Nonstoichiometric Carbide Research Articles

Synthesis of non-stoichiometric carbides and carbohydrides of Ti and Ti-Nb using self-propagating high-temperature synthesis technique

Non-stoichiometric monocarbides are of great interest because they can be used as additives at production of hard alloys. Interestingly, the non-stoichiometric carbides of IV group metals provided with a wide range of homogeneity interact with hydrogen and form ternary compounds, carbohydrides. These compounds attract the researches as the bases for creating new hydrogen containing materials resistant at temperatures above those for known binary hydrides of transition metals. They are promising as materials for biological protection in nuclear energy. In the present work the single-phase non-stoichiometric face-centered cubic (FCC) carbides and hexagonal close packed (HCP) and FCC carbohydrides based on Ti and Ti-Nb were synthesized using Self-propagating High-temperature Synthesis (SHS) method. These syntheses were implemented in one technological stage. The high hydrogen content in the synthesized non-stoichiometric carbohydrides promoted their dispersion down to particle sizes of < 5 µm and even smaller, to the nanoscale crystallite sizes of < 50 nm. The mechanism of formation of non-stoichiometric carbides and carbohydrides in SHS was discussed. Significant advantages of a highly efficient non-energy-intensive SHS method for the synthesis of non-stoichiometric titanium and titanium-niobium carbides and carbohydrides can be of commercial and industry interest.

Characterization and reaction mechanism of in-situ micro-laminated Cr2AlC coatings by plasma spraying Cr3C2/Al/Cr powder mixtures

Large-scale fabrication of MAX phase coatings such as Cr2AlC and Ti2AlC by plasma spraying is extremely challenging, owing to the ultra-high temperature and insufficient reaction time. In this research, the Cr2AlC coatings were produced by plasma spraying from Cr3C2@Al-Cr agglomerates. The influence of Al contents on microstructure and mechanics properties of coatings were investigated. Results showed the as-deposited coatings displayed a typical layered structure, mainly composed of Cr2AlC, Cr7C3, Cr23C6 and sub-stable (Cr, Al)Cx. Rapid deposition resulted in an imperfect reaction of raw powders, thus generated sub-stable (Cr, Al)Cx. The adequate Al and nonstoichiometric carbides CrCx were the main key to promote the presence of MAX-Cr2AlC phase. Nevertheless, excessive Al would create extra (Cr, Al)Cx, thus reduce the Cr2AlC content. Notably, the coatings retained high hardness (>10 GPa) and good fracture toughness (~2.5 MPa·m1/2). The micro-lamellar structure, nano-layered Cr2AlC and microcrack-toughening ensured the good mechanical properties of coatings.

Effects of titanium high energy ball milling on the solid-phase reaction Ti+C

The effect of high energy ball milling (HEBM) of titanium on the stoichiometry of TiCx under reaction diffusion in the Ti + C system was studied. It was found that under annealing simultaneous competing processes of carbon diffusion and recovery with further recrystallization stabilize nonstoichiometric products of Ti + C reaction. The process of TiC0.8-1.0 formation proceeds at a short HEBM time of Ti = 1 min. Nonstoichiometric carbides TiC0.4-0.5 are formed at HEBM times of Ti ≥ 3 min. The fraction of unreacted titanium depends on the size of the Ti grain.

Закономерности образования карбидов титана и вольфрама из продуктов электровзрывного разрушения проводников

A series of electrical explosions in propane-butane of single and strand-connected Ti and W conductors with various diameters was carried out. Electrophysical characteristics of the explosion revealed that resistive heating of conductors is characterized by two monotonically ascending sections on the voltage – current curves separated by a flat segment (plateau), which corresponds to relatively stable electrical resistivity of refractory metals in the liquid state. The energy deposited by changing the power input into the conductor during its resistive heating, which can be higher or lower than its sublimation energy and can be regulated by changing the external adjustable parameters of the discharge circuit, is a key indicator determining the structural-phase state of destructed and chemically synthesized products after the explosion. The conditions are achieved under which micro- and nano-sized powder products do not contain residual metals and consist of carbide phases completely (TiC with an average microhardness of 29580 MPa as a result of the titanium explosion and a mixture of W2C+WC1-х, in which stabilized high-temperature non-stoichiometric cubic carbide WC1-x dominates, with an average microhardness of 16770 MPa as a result of tungsten explosion).

Modelling of high-temperature order-disorder phase transitions of non-stoichiometric Mo2C and Ti2C from first principles.

