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.     

Electron-beam Treatment of Tungsten-free TiC/NiCr Cermet Ⅱ: Structural Transformations in the Subsurface Layer

The character of structural changes in the surface layer of titanium carbide (TiC) with Ni-Cr alloy binder was investigated theoretically and experimentally after electron-beam treatment of the material surface. The thermal influence of the electron-beam irradiation on the surface layer microstructure of the composite fine-grained material was mathematically analyzed. Quantitative estimations of the depth of the zone in microstructural phase transformations were carried out. The microstructure and concentration profile of Ti distribution in the metallic binder over the cross section of the surface layer with microstructural phase transformations after electron-pulse treatment of the hard metal surface were experimentally investigated.     

Electron-beam Treatment of Tungsten-free TiC/NiCr Cermet Ⅱ： Structural Transformations in the Subsurface Layer

The character of structural changes in the surface layer of titanium carbide （TiC） with Ni-Cr alloy binder was investigated theoretically and experimentally after electron-beam treatment of the material surface. The thermal influence of the electron-beam irradiation on the surface layer microstructure of the composite fine-grained material was mathematically analyzed. Quantitative estimations of the depth of the zone in microstructural phase transformations were carried out. The microstructure and concentration profile of Ti distribution in the metallic binder over the cross section of the surface layer with microstructural phase transformations after electron-pulse treatment of the hard metal surface were experimentally investigated.     

Combustion synthesis and densification of large-scale TiC–xNi cermets

Large-scale TiC–xNi cermets with 240-mm diameter were fabricated by self-propagating high-temperature synthesis and combined with pseudo heat isostatic pressing. Combustion-synthesized products consisted of TiC phase and Ni binder phase. Spheroidal TiC particles were enveloped by nearly continuous Ni binder phases. Size of TiC particles decrease with Ni content increase. Synthesized products have excellent mechanical properties and the bending strength of TiC–20Ni and TiC–30Ni is close to K151A and K152B, respectively, produced by traditional powder metallurgy technology.     

添加TiC和Ti3AlC2对燃烧合成Ti3AlC2粉体的影响

Ti、Al、C和TiC组成的Ti:Al:C=3:1.1:1.8（摩尔比）体系的燃烧合成实验结果表明，当在体系中未加入TiC时，得到的主要是TiC，但加入TiC后燃烧合成产物主要是Ti3AlC2，且Ti3AlC2量随TiC加入量的增加而增加，随燃烧反应体系温度的降低而增加；加入晶种Ti3AlC2有利于合成Ti3AlC2相物质。     

Fabrication, Structure, and Properties of TiB2–AlN Ceramics

Conductive TiB₂–AlN ceramics were prepared by self-propagating high-temperature synthesis in a filtration combustion regime at high nitrogen pressures. The effect of nitrogen pressure on the combustion rate and the Al metal content of the reaction product was studied. The combustion rate was also shown to depend on the composition of the starting mixture. The phase composition of the ceramics was determined by x-ray diffraction, chemical analysis, and microstructural examination. The porosity, electrical resistivity, and bending strength of the ceramics were measured. The resistivity of the materials was found to change sharply at a TiB₂ content of 19 vol %. This effect was interpreted as due to the formation of a fractal structure.     

High-temperature mechanical properties of WC-Co hard metals (Review)

Experimental results are discussed of the laboratory investigations into high-temperature mechanical properties of WC-Co hard metals as functions of the microstructure parameters (cobalt content, average sizes of carbide grains and cobalt interlayers), binder composition (carbon and cubic carbide contents) and the thermal and force action found in the literature. The effect of high temperatures on deformation characteristics in bending, tension and compression has been analyzed. The problems of short-and long-time strengths (high-temperature strength) and thermomechanical fatigue are discussed. The revealed mechanisms of the high-temperature deformation are considered.     

Effect of titanium on the rheological properties of MoSi<Subscript>2</Subscript>-based materials prepared by SHS

We have studied the moldability of molybdenum disilicide-based materials using free self-propagating high-temperature synthesis (SHS) compression, a method that combines combustion and high-temperature deformation processes. As a moldability criterion, we used strain. The strain of the synthesized materials has been determined experimentally as a function of the time delay before high-temperature deformation, compaction pressure, heating temperature, and metallic binder content of the starting mixture. To improve the plasticity of the synthesized material, 1–2 wt % titanium was added to the starting mixture as a metallic binder.     

