Alloy Carbides Research Articles

CoFe alloy carbide catalysts for higher alcohols synthesis from syngas: Evolution of active sites and Na promoting effect

Higher alcohols synthesis (HAS) from syngas is quite attractive but still challenging due to the unsatisfied product selectivity and stability. Here, we prepared a series of CoFe bimetallic carbide catalysts and investigated the evolution of active species. The catalysts reduced above 300 °C could ensure an adequate interdiffusion between Co and Fe species to form CoFe alloy and then achieved a high fraction of ε'-(CoxFe1-x)2.2C species during HAS reaction, which exhibited excellent catalytic performance and stability over 200 h. The ε'-(CoxFe1-x)2.2C, acting as uniformly atomic neighboring CoxC-FexC dual sites, facilitated the synergy between CO nondissociative adsorption and CO dissociation, thus boosting the selective production of higher alcohols. It is also demonstrated that adding proper content of Na (0.25 wt%) can enhance the formation of ε'-(CoxFe1-x)2.2C as well as modulate the surface concentration of intermediates. These results may provide new idea for the design of high-performance catalysts for HAS.

Investigation on Material Removal Rate and Surface Finish in Electrical Discharge Machining of AA 6061-B4C Composite Material

One of the non-conventional techniques of metal removal manufacturing processes is electrical discharge machining (EDM). The objective of this paper is to prepare a composite material consisting of a matrix of Aluminium AA 6061 alloy and Boron carbide (B4C) as reinforcement and investigate the output responses, the material removal rate, the quality of the surface formed and overcut during EDM process. The process parameters discharge current, Pulse on time and Duty cycle along with the weight % of B4C are considered for investigation to investigate the output responses such as material removal rate, surface roughness and overcut. From the experimental results, it is observed that the weight % of reinforcement has more influence on the material removal rate. The parameters discharge current and pulse-on-time plays an important role in reducing the surface roughness and overcut. Microstructural study helps in understanding the effect of process parameters on the output responses.

Structure and wear resistance of coatings produced by the short-pulse laser alloying of titanium carbohydride-based mechanocomposites

The study covers the phase composition, morphology and properties of coatings deposited on steel by means of short pulse selective laser alloying of titanium carbohydride-based mechanocomposites. Mechanocomposites were fabricated by milling of Ti and Ti–Cu powders in a liquid hydrocarbon environment. The synthesized mechanocomposites and fabricated coatings are investigated by X-ray diffraction, scanning electron microscopy and optical microscopy. The phase composition of mechanocomposites is represented by titanium carbohydride phase with a size of powder particles ranging from 2 to 30 μm for powders without copper, and from 1 to 10 μm for copper-containing powders. Coatings fabricated from powder mechanocomposites have a gradient structure. The Ti(C,H) powder coating contains 48 vol.% of the titanium carbide phase in a shell of Fe–Ti intermetallic compounds. The Ti(C,H)–Cu powder coating contains 85 vol.% of titanium carbide inclusions surrounded by Ti(Fe,Cu) and CuTi2 phases. Round-shaped carbide inclusions formed have a size of 50 to 200 nm, and dendritic ones are up to 5 μm. Coatings have a microhardness of 10 GPa and 8 GPa for compositions without and with copper, respectively. Coatings were tested for wear resistance under the conditions of dry friction in pairs with the balls made of steel and VK6 tungsten carbide alloy. Coefficients of friction for both coating types are 0.16–0.3 with the ball made of VK6 tungsten carbide alloy and 0.2–0.4 with the ball made of hardened steel. Coatings almost do not wear out under the counterbody load of 10 N and testing time of 20 min.

Study of hardness and morphology of carbide coatings obtained on complex shaped steel items by electro-spark alloying

The work investigated the possibility of forming carbide coatings by electrospark alloying on steel products of complex shape. It has been established that electrospark alloying at an AC current of 1.0 to 4.5 A makes it possible to form coatings of hard carbide alloy VK6 and T15K6 characterized by microhardness up to 11.5 GPa and by hardness HRA 86.6 and 81.5 with the nitial hardness of the steel product HRA 80.3.

