Fe3C Carbides Research Articles

Surfactant-Assisted Preparation and Magnetic Properties of Iron-Based Nanowires

Iron carbide Fe7C3 nanoparticles were prepared by decomposing pentacarbonyl complex precursors in a closed autoclave reactor. Spherical nanoparticles were obtained without adding polyethylene (10) octylphenyl ether (OP-10) as a surfactant. The addition of OP-10 induced the one-dimensional crystal growth to form the Fe7C3 nanowires. The iron metal nanowires obtained by reducing the above-mentioned carbide showed anisotropic magnetic properties with a saturated magnetization value of ∼180 emu/g.

Metal Dusting of Nickel and its Alloys

Pure nickel, Ni-Cu binary alloys, Types 304 and 310 stainless steels and Alloy 800H were all reacted with carbon- supersaturated CO/H2/H2O at 680oC. In general, the reaction led to growth of an external deposit, together with graphitization at both external surfaces and interior grain boundaries. By independently controlling the supersaturated carbon activity and pCO, it was demonstrated that the reaction of pure nickel was determined by parallel, independent reaction paths, and the carbon uptake rate was well described by Rate = k1pCOpH2 + k2p2CO - k3p2H2 leading to a maximum rate at about pCO ~ 0.7 atm. The surface graphite layer thickened, growing into the metal, while graphite particle clusters and nanofilaments formed on the surface. Each particle cluster and filament contained metal nanoparticles. Addition of copper to the nickel suppressed graphite particle cluster formation, thereby decreasing greatly the overall dusting rate. This phenomenon is interpreted in terms of graphite nucleation and copper interference with nucleation sites involving multiple nickel atoms. Reaction conditions used were such as to stabilize Fe3C and chromium carbides, Cr2O3 and spinel, but not iron or nickel oxides. Under these conditions, all three alloys, 304SS, 310SS and 800H, dusted, with 304SS experiencing the greatest metal wastage. Additions of copper had no effect on the dusting rate of 304SS, but greatly decreased the dusting of 310SS and 800H.

Development of Fe3C, SiC and Al4C3 compounds during mechanical alloying

Mechanical alloying process, as a solid-state technique, is a very useful method for fabrication of high melting point compounds like metal carbides and nitrides, which additionally have nanocrystalline structure with improved properties. In this work the development of several carbides including iron, aluminium and silicon carbides by the mechanical alloying process and the effect of subsequent heat treatment were investigated. Mixtures of elemental powders of Fe–C, Si–C and Al–C were mechanically alloyed, nominally at room temperature using a laboratory planetary ball mill. Structural changes of samples were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the aluminium carbide (Al4C3) could not be synthesized by mechanical alloying process alone, even after long milling times. A suitable subsequent heat treatment was required to allow Al–C reaction to take place kinetically. In contrast mechanical alloying of Fe–C as well as Si–C systems directly led to the formation of Fe3C and SiC carbides after sufficient milling time. In all cases the end product had a nanosized structure.

Structural peculiarities of cementite and their influence on magnetic characteristics

The iron carbide Fe3C has been studied by first-principles density-functional theory. Both prismatic andoctahedral environments of the carbon atoms have been considered. The crystal structurewith the carbon position in a prismatic environment possesses the lowest total energy. Theenergy of the magneto-crystalline anisotropy is an order of magnitude higher in theprismatic modification than in the octahedral one. The room-temperature behaviour of thecoercive force on the temperature of annealing of plastically deformed samples has beenexplained by this fact. A difference in Mössbauer parameters (electric field gradient (EFG),and the angles between the EFG and the hyperfine field) found for the two positionsof carbon in the cementite lattice looks promising for experimental detectionof the presence of carbon with octahedral environment in plastically deformedsamples.

Synthesis of Aluminium–Cementite Metal Matrix Composite by Mechanical Alloying

Cementite powder was prepared from elemental iron and graphite powder by mechanical alloying (MA) in a specially built dual-drive planetary mill. The phase evolution, particle-size distribution, and morphology of particles were studied during 40 hours grinding period. X-ray diffraction (XRD) shows formation of cementite and other iron carbides along with elemental iron after milling, whereas after annealing only cementite is present. Initially particle size increases with milling due to ductility of iron powder and then reduces with further milling. Al-cementite composite was synthesized by mixing cementite with Al powders, and then by hot pressing or cold compaction and sintering. XRD analysis of Al–Fe3C composite shows Fe3C, FeAl, Al, and other iron carbides along with Al4C3 after sintering. Scanning electron microscope (SEM) micrograph of hot-pressed samples shows excellent compatibilility between Al matrix and cementite particles.

