TiH2粉末的高能行星球磨及超细晶钛烧结

研究TiH 2粉末的高能行星球磨规律,然后使用制备出的球磨粉末开展压制?真空烧结,评价烧结样品的显微组织特征。结果表明,TiH 2粉末的高能球磨可划分为3个基本阶段：球磨初期粉末粒度迅速细化;球磨中期粉末粒度逐步趋向最小极限尺寸,同时粉末粒径的均匀性和分布集中程度明显改善;在球磨的后期,粉末粒径又会发生粗化现象。因此,TiH2粉末的高能行星球磨存在一组最佳的工艺参数。高能球磨会改变TiH2粉末的脱氢特征温度,且粉体的D 50越小,特征温度下降幅度越大,与原料粉末相比,最大差值达83℃。对烧结样品的显微组织分析显示,通过由球磨TiH 2粉末所获得的烧结钛合金的晶粒度得到了显著地细化;当采用优化的工艺开展球磨TiH 2粉末制备时,通过压制和真空烧结可获得平均晶粒度在5μm以下的超细晶钛。     

Laser alloying of metal surfaces by injecting titanium carbide powders

The process of injecting wwar resisting particles into a laser melted surface has proven to be quite versatile, being applicable to a wide range of material combinations. The present study examines the microalloying of the metal surfaces using a pulsed Nd YAG laser. Alloying was achieved by injecting fine particles of titanium carbide into the shallow laser melted metal surface. A vacuum cell was used to prevent the effect of oxygen during the alloying process. Metals which were alloyed include En58 steel, titanium, nickel, aluminium and tantalum. A carbide volume fraction of 8–16% was achieved for the metals employed in the experiment.     

Synthesis of titanium carbide nano-powders by thermal plasma

The paper presents a thermodynamic analysis for predicting the conditions for the plasma synthesis of TiC powders. The paper also investigates the effects of feeding rate and molar ratio. The experimental results show that TiC powders are synthesized by thermal plasma and the average size of the TiC powders is less than 100 nm.     

Synthesis of nano-structured titanium carbide by Mg-thermal reduction

Nano-structured titanium carbides were synthesized by liquid-magnesium reduction of vaporized TiCl₄ + CCl₄ solution. Fine TiC particles were produced by the reaction of released Ti and C atoms, and vacuum was used to remove the residual phases of MgCl₂ and excess Mg. Characterization of products was performed with various reaction parameters.     

Synthesis of high purity titanium silicon carbide from elemental powders using arc melting method

The objective of this study is to investigate the formation of Ti₃SiC₂ from Ti/Si/C powders using the arc melting method. The results show that the sample sintered at 80 s produced a near single-phase of Ti₃SiC₂ (99.2 wt.%) with a relative density of 88.9%. These results were confirmed by phase determination using XRD analysis and were supported with micrographs from FESEM/EDX analyses. The relative density and porosity of all samples were dependent on the formation of macropores in bulk samples and micropores in TiC_x grains. The proposed reaction mechanisms for the synthesis of Ti₃SiC₂ by arc melting is that Ti₃SiC₂ might be formed from TiC_x + Si, Ti₅Si₃C_x + C, and Ti₅Si₃C_x + TiC_x at early arcing time (≤ 10 s), while TiC_x + TiSi₂ take place at 15 s to 80 s. After 80 s, decomposition of Ti₃SiC₂ into TiC_x, TiSi₂ and C was observed.     

Synthesis of titanium carbide by induction plasma reactive spray

A novel method capable of sufficient mixing of titanium powder and methane of carbon source was developed in the synthesis of titanium carbide by induction plasma reactive spray. X-ray diffraction analysis, optical microscopy, scanning electron microscopy, and microhardness test were used to characterize the spray-formed deposit.The experimental results show that both primary carburization of the titanium particles inside the plasma flame and secondary carburization of the growing deposit on high temperature substrate contribute to the forming of titanium carbide. The transitional phase of TiC1-x has the same crystal structure as TiC, but has a slightly low lattice constant. The deposit consists of fine grain structure and large grain structure. The fine grain structure, harder than large grain structure, shows grain boundary fracture.     

Synthesis of high-purity ultrafine tungsten and tungsten carbide powders

High-purity ultrafine W or WC powder was prepared via a two-step process composed of the carbothermic pre-reduction of WO_2.9 and the following deep reduction with H₂ or carbonization with CH₄+H₂ mixed gases. The effects of C/WO_2.9 molar ratio and temperature on phase composition, morphology, particle size, and impurity content of products were investigated. The results revealed that when the C/WO_2.9 ratio was in the range from 2.1:1 to 2.5:1, the carbothermic pre-reduction products consisted of W and a small amount of WO₂. With changing C/WO_2.9 ratio from 2.1:1 to 2.5:1, the particle sizes were gradually decreased. In order to prepare ultrafine W or WC powder, a relatively high C/WO_2.9 ratio and a lower reaction temperature at this stage were preferred. After the second reaction, the final products of ultrafine W and WC powders with a high purity could be obtained, respectively.     

