Iron-vanadium Alloys Research Articles

Prediction and Hydrogen Acceleration of Ordering in Iron-Vanadium Alloys

Ab initio calculations of binary metallic systems often predict ordered compounds in contrast to empirical reports of solid solutions or disordered phases. These discrepancies are usually attributed to slow kinetics that retains metastable structures at low temperatures. The Fe-V system is an example of this phenomenon, in which we predict two ordered stable ground states, Fe3V and FeV3, whereas a disordered σ phase is reported. We propose to overcome this difficulty by hydrogen absorption, which facilitates metal atom mobility through vacancy formation and separation between the two elements due to their opposite affinities towards it, thus accelerating transformation kinetics. Hydrogen also increases the relative stability of the ordered structures compared with that of the σ phase without affecting the shape of the phase diagram. The hydrogen-induced formation of the ordered structures is expressed by a reversible decrease of the electrical resistivity with increasing hydrogen pressure. Such behavior has not been reported before in thin H absorbing films. Formation of the ordered structures is further substantiated by the kinetics of the resistivity changes upon variation of the hydrogen pressure, where two stages are distinguished: a fast initial stage and a much slower subsequent process in which the resistivity changes direction, associated with hydrogen dissolution and phase transformation, respectively.

Effect of vanadium neighbors on the hyperfine properties of iron–vanadium alloys

The electronic and magnetic structures of Fe–V alloys are calculated using the discrete-variational and full-potential linearized-augmented-plane wave methods. The derived hyperfine properties at Fe sites are studied against the number of Fe atoms in the neighbouring shells. As expected the magnetic hyperfine field depends strongly on the number of Fe atoms in the first and second shells of neighbours while its dependence on the variation of atoms in the third shell is weak. The calculated distribution of the magnetic hyperfine fields at the Fe sites, are compared to the experimental data of Krause et al. (Phys Rev B 61:6196–6204, 2000). The contact charge densities and the magnetic moments are also calculated. It was found that the contact charge density increases with increasing V contents and this leads to negative isomer shift on addition of V.

Mechanism and kinetics of corrosion decomposition of iron-vanadium alloys in solutions of acids and salts

Corrosion of iron–vanadium alloys containing 0 to 100% of vanadium is studied in dilute HCl and H2SO4 solutions by gravimetric method and by taking anodic polarization curves. Alloys containing no less than 30% of vanadium are readily passivated in sulfuric acid solution. Local passivity breakdowns (pitting or intergranular corrosion) are observed in HCl solutions. Apparent values of number of electrons, transfer coefficients, exchange currents, and corrosion currents are determined for active alloys. Catalytic action of V(II, III) and V(III) ions in cathodic process in aerated solutions is demonstrated.

The effect of pre-segregation of phosphorus on the dynamic embrittlement of iron-vanadium alloys at 550 °C in air

In this paper the author presents preliminary investigations of another example of dynamic embrittlement, together with the study aimed at examining the possible effect of grain boundary pre-segregation of phosphorus on the dynamic embrittlement of iron-vanadium alloys. The preliminary studies suggests that Fe-V containing alloys are susceptible to dynamic embrittlement by oxygen-containing environment. However, intergranular pre-segregation of phosphorus can suppress the dynamic embrittlement process. Efforts are currently underway to further investigate the phenomenon at different temperatures and partial pressures of oxygen.

Grain boundary segregation of phosphorus in iron-vanadium alloys

Grain boundary segregation of phosphorus has been studied in FeVP and FeVPC alloys through fracture experiments in a scanning Auger microprobe with the objective of examining the effects of vanadium on the interaction processes operative under circumstances when the structure in the interior of the grain (in the present case carbide formation) and grain boundary segregation occur simultaneously. It is understood that to predict and analyse the behaviour of an alloy, it is pertinent to consider the atomic interactions both at the grain boundaries and in the grain interior and that between the constituents and the grain boundaries. The study suggests that the determining factor for suppression or decrease in the migration of phosphorus to the grain boundaries is whether vanadium is present in the combined form (say, carbide) or is available in solid solution form. When vanadium is present in solid solution form, grain boundary segregation of phosphorus is low because of the chemical interaction of vanadium and phosphorus. However, as carbon is increasingly introduced in the alloy, vanadium now preferentially interacts with carbon in view of a higher interaction for carbon as compared to that of phosphorus. A consequence of this is an increase in the grain boundary concentration of phosphorus. In such a situation, the presence of excess carbon in addition to what is stoichiometrically required to precipitate the entire vanadium as vanadium carbides serves as a palliative with regard to the reduction in the intergranular concentration of phosphorus. This palliative behaviour is explained in terms of the site-competition model. An effort is also made to examine the behaviour of segregating elements in terms of a whole range of probable interactions (both at the grain boundaries and in the grain interior) and chemical interaction energies.

Effects of Oxygen and Nitrogen on the Rate of Nitrogen Dissolution in Iron-Chromium and Iron-Vanadium Alloys.