Fabrication of ferromagnetic iron carbide nanoparticles by a chemical vapor condensation process

Iron carbide Fe 3C nanoparticles were fabricated by a chemical vapor condensation (CVC), and their structural and magnetic properties were investigated. It was found that the formation of iron carbide nanoparticles strongly depended on reaction temperature and pressure. The iron carbide was formed only above 650 °C at 101 kPa (760 Torr) but never formed in vacuum (1.33 Pa) regardless of reaction temperature. The fabricated iron carbide nanoparticles were spherical with T c of about 240 °C. While the particles formed at 650 °C (∼30 nm) possessed a typical core–shell structure, those formed at 800 °C (∼50 nm) did not have such structure. The nanoparticles fabricated at 650 °C and 800 °C at 101 kPa, exhibited M s of ∼125 A m 2/kg whereas i H c of the former was slightly higher (∼63 kA/m) than that of the latter (∼40 kA/m).

Carbon diffusion in cementite (Fe 3C) and Hägg carbide (Fe 5C 2)

Pure iron samples were carburised at 500 ∘C in CO–H 2–H 2O–H 2S gas mixtures to study the growth kinetics of metastable iron carbides. The carbon activity was varied in the range 7 ≤ a C ≤ 4580 . Small amounts of H 2S were added to the atmosphere to prevent graphite deposition and metal dusting from occurring. The gas composition was optimised to obtain parabolic carbide layer growth over a sufficiently long time period. At activities a C < 150 parabolic growth of cementite (Fe 3C) was observed. At higher activities a C > 150 Hägg carbide (Fe 5C 2) formed on top of the cementite. From the dependence of the growth rate on carbon activity in the gas phase the self-diffusion coefficients D C ∗ of carbon at 500 ∘C were determined for both iron carbides Fe 3C and Fe 5C 2.

Structure and phase composition of a catalyst for carbon nanofiber formation

Novel carbon nanofibers (CNFs) consisting of graphite layers crossed by two systems of stripes are formed on an iron catalyst using the chemical vapor deposition method under conditions close to the phase equilibrium of the iron carbides Fe3C and Fe5C2. The formation of these CNFs is explained by the deformation caused by the decay of the carbides. It is shown that the catalyst particle, often twinned, consists of Fe3C, Fe5C2 or both. The crystallographic peculiarities of α-Fe → Fe3C → Fe5C2 transformation are discussed.

Atomic structures of iron-based single-crystalline nanowires crystallized inside multi-walled carbon nanotubes as revealed by analytical electron microscopy

Atomic structures of single-crystalline iron-based nanowires crystallized inside multi-walled carbon nanotubes during pyrolysis on silicon substrates with ferrocene as a precursor were analyzed using high-resolution analytical transmission electron microscopy and electron diffraction. Standard crystal lattices, namely body-centered cubic (bcc) α-Fe, face-centered cubic (fcc) γ-Fe and orthorhombic cementite Fe 3C, were all found to form inside the nanotubes. A fraction of α-Fe nanowires, thermodynamically favorable at room temperature, was found to be dominant. Both bcc and fcc nanowires display a wide variety of lattice planes being parallel to the nanotube walls, with none of the orientations being preferable. The minor fraction of the nanowires had unidentified long-period crystal lattices with doubled or tripled periodicities as compared to those found in the standard cubic iron phases. The crystal matching of these unusual structures to stable orthorhombic Fe 3C or less stable iron carbides, e.g., Fe 5C 2, Fe 7C 3, failed. The non-conventional phases were tentatively assigned to rarely seen silicon-doped octahedral iron carbides. Both long-period and standard cementite nanowires exhibited well-defined transient zones in the vicinity of nanowire–tube shell interfaces, where perfectly ordered carbide lattice fringes disappeared. The results suggest the non-existence of metastable equilibrium in the nanoscale Fe–C system between carbide and graphite phases during iron crystallization inside graphitic tubular channels.

Nanostructure of a cold drawn tempered martensitic steel

Abstract The carbon atom distribution in a tempered martensitic steel processed by cold drawing was investigated with a three-dimensional atom probe. Data clearly show that cementite starts to decompose at an early stage of deformation. This indicates that the driving force of cementite decomposition during plastic deformation is not related to a strong increase of the interfacial energy. Carbon atmospheres were also analysed. They probably result from pipe diffusion of carbon atoms along dislocations pinned by Fe3C carbides.