Synthesis of boron carbide powders from mono- and polysaccharides

Boron carbide powders are usually synthesized by carbothermal or magnesiothermic reduction of boron oxy-compounds, a carbon source is usually graphite or some organic phases. The aim of the work is using of mono- and polysaccharides for synthesis of boron carbide powders in which the final grains are separated from each other by an unreacted excess of carbon preventing both grain growth and formation of strong aggregates. Aqueous solutions of glucose, fructose, dextrin or hydroxyethyl starch and boric acid were dried, pyrolyzed at 850 °C for 1 h and then heat-treated at temperatures from 1300 to 1700 °C for 1 h. The amount of boron carbide in the powders depends both on the saccharide and on the temperature. The grains prepared from precursors containing monosaccharides had oval shapes whereas polysaccharides were applied they had rhombohedral shapes corresponding to the boron carbide structure. This effect can be attributed to presence of molten boron oxide.     

Preparation of titanium carbide powders by carbothermal reduction of titania/charcoal at vacuum condition

Preparation of fine TiC powders by carbothermal reduction of titania/charcoal at vacuum condition was investigated by XRD, SEM, element analysis instrument and Laser Particle Sizer. Experimental results indicate that the formation sequence of products should be Magneli phase (Ti₄O₇), Ti₃O₅, Ti₂O₃, TiC_xO_1 − x and TiC with increasing reaction temperature. The crystal grain grows up and agglomerates gradually in the initial reaction process. Then it diminishes with the liberation of much gas CO. At last, it grows up slightly with the formation of plentiful TiC at higher temperature. Fine TiC powders (D50, 2.05 μm) with low impurities were obtained at 1450 °C for 8 h when the system pressure was about 1–60 Pa.     

Synthesis of refractory metal carbide powders via microwave carbothermal reduction

The synthesis of two refractory metal carbides, titanium carbide (TiC) and tantalum carbide (TaC), has been examined via the carbothermal reduction of the oxides using microwaves as an alternative energy source. Problems with residual oxygen and exaggerated particle growth were encountered with the titanium-based compound which prevented full conversion to the non-oxide form. This was ascribed to the isomorphism of a suboxide with the carbide crystal structure. More success was observed in the case of the tantalum carbide where complete conversion was achieved, as measured by both chemical analysis and X-ray diffraction. Much faster reaction rates were also observed with higher yields being obtained at lower temperatures and in shorter times than can be achieved using conventional heating.     

Optimizing the synthesis of ultrafine tungsten carbide powders by effective combinations of carbon sources and atmospheres

Nanostructured WC powders can provide technologically attractive properties due to the fine microstructures obtained after sintering. Either W or WO₃ powders are used for the industrial production of WC. In both cases, the contact area between carbon and tungsten precursors has a critical influence on the reaction temperatures, which in turn affects grain growth and agglomeration of particles. Different methods have been studied to increase the reaction rates by enhancing the contact between reactants: carbon coating of tungsten powder, solid-gas reactions of tungsten powders with atmospheres containing CH₄, or mechanical activation followed by thermal activation of tungsten and carbon precursors.In this work WC-powders were obtained by mechanical activation of tungsten and carbon precursors followed by thermal activation of these mixes at temperatures up to 1100 °C. A systematic study has been carried out combining two dissimilar carbon sources (graphite and carbon black), with different atmosphere compositions (Ar, Ar-50H₂, Ar-10CO) and studying the evolution of phases at different stages of the synthesis. The results show how the efficiency of the interaction between carbon sources and atmospheres affects the completion of the synthesis. The synthesis of WC from WO₃ in H₂ containing atmospheres is enhanced when using carbon black sources, however in CO containing atmospheres the most effective interaction is with graphite.     

Synthesis of titanium carbide/chromium carbide multilayers by the co-evaporation of multiple ingots by electron beam physical vapor deposition

Titanium carbide and chromium carbide multilayer coatings with varying individual layer thicknesses were synthesized by the co-evaporation of titanium, chromium, and carbon (through tungsten) ingots by electron beam-physical vapor deposition. The adhesion of the multilayer coatings was found to be greater than 50 N. The hardness of the titanium carbide/chromium carbide multilayer coatings was found to increase from 1302 VHN_0.050 to 2052 VHN_0.050 by decreasing the thickness of the individual layer from 1.2 to 0.1 μm. In addition, the average grain diameter was also found to decrease from 3.315 to 0.356 μm by decreasing the thickness of the individual layers. The fracture toughness of the TiC/CrC multilayer coatings decreased from 4.179 to 1.411 MPa-m with decreasing layer thickness. Lastly, the amount of compressive stress in both the TiC and CrC layers within the multilayer coating was found to decrease with decreasing individual layer thickness. The samples were characterized by various techniques including Vicker's hardness, X-ray diffraction, scanning electron microscopy, scratch testing and fracture toughness, with the results being presented.     

Interdiffusion of hafnium carbide and titanium carbide during hot-pressing

The interdiffusion of HfC and TiC during hot pressing was investigated using concentration profiles obtained via EDS in a SEM. The results were interpreted using the Boltzmann–Matano method and the integrals and derivatives were determined via numerical means. The microstructural evolution of two materials with different HfC/TiC is then explained in terms of the rate of material movement.     

