Evolution Of Carbides Research Articles

Metallic Co2C: A Promising Co-catalyst To Boost Photocatalytic Hydrogen Evolution of Colloidal Quantum Dots

Transition-metal carbide (TMC), because of its electronic conductivity, chemical stability, and physical properties, has aroused widespread interest in catalysis. Here, we have systematically studied the photocatalytic hydrogen (H2) evolution of metallic cobalt carbide (Co2C) by a combination of theoretical and experimental investigations. In terms of intrinsic proton reduction property of the Co2C (020) facet and facile interficial electron transfer, the assembled architecture of quantum dots (QDs)/Co2C can give a rate of ∼18 000 μmol g–1 h–1 (λ = 450 nm), using TMC as co-catalysts and an apparent quantum yield of ∼2.7% of photocatalytic H2 evolution, an ∼10-fold enhancement, compared with bare QDs under identical conditions. Our results indicate that Co2C with suitable morphology and facet exposure can work as a co-catalyst to achieve photocatalytic H2 evolution.

Effects of PTFE activation and excess Al on combustion synthesis of SiC– and ZrC–Al2O3 composites

Fabrication of SiC– and ZrC–Al2O3 composites was studied in PTFE-added SiO2/Al/C and ZrO2/Al/C combustion systems. PTFE acted as not only a reaction promoter to initiate and sustain self-propagating combustion, but a carburizing agent to facilitate evolution of carbides. Excess Al was required to improve thermite reduction of oxide precursors. Based upon the dependence of flame-front velocity on combustion temperature, the activation energies of 149.8 and 104.0 kJ/mol were respectively deduced for synthesis of SiC– and ZrC–Al2O3 composites from PTFE-assisted combustion. The SiC–Al2O3 composite with the least amount of minor phases was produced from the SiO2/Al/C system with PTFE of 6.2 wt% and extra Al of 25%. The best yield of the ZrC–Al2O3 composite was obtained from the ZrO2/Al/C system with PTFE of 3 wt% and 25% excess Al.

The effect of substrate temperature on microstructural evolution and hardenability of tungsten carbide coating in hot filament chemical vapor deposition

AbstractEffect of substrate temperature on microstructural evolution and hardenability of tungsten carbide coating produced by hot filament chemical vapor deposition (HFCVD) process was studied. Annealed sheets of 316L stainless steels were used as the substrate. HFCVD technique, with substrate temperatures of 400 and 500°C, was used to deposit tungsten carbide coating on these sheets. Field Emission Scanning Electron Microscope (FE‐SEM) was used to study the evolution of microstructure. X‐Ray Diffraction spectroscopy was used to analyze the phases formed and Raman spectroscopy was employed to differentiate molecular composition of the coatings. The amount of the porosity of the coatings was measured and Vickers hardness measurement was used for hardness assessment. Results show that the tungsten carbide coatings have a honeycomb structure and increasing the temperature of the substrate increases the amount of porosity of the coating. XRD results showed that 3 different crystalline structures containing W, WC, and W2C were formed in the coating deposited on the 316L stainless steel. Increasing the temperature of the deposition has increased the intensity of the peaks in the XRD results. Raman spectroscopy results indicated the presence of a carbon in the tungsten carbide coatings. Finally, microhardness of the tungsten carbide coating increases with increasing the temperature of the substrate.

Phase Transformation and Carbide Precipitation of Functional Gradient Semi-solid 9Cr18 Steel

The unique phase transformation and carbide evolution in 9Cr18 steel were investigated during semi-solid forming and subsequent heat treatment. The functional gradient thixoforging 9Cr18 component was divided into inner area and edge area. Microstructure evolution was different at each area. After semi-solid cooling, the solid particles in the inner area were retained as meta-austenite. During annealing, M23C6 carbide began to precipitate when temperature reached 700 °C. Martensite transformation occurred when temperature reached 800 °C. The occurrence of M23C6 carbide and martensite structure would be harmful to the mechanical properties of inner area. In the edge area, the liquid underwent eutectic transformation to form bar-shape M7C3 carbide and secondary austenite after semi-solid cooling. The width of bar-shape carbide would decrease during annealing. By controlling the carbide evolution, we could tailor the functional gradient material with required property.

