂/TiC ratios of 1:2 were poorly consolidated and contained extensive cracks. Given the high energy and time efficiency, high product quality and inexpensive reactants, the SHS/DC of Ti and B₄C represents an attractive technique for the economical fabrication of TiB₂/TiC composites.     

In situ synthesis of titanium diboride composites through volume combustion

Abstract

Hard in situ synthesis of TiB₂–Fe₂B metal matrix composite (MMC) has been synthesised by volume combustion synthesis (VCS) reactions of Fe–FeTi–FeB system. VCS samples were characterised by SEM, EDX, XRD and DTA. Results show that it is possible to synthesise in situ structured MMC samples (with TiB₂ and Fe₂B phases) by VCS. Metallographic investigations show that Fe₂B and TiB₂ are found dispersed throughout the metal matrix, and other borides are present in microlevel patches dispersed in a eutectic matrix. The Fe–TiB₂ composites sintered at temperature of 1200°C consist of three different regions, i.e. α-Fe, TiB₂ and Fe₂B regions. The increase in sintering temperature to 1400°C leads to a hypereutectic microstructure of the Fe–B binary system having TiB₂ grains uniformly distributed throughout the matrix. A semiliquid phase sintering occurred by increasing eutectic phase transformation temperatures to 1400°C, which increased the efficiency of VCS. On the other hand, increasing sintering time from 1 to 3 h decreased the volume fraction of α-Fe and increased the volume fraction TiB₂ phase.