A transmission electron microscopy study of WC-10Co cemented carbides with modified hard and binder phases

The alloy design of WC-10Co cemented carbide, modified with addition of a hard carbide phase, TiC, and with Ni and Mo in the binder phase, has been highlighted by the authors in a number of publications. The present article deals with the fine microstructural features of various phases in such cemented carbides. WC grains in all the investigated cemented carbide compositions appear to develop straight facets during sintering because of their anisotropic nature. In contrast, the TiC phase is characterized by its rounded shape. Dislocations are present in both WC and TiC grains, being of lesser density in the latter. The binder phase is always associated with stacking faults. The nature of the hard phase/binder interfaces has been found to be dependent on the binder phase chemistry. The observed changes in microstructures and mechanical properties have been correlated with the wettability and solubility of the hard phases in the binder melt, and with the different strengthening mechanisms in the binder phase.     

Synthesis of the Ti<Subscript>2</Subscript>AlC MAX Phase with a Reduction Step via Combustion of a TiO<Subscript>2</Subscript> + Mg + Al + C Mixture

Technologically viable principles have been developed for the preparation of the MAX phase Ti₂AlC by self-propagating high-temperature synthesis (SHS) with a reduction step, using titanium dioxide. We have studied the influence of synthesis conditions (starting-mixture composition and ratio of reactants) on the composition, structure, and particle size of Ti₂AlC powders. The results demonstrate that an excess of magnesium in the starting mixture leads to a decrease in the percentage of MgAl₂O₄ (spinel), and carbon deficiency in the starting mixture reduces the percentage of titanium carbide in the final product. The Ti₂AlC powders prepared by SHS consist of agglomerates of layered particles differing in size: from coarse (several microns) to ultrafine and nanometer-sized particles. The composition of the powders was confirmed by chemical analysis, microstructural examination, and X-ray diffraction.     

Microstructures and Mechanical Properties of TiC_P/Al-4.5Cu-0.8Mg Composites by Direct Reaction Synthesis

Direct reaction synthesis (DRS), based on the principle of self-propagating high-temperature synthesis (SHS), is anew method for preparing particulate metal matrix composites. TiCp/Al-4.5Cu-0.8Mg composites were fabricatedby DRS. Particulate composites were fabricated with Ti carbide (TiC) particles, generally less than 1.0 μm. Thereacted, thermal extruded samples exhibit a homogeneous distribution of fine TiC particles in Al-4.5Cu-0.8Mg matrix.Mechanical property evaluation of the composites has revealed a very high tensile strength relative to the matrixalloy.Fractographic analysis indicates ductile failure.     

Al2O3—TiC—Co与Al2O3—TiC陶瓷冲蚀磨损行为的比较研究

通过特殊的化学处理方法，完成了对Ａｌ２Ｏ３及ＴｉＣ陶瓷粉末的钴包覆，将包覆后的两种粉安７０ｗｔ％Ａｌ２Ｏ３－Ｃｏ和３０ｗｔ％ＴｉＣ－Ｃｏ的比例混合烧结出硬度和韧性都较理想的Ａｌ２Ｏ３－ＴｉＣ－Ｃｏ（ＡＴＣ）精细陶瓷，通过ＳＥＭ观察研究其冲饥蚀磨损行为，并对ＡＴ３０（７０ｗｔ％Ａｌ２Ｏ３－３０ｗｔ％ＴｉＣ）和ＡＴＣ陶瓷的冲蚀行为机制进行了比较研究，与ＡＴ３０陶瓷相比，ＡＴＣ陶瓷良好的综合力学性能和细     

大尺寸硬质合金部件的燃烧合成研究

采用自蔓延高温燃烧合成结合准热等静压技术（SHS／PHIP）,成功地得到了致密性良好,直径为240mm的大尺寸TiC-Ni系硬质合金部件,燃烧合成产物由TiC和Ni两相组成,Ni粘结相基本上呈网状连续分布于球形的TiC颗粒周围,随着Ｎi含量的增加,ＴiC颗粒尺寸减小,燃烧合成产物具有良好的力学性能。     

Preparation of weakly agglomerated yttrium aluminum garnet powders by burning a mixture of yttrium aluminum hydroxynitrates, urea, and acetic acid

We have optimized the composition of a mixture of aluminum yttrium hydroxynitrates, urea, and acetic acid for the self-propagating high-temperature synthesis of yttrium aluminum garnet (YAG) powder. The powder prepared in this way offers a low degree of agglomeration, and the ceramic produced from it has a low carbon content. A technique has been proposed for carbon determination in YAG ceramics through gas chromatographic analysis of the gaseous products of carbide hydrolysis after the dissolution of the ceramic in pyrophosphoric acid.     