Effect of Tempering Temperature on the Low Temperature Impact Toughness of 42CrMo4-V Steel

Effects of tempering temperature on the microstructures and low temperature impact toughness of 42CrMo4-V steel were investigated. Microstructures of 42CrMo4-V steel after tempering at 570–720 °C for 3 h were experimentally investigated using scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The results showed that the carbide precipitation sequence of 42CrMo4-V steel is M8C7 → M3C and the absorbed energies of 42CrMo4-V steel increase greatly from 21.7 J to 132.3 J with increasing tempering temperature from 570 °C to 720 °C. The changes of impact toughness with increasing tempering temperature were attributed to the softening of matrix, the evolution of carbide precipitates and grain structures.

Characterization and in-situ formation mechanism of tungsten carbide reinforced Fe-based alloy coating by plasma cladding

The precursor carbonization method was first applied to prepare W–C compound powder to perform the in-situ synthesis of the WC phase in a Fe-based alloy coating. The in-situ formation mechanism during the cladding process is discussed in detail. The results reveal that fine and obtuse WC particles were successfully generated and distributed in Fe-based alloy coating via Fe/W–C compound powders. The WC particles were either surrounded by or were semi-enclosed in blocky M7C3 carbides. Moreover, net-like structures were confirmed as mixtures of M23C6 and α-Fe; these structures were transformed from M7C3. The coarse herringbone M6C carbides did not only derive from the decomposition of M7C3 but also partly originated from the chemical reaction at the α-Fe/M23C6 interface. During the cladding process, the phase evolution of the precipitated carbides was WC → M7C3 → M23C6 + M6C.

Evaluation of microstructure and growth kinetics of tungsten carbide ceramics at the interface of iron and tungsten

The microstructure evolution and growth kinetics of tungsten carbide (WC or W2C) ceramics at the interface of iron and tungsten during isothermal annealing process from 1100 °C to 1150 °C were investigated by X-ray diffraction (XRD), electron backscattered diffraction (EBSD) and scanning electron microscopy (SEM), respectively. The results show that carbide formation that involves W2C, WC and a small quantity of Fe2W2C, is strongly dependent on a diffusion-controlled reaction. During the evolution process of tungsten carbides, W2C emerges as an initial phase by the reaction of W and C, and WC is formed as the second phase, because the value of Gibbs free energy of formation of W2C is more negative than that of WC at the annealing temperature. The initial W2C transforms into WC until complete consumption of the W2C. Additionally, Fe2W2C between W2C and WC was formed at a relatively low temperature (1100 °C and 1125 °C) with a long annealing time (60 min). After the W2C is consumed completely, WC grows by C diffusion, where the growth kinetics of a dominant WC ceramic exhibits a parabolic growth law before metal tungsten is completely consumed. The growth activation energy of the WC layer is calculated to be achieved with a value of 208.68 kJ/mol. When the metal tungsten is completely consumed after annealing at 1150 °C for 60 min, the W2C and Fe2W2C disappear, and only the WC phase exists in the final product.

Effect of WC Additive on Microstructural Evolution and Properties of TiC Steel-Bonded Carbide

TiC base high manganese steel-bonded carbide was manufactured by powder metallurgy technique, and the effect of adding mode of WC additive on microstructural evolution and properties of the alloy was studied. SEM microstructure showed that surrounding structure appeared obviousely with WC additive; while microstructure of the alloy was well-distributed and particle shape was distinct very much. The shape of TiC particles became irregular with pure WC powder addition, and irregular with further increase of WC content, and the shape of some TiC particles became angular because of excessive reaction between TiC particles and WC additive. microstructure of the alloy was refiner and the strength, toughness were improved with the increase of WC addition. (Ti, W)C solid solution is superior to form on the surface of TiC particles through dissolution precipitation mechanism because the activity energy of WC was higher than that of TiC-WC compound carbides when WC was added in the alloy, and the particle shape of TiC became more irregular and the properties were improved significantly.

