Vanadium Carbide Powders Research Articles

Effects of high-energetic 3He+ ion irradiation on tungsten-based composites

The interaction of plasma and its encompassing materials with one another is one of the principle designing issues of fusion reactors. Tungsten is considered one of the primary candidate materials in fusion applications due to its superior properties. In this work, we doped vanadium carbide powders to the tungsten matrix to enhance the properties of tungsten alloys. Tungsten-based composites, which irradiated with 2.5 MeV 3He+ ions at room temperature, were analyzed by atomic force microscopy (AFM) in order to obtain surface morpohologies after irradiation. Helium ion irradiated tungsten-based composites were studied by using XRD, neutron diffraction technique, raman spectroscopy, and positron annihilation spectroscopy to reveal the microstructural changes. XRD and ND analyses clarified the changes in the crystal structures of the tungsten-based materials after He ion irradiation. The simulation of radiation damage and the calculation of displacements per atom (DPA) was also determined by the SRIM code. SRIM showed that the maximum helium concentration in the specimens takes place in the depth range of 58–65 nm. The crystallite size of tungsten-based composites slightly increased after helium ion irradiation. AFM results revealed that the maximum size of bubbles on the surface of tungsten-based composites shape under 100 nm. Positron annihilation spectroscopy studies of the specimens have been discussed before and after 3He+ ion irradiation.

Correlation between the Pycnometric Density and Specific Surface Area of Nanocrystalline VCy Powders

Nanocrystalline cubic VCy nonstoichiometric vanadium carbide powders differing in nonstoichiometry (0.69 ≤ y ≤ 0.76) and ranging in average particle size from 20 to 60 nm have been prepared by solid-state reactions followed by high-energy milling. Milling of the VCy powders has been shown to be accompanied by changes in their chemical composition, lattice parameter, specific surface area, average particle size, lattice strain, and density. We have derived a relation between the pycnometric density and specific surface area of the VCy powders, which has been used to determine the percentage of oxygen in carbide powder particles and their true density. Oxygen adsorption on the surface of the carbide particles has been shown to be the main cause of the decrease in the pycnometric density of the powders with increasing specific surface area.

Microstructural Characteristics and Properties of Adding Vanadium Carbide Powders to Vanadis 4 Tool Steel through Vacuum Sintering and Heat Treatments

Microstructural Characteristics and Properties of Adding Vanadium Carbide Powders to Vanadis 4 Tool Steel through Vacuum Sintering and Heat Treatments

Microstructural characterization of a V2C and V8C7 ceramic-reinforced Fe substrate surface compound layer by EBSD and TEM

Microstructural characterization of a vanadium carbide (V2C and V8C7) surface compound layer was performed by electron backscatter diffraction (EBSD), scanning electron microscopy and transmission electron microscopy. The phase composition and phase distribution of the compound layer and the morphologies, grain sizes and crystallographic orientations of the vanadium carbides were investigated through elemental distribution and crystallographic analyses. The results indicated that the compound layer was composed of four distinct zones consisting of a compact and uniform V2C layer (thickness of 3.2 μm) over the topmost surface followed by a nonuniform V + V2C (2.4 μm) zone, a V8C7 layer (10.7 μm) and a relatively deep V2C + V8C7+α-Fe + Fe3C zone (23.2 μm). The V2C layer was found in the V-rich region, and the EBSD results revealed that this phase grew mainly along the {0001} and {11–20} orientations. However, the V8C7 phase appeared to be nearly random, which indicates that the growth of this phase had no specific orientation. The V2C and V8C7 phases exhibited low-angle grain boundaries, and their misorientation angle distributions were mainly concentrated within the range of 0–5°. Furthermore, nanoindentation tests demonstrated that the hardnesses of the V2C and V8C7 layers were 22.16 ± 0.52 and 32.42 ± 0.61 GPa, respectively. Examination of the vanadium carbide powders obtained by chemical extraction indicated that the V2C grains with a hexagonal structure exhibited equiaxial and short rod-like morphologies, and the V8C7 particles with a cubic structure possessed short rod-like and spherical morphologies.

