Nitrogen In Α-iron Research Articles

Effect of Alloying Elements on Grain Boundary Segregation of Nitrogen in α-Iron

Effect of Alloying Elements on Grain Boundary Segregation of Nitrogen in α-Iron

Size-Dependent Transformation of α-Fe into γ′-Fe4N in Nanocrystalline the Fe–NH3–H2 System

On the basis of the results of experiments concerning nanocrystalline iron nitriding with ammonia–hydrogen mixtures, it was found that α-iron crystallites transform to iron nitride γ′-Fe4N in their whole volume in the order of their size, from the largest to the smallest ones. During the phase transition, the saturated solid solution of nitrogen in α-iron, being in the chemical equilibrium state, changes into an unsaturated solid solution of nitrogen in γ-iron, being in the transition state, and then to γ′-nitride, being again in the chemical equilibrium state. The phase transition is an adiabatic and isothermal process, and thus the change in Gibbs energy of the system is zero. On that basis the energy balance of the system was performed in this paper. The Gibbs energy required for phase transition was supplied to the iron nanocrystallite by the nitriding potential of the ammonia–hydrogen mixture. This energy depends on the ratio of the nanocrystallite active surface to its volume ratio and is the lowest...

Many-body potential for nitrogen in α-iron

N atom is one of the most frequent foreign interstitial atoms in α-iron along with C atoms. The Fe–C potential has been well-developed and can reproduce many significant interactions of C with point defects present in α-iron. However, there exists no satisfactory Fe–N potential to describe the interactions of N with point defects. Here, we develop a many-body potential for N in α-iron. The potential parameters are determined by fitting to ab initio data, which includes energetics, configurations, and relaxations of Fe atoms close to N atom. This potential successfully describes the interactions of Fe–N across a wide range of defect environments. The potential employs the embedded atom method form and hence is appropriate for large-scale molecular dynamics simulation.

Nitriding of Interstitial Free Steel in Potassium–Nitrate Salt Bath

A study has been made of nitriding of interstitial free (IF) steel in the potassium-nitrate salt bath at temperatures ranging from 400 to 650°C. The salt is decomposed to generate nitrogen and oxygen. Nitrogen diffuses into steel, or steel is nitrided, while oxygen reacts on steel surface to form the oxide scale. The oxide scale thickness is much smaller than the nitriding thickness. Most of nitrogen resides in steel as a form of interstitial solid-solution. For nitriding at higher temperatures, nitride precipitates (γ'-Fe 4 N and ζ-Fe 2 N) exist mostly in grain boundaries and partly in grains of the steel. The nitrate nitriding gives rise to much larger nitriding depth than other nitriding methods at similar nitriding temperature and time. The nitrate nitriding of steel substantially increase its tensile strength as well as hardness, e.g., an IF steel specimen nitrided at 650°C for 1.5 h shows a tensile strength of 916 MPa, which is 2.2 times higher than that of non-nitrided IF steel specimen, and an elongation of 20% at 70°C. Severe serrations are observed in flow curves of nitrided steel specimens, mainly due to dynamic strain aging that occurs because of interaction between dissolved nitrogen and moving dislocations. The effective diffusion coefficient of nitrogen D N obtained from the nitriding data, D N =D 0 expl(-Q/RT) with D 0 =3.789x10 -7 m 2 .s -1 and Q=76.62 kJ mol -1 , is approximately the same as that for diffusion of nitrogen in α-iron.