Effect of traceable nitrogen from low-pressure plasma nitriding on diamond growth over WC-co cemented carbides

Cemented tungsten carbide alloys with 6% and 12% Co were plasma-nitrided at 600 °C in a 6 Pa vacuum in different working gases using a hot filament ion source. Diamond was directly coated onto the nitrided specimens. Surface morphologies and microstructures of the nitrided and diamond-coated specimens were characterized using scanning electron microscopy, electron probe microscopy, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. The adhesion between the coating and the substrate was evaluated using Rockwell indentation and the standard DIN-VDI 3198. Selective ion-etching of the surface Co during the low-pressure plasma nitriding introduced roughness and craters on the nitrided surface and created a surface conversion layer with a fixed structure. The nitrided surface is covered with a thin Co4N layer that produces a traceable amount of N and can effectively depress the graphitization catalysis of Co species. Diamond coating with adhesion level HF 2–4 can be deposited onto the 6% and 12% WC-Co alloys without chemical etching after the nitriding process.

Surface Phenomena at the Interface between Silicon Carbide and Iron Alloy.

The paper discusses a potential composite produced using the casting method, where the matrix is gray cast iron with flake graphite. The reinforcement is provided by granular carborundum (β-SiC). The article presents model studies aimed at identifying the phenomena at the contact boundary resulting from the interaction of the liquid matrix with solid reinforcement particles. The scope of the research included, primarily, the metallographic analysis of the microstructure of the resulting composite, carried out by using light (LOM) and scanning electron (SEM) microscopy with energy dispersive X-ray spectroscopy (EDS) analysis. The occurrence of metallic phases in the boundary zone was indicated, the contents and morphology of which can be optimized in order to achieve favorable functional properties, mainly the tribological properties of the composite. In addition, the results obtained confirm the possibility of producing similar composites based on selected iron alloys.

Evaluation of mechanical properties of Al-B4C and Al-SiC metal matrix composites – A comparison

This paper deals with the comparison of Mechanical properties of aluminum alloy (LM 25) with boron carbide and aluminum alloy with silicon carbide metal matrix composite. The boron carbide of particle size 35 to 40 µm and silicon carbide of similar particle size were used as reinforcements. This addition gives excellent hardness and good impact strength. The fabrication was done by using stir casting., The samples are prepared and mechanical testing like tensile, hardness, and impact was done. It was found that aluminum alloy with boron carbide showed better tensile and impact strength whereas aluminum alloy with silicon carbide showed better hardness. The structure was characterized by using optical microscope.

Molten Ga-Pd alloy catalyzed interfacial growth of graphene on dielectric substrates

Direct growth of graphene on insulating substrate is highly desired for practical applications in two-dimensional electronics and optoelectronics. However, the controllable synthesis of large scale graphene films on dielectrics is still limited in thickness and crystallinity by the absence of catalyst. Here we develop a transfer-free method to synthesize continuous graphene films on various insulating substrates by using molten Ga-Pd alloy and silicon carbide (SiC) as catalyst and carbon source. The catalytic Ga-Pd alloy induces the decomposition of SiC bonds and promotes the formation of C-C sp2 at relatively low temperature around 1000 °C. The suitable carbon solubility of Ga-Pd alloy could also promote the fabrication of few-layer graphene films on the interface between alloy and SiC substrate as well as the upper surface of alloy. Furthermore, this strategy could be extended to the interfacial growth of graphene on other dielectrics by simply placing the substrate face down to the surface of molten alloy. The progress of this work may help to understand the mechanism of molten Ga-Pd alloy catalyzed interfacial growth of transfer-free graphene and benefit the application of graphene-based electronics and photonics in the future.