Microstructural characterisation of Fe–Cr–P–C powder mixture prepared by ball milling

In an attempt to prepare amorphous Fe 77Cr 4P 8C 11 alloy by mechanical alloying, powder mixtures of Fe, Cr, P and C (activated carbon) with a purity of 99.9% have been ball milled, under argon atmosphere for several periods up to 90 h, in a planetary ball mill (Fritsch P7). The structure and microstructure of the milled powders have been characterised, as a function of milling time, by X-ray diffraction and Mössbauer spectrometry. During the first stage of milling (up to 12 h), detailed analyses of the diffraction patterns and the Mössbauer spectra reveal a complete dissolution of the elemental powders and the formation of Fe x P (1 < x < 2), Fe 3P phosphides and Fe 3C carbide in addition to α-(Fe,Cr) phase. The amorphous like state is reached after 32 h of milling. Further milling leads to the appearance of new carbides such as (Fe,Cr) 7C 3 and ɛ′-Fe 2.2C with the phosphide (Fe,Cr) 3P type phase embedded in the amorphous matrix.

Microprocesses of metal dusting on iron‐nickel alloys and their dependence on the alloy composition

AbstractMetal dusting of iron proceeds via the formation and disintegration of the metastable carbide Fe3C, and the resulting fine Fe particles in the coke further catalyse carbon deposition. By contrast, nickel disintegrates directly, and larger grains are released. As revealed by TEM and AEM techniques, in both cases the disintegration proceeds by inward growth of thin graphite filaments, the atomic basal planes of which being oriented perpendicular to the surface thus effecting a high reactivity at the growth front.Consequently, successive alloying of iron with nickel should lead to a change over from one disintegration mechanism to the other, and, in fact, we could evidence that the carbide formation takes place only up to a nickel content of about 5 wt.%. Already at a Ni concentration of 10 wt.% a direct disintegration of the metal proceeds, as it is typical for pure nickel. Furthermore, in all investigated Ni‐Fe alloys a surface‐near enrichment of Ni was observed which indicates a selective corrosion of Fe, decreasing with increasing Ni content of the basic alloy.

Effect of Structure and Phase Composition on Coercive Force and Saturation Magnetization of Mechanically Alloyed Fe100 – xCx (x = 25, 32) Powders

Mechanical fracture of iron and graphite powders leads to formation of the amorphous Fe–C phase and subsequent precipitation of the Fe3C carbide in the Fe(75)C(25) system and Fe3C→Fe7C3 carbide in the Fe(68)C(32) system, which are in strongly strained states. After annealing, carbides with undistorted structures are formed, and their content is higher. On the initial stage of this mechanical alloying, the magnetic characteristics of these structures are determined by their defectiveness, and on the subsequent stages of fracturing and annealing by the types, quantities, and structural conditions of the resulting carbide phases.

Anodic Dissolution of Armco Iron in a Mixed Kinetics Mode: Effect of Macroscopic Heterogeneity

Dependence of the current density of dissolution of an RDE made of Armco iron on the solution agitation rate is studied. Experiments are performed in weakly acid nitrate solutions of pH 1 containing 1–3 M of nitrate ions, at different charges passed, in a prepassive region. On the basis of these dependences, the effect the macroscopic heterogeneity (accumulation of passive phase of iron carbide Fe3C during dissolution) exerts on the dissolution is analyzed. The anomalous behavior of the dissolution rate (which increases with decreasing rotation rate ω of RDE in a certain ω interval) is due to transition from a mixed diffusion–kinetic control of dissolution rate at a macroscopically heterogeneous electrode to a diffusion control, when the salt film that forms during dissolution screens the heterogeneity (passive areas). Heterogeneity parameters under mixed control may quantitatively be analyzed at high ω, if at low ω one observes a diffusion dissolution connected with a complete surface coverage by the formed salt film.

Diamond formation using Fe 3C as a carbon source at high temperature and high pressure

Fe 3C carbide was treated at a high temperature of 1630 K and a high pressure of 5.3 GPa (HPHT) in a cubic anvil apparatus. Diamond is found to form from iron carbide at high temperature–high pressure (HPHT). The formation of the diamond can be confirmed by laser Raman spectra. This work opens a new way to synthesize diamond at HPHT and provides direct evidence about the diamond formation mechanism in the presence of a transition metal catalyst using graphite as the carbon source at HPHT.