Effects of Oxygen and Nitrogen on the Rate of Nitrogen Dissolution in Iron-Chromium and Iron-Vanadium Alloys.

Mixing enthalpies and calorimetric investigations of liquid iron-vanadium alloys

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High-temperature thermopower of iron and dilute iron-vanadium alloys

The thermopower of pure iron and dilute iron-vanadium alloys (0-1.8 at.% V) has been measured in the temperature range 300-1750 K. The temperature dependence of the thermopower for the BCC phases above and below the FCC regime has been clearly established for the first time. The large differences between the BCC and FCC values reflect the difference in band structures, and the large value for the lattice indicates the presence of fine structure in the energy spectrum and hence a strong energy dependence of scattering.

Polarised neutron study of iron-vanadium alloys

The authors have made polarised neutron diffuse-scattering measurements on ferromagnetic alloys Fe1-cV/sub /c(0.01<c<0.2). The nuclear magnetic cross sections are analysed within Marshall's formalism, using the short-range order parameters previously measured. These new results show a negative constant moment of about -1 mu B on vanadium sites up to 10 at.% V. In particular they observe no steep enhancement of the vanadium moment at dilute concentrations. The iron and vanadium moments are in good agreement with theoretical predictions.

Diffusitivity of carbon in Fe-V alloys

Tracer diffusion of carbon in iron—vanadium alloys containing 2.45, 0.5 and 0.1 at% vanadium has been measured in a temperature range from 900 to 1325 K. No effect of magnetic transforms is observed on the C14 diffusivity in the α-stabilized Fe-2.4 at% V alloy, as has been reported for α-iron. Trapping behaviour of vanadium in slowing down the carbon diffusivity in iron shows a reasonable fit with the model predicted by Koiwa. However, the effect of these traps appears not to be felt during lowtemperature internal friction measurements in Fe-2.4 at% V alloy.

Magnetic moment distribution in iron vanadium alloys

Neutron diffuse scattering measurements were performed on FeV alloys from 1 to 20 at. % vanadium. The nuclear cross sections were previously investigated in comparison with NMR experiments and exhibit a very strong atomic short range order (SRO). The magnetic cross sections are analyzed here in terms of the localized moment model of Marshall using the known atomic SRO parameters. The data indicate a strong negative moment on the impurity site, almost constant at about −1.1μB up to 5 at. % V. It then tends to zero as the Vanadium concentration increases. The moment values are in very good agreement with previous unpolarized neutron experiments on dilute alloys. The magnetic perturbation terms on the neighbor sites are shown to have similar trends as shown by the SRO parameters. The values of the magnetic cross sections for K = 0 are in good agreement with the values of (dμ/dc)2 derived from magnetization measurements when c⩽4 at. %V. They differ strongly for higher concentrations, which suggests that neutrons probe a more restricted region around the impurity than magnetization measurements.

Polarised neutron study of the concentration dependence of magnetic moment distribution in iron-vanadium alloys

A polarised neutron diffraction study of three iron-vanadium alloys containing 5, 15 and 22 at.% of vanadium has been carried out. The localised 3d spin moment, the diffuse negative moment and the asphericity parameter have been determined from these measurements, and are compared with results from bandstructure calculations.

The Thermochemical Properties of Solid and Liquid Iron-Vanadium Alloys

The Thermochemical Properties of Solid and Liquid Iron-Vanadium Alloys

Hydrogen permeability of ironvanadium alloys

Hydrogen permeability of ironvanadium alloys

Electrical resistivity of iron-vanadium alloys between 78 and 1200 K

Electrical resistivity (ϱ) of FeV alloys containing 0.5, 0.9, 2.7, and 6.1 at% V has been measured as a function of temperature ( T) between 78 and 1200 K. The ϱ vs. T curves exhibit a change in the slope at the ferromagnetic Currie temperature ( T c). The d ϱ/d T vs. T curves in the neibhorhood of T c are similar to the corresponding plot for pure Fe. Our studies confirm the previously observed anomalous effect of V on T c of Fe, i.e., that T c increase with small additions of V to Fe. The critical index λ + associated with the power law of d ϱ/d T just above T c has been determined as a function of V concentration.

Specific Heat of Iron-Rich Iron-Chromium, Iron-Aluminium, and Iron-Vanadium Alloys between 600 and 1200 °K