Effect of Ni addition on bainite microstructure of low-carbon special bar quality steels and its influence on CCT diagrams

This work presents the effect of Ni content on the bainite morphology and in the mechanical properties of low-carbon special bar quality steels. Four steel ingots with the same base chemical composition, having only Ni content as their main difference, were melted in a pilot scale vacuum induction melting furnace, hot forged into square bars followed by cooling in still air to room temperature. Samples for Charpy-V notch tests of every steel were tempered to equalize their hardness level. Through dilatometry tests, it was found that Ni slows down the transformation kinetics of ferrite and pearlite phases, and, acts in favor of bainite and martensite formation. In low-carbon bainitic steels, Ni also favors the formation of lower bainite at the expanse of upper bainite. The steels with lower Ni contents, 0.03 % and 0.70 %, have their microstructures mostly composed of upper bainite, with clusters of alloy carbides precipitated in the boundaries of the bainitic ferrite packages. The clusters of hard carbides act as stress raisers, facilitating the propagation of cracks, and, reducing the toughness of the steels. The other two steels studied with higher Ni, 1.40 % and 2.75 %, showed superior hardenability and predominant microstructures of lower bainite and martensite, respectively. The steels composed of lower bainite and martensite have higher toughness than the steels with upper bainite due to their more refined microstructure and homogenous dispersion of the alloy carbides in the steel matrix. It was also observed in the bainitic steels that tempering does not influence the location where the alloy carbides precipitated during bainite transformation. After tempering, these carbides remain at the bainitic ferrite boundaries or dispersed in the bainitic ferrite packages depending on the type of bainite formed.

Upper Bound Analysis of Closed-Die Forging of Eccentrically-Located SiCp AMC Preforms

ABSTRACT The paper presents analysis of closed-die forging of eccentrically-located SiCp AMC cylindrical preforms at cold conditions using ‘UpperBound0 approach. The deformation has been considered in two subsequent stages, i.e. free barreling and constrained deformation stages. Second stage was again divided into two modes, i.e. unilateral and bilateral constrained deformations. For basic experimental analysis, the preforms were fabricated via liquid metal stir casting manufacturing route using LM6 Aluminium alloy and Silicon Carbide particles as reinforcements. These preforms were located eccentrically in the closed-die with respect to die axis and subsequently forged into double-hub flange components. The generalized expressions for velocity field, strain rates, various energy dissipations and average forging load were formulated for all the above deformation stage and results were compared with the experimental findings. It is expected that the present work will be useful for the analysis of the precision net-shape flashless closed-die forging operations at cold conditions.

Core size effects of laser fusion subcritical research reactor for fusion engineering research

A multi-purpose high repetition laser facility, the so-called Japan establishment for power-laser community harvest (J-EPoCH) is proposed as a next generation laser facility. J-EPoCH will operate at the maximum rate of 100 Hz. The omnidirectional 12 laser beams with 8 kJ would yield ∼1013 neutrons with a large high aspect ratio target. As one of the applications of J-EPoCH, a laser fusion subcritical research reactor has been conceptually designed based on existing technologies. Moreover, a variety of fusion engineering studies: energy conversion, tritium breeding, neutron irradiation effects, etc, can be conducted. The feasibility of the subcritical research reactor is considered in terms of neutron-thermal (n-t) conversion and tritium breeding. Lead–lithium alloy (Li17Pb83) and boron carbide (B4C) have the potential to be studied for preliminary fusion power generation. The subcritical reactor will generate 21.4 W and 20.0 W of the thermal fusion power with the Li17Pb83 and the B4C layers of the thickness of 80 cm, respectively at 1 Hz operation. The Li17Pb83 layer of a 5 mm thickness will achieve the temperature rise of 0.203 mK per shot. The thermal fusion energy is detectable with conventional measurement techniques. The core with the Li17Pb83 layer thickness of 100 cm will yield more than one tritium from a deuterium–tritium fusion neutron. However, laser windows reduce the efficiency of n-t conversion and tritium yield.

Effects of solution annealing on the precipitation of dendrite-like carbides during continuous cooling in Alloy 690

The precipitation characteristics of intergranular carbides generally have a significant impact on the microstructural homogeneity and properties of Alloy 690. M23C6 carbides with distinct dendritic morphology are found precipitating in the continuously cooled Alloy 690, and this study focuses on the effects of solution heat treatment on the morphology evolution of M23C6 carbides during cooling. Results show that if the solution heat treatment is sufficient enough, carbides tend to precipitate in a dendritic morphology during cooling, otherwise rod-like carbides would be dominant on the grain boundary. The precipitation morphology of carbides basically depends on the critical nucleation radius R∗ and the relative supersaturation S which is closely related to the solute concentration on the grain boundary. Insufficient solution annealing will leave undissolved original carbides on the grain boundary, which can grow prior to the precipitation of new carbides and consume lots of solute elements in advance. This earlier solute consumption and the consequent element diffusion could prevent the formation of dendritic carbides, and then rod-like carbides form instead. This study will help to regulate the precipitation morphology of grain boundary carbides in Alloy 690 during continuous cooling, so as to obtain a microstructure conducive to properties.