Ultrasonic absorption in ultra-low carbon steel

The laser-based reverberant technique is used to measure ultrasonic absorption spectra in the 2 to 45 MHz frequency range. This technique, being contactless, allows measurements at high temperature. The absorption spectra of ultra-low carbon steel samples are studied at room temperature in a magnetic field (in order to suppress the magnetoelastic contribution) and in a high temperature furnace (20–1200 °C) without magnetic field. Small steel samples (about 10×10×1 mm3) are used. At room temperature, two main contributions to the ultrasonic absorption are identified: microeddy currents (magnetoelastic contribution) and absorption caused by dislocations (deformation contribution). A typical microeddy current peak is observed and yields a reasonable estimate of the magnetic domain size. Above 10 MHz, the nonmagnetic contribution to the total absorption follows the classical vibrating string model. However, other phenomena also contribute to the absorption spectra. Below 10 MHz, an amplitude-independent damping background is observed. In addition, a small frequency-independent contribution to the absorption is observed at room temperature and is attributed to a thin surface layer. The absorption at high temperature is dominated below the Curie point by the magnetoelastic contribution. Two internal friction peaks are also detected. The first one, at 100 °C, is related to the dislocation kink motion. The second one, measured at 330 °C and 10 MHz, is attributed to the Snoek relaxation of carbon and/or nitrogen in α-iron. The Curie transition as well as the ferrite-austenite transition strongly affect the internal friction spectra.

Magnetic After-Effect and Internal Friction as Quantitative Tools for the Analysis of Carbon and Nitrogen in α-Iron

The high sensitivity of the magnetic after-effect (MAE) and internal friction (IF) on the presence of interstitially dissolved carbon (C) and nitrogen (N) in α-iron (Fe) is utilized for the quantitative analysis of corresponding impurity concentrations. The calibration of respective relaxation strengths is achieved, following precise chemical analyses of identically C- and N-doped Fe samples, under consideration of method-specific influences such as, i.e., texture of polycrystalline Fe samples (IF), geometry- and frequency-dependent demagnetizing factors and skin-effect (MAE). In agreement with theoretical calculations, the following strength parameters for IF and MAE of C and N, respectively, are determined: Q/cC ≃ Q/cN = 1.8 × 10−5 at ppm−1, and (Δr∞/r0)C/cC = 1.3 × 10−2 at ppm−1, (Δr∞/r0)N/cN = 0.65 × 10−2 at ppm−1, yielding detection limits, in the case of the MAE, of <0.1 at ppm. Die hohe Empfindlichkeit von magnetischer Nachwirkung (MNW) und innerer Reibung (IR) auf interstitiell gelosten Kohlenstoff (C) und Stickstoff in α-Eisen (Fe) wird zur quantitativen Konzentrationsbestimmung dieser Verunreinigungsatome eingesetzt. Die Eichung der entsprechenden Relaxationsamplituden erfolgte mit Hilfe praziser chemischer Analysen an identisch dotierten, polykristallinen Fe-Proben, unter Beachtung verfahrensspezifischer Einflusse, wie z. B. der Kristalltextur (IR) sowie der geometrie- und frequenzabhangigen Entmagnetisierungsfaktoren und Wirbelstromverluste (MNW). In Ubereinstimmung mit theoretischen Rechnungen werden fur IR und MNW von C und N jeweils folgende Amplitudenparameter bestimmt: Q/cC ≃ Q/cN = 1.8 × 10−5 at ppm−1, und (Δr∞/r0)C/cC = 1.3 × 10−2 at ppm−1, (Δr∞/r0)N = 0.65 × 10−2 at ppm−1, woraus sich - im Falle der MNW - eine Nachweisempfindlichkeit <0.1 at ppm ergibt.

Octahedral defects in a b.c.c. lattice examined by lattice theory

Displacements of atoms, induced by single or multiple interstitials located at octahedral sites in a b.c.c. lattice, were examined using an eigenstrain method in lattice theory with the harmonic approximation. It is shown that the assignment of proper eigenstrains yields the correct magnitude of the potential energy for hydrogen, carbon and nitrogen in α-iron. It is also shown that the precipitate structure of Fe 16N 2 proposed by Jack takes a minimum potential energy among various kinds of plausible interstitial agglomerates.

Diffusivity of nitrogen in α-iron at high nitrogen potential at 773 k

The diffusion coefficient of nitrogen in α-Fe has been well-documented in the literature. However such measurements under high nitrogen potentials have rarely been reported. In this investigation, the diffusivity of nitrogen in α-Fe at various high nitrogen potentials, attained by use of hydrogen—ammonia gas mixtures were measured by a thermogravimetric technique.