Microstructure evolution and micro-mechanical behavior of secondary carbides at grain boundary in a Fe–Cr–W–Mo–V–C alloy

The microstructure evolution and micro-mechanical behavior of secondary carbides at grain boundary in a Fe–Cr–W–Mo–V–C alloy for cold work roll were systematically investigated in this study. The typical microstructures at the characteristic temperature of 1240°C, 1200°C and 1150°C were observed by Optical Microscope and Field Emission Scanning Electron Microscope. The hardness values of secondary carbides were predicted and measured by first-principles calculation, Vickers hardness tester and nanoindentation technique. The fracture toughness (KC) values were calculated by a method known as Indentation Microfracture. Single-pass scratch tests were carried out to investigate the micro-scale wear behavior of secondary carbides. The macroscopic pin-on-disk test was also performed. The results show that the secondary carbide at grain boundary contains secondary MC, M2C and M7C3. The formation of secondary MC and M7C3 belongs to the precipitation and growth process, while secondary M2C is the result from the growth of eutectic M2C. In the studied alloy, M7C3 is a dominant carbide in quantity, and has higher hardness than secondary M2C and the matrix, and processes the better toughness than secondary MC, whose hardness almost reaches 30GPa, and can also effectively resist the crack initiation and propagation, which therefore makes a significant contribution to the wear resistance of the alloy.

Evolution of carbide precipitates in Hastelloy N joints during welding and post weld heat treatment

The effects of welding and post weld heat treatment (PWHT) on the morphology, composition and lattice parameter of the precipitates in the welding joints of Hastelloy N alloy have been investigated. Joint samples were prepared using a gas tungsten arc welding process with ERNiMo-2 fillers, and then solid-solution treated at 1100°C for 20min. After welding, eutectic M6C carbides can be observed in the heat affected zones (HAZ) and weld zones (WZ). These eutectic M6C carbides become spheroidized during the following PWHT. The concentration of Si decreases during the transformation from the primary ones to eutectic ones, and then increases during the spheroidizing process. The total concentration of Si+Cr in M6C carbides remains stable during welding and PWHT. At the same time, the heat leads to the outward diffusion of Mo from the M6C carbides, which results in a decrease of the lattice parameter.

Microstructure engineering by dispersing nano-spheroid cementite in ultrafine-grained ferrite and its implications on strength-ductility relationship in high carbon steel

Thermo-mechanical processing is performed to engineer the microstructure comprising of nano-spheroidized cementites in ultrafine-grained ferrite in high carbon steel to enhance strength-ductility relationship. Spheroidization is achieved through heavy warm rolling (4-passes of 30% reduction at 823K and 873K) followed by extended annealing (1h and 2h) at the respective deformation temperatures. The influence of annealing temperature and time on the evolution of carbide precipitates, the extent of spheroidization and ferrite softening is investigated employing scanning electron microscopy, electron back scatter diffraction and transmission electron microscopy techniques. A near-complete spheroidization is achieved following heavy warm rolling and subsequent annealing for 2h at 873K (WR873K-2H). Although ferrite grain size increases with time and temperature of annealing, it ceases to cross the ultrafine regime (470–750nm) due to the pinning effect of the carbides that restricts the migration of ferrite grain boundaries. A simultaneous increase in strength and ductility is achieved following heavy warm rolling and subsequent annealing for 1 to 2h at 873K. Maximum elongation (~30%) is achieved in the WR873-2H specimen in contrast to ~20% elongation in as-received specimen. Such an increase in ductility is due to the near-complete spheroidization as revealed by the ductile mode of fracture in fractrographic analysis.

Carbides Evolution and Tensile Property of 4Cr5MoSiV1 Die Steel with Rare Earth Addition

Studies of 4Cr5MoSiV1 die steel suggest that under appropriate conditions, additions of rare earth (RE) can enhance tensile property. This improvement is apparently due to the more uniform distribution of carbides and the enhancement of precipitation strengthening after RE additions. In this present work, the effect of the RE addition on the carbides evolution and tensile property of 4Cr5MoSiV1 steel with various RE contents (0, 0.018, 0.048 and 0.15 wt %) were systematically investigated. The two-dimensional detection techniques such as optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) were used to investigate the carbides evolution of as-cast, annealed and tempered with RE addition. The results indicated that the carbides in 4Cr5MoSiV1 steels were modified by adding the suitable amount of RE. The eutectic structure and coarse eutectic carbides were all refining and the morphology of the annealed carbides initiated change from strip shape to ellipsoidal shape compared with the unmodified test steel (0RE). In addition, the amount of the tempered M8C7 carbides increased initially and then decreased with the alteration of RE addition from 0.018 to 0.15 wt %. Notably, the tensile test indicated that the average value of ultimate tensile strength (UTS) and elongation rate of 0.048RE steel increased slightly to 1474 MPa and 15%, higher than the 1452 MPa and 12% for the unmodified test steel (0RE), respectively. Such an addition of RE (0.048 wt %) would have a significant effect on the carbides evolution of as-cast, annealed and tempered and resulting in the tensile property of 4Cr5MoSiV1 die steel.