High-temperature order-disorder phase transitions play an important role in determining the structure and physical and chemical properties of non-stoichiometric transition metal carbides. Due to the large number of possible carbon vacancy arrangements, it is difficult to study these systems with first-principles calculations. Here, we construct a simple atomistic potential capable of accurately reproducing the energetics of the carbon vacancy arrangements in cubic Mo2C and Ti2C obtained from density functional theory calculations. We show that this potential can be applied to correctly predict the transition temperatures between the ordered and disordered states in Monte Carlo simulations on large supercells and reveal the extend of local order in the disordered phases of Mo2C and Ti2C that show interesting physical and chemical properties. We find that even the high-temperature disordered phase exhibit a relatively high degree of local order as indicated by the relatively small change in the root mean square number of C atom neighbours of Mo/Ti compared to the ordered phase (from 3.0 to 3.1-3.2). This atomistic potential enables the study of how the structure of these carbides can be tuned through the synthesis temperature to control the properties of carbide materials that are related to the degree of disorder in the system such as catalytic activity and electrical conductivity and play an important role in applications of these carbides. Fundamentally, the successful modelling of these carbides suggests that despite the presence of metallic, covalent and ionic interactions, bonding in carbides can be modelled by simple and physically intuitive interatomic potentials.

Modulated and Incommensurate Superstructures of Atomic–Vacancy Ordering in Refractory Transition Metal Carbides

The formation of partially disordered modifications in refractory nonstoichiometric carbides prone to M6X5-type ordering is considered. In M6X5 superstructures the ray $${\mathbf{k}}_{9}^{{(3)}}$$ of the Lifshitz star {k9} with a current parameter μ9 = 1/2 in the disorder–order phase transition channel is replaced by the rays $${\mathbf{k}}_{5}^{{(6)}}$$ and $${\mathbf{k}}_{5}^{{(5)}}$$ of the non-Lifshitz star {k5} with a variable current parameter 0 < μ5 < 1/2. Depending on the specific value of the index μ5, this replacement leads to various modulated structures that differ by the concentration of vacancies in the defective ( $$1\bar {1}1$$ ) planes of the carbon fcc sublattice and by the modulation period. In the diffraction spectra the positions of the superstructure reflections due to the star {k5} at μ5 ≈ 0.473 correspond to the incommensurate ordered phase experimentally detected in a nonstoichiometric tantalum carbide. The incommensurate phase is close in short-range order in the first coordination sphere to the initial M6X5 superstructures with a reduced long-range order parameter η ≈ 0.6.

Intrinsic defects, Mo‐related defects, and complexes in transition‐metal carbide VC: A first‐principles study

AbstractIntrinsic defects and Mo‐related defects in vanadium carbide VC, as well as the defect complexes between vacancies and Mo defects were investigated by means of first‐principles calculations within the framework of density functional theory. In addition, Mo diffusion in VC was also studied using LST/QST method. The formation energies of defects have clearly shown that except C vacancy (VC) all other point defects are not energetic favorable compared to perfect VC. VC can exist in the lattice forming nonstoichiometric carbide VCx (x &lt; 1), and also can stabilize the Mo‐related defects (SMo‐V, SMo‐C, and TMo). Free Mo atoms have the strong tendency to enter the already formed VV and occupy the lattice position of V atoms. Meanwhile, Mo atom in C lattice (SMo‐C) and interstitial Mo (IMo) atom can also enter the VV position stabilizing the lattice structure. SMo‐C + VV will transform into SMo‐V + VC and IMo + VV will transform into SMo‐V during optimization, and large binding energy makes Mo atom tend to exist in the interstitial position. From the perspective of energy, Mo atom tends to diffuse through the interstitial position.

High-energy ball milling of nonstoichiometric compounds

The preparation of nanocrystalline powders of nonstoichiometric compounds such as carbides and oxides by high-energy milling is considered. The modern state of milling models for nonstoichiometric compounds is described. The influence of nonstoichiometry on the particle size of the produced nanopowders is discussed. The model dependences of the size of nanopowder particles on the duration of milling and the composition of nonstoichiometric compounds are compared with the literature experimental results on the milling of nonstoichiometric carbides and oxides. Diffraction methods for the certification of nanopowders according to the size of the nanoparticles, the magnitude of microstrains, and the uniformity of the microstructure are considered in detail. The importance of taking into account the anisotropy of microstrains in milled nanopowders is shown.

