Self-propagating high-temperature synthesis of composite material TiB2-Fe

Results of an investigation of the microstructure and phase composition of materials of the Ti-B-Fe system, obtained by self-propagating high-temperature synthesis (SHS) are presented.Investigations were conducted for two types of powder mixtures; the first was a mixture of elemental powders Ti, B, Fe and the second was a mixture of ferroboron alloys, FeB_n, with titanium. The possibility of using commercial ferroalloys to obtain of boron-containing MMC_s is shown. The combination of SHS with force effect rooling leads to a dense product with high operational properties. More efficient fields of the application of SHS composite to produce TiB₂-Fe are shown.     

Self-propagating high-temperature synthesis of ultrafine and nanometer-sized TiC particles

The feasibility of preparing ultrafine and nanometer-sized titanium carbide particles by self-propagating high-temperature synthesis (SHS) has been studied. Data are presented on the structure formation of TiC powders during SHS with a reduction step. Basic to this process is an exothermic reaction between titanium dioxide, magnesium metal, and carbon. The effects of the composition of the starting mixture, relationship between its components, and the morphology and particle size of the starting TiO₂ powder on the particle size of the forming material have been investigated. The TiC powder was recovered from the sinter cake by chemical dispersion, a chemothermal treatment of the synthesis product in different solutions. The results demonstrate that treatment of the sinter cake with appropriate solutions removes impurities and causes imperfect intergranular layers to dissolve. As a result, the cake breaks down into homogeneous single-crystal particles. Subsequent treatment in different solutions further reduces the particle size of the powder. The effect of the composition of the dispersing solution on the particle size of the TiC powder has been studied. Our results made it possible to identify conditions for the preparation of titanium carbide powders containing up to 70% of particles less than 0.3 μm in size by SHS followed by chemical dispersion.     

Self-propagating high-temperature (combustion) synthesis (SHS) of powder-compacted materials

Self-propagating high-temperature synthesis (SHS) of powder compacts is a novel processing technique currently being developed as a route for the production of engineering ceramics and other advanced materials. The process, which is also referred to as combustion synthesis, provides energy- and cost-saving advantages over the more conventional processing routes for these materials. At the same time, the rapid heating and cooling rates provide a potential for the production of metastable materials with new and, perhaps, unique properties. This paper reviews the research that has been, and is being, undertaken in this exciting new processing route for high-technology materials and examines the underlying theoretical explanations which will, eventually, lead to improved control over processing parameters and product quality.     

Structural and mechanical properties of TiB2 and TiC prepared by self-propagating high-temperature synthesis/dynamic compaction

Titanium-diboride and titanium-carbide compacts with diameters of 100 mm and thicknesses of 25 mm were fabricated by self-propagating high-temperature synthesis/dynamic compaction (SHS/DC) of the elemental powders. Under the best conditions, the densities were greater than 99% and 96.8% of the theoretical densities for TiB₂ and TiC, respectively. The microhardness, compressive strength, and elastic modulus of the TiB₂ prepared by the SHS/DC method were comparable to reported values for hot-pressed TiB₂. While the microhardness and elastic modulus of the TiC compacts were comparable to those for hotpressed TiC, the compressive strength was lower due to extensive cracks in the compacts. The TiB₂ prepared using a low-purity boron powder (1–5% carbon impurity) compacted to higher densities and had less cracking than that prepared using a high-purity boron powder (0.2% carbon). This result could have an impact on the cost of producing TiB₂/TiC structural components by the SHS/DC process.     

The microstructure and mechanical properties of TiC and TiB2-reinforced cast metal matrix composites

TiC and TiB₂ particles have been spontaneously incorporated into commercial purity aluminum melts through the use of a K-Al-F-based liquid flux that removes the oxide layer from the surface of the melt. The combination of spontaneous particle entry and close crystal structure matching in the Al-TiB₂ and Al-TiC systems, results in low particle-solid interfacial energies and the generation of good spatial distributions of the reinforcing phase in the solidified composite castings. The reinforcement distribution is largely insensitive to the cooling rate of the melt and the majority of the particles are located within the grains. Modulus increases after TiC and TiB₂ particle additions are greater than those for Al₂O₃ and SiC. It is thought that interfacial bonding is enhanced in the TiC and TiB₂ systems due to wetting of the reinforcement by the liquid and particle engulfment into the solid phase. TiC-reinforced composites exhibit higher stiffnesses and ductilities than TiB₂-reinforced composites. This has been attributed to stronger interfacial bonding in the Al-TiC system, due to the increased tendency for nucleation of solid on the particle surfaces.     