Titanium with controllable pore fractions by thermoreversible gelcasting of TiH2

Thermoreversible gelcasting, a near-net-shape processing technique, is demonstrated here for titanium. The gelcasting system is composed of TiH₂ particles suspended in a triblock copolymer gel that behaves as a viscous liquid above 56 °C and an elastic solid at room temperature, a temperature-dependent transition that is fully reversible when solvent is present. Organic pyrolysis to remove the gel followed by vacuum sintering to densify the Ti powders (produced by decomposition of the hydride) results in titanium with near full density and low contamination. Incorporation of polypropylene and poly(methyl methacrylate) space-holder particles into the gel results in titanium with controlled porosities up to 44 vol.% and with low contamination. These foams exhibit tailorable stiffness and strength, together with excellent compressive ductility and energy absorption.     

Synthesis mechanism of titanium carbide nano-fiber during self-propagating high temperature synthesis

Titanium carbide nano-fiber was synthesized by self-propagating high temperature synthesis (SHS) method. The final products after the SHS reaction were titanium carbide containing excess carbon and metallic titanium, which were removed by additional leaching process. TEM observation revealed that the average diameter is about 20 nm. Neutron diffraction analysis was carried out to study non-stoichiometric number of the titanium carbide. The non-stoichiometric numbers of the titanium carbide were 0.89–0.94. The Rietveld refinement of each patterns converged to good agreement (÷2=0.49–1.34). The formation mechanism of the carbide is related to a liquid-solid reaction including the preferential diffusion process of carbon atom into liquid titanium. This article is based on a presentation made in the 2002 Korea-US symposium on the “Phase Transformations of Nano-Materials,” organized as a special program of the 2002 Annual Meeting of the Korean Institute of Metals and Materials, held at Yonsei University, Seoul, Korea on October 25–26, 2002.     

Size-controlled synthesis of nano Mo powders via reduction of commercial MoO3 with carbon black and hydrogen

In the present work, a novel method was proposed to synthesize pure nano Mo powder via temperature programmed pre-reduction of commercial MoO₃ by carbon black, followed by deep deoxidation with hydrogen. It was found that the carbothermal reduction of MoO₃ included three stages: MoO₃ → MoO₂ → MoO₂ + Mo₂C → Mo. It can be concluded that the morphology and size of the obtained Mo particles are mainly determined at the carbothermic reduction stage. The dispersed nucleation and controlled growth are critical issues for controlling the particle sizes of Mo, which could be achieved at the carbothermic reduction stage by using the raw materials of the MoO₃ and carbon black. Whereas, when the MoO₃ was directly reduced by hydrogen or active carbon with a larger particle size, or commercial MoO₂ were reduced with the carbon black, the dispersed nucleation and controlled growth of products can't be achieved. Meanwhile, to reduce the residual carbon content in the final Mo product, in the carbothermic pre-reduction process, the small amount of MoO₂ was kept in the pre-reduced Mo powders, which was removed by hydrogen at the deep deoxidation stage. The content of residual carbon content in the final produced Mo nanopowders can be reduced to about 0.02%. This method has the advantages of simplicity, low cost, and high efficiency. Therefore, it has great potential for the industrial preparation of nano Mo powders in a large-scale.     

Synthesis of nanocrystalline titanium carbonitride during milling of titanium and carbon in nitrogen atmosphere

Mixtures of elemental titanium and carbon with Ti/C atomic ratio of 1:0, 1:0.125, 1:0.25, 1:0.5 and 1:0.75 were milled in nitrogen atmosphere using a high-energy ball mill, Spex8000. Nanocrystalline titanium carbonitride powders have been synthesized when milling the mixtures with Ti/C atomic ratio not less than 0.25. The formation of the titanium carbonitride phase has been found to undergo an abrupt reaction. It is suggested that this reaction appears to be a self-propagating high-temperature synthesis (SHS) process. During the incubation duration of the SHS reaction, small amount of titanium carbonitride phase has been detected by X-ray diffraction (XRD). The composition and lattice parameter of the synthesized titanium carbonitride phase can be adjusted by changing the composition of the starting mixture of elemental titanium and carbon.     

Zirconium boride and tantalum carbide coatings sprayed by electrothermal explosion of powders

Refractory zirconium diboride and tantalum monocarbide ceramic powders were sprayed using an electrothermal explosion caused by a high-voltage electric breakdown and large-current discharge heating. This spray technique was improved using a purpose-designed powder container, which made it possible to melt the powder completely and accelerate it to impinge on substrates. The electrical energy applied to the powder was estimated to be about twice the energy theoretically needed to melt just the powder. Although the ceramics used in this work are hard-sintered materials by nature, they could be sprayed and deposited to form coatings on metal substrates without additives and sintering agents. The coatings formed exhibited no chemical decomposition in the boride, and only small amounts of decarburization in the carbide due to its nonstoichiometry. The tantalum carbide coating mixed with iron and aluminum substrates in the range of 10 μm to several tens of micrometers.