Experimental approach to determine the impact of the droplet transfer mode on the degradation of fused tungsten carbides during GMAW

The application of fused tungsten carbides (FTCs) in nickel-based alloys is important for improving the wear resistance of tooling equipment in the mining industry. However, FTCs are thermally unstable and will dilute under excessive energy input during welding. The parameters affecting dilution in this context are diverse and not yet completely understood. To date, the existing scientific literature focuses on the impact of the melt bead characteristics to explain the degradation during gas metal arc welding (GMAW). The degradation-promoting influence of the droplet transfer mode has not yet been considered. A methodology was developed to experimentally quantify the dependence of the degradation kinetics of FTCs on the droplet transfer mode. The established experimental model demonstrated that the globular transfer mode leads to increased degradation of FTCs in comparison to that of the short-arc mode, which can be attributed to the higher process power and hence higher droplet temperature. In this context, the quantifiable impact of the droplet transfer mode was determined.

Performance of Carbide Alloy Compounds in Carbon Doped MoNbTaW

In this work, the performance of the carbon doped compositionally complex alloy (CCA) MoNbTaW was studied under ambient and high pressure and high temperature conditions. TaC and NbC carbides were formed when a large concentration of carbon was introduced while synthesizing the MoNbTaW alloy. Both FCC carbides and BCC CCA phases were detected in the sample compound at room temperature, in which the BCC phase was believed to have only refractory elements MoNbTaW while FCC carbide came from TaC and NbC. Carbides in the carbon doped MoNbTaW alloy were very stable since no phase transition was obtained even under 3.1 GPa and 870 °C by employing the resistor-heating diamond anvil cell (DAC) synchrotron X-ray diffraction technique. Via in situ examination, this study confirms the stability of carbides and MoNbTaW in the carbon doped CCA even under high pressure and high temperature.

42KhNM Alloy and Silicon Carbide as Material for Accident-Tolerant Fuel-Rod Cladding

42KhNM Alloy and Silicon Carbide as Material for Accident-Tolerant Fuel-Rod Cladding

Structure, magnetism, electronic properties and high magnetic-field-induced stability of alloy carbide M7C3

Previous experiments have proven that a high magnetic field can make alloy carbide M7C3 (M = Fe, Cr) precipitate ahead of time at intermediate temperatures. First-principles calculations are employed to search the source of magnetic-field-induced alloy carbides precipitation behaviors of orthorhombic M7C3. The basic building units for M7C3 crystals are polyhedrons formed by metal atoms and C atoms. The framework structure of o-M7C3 consists of metal tetrahedrons, metal octahedrons and triangular metal prisms. Magnetic calculations show that the substitution of Cr atoms causes the magnetic moments of the Fe atoms to decrease to different extents. Moreover, Cr atoms also have a magnetic moment antiparallel to the Fe atoms. Electronic structure calculations indicate that the bonds in M7C3 are a mixture of ionic, covalent and metal bonds. The substitution of Cr atoms weakens the ionic and covalent bonds between the Fe and C atoms; however, strengthen the metal bonds between the Fe and Cr atoms. The magnetic free energy change of M7C3 is larger than that of M2C and M3C at 823 K with a 12 Tesla field, which agrees well with the experimental results for a magnetic field promoting the precipitation of M7C3. This study provides a theoretical basis for the precipitation behaviors induced by magnetic fields in steels.