Development of feedstocks based on steel matrix composites for metal injection moulding

Feedstock behaviour during processing via powder injection moulding (PIM) has a critical influence on the properties of the final parts. The objective of the present study is to analyze the effect of vanadium carbide (VC) addition (1, 3, 6, 10 and 14wt.% VC) on the rheological response and stability of AISI M2 high speed steel PIM feedstocks. A binder system consisting of high density polyethylene (HDPE) and paraffin wax (PW) is proposed. The effects of VC concentration, powder loading, apparent viscosity and temperature have been investigated via capillary rheometer. The feedstocks with a 14wt.% of VC, which exhibited a variable flow behaviour was discarded. The rest of the feedstocks showed a pseudoplastic behaviour which is suitable for injection moulding and in particular, those reinforced with 6 and 10wt.% of VC exhibited the best rheological responses. In order to maximize the powder loading and ensure safety conditions, the feedstocks reinforced with 6wt.% (5.9vol.%) and 10wt.% (9.8vol.%) of VC with 68vol.% of powder loading and 3wt.% (2.8vol.%) of VC with 62vol.% of powder loading were selected to continue the rheological analysis. Rheological parameters such as flow behaviour index (n), yield stress (ơy), flow activation energy (Ea) and the general mouldability index (αstv) have been analyzed for the above mentioned feedstocks and showed suitable values for the injection moulding. The green parts produced using the three feedstocks have constant weights and dimensions, good mouldability, density, shape retention and stiffness. Therefore, the developed feedstocks are suitable for PIM.

Features of Brittle Material Powder Compaction During Pressing

The compaction of (i) titanium, vanadium, and molybdenum carbide powders, (ii) mixtures of these powders with the addition of nickel, chromium, and niobium carbide powders, and (iii) coarse titanium and tungsten carbides as well as diamond powders clad with cobalt under compressive loading with a constant rate on the powder in a die at room temperature is studied. Based on the variation of the relative density of the compacts with current pressure, the variation of stresses in the matrix (forming a porous body) with its strain is determined. This enables to reveal the features of compaction, strain hardening, and fracture of brittle powder particles during pressing. It is established that for the molybdenum carbide powder, consisting of powder particles with pronounced irregular shape, the initial elastic deformation of the matrix abruptly transits into the stage of plastic deformation with almost linear strain hardening. At the final stage of the powder compaction, the particles are fractured. At the initial stage of pressing, an increase in the packing density of the powder particles and in their strain hardening occurs for titanium and vanadium carbide powders, whose particle shape is close to the rounded. With the increase in the density of the porous body, it is observed an almost linear strain hardening of the matrix, which changes into decay with decreasing shear stress, when the powder body approaches its non-porous state. As a result of pressing, the size of the coherent X-ray scattering areas decreased to 62 nm and the dislocation density in the titanium carbide particles grew to 8.3 · 1010 cm–2. With an increasing content of plastic metallic particles in the matrix with the particles of brittle materials, it is the metallic particles that predominantly undergo the plastic strain with strain hardening. This causes a sharp increase in the total strain hardening, reducing the density of the porous body during pressing. It is established an effective compacting of coarse titanium and tungsten carbide powders with cobalt-clad particles 200–600 μm in size. In this case the variation of the stress in the matrix with the strain of the matrix has a sharp yield point associated with high elastic limit, significantly exceeding the yield stress, and the time delay in the transition from a purely elastic deformation of the body to its plastic flow.

