Effect of carbon sources on the combustion synthesis of TiC

The effect of carbon sources, i.e. graphite and amorphous carbon, on the reaction mechanism, product morphology, and the rate of combustion reaction between Ti and C to form TiC were studied. A reaction mechanism was proposed for each carbon source from the activation energy of combustion reaction. The microstructure and the composition of reaction products were also investigated. It was observed that graphite fissured in a layered form during the combustion reaction and the reaction between graphite and liquid titanium was accomplished mainly on the surface of the thinly fissured layer. Graphite was found to be more reactive with titanium and titanium carbide synthesized with graphite contains less amount of unreacted carbon and is more close to the stoichiometric TiC.     

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.     

A novel fabrication method for TiC–Al2O3–Fe functional material under centrifugal acceleration

Compacted powders of titanium (Ti) and carbon (C) in form of pellets were exposed to a massive amount of heat generated from the thermite reaction of Fe₂O₃ and Al in a graphite–steel tube mounted in a developed centrifugal accelerator machine. The centrifugal force facilitated the formation of multi-component products during the process. Titanium carbide (TiC) product is joined to an Al₂O₃–Fe layer, which are the products of the thermite reaction. The existence of centrifugal acceleration had a significant effect on both metallurgical alloying and mechanical interlocking between different layers of the sample to form a functional material. A mathematical model developed for this experiment to describe the speed rate of iron infiltration inside the TiC product as well as viscosity rate variation was presented. The composition, microstructure and mechanical properties confirmed the model.     

含Zr多组元掺杂石墨材料的性能研究

以天然石墨为原料，通过热压工艺，制备了含Zr多组元掺杂石墨材料。研究了掺杂元素对材料性能的影响。实验结果表明：随着Zr含量增加，基体石墨的强度、导电和导热性成线性增加；但是过量的ZrO2会消耗基体炭原子，生成金属Zr蒸汽逸出基体，形成孔隙和缺陷，导致材料的性能下降，因此应控制ZrO2的加入量。另外，采用SEM、XRD等分析手段研究了材料微观结构，探讨了微观结构对其性能的影响。     

Inductive hot-pressing of titanium and titanium alloy powders

Inductive hot-pressing is a field-assisted sintering process (FAST) in which an electrical current is used to enhance the densification of the powder. Inductive hot-pressing could be employed to enable titanium powder to reach a higher density in less time than the pressing and sintering process. In this study, titanium and titanium alloy powders with different features were processed by means of inductive hot-pressing. The influence of processing temperature on density, microstructure, quantity of interstitial elements and hardness was investigated. Generally, practically fully dense materials were obtained without any carbon pick-up, even if the materials were in contact with the graphite matrix during processing. Nevertheless, there was an increment of the nitrogen content and some oxygen pick-up, especially for the powders with smaller particle size. Hardness is not significantly affected by the pressing temperature, but it strongly depends on the amount of interstitials.     

Effects of calcium nitride and calcium carbonate gasifying agents on the combustion synthesis of Ni3Ti-TiC composites

The synthesis of nickel-titanium (NiTi) intermetallic composites is of considerable interest due to the ability to create a porous material with high strength and improved wear resistance. The effects of adding a carbon reactant to modify the reaction products and exothermicity have been studied using two reaction stoichiometries involving elemental nickel, titanium and carbon (graphite). The present study examines the synthesis of porous Ni₃Ti intermetallic composites in the presence of calcium nitride or calcium carbonate gasifying agent. Both gasifying agents show significant effects on the ignition characteristics of the reaction, burning velocities and TiC particle sizes present in the final product, but do not affect the combustion temperatures.     

Investigation of Brazing Structure of Bulk Graphite to a W-Re Substrate

The brazing structure of bulk graphite to a W-Re substrate has been investigated in the present paper. The brazing specimens used were a W-15Re substrate with a sintered density of 17.97g/cm³, and bulk graphite with a density of 1.86g/cm³. Titanium foil with a thickness of 0.05–1.5mm was used as the brazing agent. W-Re substrate and bulk graphite were brazed in a vacuum furnace. After brazing, the phase composition and structure of brazed layer were analyzed. The test results show that W-Re substrate and bulk graphite can be joined together when titanium was used as the brazing agent. Titanium and graphite react and form TiC, which has a higher melting point during brazing. The phase compositions of the brazed layer were a W-Re solid solution, TiC, and residual carbon. The metallographic structure of the brazed layer was made up of a W-Re solid solution and TiC.     

Direct combustion synthesis of SiC powders

In this study, we examine the possibility to direct combustion synthesize SiC powders from elements using four reactant combinations (Si and graphite; Si and carbon black; ground Si and carbon black; both Si and carbon black ground) under two reacting environments (heated by an oxy-acetylene torch in air, and heated by tungsten coils in vacuum). The experimental results showed that high-purity SiC product with no oxide present was obtained if pellet, made of Si and carbon black, was burned by an oxy-acetylene torch in air. Also, ground reactant particles yielding deteriorated effect on product conversion is also discussed.     