Effect of alloying element content on ion-nitriding behavior of Fe-Ti alloys

The ion-nitriding behavior of iron alloys with a titanium content of between 1.07 and 2.58 wt.% was investigated in the α-phase region. The behavior was found to be analogous to the internal oxidation behavior of iron alloys: An internal-nitriding layer, where small TiN precipitates are dispersed, as well as a very thin surface layer of γ′-Fe4N were formed. A parabolic rate law holds for growth of the internal-nitriding layer. The kinetics of growth of the internalnitriding layer is discussed according to the rate equation of internal-oxidation, giving the diffusion coefficient of nitrogen, D N app , in the layer. The measured D N app decreases as the volume fraction of TiN, f, increases, indicating that the diffusion of nitrogen is apparently inhibited by the existence of TiN precipitates. Furthermore, the diffusion coefficient of nitrogen in α-iron was evaluated by extrapolating D N app to f=0, being in good agreement with that reported previously. The f-dependence of D N app is discussed in terms of the effective area for diffusion of nitrogen in α-iron.

Ion-nitriding behaviour of Fe-Ti alloys in the?-phase region

The ion-nitriding behaviour of four iron alloys containing between 0.11 and 1.48 wt% titanium was investigated in theα-phase region to discuss kinetics of the growth of the nitriding layer. The ion-nitriding experiments have been made at 823 K. Two nitriding layers were observed: a thin surface layer which mainly consists of Fe4N; an internal nitriding layer beneath the surface layer, where the nitride formed was found to be TiN. The growth of the internal nitriding layer is controlled by a diffusion process of nitrogen in the matrix metal. The apparent diffusion coefficient of nitrogen in the nitriding layer, evaluated using the rate equation proposed for internal oxidation, increases linearly with the volume fraction of titanium nitride. Furthermore, by excluding the effect of the titanium nitride from the apparent diffusion coefficient, the diffusion coefficient of nitrogen inα-iron was calculated, being in good agreement with that reported so far. In addition, the increase in hardness in the internal nitriding layer has been discussed.

Magnetic Anisotropy Energy Caused by Nitrogen in α-Iron

The magnetic anisotropy energy due to interstitial nitrogen in α-iron is measured by means of the magnetostriction after-effect. The after-effect strength is measured as a function of concentration of nitrogen. The anisotropy energy is measured to be 4.3×1010J/m3 per unit fraction of nitrogen, or 5.1×10-16 erg per atom.

A contribution to the resolution of purity and grain-size effects on the relaxation strength of interstitial nitrogen in α-Iron

An analysis of the experimental data published on the inelastic behaviour of the interstitial N-αFe solid solution shows that grain-size and purity effects on the stress-induced ordering relaxation strength can be resolved into two independent phenomena. The grainsize effect is discussed in terms of grain boundary segregation: the necessity of assuming a strong enrichment of interstitials at intercrystalline ridges is proved. The influence exerted by purity on the relaxation strength is quantitatively analysed on the basis of the strain produced by substitutional impurities in the parent lattice. The results obtained make it possible to evaluatea priori the inelastic reponse of a N-αFe solid solution of known chemical composition and grain size. The distortion of the Snoek-peak profile, experienced in less pure materials, is to be connected with a relaxation-time spectrum.

&amp;alpha;鉄中の窒素の溶解度および拡散におよぼすニッケルの影響

The effect of nickel on the solubility of Fe4N and the diffusion of nitrogen in α-iron was examined by an internal friction technique. The used materials were pure iron and iron alloys containing 0.5, 1.0, 2.1 and 4.7 wt% nickel. The experimental results are as follows:(1) During measuring the Snoek peak of internal friction, it is necessary to apply the magnetic field of more than about 80 Oe to the specimen in order to eliminate the magnetoelastic damping.(2) The Snoek peak due to nitrogen in iron-nickel alloys takes place by the single relaxation mechanism.(3) The diffusion of nitrogen in α solid solution of iron and iron-nickel alloys was examined in the frequency range from 0.5 to 2.0 cps. Consequently, the diffusion coefficent of nitrogen in these alloys is represented by D=2.2×10−3exp(−18000⁄RT) and the effect of nickel on diffusion is negligible.(4) The solubility of Fe4N in α-iron is given by the equation log(N%)=−1230⁄T+0.36, and the heat of solution of Fe4N for α-iron is about 5600 cal/mol. The increase of nickel content dissolved in α solid solution has an effect of decreasing the solubility of Fe4N and has an effect of increasing the heat of solution of Fe4N.