The self-propagating high-temperature synthesis (SHS) of ultrafine high-purity TiC powder from TiO2+Mg+C

The self-propagating high-temperature synthesis (SHS) method has been applied to the preparation of TiC powder from mixtures of TiO₂, Mg, and C. The TiC/MgO product is leached by hydrochloric acid to remove the MgO. The optimum mixing ratio was TiO₂MgC=12.21.5. The final product TiC had > 99.9% purity and relatively uniform particle size of 0.3–0.4 m.     

On the nanocrystalline structure of TiC and TiB2 produced by laser ablation

Thin films of TiC and TiB₂ have been deposited on silicon substrate by pulsed laser deposition technique, under substrate temperatures 25–600 °C. Transmission electron microscopy revealed the nanocrystalline structure of the coatings. The grain size for the TiC film was between 10 and 70 nm and for the TiB₂ film was between 10 and 50 nm.     

SHS法制备TiC-Ni(Cr)金属陶瓷复合材料组织与性能

采用自蔓延燃烧合成技术在7A52铝合金表面制备了TiC-Ni(Cr)金属陶瓷复合涂层。利用扫描电镜(SEM)、能谱测试(EDS)、X射线衍射分析(XRD)、显微硬度测量、抗热震性检测等对涂层的组织与性能进行了研究。实验结果表明,涂层主要由陶瓷相TiC和金属相Ni组成,涂层内部各物相之间结合较紧密,涂层与基体结合处为高强度的冶金结合,其显微硬度约为基体的10倍,并通过抗热震性实验进一步验证了涂层与基体之间结合良好。实验证明在过渡层处加入与基体润湿性良好的Cr、Ni可较好地提高涂层与基体之间的结合强度。     

Self-propagating high-temperature synthesis (SHS) of a pyrochlore-based ceramic for immobilization of long-lived high-level waste

A Y₂Ti₂O₇-based ceramic of pyrochlore-type structure as a host material for immobilization of actinide-containing HLW was prepared by SHS compaction. Up to 25 at. % Zr was introduced into the pyrochlore structure instead of Ti to enhance the chemical and radiation resistance. The ceramic matrices exhibit low porosity, good water resistance, and high mechanical strength. The study was performed with simulated HLW.     

The influences of trace TiB and TiC on microstructure refinement and mechanical properties of in situ synthesized Ti matrix composite

Ti–5Al–5Mo–5V–1Fe–1Cr Ti alloy and Ti–5Al–5Mo–5V–1Fe–1Cr Ti matrix composites containing different weight fractions of trace TiB and TiC are fabricated via in situ synthesis method. The as-cast ingots are subjected to thermo-mechanical processing and heat treatment. The Widmannstatten structure is obtained after the heat treatment. The microstructure length scales of the materials are identified. The identification indicates that 0.4 wt.% TiB and 0.1 wt.% TiC can reduce the average size of the β grains by more than 50%. Whereas the extent of the microstructure refinement gradually decreases while increasing the weight fraction of the trace reinforcements. The influences of weight fraction and morphology of the trace TiB and TiC on microstructure refinement are researched in this work. Moreover, the tensile properties of the heat-treated materials are examined. It is revealed that Hall–Petch mechanism plays an identically important role in improving the mechanical properties of the composites comparing with the load bearing and dispersion strengthening of the trace reinforcements.     

The influences of trace TiB and TiC on microstructure refinement and mechanical properties of in situ synthesized Ti matrix composite

Ti–5Al–5Mo–5V–1Fe–1Cr Ti alloy and Ti–5Al–5Mo–5V–1Fe–1Cr Ti matrix composites containing different weight fractions of trace TiB and TiC are fabricated via in situ synthesis method. The as-cast ingots are subjected to thermo-mechanical processing and heat treatment. The Widmannstatten structure is obtained after the heat treatment. The microstructure length scales of the materials are identified. The identification indicates that 0.4 wt.% TiB and 0.1 wt.% TiC can reduce the average size of the β grains by more than 50%. Whereas the extent of the microstructure refinement gradually decreases while increasing the weight fraction of the trace reinforcements. The influences of weight fraction and morphology of the trace TiB and TiC on microstructure refinement are researched in this work. Moreover, the tensile properties of the heat-treated materials are examined. It is revealed that Hall–Petch mechanism plays an identically important role in improving the mechanical properties of the composites comparing with the load bearing and dispersion strengthening of the trace reinforcements.     