Nanocrystalline ordered vanadium carbide: Superlattice and nanostructure

The crystal structure, micro- and nanostructure of coarse- and nanocrystalline powders of ordered vanadium carbide V8C7 have been examined by X-ray and neutron diffraction and electron microscopy methods. The synthesized coarse-crystalline powder of ordered vanadium carbide has flower-like morphology. It was established that the real ordered phase has the composition V8C7-δ (δ ≅ 0.03) deviating from perfect stoichiometric composition V8C7. The vanadium atoms forming the octahedral environment □V6 of vacant sites in V8C7-δ are displaced towards the vacancy □. The presence of carbon onion-like structures was found in the vanadium carbide powders with a small content of free (uncombined) carbon. The nanopowders of V8C7-δ carbide with average particle size of 20–30 nm produced by high-energy milling of coarse-crystalline powder retain the crystal structure of the initial powder, but differ in the lattice deformation distortion anisotropy.

Nanocrystalline VC y powders in the homogeneity range of a disordered cubic phase

Nonstoichiometric vanadium carbide nanopowders ranging in average particle size from 20 to 40 nm have been prepared by high-energy milling and then vacuum-annealed at temperatures from 600 to 1200°C. The crystal structure, microstructure, morphology, and particle size distribution of the starting, milled, and annealed vanadium carbide powders have been studied by X-ray diffraction, laser diffraction, and scanning electron microscopy. The results demonstrate that vacuum annealing of the VCy nanopowders at temperatures from 600 to 800°C leads to partial carbon loss, compositional changes within the homogeneity range of the disordered cubic phase, and a slight increase in nanoparticle size. Vacuum annealing of the nanopowders at t ≥ 1000°C leads to a considerable carbon loss, the formation of the hexagonal phase V2C along with the cubic carbide VCy, and an appreciable increase in VCy particle size. Thus, vacuum annealing of the nanopowders at 800°C or a lower temperature allows one to vary their composition from VC0.70 to VC0.81, while maintaining the average particle size in the range 50 to 20 nm.

V8C7–δ superstructure in nonstoichiometric vanadium carbide powders

Neutron and X-ray diffraction methods have been applied jointly to study the crystal structure of the ordered phase of V8C7–δ in macro- and nanocrystalline vanadium carbide powders. The deviation of the composition of the ordered V8C7–δ phase from the stoichiometric composition V8C7 has been established. The magnitudes and directions of the displacements of C and V atoms have been determined. It has been shown that vanadium atoms are displaced towards a vacancy, increasing the electron density of the vacancy position of the carbon sublattice.

Initial stage of Free Pressureless Spark-Plasma Sintering of vanadium carbide: Determination of surface diffusion parameters

Inter-particle neck growth kinetics of vanadium carbide powder during initial stages of conventional and spark-plasma sintering has been investigated. Vanadium carbide (V8C7) micron-size powder has been subjected to both Free Pressureless Spark Plasma Sintering (FPSPS) and conventional sintering dilatometry. While the latter resulted not to be effective in the consolidation process, FPSPS procedure proved to be successful in terms of the initial stages of densification and inter-particle neck evolution, even without applying an external load. FPSPS was conducted with and without holding time, in order to obtain experimental data on the inter-particle contact growth for different temperature regimes. The neck sizes derived from scanning electron microscopy observations were utilized in the inverse regression of the surface diffusion equation and led to the calculation of the value of the surface diffusion activation energy for the vanadium carbide powder.

Effect of iron catalyst on the microstructure and electrochemical properties of vanadium carbide-derived carbons

Carbide-derived carbons (CDCs) are obtained from vanadium carbide powders in the presence or absence of iron catalyst at chlorination temperatures of 400, 600, 800 and 1000 °C. The structural differences of the resulting carbons are characterized by low-temperature nitrogen sorption, Raman spectroscopy, X-ray diffraction, and transmission electron microscope techniques. Unlike monotonous increase in the degree of order for the CDC synthesized without addition of iron catalyst, a various decrease for the CDC synthesized with the catalyst is observed. This difference is due to the formation of many nano-diamonds during the CDC processing in the presence of iron catalyst. Variation in the specific surface area is associated with the degree of order in the CDC. CDC synthesized with iron catalyst at chlorination temperature of 800 °C shows the highest specific surface area, as well as the best electrochemical performance.