The specific heats of iron alloys with up to 21 at% aluminium, 30 at% chromium, and 30 at% vanadium show sharp λ-peaks even for alloys with high non-ferromagnetic impurity concentration. Only the alloy with 40 at% chromium has a rounded peak. The temperature dependence of the magnetic specific heat cM follows frequently a function of the form cM ∝ In ϵ over a range 10-1 > ϵ > 10-3 above the Curie temperature Tc. ϵ is defined as ϵT TC - 1 ∣. A logarithmic dependence on ϵ for cM is found below Tc only in intervals of the order of 10-1 > ϵ > 10-2 for some alloys. Magnetic energy and entropy values are compared with theoretical predictions from series expansion calculations. It is frequently not possible to associate magnetic specific heats uniquely with either the Heisenberg model or the Ising model. Die spezifische Warme von Eisenlegierungen mit bis zu 21 At% Aluminium, 30 At% Chrom and 30 At% Vanadium zeigt scharfe λ-Maxima sogar fur Legierungen mit hoher nichtferromagnetischer Storstellenkonzentration. Nur die Legierung mit 40 At% Chrom besitzt ein abgerundetes Maximum. Die Temperaturabhangigkeit der magnetischen spezifischen Warme cM folgt haufig einer Funktion der Form cM ∝ In ϵ uber einen Bereich 10-1 > ϵ > 10-3 oberhalb der Curie-Temperatur TC- ϵ wird als ∣T / Tc - 1 ∣ definiert. Eine logarithmische Abhangigkeit von ϵ fur cM wird unterhalb Tc nur in Intervallen der Grosenordnung 10-1 > ϵ > 10-2 fur einige Legierungen gefunden. Die Werte der magnetischen Energien und Entropien werden mit theoretischen Vorhersagen aus Reihenentwicklungsrechnungen verglichen. Es ist haufig nicht moglich, die magnetischen spezifischen Warmen mit dem Heisenbergmodell oder mit dem Isingmodell eindeutig zu verknupfen.

Determination of oxygen activities in iron-vanadium alloys by a levitation melting procedure

The effect of vanadium on the activity of oxygen in liquid iron has been determined by a new method based on levitation melting. With water-vapour/hydrogen mixtures to control the oxygen potential of the gas phase, thermal diffusion effects caused uncertainty in the actual gas composition at the droplet interface. For this reason the data have not been used to determine equilibrium constants directly although it has been shown that they can be evaluated by conducting experiments with the addition of an inert gas to change the mean molecular weight of the gas mixture. By expressing the data in terms of an apparent equilibrium ratio vanadium-oxygen interaction parameters have been calculated for temperatures between 1550° and 1750°C. The results agree well with data obtained from crucible experiments where thermal diffusion effects were eliminated by bubbling the gas through the melt. Résumé L'effet de vanadium sur l'activite de l'oxygéne dans le fer liquide a été déterminé en utilisant une nouvelle méthode fondée sur la lévitation électromagnetique du spécimen en fusion. Le potentiel d'oxygène de la phase gazeuse a été contrôlé au moyen de mélanges de vapeur d'eau et d'hydrogene. Les auteurs n'ont pas déterminé les constantes d'équilibre vu l'existence du phénomène de diffusion thermique qui introduisait une incertitude quant à la composition du mélange gazeux à l'interface du spécimen. Ces constants pourraient être déterminées au moyen de manipulations qui consisteraient à faire varier le poids moléculaire moyen du mélange gazeux par addition d'un gaz inerte. Les paramètres d'interaction entre le vanadium et l'oxygène ont été calculés pour des températures variant de 1550°C a 1750°C en exprimant les données en fonction d'une constante apparente d'équilibre. Les résultats sont en bon accord avec les résultats obtenus à partir de manipulations utilisant des creusets où la diffusion thermique avait été éliminée en faisant passer le mélange gazeux à travers le bain.

Determination of oxygen activities in iron-vanadium alloys by a levitation melting procedure

Determination of oxygen activities in iron-vanadium alloys by a levitation melting procedure

THE ZENER RELAXATION IN SOME HIGH PURITY IRON-VANADIUM ALLOYS

The Zener relaxation has been studied in three high purity iron vanadium alloys containing 12, 16 and 20 wt. % vanadium. The measurements have been made in an inverted torsion pendulum under high vacuum in the frequency range 0.1-10 Hz. The relaxation time (τr) in the temperature range 650 to 720 °C can be described by the equations Fe-12 % V τr = 1.1 x 10-22 exp(93,500/RT) Fe-16 % V τr = 1.6 x 10-23 exp(95,500/RT) Fe-20 % V τr = 1.9 x 10-24 exp(96,700/RT). A relation between the relaxation parameters τr0 and ƊHr and the diffusion parameters of the faster diffusing specie is assumed, it is found that diffusion rates increase with increasing vanadium content. The results are discussed in terms of current theories of the Zener relaxation and reasons for the high activation energy of the diffusion process are given.

Mechanism and Transformation Kinetics of the Alpha → Sigma Phase Transformation in Iron-Vanadium Alloys

Abstract Electrical-resistance measurements of the alpha (α) → sigma (σ) phase transformation in body-centred cubic iron-vanadium alloys containing tantalum, tungsten, manganese, rhenium, and zirconium suggest that the degree of long-range order present in the a phase before the transformation is a controlling factor in the reaction kinetics. The influence of ternary alloying elements on the transformation is probably dependent on the manner in which they affect the longrange order. Twinning has been observed in the bcc iron-vanadium phase which shows long-range order.