二次热压烧结对燃烧合成TiB2-Cu-Ni复合材料组织及性能的影响

采用自蔓延高温燃烧合成技术制备了相对密度为90%左右的TiB2-40Cu-8Ni金属-陶瓷复合材料.为了进一步提高复合材料的力学性能,分别在1 200,1 250,1 300℃温度下对复合材料进行二次热压烧结,详细研究了TiB2-40Cu-8Ni复合材料液-固两相区间的变形行为、组织特征及力学性能的变化.结果表明:经过二次热压后,材料的相对密度和弯曲强度有了较大幅度的提高,在1 300℃时,材料的相对密度提高了5%,弯曲强度达到608 MPa.复合材料中TiB2颗粒的形貌也发生了改变,大部分由原来的等轴状转变为长棒状,同时根据TiB2晶体结构特征,分析了复合材料中TiB2的晶体学上的生长机制.     

Hot deformation behavior of in situ synthesized Ti composite reinforced with 5 vol.% (TiB + TiC) particles

In this paper, 5 vol.% (TiB + TiC)/Ti-1100 composite was fabricated using in situ techniques. Hot-deformation behavior of the composite was studied by hot compression tests in the temperature range 1,000–1,150 °C under different strain rates. It was found that solid solute C element in Ti matrix has obvious effect on the hot-deformation behavior of the composite by increasing the (α + β)/β transus temperature. Variation in the volume ratio of α/β phase and the effect of reinforcements result in a change in hot-deformation behavior of the composite. The effect of reinforcements on hot deformation behavior of the composite is more obvious in the (α + β) phase field than in the β phase field.     

The formation of TiC/Al2O3 microstructures by a self-propagating high-temperature synthesis reaction

The effect of processing variables on reaction rate and ceramic microstructure are examined for the self-propagating high-temperature synthesis reaction 3TiO₂+4Al+3C 3TiC+2Al₂O₃. Reaction controlling methods used are reactant particle size, the use of diluents (to lower the combustion temperature) and the use of reactant preheating (to increase the combustion temperature). The ceramic microstructure has an unusual laminar structure which is generally only observed during unstable combustion wave propagation.     

Development and characterisation of SiAlON composites with TiB2, TiN, TiC and TiCN

30 vol% of TiB₂, TiCN, TiN or TiC was added to a sialon matrix with an X-phase sialon (Si₁₂Al₁₈O₃₉N₈) and an Al₂O₃–Si₃N₄ (77/23 wt%) starting powder composition and hot pressed at 1650°C in vacuum. The microstructures of the obtained composites were characterised by means of X-ray diffraction and electron microscopy, and the mechanical properties; E-modulus, hardness, bending strength and fracture toughness were measured and evaluated.Fully dense composites with an X-phase sialon or a polyphase Al₂O₃–-sialon–X-sialon matrix with 30 vol% of TiB₂, TiN and TiCN were obtained. TiC, added as a dispersed phase, however reacts with the nitrogen from the Si₃N₄ during liquid phase sintering, with the formation of TiC_1–x N _x , SiC and a changed sialon matrix composition. In the case of the X-phase sialon starting composition, a mullite matrix is obtained after sintering. The microstructural observations with respect to the sialon-TiC composites are found to be in agreement with the thermodynamic calculations.     

Self-propagating high-temperature synthesis of SrTiO3 and Sr x Ba y TiO3 (x+y=1)

Thermal initiation of mixed powders of SrO₂ and titanium and/or TiE (E = O₂, Cl₃) induces a self-propagating reaction. Trituration of the product with water yielded fine crystalline particles of cubic SrTiO₃. Stepwise replacement of SrO₂ with BaO₂ in the reaction mixture generates a solid solution Sr _x Ba _y TiO₃ (x+y=1). Products were characterized by X-ray powder diffraction, scanning electron microscopy, energy-dispersive analysis of X-rays and Fourier transform-infrared spectroscopy. Thermal studies were monitored by thermogravimetric analysis and differential scanning calorimetry measurements.