Preparation and microstructure of VC0.875 nanopowder

Nanocrystalline VC0.875 nonstoichiometric vanadium carbide powder has been prepared by high-energy milling. The crystal structure, microstructure, morphology, and particle size distribution of the starting and milled powders have been studied by X-ray diffraction, laser diffraction, and scanning electron microscopy. The particle size of the VC0.875 nanopowder has been calculated as a function of milling time in a model of the milling process. Comparison of experimental data and calculation results demonstrates that 10-h milling of vanadium carbide powder with an average particle size 6 μm yields nanopowder 40 to 80 nm in average particle size.

Microstructure of nanocrystalline nonstoichiometric vanadium carbide VC0.875

A nanocrystalline powder of nonstoichiometric vanadium carbide VC0.875 has been prepared by the high-energy ball milling method. The crystal structure, microstructure, morphology, and size distribution of particles of the initial and milled powders have been investigated using X-ray diffraction, laser diffraction, and scanning electron microscopy. For vanadium carbide, the model calculation of the particle size of a VC0.875 nanopowder as a function of the milling duration has been performed for the first time. A comparison of the experimental and theoretical results has demonstrated that a nanopowder with an average particle size of 40–80 nm can be obtained by a 10-h high-energy ball milling of the initial vanadium carbide powder with an average particle size of ∼6 μm.

Room-temperature ferromagnetism of vanadium-doped 6H–SiC

Semiconductors doped with magnetic ions, also known as dilute magnetic semiconductors, reveal both semiconducting and ferromagnetic. In this Letter, we prepared vanadium-doped 6H–SiC single crystals for exhibiting ferromagnetism characteristic measured by superconducting quantum interference device (SQUID) magnetometer. Our work demonstrates that moderate doping of vanadium carbide powder into 6H–SiC is a novel attempt for finding ferromagnetism feature. And the testing results show that the observed magnetic signal is possibly contributing to V element. This phenomenon can be explained by sp-d exchange mechanism, and offers an experimental data for possibility of doping nonmagnetic atoms to control the local moments in wide band-gap semiconductors.

Comparison of Mechanical Properties of the 32CrMoV12-28 Hot Work Tool Steels Alloyed with WC, VC and TaC Powder Using HPDL Laser

This work presents the investigation results of laser remelting and alloying especially the laser parameters and its influence on the structure and properties of the surface of the 32CrMoV12-28 hot work steel, using the high power diode laser (HPDL). As a result structure changes in form of fragmentation were determined. The reason of this work was to determine the optimal laser treatment parameters, particularly the laser power to achieve good layer hardness for protection of this hot work tool steel from losing their work stability and to make the tool surface more resistant to action in hard conditions. For alloying the tantalum carbide, tungsten carbide and vanadium carbide powders were used. For investigations hardness measurements of the different remelting areas were performed. The remelted layers which were formed in the surface of investigated hot work steel were examined metallographically and analyzed using light and electron microscope. Three phases of carbides, TaC, VC and WC, were observed.

Effect of tempering on hardness improvement in a VC/steel surface-alloyed material fabricated by high-energy electron-beam irradiation

Abstract The present study is concerned with the tempering effect in improving the hardness of a vanadium carbide (VC)/carbon steel surface-alloyed material fabricated by high-energy electron-beam irradiation. The mixture of VC powders and flux (50%MgO–50%CaO) was placed on a plain carbon steel substrate, and then electron beam was irradiated. The surface-alloyed layer of 1.8 mm in thickness was homogeneously formed without defects. The microstructural analysis indicated that coarse VC particles were formed along solidification cell boundaries, and the matrix inside cells was mostly composed of lath-type martensite and fine cuboidal VC particles. A large amount of these VC particles in the lath-type martensitic matrix provided hardness four times greater than that of the substrate. When the VC/steel surface-alloyed material was tempered, fine VC particles precipitated in the tempered martensitic matrix, thereby leading to additional hardness increase. In addition, reduction of residual stress and an increase in fracture toughness could be expected.