The mechanical damping of iron from room temperature to 400°c at 7 megacycles/sec

The pulse-echo method has been used to measure the Snoek damping peaks of nitrogen and carbon in α-iron at 6.65 Mc/sec. The diffusion peaks occurred at 326° ± 2° C for nitrogen and at 353° ± 4° C for carbon. The diffusion coefficient of nitrogen so determined was in good agreement with a determination by other workers at nearly the same temperature by a bulk desorption method. The carbon diffusion coefficient so determined fell on the Arrhenius line extrapolated upward from the low temperature data, but the present data accentuates the anomaly reported by both Smith and Homan. Consideration of all the available diffusion data for nitrogen in α-iron (and also δ-iron) shows that the diffusion coefficient can be represented from −50° to 1470°C by a single Arrhenius relation, namely: D = 4.88 × 10 −3 exp[ −(18,350) RT ] cm 2/ sec . The diffusion coefficient of carbon may be represented from −40° to at least 350°C by: D = 3.94 × 10 −3 exp[ −(19,160) RT ] cm 2/ sec . This new value for carbon removes the discrepancy between D 0 for carbon and nitrogen and according to the ideas of Zener, shows that the strain energy is only a part of the total activation free energy for diffusion. The work on the carbon Snoek peak indicates that supersaturated solutions of carbon in α-iron may be quite persistent around 350°C. This could possibly account for the discrepancy between the heat of solution of cementite as measured by equilibria and by internal friction methods. The orientation dependence of the nitrogen Snoek peak height was observed with longitudinal waves propagated in the [100], [110] and [111] directions in single crystals, with results confirming Polder's theory. The tem perature dependence of this relaxation in single crystals gives a possible indication that nitrogen atoms resist martensite-type ordering to a great extent in dilute iron-nitrogen alloys. Magnetic damping peaks due to carbon and nitrogen, similar to those observed by Maringer near 1 c/s, have been observed at 6.65 Mc/sec. The small nitrogen Maringer peak, however, occurs at a lower temperature than the corresponding Snoek peak.

A kinetic theory of nucleation in dilute solid solutions; Application to the precipitation of carbon and nitrogen in α-iron

A kinetic theory of nucleation and growth of precipitate particles in solid solutions is derived. This theory, which is applicable at low solute concentration and rather low solute-solute binding energy, is obtained by application of equilibrium conditions to the reversible chemical rate equations of cluster formation. The equilibrium approximation permits sufficient cancellation of terms in the equations so that the remaining equations are integrable. Computer solutions for a variety of parameters of the kinetic equations were used to check the range of validity of the approximation necessary to obtain the analytic expression. The analytic expression describes the experimentally observed time dependence of the precipitation of carbon and nitrogen in α-iron. The analytic theory also predicts an average binding energy of precipitate atoms in a cluster which is in reasonable agreement with experimentally derived carbon-carbon and nitrogen-nitrogen binding energies in iron. The size of the fundamental precipitate nucleus, as well as the distribution of sizes of precipitate particles, is also predicted, but no experiments exist to check these predictions.