Investigation of evaporation of nonstoichiometric hafnium carbide up to 4500 K using laser heating

Investigation of evaporation of nonstoichiometric hafnium carbide up to 4500 K using laser heating

Thermal expansion of non-stoichiometric zirconium carbide up to 3200 K

Thermal expansion of non-stoichiometric zirconium carbide up to 3200 K

STRUCTURAL-PHASE COMPOSITION OF TiC/Ti COMPOSITES AND POWDERS OBTAINED BY COMBUSTION OF Ti-C GREEN MIXTURES WITH A TITANIUM EXCESS

The formation of the structure of the TiC/Ti metal matrix composite in the process of wave combustion has been discussed, taking into account the occurrence of possible reactions. An analysis of the experimental data shows that under this high-temperature process, a set of nonstoichiometric titanium carbides is formed, where, at certain ratios of the reacting components, low-dimensional TiC0.5 structures are formed, which can be attributed to the so-called MXenes. The study was carried out using electron and optical microscopy, X-ray diffraction and Raman spectroscopy. It was shown that the use of this set of experimental techniques makes it possible to reveal, by indirect signs, the presence of nonstoichiometric carbide phases, similar to MXenes, based on structural and morphological features.

Ordering Sequence in Strongly Nonstoichiometric Niobium Carbide with the Formation of Nb6C5-Type Superstructures

Possible Nb6C5-type superstructures of nonstoichiometric niobium carbide NbC0.83 are found by the evolutionary crystal structure prediction algorithm. A symmetry analysis is carried out of Nb6C5-type model superstructures, the formation of which is possible in nonstoichiometric cubic niobium carbide with B1 structure. Channels of order–disorder transition NbCy → Nb6C5 are found. It is shown that niobium and carbon atoms, which form the immediate environment of vacant sites in Nb6C5 superstructures, experience the maximum displacement and are shifted away from structural vacancies (the vacant sites of the carbon sublattice). As temperature decreases, two physically admissible sequences of transformations associated with the formation of Nb6C5 superstructures are possible in NbC0.83 carbide. The transformation sequences obtained are in agreement with the calculation of the ground states of the superstructures.

A new method for solid‐state diffusion of boron atoms into powders of a multicomponent carbide

AbstractMulticomponent boron‐containing carbide (ie, Zr‐Ti‐C‐B) composites show good ablation resistance. The present work is the first report to introduce the powder fabrication of Zr‐Ti‐C‐B using a new method for solid‐state diffusion of boron atoms. First, the nonstoichiometric carbide (ie, Zr0.8Ti0.2C0.8) with carbon vacancies was fabricated by free‐pressureless spark plasma sintering. Different boron sources such as B2O3, B, and B4C were used to react with the nonstoichiometric carbide. The Zr0.81Ti0.19C0.86B0.14 can be finally generated through the solid‐state diffusion of boron atoms using the B2O3 boron source at 1300°C followed by carbon thermal reduction using the phenolic resins at 1600°C.

Microinhomogeneity of the Structure of Nanocrystalline Niobium and Vanadium Carbides

The evolution of the microstructure of nonstoichiometric niobium carbides NbCy (y = 0.77, 0.84, 0.96) and vanadium carbide V8C7 subjected to high-energy milling has been studied by the time-of-flight neutron diffraction method. It has been found that milled nanocrystalline powders have microinhomogeneous structure: two fractions with different sizes of particles have been identified in them. The content of the nanofraction is more than 90 wt %; the size of particles of this fraction varies from 90 to 250 A, depending on the composition of the initial carbide and the duration of milling. The size of particles of the second fraction is more than 2000 A. Anisotropic deformation distortions have been revealed. The mean size of coherent scattering regions and microstrains in nanocrystallites have been estimated.

Особливості структуроутворення вторинного карбіду кремнію, синтезованого в умовах взаємодії нанорозмірного нестехіометричного карбіду кремнію з оксидом заліза

Особливості структуроутворення вторинного карбіду кремнію, синтезованого в умовах взаємодії нанорозмірного нестехіометричного карбіду кремнію з оксидом заліза

Characterization of thin carbon films formed on the iron surface by magnetron sputtering with ion-beam mixing

The chemical composition, interatomic chemical bonds, and specific features of the atomic structure of nanosized carbon films on the surface of iron, which were prepared by magnetron sputtering before and after ion-beam mixing, have been investigated using X-ray photoelectron spectroscopy, X-ray diffraction analysis, extended electron energy loss fine structure spectroscopy, and Raman spectroscopy. It has been found that, in the ion-beam mixing region, there are structural inhomogeneities and nonstoichiometric iron carbides. The parameters of the local atomic structure of an ultrathin surface layer and the “carbon film–metal” transition region have been determined.