Thermal Fatigue Characteristics of Pta Hardfaced Steels

Many types of hard material are coated on the surface to improve their wear resistance. Addition of vanadium carbide to Co based alloys (stellite no. 21) as a hard material powder is one of the ways to improve the wear resistance characteristics of the surface layer. The plasma transfer arc (PTA) welding process was introduced as a coating technology for elevated temperature surface modification. This process has recently generated interest in the surface modification field owing to its operability, low initial cost of equipment, high deposition rate, and small dilution rate. Coated layers produced by PTA considerably improve the hardness and wear resistance of surface layers for elevated temperature applications. Vanadium carbide (VC) addition into stellite powder showed a significant improvement in wear resistance. However, alloys containing VC showed pronounced sensitivity to hot cracking under repeated heating and cooling environments. This study clarifies the cause of thermal fatigue cracking in Co based alloy deposits with VC powder additions. Cracks result from the difference in thermal expansion coefficient between the matrix and the carbides. Cracks initiate in the central part of the surface region and grow in a perpendicular direction towards the surface. The tendency for thermal fatigue crack initiation seems to increase with increasing carbide volume fraction and decrease as the volume fraction of the dendritic region decreases.

Microstructural analysis of vanadium carbide/steel surface-alloyed materials fabricated by high-energy electron-beam irradiation

This study is concerned with the microstructural analysis of vanadium carbide (VC)/steel surface-alloyed materials fabricated by high-energy electron-beam irradiation. The mixtures of VC powders and MgO-CaO flux were deposited on a plain carbon steel substrate, and then electron beam was irradiated on these mixtures using an electron-beam accelerator. Microstructures of the irradiated surface regions were examined by optical microscopy, scanning electron microscopy, and transmission electron microscopy. Residual pores were found in the specimen processed without flux, but hardly found in the specimens processed with a considerable addition of flux. As a result of irradiation, vanadium content was homogeneously maintained throughout the melted region, and fine vanadium carbides were formed in the melted region. These microstructural modification including the formation of vanadium carbides greatly improved hardness, especially high-temperature hardness up to 500 °C.

Oxidation behavior of vanadium carbide powder.

In this study, the oxidation of vanadium carbide (VC) powder at high temperatures was investigated in order to determine the possibility of its high-temperature use and additional material for improvement of the oxidation resistance of carbon-carbon (C/C) composites. VC powder with an average particle size of about 10μm was used in this experiment. The sample was oxidized at 300 to 1000°C for 5min to 25h in air. The weight changes were measured to estimate the oxidation resistance. The oxidation of sample for short oxidation times of 5min started at 300 to 400°C and weight gain change was measured with increasing oxidation temperature. The sample at the oxidation time of 0.5 to 1h, exhibited a weight gain change with increasing oxidation temperature. VC is oxidized to V2O5 at about 600°C.

Synthesis of Vanadium Carbide by Temperature Programmed Reaction

Vanadium carbide powders are prepared with moderate surface areas of 60 m 2 g -1 (particle size 17 nm) by a temperature programmed reaction between solid vanadium pentoxide (19 m 2 g -1) and a methane-hydrogen mixture. The synthesis involves two steps. In the first step a single suboxide intermediate, V 2O 3, is formed by reduction of V 2O 5 by hydrogen at 800 K. In the second step the V 2O 3 is reduced and carburized by methane with production of CO at 1180 K. In the early stages, the synthesis is found to be limited by the activation of hydrogen as found from experiments with Pt/V 2O 5. The transformation is accompanied by retention of external shape and size, and so is pseudomorphic, but does not conserve orientation of crystallographic planes, so is not topotactic. The results are compared and contrasted to those of nitridation with ammonia and reduction by pure hydrogen.