Tempering stability of Fe–Cr–Mo–W–V hot forging die steels

The tempering stability of three Fe–Cr–Mo–W–V hot forging die steels (DM, H21, and H13) was investigated through hardness measurements and transmission electron microscopy (TEM) observations. Both dilatometer tests and TEM observations revealed that DM steel has a higher tempering stability than H21 and H13 steels because of its substantial amount of M2C (M represents metallic element) carbide precipitations. The activation energies of the M2C carbide precipitation processes in DM, H21, and H13 steels are 236.4, 212.0, and 228.9 kJ/mol, respectively. Furthermore, the results indicated that vanadium atoms both increase the activation energy and affect the evolution of M2C carbides, resulting in gradual dissolution rather than over-aging during tempering.

Evolution of grain boundary carbides in thermal exposed Ti-modified alloy N

The evolution of grain boundary MC carbides during the thermal exposure has been investigated in the Ti-modified Hastelloy N alloy. MC carbides precipitate at the grain boundaries after exposed at 650°C and 750°C for 500hours (h). The morphology of MC carbides at the grain boundaries was characterized as separated particles with a cube-on-cube orientation relationship with the matrix. The MC/Matrix interface exhibits a semi-coherent character, on which misfit dislocations are repeated in every sixth layer of (111)MC. With the further thermal exposure, the size of MC carbides remain stable at 650°C. By contrast, M2C carbides undergo the initial coarsening and subsequent re-dissolution at 750°C. At the same time, the deviation angles from the cube-cube orientation relationship (OR) between MC carbides and the matrix become larger at both 650°C and 750°C, which indicate the semi-coherent interfaces are transforming to the incoherent ones.

Morphology evolution of Ti3AlC carbide precipitates in high Nb containing TiAl alloys

This work elucidates the morphology evolution of perovskite Ti3AlC carbides in a Ti-45Al-5Nb-0.75C alloy during ageing at 800 °C. The carbides in the γ matrix are initially needle-shaped with their long axis parallel to the [001] lattice direction of the γ matrix. After extended annealing, they decompose into small carbide sub-particles. By combining different transmission electron microscopy characterization methods and atom probe tomography it has been verified that the carbides first split into several parallel needles that are aligned along the [001]γ lattice direction. Later these parallel needles further decompose into numerous small sub-particles, while the matrix phase region between the sub-particles crystallographically reorients. To the authors' knowledge this is the first work which demonstrates such a precipitate splitting process in a matrix with a tetragonal crystal structure. It is proposed that the decomposition into small sub-particles is energetically favourable owing to the elastic interaction energy between the split sub-particles.

Evolution of carbides and its characterization in HAZ during NG-TIG welding of Alloy 617B

The carbides evolution affected by heating will have great effect on the mechanical property of welded components. The paper focused on the microstructure characterization and carbides evolution under different thermal cycle in narrow-gap tungsten inert gas welding (NG-TIG) on Alloy 617B. The carbides were characterized to be M23C6 by TEM in 617B base metal (BM), which distribute along the grain boundaries and within the grains in roughly similar size and characteristics. However, many kinds of multi-carbides and various characteristics were found in heat affected zone (HAZ) with different distance to fusion line, which contributed to different heating peak temperatures (Tp). As the Tp reaches the solvus temperature of M23C6 carbides, M23C6 carbides show partial dissolution and M6C will form at the interface γ/M23C6 due to the segregation of element Mo. Upon the eutectic temperature, the surviving M23C6 carbides react with the surrounding γ matrix and then it leads to the formation of liquid film. Some phases with lamellar structure composed of M6C and M23C6 multi-carbides and islands of γ-austenite will occur as the temperature decreases. In addition, a large number of fine granular Cr-rich second phases like texturing morphology, scattered around each new kind of multi-carbide after the post weld heat treatment (PWHT). Basing on the analysis result, it can be seen that the evolution behavior of carbides is determined by the thermal input during NG-TIG welding process. No adverse effects, such as liquation cracking, have been brought into the welded joint due to the low thermal input, proving that NG-TIG is a suitable welding method to join Alloy 617B.