Characterizing Atomic Interactions in Interstitial Non-Stoichiometric Compounds by Statistical Thermodynamics: Engineering Usage of Estimated Values of Statistical Thermodynamic Parameters

Statistical thermodynamics allows us to estimate atomistic interactions in interstitial non-stoichiometric compounds MXx through analysis of experimentally determined pressure-temperature-composition (PTC) relationships for MXx being in equilibrium with X2 in gaseous state (X=H,N,P or S) or for non-stoichiometric carbide MCx being in equilibrium with excess C. In case of analysis for MCx, chemical activity a(C) of C in place of partial pressure p(X2) of X2 gas must be known. On statistical modelling of crystal lattice structure for MXx, an a priori assumption of constant nearest-neighbour X-X interaction energy E(X-X) within a homogeneity composition range at arbitrary temperature T was accepted to determine number θ of available interstitial sites for occupation by X atoms per M atom. Values of interaction parame-ters estimated as such appear rational and realistic noting consistency of the values for M’s in the same group in the Periodic Table of the Elements and compatibility with enthalpy values evaluated by conventional thermodynamic approach. Engineering insights gained for MXx through analysis of atomistic interaction parameter values evaluated by the statistical thermodynamics are reviewed comprehensively in this paper. M might be substitutional alloy A1-yBy composed of constituents, A and B, or MZz containing another interstitial constituent Z besides X. Insights acquired from this line of statistical thermodynamic analysis appear to be of pragmatic use for advanced alloy design as shall be demonstrated hereafter.

PHASE COMPOSITION AND STRUCTURE OF TIC – HSS STEEL BINDER PRODUCED BY SHS METHOD

The paper describes the production of «TiC – steel binder» metal matrix composites by self-propagating high-temperature synthesis (SHS) in reaction powder mixtures of titanium, black carbon (soot), and HSS powders in laminar burning mode. Composite powders with various steel binder contents were prepared by milling and screening the synthesis products. The synthesis products were studied by optical and scanning electron microscopy, X-ray diffraction analysis, and electron probe microanalysis. It was found that an average size of carbide inclusions in the structure of the metal matrix composite depends on the content of the heat inert steel powder in reaction mixtures and can be controlled over a wide range. The lattice parameter of the titanium carbide formed in the SHS process is smaller than that of equiatomic TiC. The main reason for decrease in the lattice parameter is the non-stoichiometric carbide composition preconditioned by the carbon deficit. According to the results of the electron probe microanalysis, titanium carbide inclusions in the composite structure additionally contain up to 1 at% of iron and other alloying elements. The dissolution of iron and alloying elements leads to a certain increase in the carbide lattice parameter, which partially compensates for decrease in the lattice parameter caused by the carbon deficit. According to the results of the X-ray microanalysis, ferrite as a main phase in the metal binder has an ultra-equilibrium content of alloying elements. SHS products annealed at700 °Cresult in decomposition of retained austenite and dissolution of alloying element carbides in ferrite.

Microstructure of nanocrystalline powders of nonstoichiometric vanadium VC0.875 and niobium NbC0.93 carbides

The evolution of the microstructure of nonstoichiometric vanadium VC0.875 and niobium NbC0.93 carbide powders subjected to high-energy ball milling is investigated by neutron diffraction. It is established that milling produces non uniform powders and two distinct fractions with differing microstructure can be identified in them. It is shown that the time-of-flight neutron-diffraction technique is promising for studying the microstructure of highly deformed nonstoichiometric carbides and for quantitative determination of the anisotropy of microstrains.

Effect of small particle sizes on the measured density of nanocrystalline powders of nonstoichiometric tantalum carbide TaC y

Nanocrystalline powders of the nonstoichiometric tantalum carbide TaCy (0.81 ≤ y ≤ 0.96) with an average particle size in the range from 45 to 20 nm have been prepared using high-energy ball milling of coarse-grained powders. The density of the initial coarse-grained and prepared nanocrystalline powders of TaCy has been measured by helium pycnometry. The sizes of particles in tantalum carbide powders have been estimated using the X-ray diffraction analysis and the Brunauer–Emmett–Teller (BET) method. The density of TaCy nanopowders measured by helium pycnometry is underestimated as compared to the true density due to the adsorption of helium by the highly developed surface of the nanocrystalline powders. It has been shown that the difference between the true and measured densities is proportional to the specific surface area or is inversely proportional to the average particle size of the nanopowders. The large difference between the true and measured pycnometric densities indicates a superhydrophobicity of the tantalum carbide nanopowders.