Fractal characterization of fractured surfaces of a steel containing dispersed Fe 3C carbide phase

Fractals are mathematical models of very irregular sets. In particular, the one-dimensional statistical fractals on a plane are useful models for fracture profiles. The vertical section method was used to obtain the fractal dimension D. The investigation were performed on a high carbon steel with ca. 1% C after spheroidization annealing. The fracture surfaces were obtained by fracturing of Charpy V notch specimens, at the following testing temperatures: −20°C, −45°C, and −196°C. From the results obtained it may be concluded that the analyzed fractured surfaces have weak fractal properties, which are not dependent on the impact test temperature ( D=1.09–1.10, by the yardstick method).

Phase Composition and Distribution of Ni-, Co-, Fe-, Mn-, and Cr-Based Inclusions in Carbonado Polycrystals

The phase composition and distribution of inclusions of the Ni, Co, Fe, Mn, and Cr metal catalysts in carbonado polycrystals 8 mm in diameter were studied. The polycrystals (>90 wt % diamond) were found to contain inclusions of solid solutions of carbon in Ni and Co and the carbides Fe3C, Mn5C2 , and Cr3C2 . The concentration of inclusions was shown to increase with distance from the catalyst insert. The results are interpreted in terms of the model for the growth of polycrystalline diamond via corrugation of graphite layers, their dissolution in the catalyst melt owing to structural matching with its fcc lattice, and stabilization of the diamond structure at high supersaturations in the p–T stability region of diamond.

Mechanochemical nitridation by ball milling iron with pyrazole: a structural investigation

Nitrides of iron have been obtained by mechanochemical treatment of Fe powder with the amine compound, pyrazole (C 3H 4N 2). The structural developments which occur during the milling of the Fe and pyrazole powder mixture have been monitored by x-ray diffraction. The hexagonal ɛ-Fe 3N x phase with x ~ 1.1–1.2 was found to be the main milling product. The decomposition of the nitride and the formation of iron carbide Fe 3C have been observed after transient heating at 550 °C. Analysis of x-ray diffraction data for the as-milled and thermally treated powders has led to information on the locations of the carbon and the hydrogen stemming from the pyrazole compound.

Carbon in the core

Carbon is extremely abundant in the solar system (10 × Si, 20 × S) in Cl carbonaceous chondrites (3.2 wt%) and it dissolves readily in liquid Fe at low pressures (4.3 wt% at 1420 K). Despite these properties it is rarely considered a potential light element in the Fe-rich core, because it is volatile, even at low temperatures as CO. In this paper I show that carbon volatility is a strongly pressure-dependent phenomenon and that it applies during condensation from a solar gas ( ∼ 10 −3 atm), but not at the pressures and temperatures generated during planetary accretion and differentiation (0.01–5 GPa). Thus, impact heating and degassing of the protoearth should have led to an Fe-rich melt with around 2–4 wt% carbon, compared to the 0.01–0.6 wt% in iron meteorites and 0.3–3 ppm C predicted to be present in Fe condensed from the solar gas. Experiments (to 9 GPa) and thermodynamic calculations on the systems Fe C and Fe C S show that carbon solubility in Fe melt increases slightly with pressure but that carbon could not conceivably constitute more than half the light element content of the core. The addition of even very small amounts of carbon ( < 1%) to liquids containing Fe and a light element such as S has, however, a dramatic effect on the properties of the system. At 330 GPa (inner core-outer core boundary) 0.3% of carbon is sufficient to stabilise iron carbide Fe 3C, rather than ε-Fe as the first phase to crystallize in melts with around 10% S. Thus, for most conceivable ratios of S/C, the inner core would be expected to be crystallising Fe 3C, rather than ε-Fe or Fe Ni alloy. Given probable inner core temperatures of around 5000–6000 K, both ε-Fe and the higher pressure α′-Fe are too dense to explain the inner core density of 12.85 Mg m −3. The stability of iron carbide provides a possible solution. I show that, given a plausible equation of state for Fe 3C, this phase acquires the inner core density in the right pressure-temperature range.

Some polytypes of Fe7C3 carbides

Abstract Reduction of the Fe7C3 unit cell performed by other authors (Herbstein and Snyman, 1964; Fruchart, Senateur, Bouchaud and Michel, 1965) leads to the conclusion, that the unit cell of this carbide is primitive orthorhombic with lattice constants: a = 688 pm, b = 595 pm and c = 452 pm. In Fruchart's (1965) data the lattice constant “b” is twice as large. The analysis performed has shown that the Fe7C3 unit cell produced as a result of ordered translations differs only in the “a” parameter and the configuration of atomic layers. In agreement with the polytype definition given in the work of Verma and Krishna (1966) these unit cells are polytypes of the Fe7C3 carbide with the monoclinic or orthorhombic structure. These polytypes have been characterized by the Ramsdell (1943) symbol and lattice parameters.