Self-propagating high temperature synthesis (SHS) of titanium carbide

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

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.     

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.     

TiB/Ti复合材料自蔓延高温燃烧合成的研究

采用自蔓延高温燃烧合成－准热等静压工艺（SHS／PHIP）制备了TiB-Ti体系复合材料,理论计算了该体系的绝热温度,测量了燃烧温度和燃烧速度。结果表明,绝热温度、燃烧温度和燃烧速度均随Ti含量的增加而降低。对合成产物的分析发现：反应产物主要由TiB和Ti两组组成,TiB相分布均匀,主要有棒状和块状两种形态,并且随Ti含量的增加,TiB尺寸减小;部分产物中还有少量TiB2相存在。合成产物具有高的致密度和硬度,其相对密度超过94％,硬度HRA＞82。     

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.     

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.     

Self-propagating high-temperature synthesis of titanium borides

The application of self-propagating high-temperature synthesis (SHS) to prepare a few borides of titanium was investigated. Using the plane wave propagation mode, the synthesis of titanium borides in the cold-pressed cylindrical specimens of the component powder mixtures was effected and was studied as a function of boron content in the initial mix and the specimen size. SHS reaction in compacts having diam. of 6 mm or less and high bulk density could not be initiated and/or sustained and was considered to be a result of rapid heat dissipation.     

Self-propagating high temperature synthesis of high purity single-crystalline SnO2 nanobelts

A new application of conventional self-propagating high temperature synthesis technology was developed to synthesise high purity single-crystalline SnO₂ nanobelts through a simple, convenient and efficient method. The outcome of the research, for which an experimental setup was designed and manufactured, turned out to be successful and propagable. High purity SnO₂ nanobelts could be obtained within minutes. The growth mechanism of these SnO₂ nanobelts is discussed in this article.     

Processing, properties and arc jet oxidation of hafnium diboride/silicon carbide ultra high temperature ceramics

The processing and properties of HfB₂-20 vol%SiC ultra high temperature ceramics were examined. Dense billets were fabricated by hot-pressing raw powders in a graphite element furnace for 1 h at 2200°C. Specimens were then tested for hardness, mechanical strength, thermal properties and oxidation resistance in a simulated re-entry environment. Thermal conductivity of the current materials was found to be less than previous work had determined while the strength was greater. Oxidation testing of two flat-face models was conducted, at two conditions, for two 10-min durations each. It was concluded that passive oxidation of SiC plays a role in determining the steady-state surface temperatures below 1700°C. Above 1700°C, temperatures are controlled by the properties of a thick HfO₂ layer and active oxidation of the SiC phase.     

A study on the high temperature reaction of niobium pentoxide with niobium carbide and titanium carbide

The high temperature reaction of niobium pentoxide with niobium carbide and titanium carbide has been studied to investigate the feasibility of direct preparation of NbTi alloys and the titanium deoxidation tendency of niobium. Starting with oxide-carbide charges with o:c mole ratio varying from 1.01 to 1.10 and titanium content varying from 4 to 56 wt. pct., the reaction was carried out at 1900°C under high vacuum in a vacuum induction furnace followed by treatment in the electron beam melting furnace to obtain the final alloys. The material balance of the charge indicates that substantial amount of oxygen is removed as TiO(g) besides as CO during the reaction. This results in low titanium and oxygen contents and high residual carbon content in the alloy, particularly when the initial titanium content in the charge exceeds 4 wt. pct. The results obtained in the present investigation have been explained from the available/estimated thermodynamic data for the NbTiCO system.     

Synthesis of titanium diboride TiB2 and Ti-Al-B metal matrix composites

Titanium diboride TiB₂ and TiAl aluminide composites reinforced with in situ borites have been synthesized from the elemental powders of Ti and B, and Ti, Al and B respectively using mechanical alloying technique. No progressive diffusion between Ti and B was observed. The formation of TiB₂ was found to be governed by strong and fast exothermic heat release. This indicates that the formation of TiB₂ compound in local area of mechanically alloyed powder generated high energy which in turn ignited and promoted the formation of new compound in the rest of the area. Because of the presence of Al in Ti-Al-B system, the concentration of Ti or B was diluted. The exothermic reaction between Ti and B was consequently delayed. However, grain refinement of Ti and Al in this system down to nanometer scale is faster than that in Ti-Al system due to the contribution of B. Using X-ray analysis, strong but broad TiAl, and weak TiB and TiB₂ peaks had been detected at 50 h of mechanical alloying indicating the formation of nano TiAl composite reinforced by TiB and TiB₂. However, TiB was, however, not a stable phase; it later was transformed into equilibrium phase of TiB₂ after annealing at 800 °C.     

Self-propagating combustion synthesis of intermetallic matrix composites in the ISS

Combustion Synthesis experiments have been performed on the ISS (International Space Station) during the Belgian taxi-flight mission ODISSEA in November 2002, in the framework of the ESA-coordinated project COSMIC (Combustion Synthesis under Microgravity Conditions). The main objective of the experiments was to investigate the general physico-chemical mechanisms of combustion synthesis processes and the formation of products microstructure. Within the combustion zone, a number of gravity-dependent phenomena occur, while other phenomena are masked by gravity. Under certain conditions, gravity-dependent secondary processes may also occur in the heat-affected zone after combustion. To study the influence of gravity, a specially dedicated reactor ensemble was designed and used in the Microgravity Science Glovebox (MSG) onboard the ISS. In this work, the experiment design is first discussed in terms of the experimental functionality and reactor ensemble integration in the MSG. To investigate microstructure formation, a sample constituted by a cylindrical portion followed by a conical one, the latter being inserted inside a massive copper block, is used. The experiment focused on the synthesis of intermetallic matrix composites (IMCs) based on the Al-Ti-B system. Depending on the composition, different intermetallic compounds (TiAl and TiAl₃) can be formed as matrix phase while TiB2 represents the reinforcing particulate phase. During the ISS mission, six samples with a relatively high green density of 65%TD have successfully been processed. The influence of the composition on the combustion process will be examined.     

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.     

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.     

Combustion synthesis of titanium carbide: Theory and experiment

The combustion synthesis of titanium carbide from elemental powders has been theoretically and experimentally studied as a model system for self -propagating high temperature synthesis (SHS) of refractory compounds. Calculations of the adiabatic temperature of combustion of graphite and titanium powders to form TiC _x have been made to show the effects of stoichiometry, dilution and the initial temperature of the reactants. Experimental observations on the stability of the combustion front, combined with theoretical predictions, lead to an estimated activation energy of 117 kJ mol^–1 for the process. This value is at least a factor of about four too low to correspond to a diffusion-controlled process. The combustion of graphite and titanium powders was accompanied by the evolution of gases whose primary constituent was found to be hydrogen. This observation was attributed to the reaction of adsorbed moisture with titanium powder. The titanium carbide phase resulting from the combustion of compacted mixed powders of the elements was highly porous ( 50% porosity). It can be obtained in high density (5% porosity) when pressure is applied during the combustion process.     

Plasmachemical synthesis of titanium carbide on copper substrates

The properties of coatings synthesized using titanium deposition from metal plasma generated by vacuum-arc discharge in benzene vapor have been studied. The plasma composition, as well as the composition and structure of titanium carbide coatings formed on copper substrates, have been determined. Questions pertaining to the process the penetration of plasma into a cylindrical cavity are considered. We discuss how the titanium-carbide coating is formed on the anodes of the oscillator tubes.