REACTIVE DIFFUSION BETWEEN Si AND NbC AT 1300℃

ＲＥＡＣＴＩＶＥＤＩＦＦＵＳＩＯＮＢＥＴＷＥＥＮＳｉＡＮＤＮｂＣＡＴ１３００℃Ｃ．Ｒ．Ｋａｏ（ＤｅｐａｒｔｍｅｎｔｏｆＣｈｅｍｉｃａｌＥｎｇｉｎｅｅｒｉｎｇＮａｔｉｏｎａｌＣｅｎｔｒａｌＵｎｉｖｅｒｓｉｔｙＣｈｕｎｇＬｉ，Ｔａｉｗａｎ）Ｊ．Ｗｏｏｄｆ...     

难熔金属碳化物Cr_3C_2、VC、WC、TaC、NbC及Ti(C、N)粉中硫的测定

研究了燃烧──—电导法测定金属陶瓷添加剂Cr3C2；VC；WC；TaC；NbC及Ti（C、N）粉中硫的测定条件，建立了分析方法．方法相对标准偏差5％左右，仪器的灵敏度为3X10－6，最低检测限为1ppm．     

TEM Investigations of Fine Niobium Precipitates in HSLA Steel

Commercially produced 0.03 % C, 0.08 % Nb, 0.01 % Ti high strength low alloyed (HSLA) steel in the form of 20 mm thick plates was investigated. The steel was thermomechanically processed and the mechanical properties of the steel were evaluated by tensile testing. Using analytical and high resolution transmission electron microscopy the distribution, morphology, composition, crystal structure and particle size of niobium and titanium carbonitrides were observed and identified in these steels. The distribution of the precipitates was found to be nearly random, with occasional occurrence of precipitation free zones. Complex agglomerates with a cubic TiN seed crystal overgrown by a cubic NbC particle were the most commonly observed precipitates. Further TEM analysis in the accelerated cooled and tempered specimens in 1/4 plate thickness did not reveal any evidence that additional precipitation in the ferrite occurred. Precipitation in ferrite was only detected after subsequent cold deformation and tempering of the same samples. By a combination of EFTEM, STEM, HRTEM in addition to EDX spectroscopy, a large population of strain induced NbC precipitates with fcc crystal structure ranging in size down to 2 nm were identified in the ferrite matrix.     

Effect of Niobium on Microstructure of Cast AISI H13 Hot Work Tool Steel

The effect of niobium addition on the microstructure of cast AISI H13 hot work tool steel was evaluated by using EDX analyzer attached to the scanning electron microscope. The volume percent of eutectic area and eutectic cell size and also volume percent of different carbides of new steel, which is heat treated under different conditions, are also determined. The results show that the a niobium addition modifies the cast structure of Nb-alloyed hot work tool steel, and reduces the size and volume of eutectic cells, and increases the maximum hardness of the steel.     

Preparation and characterization of biocompatible Nb-C coatings

Nb-C composite films, obtained by DC magnetron sputtering method, were investigated as possible candidates for the protective layers used in medical implants. Coatings of different carbon/niobium ratios were prepared and analyzed for elemental and phase composition, crystallographic structure, texture, corrosion behavior, and cell viability. The coating with the highest C/Nb ratio (≈ 1.9) was found to have a nanocomposite structure, in which NbC nanocrystalline phase coexists with an amorphous a-C one. The coated samples exhibited an improved corrosion resistance as compared with the Ti alloy. Cell viability measurements proved that human osteosarcoma cells are adherent to the coating surfaces, the highest viability being found for the film with the highest carbon content.     

Microstructural characteristics of PTA-overlayed NbC on pure Ti

This study uses the technique of PTA (plasma transferred arc) to overlay NbC reinforcing particles on the surface of commercially pure Ti in order to investigate the microstructural features of the overlayer and the interface between the overlayer and base metal by changing the overlaying current. The results indicate that the matrix phase of the overlayer was -Ti containing about 10 at.% Nb and 1 at.% C. NbC and precipitated TiC produced by dissolved NbC reacted with Ti dispersed in the matrix. The microstructure of the cross-section of the overlayer (from surface to base metal), which was composed of -Ti, can be separated into three layers: an upper overlayer with TiC, a middle overlayer with TiC and NbC, and a lower overlayer (interfacial layer and heat affected zone, HAZ). Due to solidification beginning at the interface and the effect of dilution, the TiC in the interfacial layer was finer. Owing to faster solidification under low-current conditions, the TiC particles were finer than under high-current conditions. Also, dendritic TiC under a low-current in the upper overlayer was also finer than under a high-current. Meanwhile, TiC precipitate that resulted from heterogeneous nucleation and Gibbs' free energy was also found around NbC. This NbC diffusion layer between TiC and NbC may have been βNb₂C phase.     

原位生成NbC颗粒增强镍基激光熔覆层

激光熔覆技术是金属材料表面强化和改性的有效方法之一。利用该技术,在A3钢表面激光熔覆预置涂层,成功制备出了原位生成NbC颗粒增强的镍基复合涂层,并进行了硬度、摩擦性能测试,X射线衍射(XRD)和显微组织分析。扫描电镜(SEM)、能谱分析(EDS)和X射线衍射分析结果表明,原位生成NbC颗粒增强的镍基复合涂层与基材呈现良好的冶金结合,熔覆层的组织为先共晶析出的树枝晶(Cr,Fe碳化物相)和原位生成的NbC颗粒相均匀分布在γ(Ni Fe)基体中。硬度测试和摩擦磨损实验表明,激光熔覆原位生成NbC颗粒增强镍基复合涂层平均硬度高达HV0.31200,耐磨性是纯Ni60激光熔覆层的2.5倍。分析认为,其硬度和耐磨性提高的原因在于涂层中形成了大量的、原位生长的NbC颗粒增强相,且均匀分布于基体中。     

XAFS Analysis for Niobium Carbide Particle Growth on Silica Support During Preparation Process

SiO₂-supported NbC catalysts were prepared by using temperature-programmed reactions (TPR). XAFS analysis confirmed that Nb₂O₅ was reduced to NbO₂ in the first TPR stage and converted into NbC in the second TPR stage. Nb particles grew only in the second TPR stage. Formation of highly dispersed NbC particles on SiO₂ surfaces was achieved.     

Facile route to prepare TaC,NbC and WC nanoparticles

By a novel solid-state reaction process using amorphous C3N4 （α-C3N4） and transition metal oxides as starting reagents, cubic TaC, NbC and hexagonal WC nanoparticles were successfully synthesized at 1150 . The products were characterized by power X-ray diffraction （XRD）, field-emission scanning electron microscope （FE-SEM）, energy-dispersive X-ray spectroscopy （EDX） transmission electron microscopy （TEM） and high-resolution TEM （HRTEM）. The experimental results show that a-C3N4 obtained by the reaction between C3N3C13 and Li3N is a highly efficient carburation reagent and the transition metal oxides are completely transformed into the corresponding metal carbide nanoparticles at 1150 ℃, respectively, which is significantly lower than that reported for the traditional preparation of carbides, typically〉 1600 ℃. The TaC, NbC and WC nanoparticles are found to have an average particle size of 10 nm, 15 nm and 8 nm by TEM observation, respectively.