Microstructure and local strains in GH3535 alloy heat affected zone and their influence on the mechanical properties

GH3535 alloy plates were welded by Gas Tungsten Arc Welding in order to study the evolution of microstructure and mechanical properties in the heat-affected zone (HAZ). Our results suggest that welding thermal cycles induced the morphology evolution of M6C carbides from block to eutectic near the fusion line in the HAZ. Electron backscatter diffraction (EBSD) results show that significant amounts of plastic strains occurred in the HAZ after welding. In addition, local coherent twin boundaries (Σ3) and dislocations were observed to decrease with the distance from the fusion line. Mechanical tests indicate that the hardness, yield strength and ultimate strength in HAZ are higher than those in base metal, and their values decrease with the distance from the fusion line. However, the elongation increases as the strengths decrease. The higher strength and lower elongation in the HAZ are mainly attributed to residual strains with the function of strain-hardening. Moreover, the change of Σ3 boundary which is in good agreement with that of elongation suggests a positive influence on the plastic deformation.

Effect of Ni alloying on the microstructural evolution and mechanical properties of two duplex light-weight steels during different annealing temperatures: Experiment and phase-field simulation

This paper presents a study of two lightweight steels, Fe-15Mn-10Al-0.8C-5Ni and Fe-15Mn-10Al-0.8C (in wt.%) where strength is dependent upon the microstructure of 2nd phase precipitates. We investigate the effects of annealing temperature from 500 °C to 1050 °C on the precipitation of ordered phases size and morphology through phase-field modelling and experimental studies based on laboratory scale annealing and characterization. The chemical composition of carbides and B2 compounds as a function of isothermal annealing temperature and the matrix within which they formed are elucidated in this study. It is found that nano-sized disk-shaped B2 particles form at higher annealing temperatures (e.g. 900 °C and 1050 °C). The simulation results on carbides demonstrated the effects of energetic competition between interfacial energy and elastic strain energy on the morphological evolution of carbides. In addition to that, different ordering behaviours observed depending on the Ni content into the steel. The results demonstrate processing route designed through the phase-field simulations led to a better combination of strength and ductility. The tensile testing results indicate an increase in the strength and elongation when B2 precipitate morphology changes from micro-size faceted shape to nano-size disk-like particles.

Carbide evolution and its potential reduction methods in Ti-22Nb based alloys prepared by metal injection moulding

Carbide evolution was investigated in order to find methods to reduce the carbide precipitation because it lowers the ductility of metal injection moulded <beta>-Ti alloys. Comparing Ti-22Nb and Ti-22Nb-2/10Zr alloys, the crystal structure of carbides is unchanged, but the lattice parameter of <beta>-Ti increases, then carbide fraction slightly decreases. Dissolution of carbides occurs fast around <beta>-transus temperature via short circuit diffusion but it becomes slow with decreasing temperature because of uniform atomic detachment. Based on these results, a multiple step treatment was proposed which in conjunction with grain refinement by adding Y/Y2O3, can significantly decrease the carbide fraction of Ti-22Nb-10Zr-xY/Y2O3.

Molybdenum Carbide-Derived Chlorine-Doped Ordered Mesoporous Carbon with Few-Layered Graphene Walls for Energy Storage Applications.

In this work, we propose a one-step process to realize the in situ evolution of molybdenum carbide (Mo2C) nanoflakes into ordered mesoporous carbon with few-layered graphene walls (OMG) by chloridization and self-organization, and simultaneously the Cl-doping of OMG (OMG-Cl) by modulating chloridization and annealing processes is fulfilled. Benefiting from the improvement of electroconductivity induced by Cl-doping, together with large specific surface area (1882 cm2 g-1) and homogeneous pore structures, as anode of lithium ion batteries, OMG-Cl shows remarkable charge capacity of 1305 mA h g-1 at current rate of 50 mA g-1 and fast charge-discharge rate within dozens of seconds (a charge time of 46 s), as well as retains a charge capacity of 733 mA h g-1 at a current rate of 0.5 mA g-1 after 100 cycles. Furthermore, as a promising electrode material for supercapacitors, OMG-Cl holds the specific capacitances of 250 F g-1 in 1 M H2SO4 solution and 220 F g-1 at a current density of 0.5 A g-1 in 6 M KOH solution, which are ∼40% and 20% higher than those of undoped OMG electrode, respectively. The high capacitive performance of OMG-Cl material can be due to the additional fast Faradaic reactions induced from Cl-doping species.