Calculations of the energy and migration characteristics of carbon and nitrogen in α-iron and vanadium

Various properties of carbon and nitrogen in iron, and nitrogen in vanadium have been investigated with the aid of computer techniques. The metal-metal interaction, represented by a two-body central force which matches the elastic moduli, is sharply repulsive at close separation, and goes to zero midway between the second and third neighboring atoms. 531 atoms surrounding the defect were treated as individual particles, while the remainder of the crystal was treated as an elastic continuum with atoms imbedded in it. The iron-carbon interaction was represented by a cubic equation with the parameters chosen to yield the experimental value for the carbon migration energy, the activation volume (effectively zero), and the binding energy of a carbon atom to a vacancy. With this model the approximate experimental value of the energy of a carbon atom in solution in iron relative to Fe 3C was obtained. The model also gives the correct relaxation strength for internal friction, but does not reproduce closely the formation volume. The migration path for carbon, on the basis of this calculation, is a straight line from octahedral to octahedral position with the tetrahedral position as the saddle point. The behavior of nitrogen in iron is almost identical. The same method of calculation for nitrogen in vanadium reproduced closely the sizable experimental activation volume for this system. Some calculations of di-carbon formation and motion are also reported.

The application of the Snoek damping method to the determination of gas desorption from metals

As an example for an investigation on the desorption of gases from metals into a high vacuum, wires made of highest purity iron, containing small amounts (up to .07 per cent by weight) of nitrogen were exposed for several hours to temperatures between 500° and 800°C. The exposure took place either in high vacuum (10 −6 torr) or in an atmosphere of hydrogen at pressures between 10 −4 and 10 torr. The resultant desorption of gas originally dissolved in the metal was determined from the reduction in the nitrogen in the α iron with the help of the Snoek (named Snoek after its discoverer 2) damping method. Whilst the desorption rate at hydrogen pressures ranging from 10 −2 to 10 −6 torr is practically independent of the pressure, the desorption of the nitrogen from the iron is markedly increased of the presence of hydrogen at higher pressures. Experiments were carried out at a pressure of 10 −6 torr and a temperature of 700°C with wires of various diameter and for various periods. It appeared that the rate of reduction of the concentration of of the nitrogen in α-iron is proportional to the square of the corresponding nitrogen concentration in the iron. This relationship allows us to conclude that thegaseous atoms dissolved in the metal recombine to form molecules at the metal surface before the gas can desorb into the vacuum. The activation energy for the desorption process of the nitrogen from the interior of the α-iron into a high vacuum can be determined by varying the temperature of the process. It amounts to 28900 (±1200) cal/mol.

The influence of pressure on the mean time of stay of interstitial carbon in iron

The influence of the hydrostatic pressure on the mean time of stay of interstitial carbon atoms in α-iron was determined by measuring the time decrease of permeability. This influence was found to be very small at temperatures from 239°K up to 253°K. The corresponding activation volume is negligible. It is concluded that Keyes' semi-empirical relationship between activation energy and activation volume does not hold for this case, no more than for solid solutions of nitrogen in α-iron.

Diffusion of Interstitial Solutes in the Group V Transition Metals

Mechanical relaxation measurements are used extensively to obtain information on the diffusion rate of interstitial solute atoms in body-centered cubic metals. Such studies have been stimulated by a model, developed by J. L. Snoek, which yielded a relationship between a relaxation time, an experimental parameter, and the diffusion coefficient of the solute atom. Although Snoek's model was confirmed very well for solid solutions of carbon or nitrogen in α-iron, a number of anomalies were observed when relaxation studies were extended to the group V transition metals. An extensive experimental study has been made of the factors that influence relaxation times. The anomalous behavior of the group VA metals can be accommodated within the framework of Snoek's model by taking account of the specific nature of solid solutions based on these metals. Diffusion data obtained by a variety of relaxation techniques are presented for oxygen, nitrogen, and carbon in vanadium, niobium (columbium), and tantalum. These data are in agreement with those obtained by the more conventional concentration gradient techniques in the few instances where such information is available. The pattern of activation energies suggest that lattice strain considerations alone are insufficient to explain the activation process involved in interstitial diffusion.

The influence of pressure on the mean time of stay of interstitial nitrogen in iron

Abstract The influence of the hydrostatic pressure on the mean time of stay of interstitial carbon atoms in α-iron was determined by measuring the time decrease of permeability. This influence was found to be very small at temperatures from 239°K up to 253°K. The corresponding activation volume is negligible. It is concluded that Keyes' semi-empirical relationship between activation energy and activation volume does not hold for this case, no more than for solid solutions of nitrogen in α-iron.