Investigation of joining Al–C–Ti cermets and Ti6Al4V by combustion synthesis

The paper has addressed a route for the welding of titanium alloy (Ti6Al4V) and Al–C–Ti powders by the combustion synthesis (CS) method. Al–C–Ti powders were compressed in the titanium alloy pipes with relative densities of 65%, and then the powder compact was sintered by two reaction mode at the same time as the annulus of titanium alloy and the synthesized product were joined. The paper has studied the effects of reaction mode and Al content in starting powders on the structure and property of the welded joints. And it has also discussed the microstructure of welded joints by laser-induced combustion synthesis (LCS). The mechanical properties of the welding seam have been also tested. The results show that LCS welding has realized fusion welding and the welding seam has good mechanical properties. Furthermore, SEM analysis has indicated that nano-size grains of TiC were formed in the joint layer.     

感应熔覆原位自生TiC/Ni基复合涂层的组织与耐磨性研究

为了提高16Mn钢的干滑动磨损耐磨性能,以Ni60、钛粉和石墨粉为原料对16Mn钢表面进行感应熔敷处理,制备出以TiC颗粒为增强相的原位自生复合涂层,利用金相、SEM、XRD等技术分析了涂层的显微组织,在室温干滑动磨损试验条件下测试了涂层的耐磨性.结果表明:涂层中TiC颗粒均匀分布于共晶基体上,整个涂层组织均匀、无气孔、无裂纹;涂层与基材形成了良好的冶金结合,涂层具有很高的硬度,在室温干滑动磨损试验条件下具有优异的耐磨性能.     

A study of combustion synthesis reaction in the Ti + C/Ti + Al system

Combustion synthesis (SHS) of the Ti + C/Ti + Al system was investigated by using titanium, graphite and aluminum powders as reactants. These powders were thoroughly mixed and pressed into cylindrical compacts, and heated in an argon atmosphere. The effects of the reactant composition and the heating rate were studied. The phase identification and morphology observation of the products were carried out by X-ray diffraction (XRD) and scanning electronic microscopy (SEM), respectively. XRD analysis showed that the products, in addition to the expected TiC/TiAl phases, also contained an appreciable amount of the ternary carbide (i.e., Ti _x AlC). The heating rate was found to strongly affect the extent of the combustion reaction. A possible reaction mechanism based on the experimental results was proposed to describe the whole process of the SHS reaction and the characteristic product morphology. It was considered that the ternary carbides may be formed by the peritectic reaction between TiC and the Ti–Al melt during the cooling stage after combustion.     

Densification process of TiCx–Ni composites formed by self-propagating high-temperature synthesis reaction

Densification behaviour of the TiC-Ni system formed by self-propagating high-temperature synthesis (SHS) were studied to develop the structural metal-matrix composites. The composites were prepared by two routes: (1) consolidation during the SHS reaction, and (2) consolidation process after the SHS reaction. The final phases of the stoichiometric reactant mixture of titanium and graphite with 50 wt% nickel produced by simultaneous combustion reactions, were titanium carbide in a nickel-rich solid solution containing carbon and titanium. The density of the products was relatively low, with a value of 90% theoretical density. In the second approach, liquid infiltration and liquid-phase sintering were applied for the titanium carbide-nickel mixture. Densification rates were reduced due to the excess carbon in the combustion products of titanium carbide. The densities of the liquid-phase sintered samples were more than 97% theoretical density. This revised version was published online in November 2006 with corrections to the Cover Date.     

Graphite and boron carbide composites made by hot-pressing

Composites consisting of graphite and boron carbide were made by hot-pressing mixed powders of coke carbon and boron carbide. The change of relative density, mechanical strength and electrical resistivity of the composites and the X-ray parameters of coke carbon were investigated with increase of boron carbide content and hot-pressing temperature. From these experiments, it was found that boron carbide powder has a remarkable effect on sintering and graphitization of coke carbon powder above the hot-pressing temperature of 2000° C. At 2200° C, electrical resistivity of the composite and d(002) spacing of coke carbon once showed minimum values at about 5 to 10 wt% boron carbide and then increased. The strength of the composite increased with increase of boron carbide content. It was considered that some boron from boron carbide began to diffuse substitutionally into the graphite structure above 2000° C and densification and graphitization were promoted with the diffusion of boron. Improvements could be made to the mechanical strength, density, oxidation resistance and manufacturing methods by comparing with the properties and processes of conventional graphites.     