High Pressure Sintering of Silicon Carbide with Mg-Cr<sub>3</sub>C<sub>2</sub> Composite Additive

Porous silicon carbide was sintered at 1300 °C/30 MPa for 2 h with 4 wt.% magnesium alloy and 4 wt.% chromium carbide composite additives. The sintered ceramic presented density of around 92% of the theoretical density. No new phase was observed after sintering. Mg segregates around chromium carbide particles in sintered ceramic. The silicon carbide particles were mainly bonded by melt magnesium alloy and chromium carbide diffused in solid state. The voids existed in the sintered ceramic, but much more fracture occurred in silicon carbide particles during bending due to high bonding strength of sintering necks. Some voids existed in the ceramic, which act as crack sources during fracture.

Precipitation behavior of carbides in a low-carbon NiCoFeCr high-entropy alloy at 800 °C

Previous studies have reported dramatical strengthening of carbides in NiCoFeCr high-entropy alloy. The feature of carbides, especially species, morphology and distribution, plays an important role in its properties. Hence, their precipitation behavior in the compositionally complex NiCoFeCr HEA deserves to explore. Here, the carbide precipitation behavior in a (NiFeCoCr)99.75C0.25 alloy at 800 °C was studied. Intergranular and intragranular M23C6 carbides form with quite different morphologies and change gradually with the prolonging time. Intragranular carbides with needle-like and rhombus morphologies have a parallel OR of (100)M23C6//(100)matrix and [013]M23C6∥[013]matrix with matrix, which were formed on (111) plane and (100) plane respectively.

Role of carbides on hot deformation behavior and dynamic recrystallization of hard-deformed superalloy U720Li

Cracks are easy to occur during cogging of U720Li ingots due to the high deformation resistance and poor hot ductility. In this paper, the low-carbon alloy (0.014 wt% C) and high-carbon alloy (0.071 wt% C) ingots were fully homogenized. The effect of carbides on hot deformation behavior and dynamic recrystallization (DRX) of as-homogenized U720Li alloy was investigated with aim of providing some guidance for improving hot working performance of such hard-deformed superalloys. It has been found that the blocky MC carbide is the major form of carbon in both alloys. Compared with low-carbon alloy, the size, number density and area fraction of MC carbides in high-carbon alloy are much greater, but those of γ′ are similar. The existence of more MC carbides leads to finer initial grains in the high-carbon alloy. During compression deformation of both alloys at 1060 °C, intergranular cracking occurred preferentially along initial grain boundaries (GBs), but cracking of the high-carbon alloy occurred at larger strains than that of low-carbon alloy. Before the occurrence of cracking, the yield and flow stress of high-carbon alloy were obviously lower than those of low-carbon alloy. An interesting phenomenon was discovered that the DRX always occurred preferentially around the MC carbides. Based on this phenomenon and electron backscatter diffraction analysis, the MC carbides were assumed to act as nucleation sites during initiation of DRX by a particle stimulated dynamic recrystallization mechanism. In addition, initial GBs are also preferential sites for DRX initiation. The greater size and number density of MC carbides and finer initial grains significantly increased the stored energy and nucleation sites, which distinctly promoted DRX in the high-carbon alloy. The faster DRX is the major contributor in reducing deformation resistance and cracking of the high-carbon alloy.

Austempering and Cryogenic treatment of Vanadium modified Carbidic Ductile Iron

ABSTRACT Conventional Austempered Ductile Irons (ADI) are lacking in adequate wear resistance. Introduction of hard alloy carbide phase in the microstructure by strong carbide formers imparts wear resistance. Deep cryogenic treatment subsequent to austempering is another way to improve the wear resistance without a major reduction in toughness. The present study encompasses the microstructural evolution following deep cryogenic treatment and the resultant bulk hardness, Charpy impact toughness and dry sliding wear of two vanadium contained (0.1% and 0.3%) carbidic austempered ductile irons. The irons were austempered at two temperatures viz. 3000 C and 3500 C for 30, 45, and 60 minutes and which is accompanied at −1960 C either by deep cryogenic treatment or air-cooled. EDS & XRD (X-ray Diffraction), Optical and Scanning Electron Microscopy analysis have characterized the microstructural features. Increase in vanadium content and cryo-treatment result in a prominent combination of wear resistance and impact toughness.