Microstructures and Mechanical Properties of TiCp／A1-4.5Cu-0.8Mg Composites by Direct Reaction Synthesis

Direct reaction synthesis (DRS), based on the principle of self-propagating high-temperature synthesis (SHS), is a new method for preparing particulate metal matrix composites. TiCp/Al-4.5Cu-0.8Mg composites were fabricated by DRS. Particulate composites were fabricated with Ti carbide (TiC) particles, generally less than 1.0μm. The reacted, thermal extruded samples exhibit a homogeneous distribution of fine TiC particles in Al-4.5Cu-0.8Mg matrix.Mechanical property evaluation of the composites has revealed a very high tensile strength relative to the matrixalloy. Fractographic analysis indicates ductile failure.     

Synthesis and characterization of polycrystalline sintered compacts of cubic boron nitride

Synthetic cubic boron nitride (CBN) compacts are an important tool material used extensively for the machining of hardened steels. The paper describes work on the synthesis and characterization of CBN compacts. A 200 tonne cubic press has been used for the generation of high pressures. Solid solutions of TiN and TiC have been used as the binder material. The CBN powder and the binder are homogenously mixed and the mixed powder is pressed in a steel die under a pressure of about 3 kbars (300 M Pa). The pellets so formed are the starting material for synthesizing the compacts. Compacts both with and without a tungsten carbide substrate have been synthesized. The best compacts are formed at 58kbar (5.8 G Pa) and 1450° C. The sintered compacts after grinding and polishing are characterized by using powder X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray analysis. Knoop hardness measurements made on the best CBN compacts give a hardness of 3400HK _0.5.     

Assessment of Microwave Heating for Sintering of Al/SiC and for in‐situ Synthesis of TiC

This work describes sintering of SiC‐reinforced Al‐matrix composites and in‐situ synthesis of TiC in a powder mixture of Ti and C. In the first case, microwave energy is absorbed by SiC grains, heating the metal matrix to sintering and even melting temperature. The composite is processed at <1 kW microwave power. Microwave absorption and the heating rate increase with decreasing SiC particle size. Composites with high SiC content (70 vol.‐%) are processed at 650 °C/1 h in the microwave furnace, whereas conventional resistive heating at the same temperature did not allow sintering of the sample. In the second case, radiative energy allowed the heating of Ti/C samples up to 950 °C, and microwave assistance enhanced the reaction sintering of Ti/C powder mixtures forming TiC at the border of the Ti particles. The results are compared with conventional processing. Optical images and XRD patterns confirmed the formation of TiC for both techniques.     

Mathematic modeling of diffusion processes of the formation of rim-core structure at sintering TiC–Ni alloys with alloying carbide additions

Using mathematical modeling the kinetics of the rim-core structure formation of TiC grains in an isothermal liquid-phase sintering of TiC–Ni hard alloy, doped with a transition metal carbide has been investigated. The algorithm that has been used to study the nature of the concentration distribution of the diffusant in a TiC grain allows for the structural characteristics of the alloy; diffusion coefficients of alloying additives in the metal melt and a grain, boundary solubility of the diffusant in the binder, and its concentration in the alloy as well as the sintering temperature and time.     

Combustion synthesis and subsequent sintering of titanium-matrix composites

Titanium-matrix composites containing up to 50 wt% TiC particles were prepared by combustion synthesis using elemental powders. The products were subsequently sintered at 1160°C for various periods of time. SEM and X-ray diffraction were used to assess the changes that took place during the two stages of processing. Products of combustion synthesis containing in excess of 25%TiC contained large cracks as well as agglomerates of carbide particles that were undesirable from the point of view of reinforcing the metal. The carbide obtained by combustion synthesis had a higher carbon content than that expected according to the Ti-C equilibrium phase diagram, due to the non-equilibrium nature of the reaction. During the sintering stage, the carbide of non-equilibrium composition reacted with titanium to yield the carbide of equilibrium composition. The composition changes were investigated and their significance on the sintering process is discussed.     

Sintered porous cermets based on TiB2 and TiB2–TiC–Mo2C

TiB₂ powder, with different binders (Ni and Ni/Mn), after milling were cold compacted (300 MPa) and sintered in H₂ at 1300 and 1350°C for 1 h. To improve the sintering behaviour, TiC/Mo₂C alloy carbide was added and the milled charge along with the same binders (Ni and Ni/Mn) was cold compacted and sintered under similar conditions. Sintered density, porosity, transverse rupture strength (TRS), grain size and lattice parameter of binder and hard phases were measured. Better densification was observed with Ni/Mn binder as compared to Ni binder for either hard phase based systems. Maximum value of TRS was noted for TiB₂–TiC–Mo₂C–40 wt.% Ni/Mn cermet. Melt exudation was observed for either hard phase based systems with Ni binder.