掺钛石墨导电性及其微观结构的研究

研究了不同钛掺杂量的再结晶石墨的电阻率与微观结构。以探讨掺杂组元对材料导电性能的影响。实验结果表明,与相同工艺条件下制得的纯石墨相比,掺钛石墨具有高密、高石墨化程度以及极低的电阻率的特点,微观结构分析表明,钛对材料的石墨化过程中具有催化作用,掺杂石墨材料中钛掺杂量对材料的石墨化程度有很大影响：少量钛掺杂量即可使材料达到高的石墨化程度,过多钛掺杂量不利于其电阻率的降低,分析表明钛元素在材料中是以碳化钛形式存在,但在制备过程中,有少量钛逸失,雁而在材料表面形成孔隙,而这些对材料的性能有不利的影响。     

Characteristics of the combustion synthesis of TiC and Fe-TiC composites

The combustion synthesis of TiC, using the thermal explosion mode, was investigated by varying some of the process parameters including the reactant particle size, the pre-compaction pressure, and the heating rate. Based on these observations, a combustion model for the reaction was developed. When iron was added to titanium and carbon black powders, the ignition temperature was dictated by the eutectic temperature of the Fe-Ti system. Iron acted as a moderator for the reaction and led to a decrease in the combustion temperature.     

双组元掺杂钛硅再结晶石墨传导性能的研究

用煅烧石油焦作填料,煤沥青作粘结剂,钛粉和硅粉作添加剂,采用热压工艺制备了一系列双组元掺杂再结晶石墨.考察了不同质量配比的添加剂对再结晶石墨的热导率、电阻率和抗弯强度的影响以及微观结构的变化.实验结果表明,与相同工艺条件下制备的纯石墨材料相比较,掺杂15wt％钛粉再结晶石墨的传导以及力学性能有较大幅度的提高.在掺杂钛粉15wt％、硅粉<2wt％时,双组元再结晶石墨的常温热导率随着硅粉的掺杂量的增加有所提高.当掺杂钛粉及硅粉分别为15wt％和2wt％时,再结晶石墨RG-TiSi-152的常温热导率可达494W/m·K.但是当掺杂钛粉15wt％、硅粉>2wt％时,随着硅粉的继续增加,再结晶石墨的常温热导率反而降低.而双组元掺杂钛硅再结晶石墨的导电以及力学性能却随着硅粉的掺杂量的增加而降低.XRD分析表明,对于双组元掺杂钛硅再结晶石墨而言,钛元素最终在材料中以碳化钛形式存在,而硅元素则大都以气态形式被逸出,XRD物相图谱中未发现硅及其碳化物的存在.材料RG-TiSi-152的微晶尺寸La以及晶面层间距d₀₀₂分别为864和0.3355nm.     

Effect of aluminium addition on the combustion reaction of titanium and carbon to form TiC

Aluminium was incorporated into the reactant mixture with a molar ratio of Ti/C of 1.0 to study the effect of its addition on the combustion reaction between titanium and carbon to form TiC. Thermal analysis of the reactant mixture and component analysis of the reaction product suggest that the combustion reaction of the Ti-C-Al system proceeds in such a way that aluminium initially reacts with titanium to form titanium aluminide compounds of TiAl₃, Ti₂Al, and TiAl with heat evolution, and then the reaction between titanium and carbon and the decomposition of titanium aluminide to titanium and aluminium is subsequently followed. As the amount of aluminium incorporated was increased over the range of 0 to 40 wt%, the grain size of TiC decreased from approximately 15 m to 0.4 m. It was also observed that most of the aluminium in the TiC-Al composite was distributed on the surface of the spherical TiC grain.     

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.     

Ignition phenomena and reaction mechanisms of the self-propagating high-temperature synthesis reaction in the Ti+C system

The ignition phenomena and the reaction mechanism of the self-propagating high-temperature synthesis reaction of titanium and carbon powders were experimentally investigated. When using coarse graphite powders (<325 mesh) as the carbon source, the ignition temperature ranged from 1650–1720C and was independent of the C/Ti ratio. The ignition temperature could be significantly lowered by using finer graphite powders (e.g. 1400C for <1 m powder). When using carbon black as the carbon source, the ignition temperature ranged from 1050–1475C and was dependent on the C/Ti ratio. The ignition was confirmed in this study to be controlled by the rate of the surface reaction between titanium and carbon which, in turn, was determined by the contact surface area between them. The fractured surfaces of the products showed two different types of morphology, i.e. groups of grains similar to sintered bodies and agglomerated fine particles. The relative quantities of the two types of morphology depended on the type of carbon used, the C/Ti ratio, the particle size of graphite and the density of the reactant pellet. Possible reaction mechanisms have been proposed on the basis of the experimental observations of the ignition phenomena and the product morphology.     

Effect of form of carbon on the microstructure of in situ synthesized TiC-reinforced iron-based composite

TiC-reinforced iron-based composites were synthesized by aluminothermic reduction of siliceous sand (a waste product of aluminium extraction plant containing oxides of Fe, Ti, Si, etc.) in the presence of carbon. The carbon was used either in the form of only graphite, mostly cast iron or graphite plus cast iron. Results show that the microstructure as well as the volume fraction of TiC strongly depends upon the form of carbon.