Pure Liquid Iron Research Articles

High-temperature viscosity of iron‑carbon melts based on liquid structure:The effect of carbon content and temperature

Iron-based melts throughout the steel production process. Its viscosity dramatically affects the efficiency of ironmaking, steelmaking, and continuous casting processes. The viscosity was a structure-sensitive physical property and the macroscopic expression of its liquid structure. Molecular dynamics simulations were carried out to study the interaction mechanism between the liquid structure of iron‑carbon melts and its viscosity, to identify the essential factors influencing the viscosity. The results showed that pure iron's liquid structure changed from a thirteen-sided body to a deformed dodecahedron at 1873 K ~ 1923 K. With the increasing of carbon content, the nearest neighbor atomic distance increased from 0.255 nm to 0.261 nm, and the coordination number increased from 13 to 15. While the correlation radius decreasing from 1.450 nm to 0.930 nm, the number of atoms in an atomic cluster decreased from 980 to 300. The liquid structure evolved from medium-range order to short-range order, and the dominant polyhedral structural unit connection transformed. Melts' free volume increased, resulting in a decrease in viscosity. The average influencing degree of carbon content and temperature on iron‑carbon melts viscosity were 15% and 30%, respectively. The short-range structure was largely influenced the temperature. As the temperature increasing, the nearest neighbor atomic distance increased significantly; the correlation radius remained almost unchanged; the coordination number and the number of atoms in an atomic cluster decreased. The increase in free volume and motility of atomic clusters led to a decrease in melt viscosity. Finally, based on kinetic analysis, a quantitative relationship between the viscosity and temperature of pure liquid iron and iron‑carbon melts was constructed.

Thermodynamic Estimation of the Solidification of Borides and Carbides from Boron-Containing Iron–Carbon Melts

Abstract—The probability of the formation of solid iron borides and boron and iron carbides from Fe–C–B melts is thermodynamically estimated. The Gibbs energy of a chemical reaction is taken as a criterion for the solidification of these phases. The following standard states are used: for iron, pure liquid iron; for carbides and borides, a pure solid at temperature T; for boron and carbon, a pure liquid supercooled (metastable) element. The results of experimental diagnostics of the structure and composition of experimental castings made of carbon steels with surface alloying with boron-containing components (ferroboron, boron carbide) are presented. The solidification of borides and carbides from the melts under study was shown to occur at high boron and carbon concentrations and low temperatures.

Isotope Exchange Measurements of the Interfacial Reaction Rate Constant of Nitrogen on Fe-Mn alloys and an Advanced High-Strength Steel

Precise control of dissolved nitrogen is required for producing Third-Generation Advanced High-Strength Steels (AHSS). Compared with well-documented thermodynamic data, the data on the kinetics of nitrogen dissolution in these steels are limited. The isotope exchange technique (15N–14N exchange reaction) was applied to measure the nitrogen reaction rate at different temperatures, on pure liquid iron, liquid iron containing different levels of manganese, and liquid Fe-Mn-Al-Si steel with a Third-Generation AHSS composition. The increase in the interfacial reaction rate with increased temperature corresponded to an activation energy of 144.2 kJ/mol for pure iron. For Fe-Mn alloy, the interfacial reaction rate increased with the increased temperature and increased manganese content. For liquid AHSS (Fe-Mn-Al-Si), the interfacial reaction rate was approximately one third of the rate for pure liquid iron, at the same temperature, with a corresponding activation energy of 160.5 kJ/mol.

The top-down crystallisation of Mercury's core

The regime governing the growth of Mercury's core is unknown, but the dynamics of core growth are vital to understanding the origin and properties of the planet's weak magnetic field. Here, we use advanced first-principles methods, which include a magnetic entropy contribution, to investigate the magnetic and thermo-elastic properties of liquid Fe-S-Si and of pure liquid iron at the conditions of Mercury's core. Our results support a ‘top-down’ evolution of the core, whereby solid iron-rich material crystallises at shallow depths and sinks. This process would likely result in a compositionally driven dynamo within a stably stratified uppermost liquid layer, providing an explanation for the observed properties of the weak magnetic field of Mercury.

Experimental determination of oxygen diffusion in liquid iron at high pressure

Oxygen diffusion experiments in liquid iron have been performed at 3–18 GPa and 1975–2643 K using a multi-anvil apparatus. Diffusion couples consisted of a pure iron rod and a sintered disk of Fe0.85O0.15 placed end-to-end in a vertical orientation. Images and chemical spot analyses were acquired along the full length of the quenched sample on lines perpendicular to the diffusion interface. Exsolution features that formed during quenching consist mostly of spherical oxide blobs of at least two size populations, as well as feathery dendritic textures in more oxygen-rich regions near the top of the samples. Diffusion during heating (i.e. prior to reaching the peak annealing temperature, Tf) is treated numerically to refine Arrhenian parameters from simultaneous least-squares fits to several concentration profiles obtained from experiments at constant pressure and variable Tf. Diffusion coefficients range from ∼6×10−9 to ∼2×10−8 m2s−1 over the P–T range of the study, with activation enthalpies of less than 100 kJ mol−1. We find a very weak effect of pressure on oxygen diffusion with an activation volume of 0.1±0.1 cm3mol−1, in agreement with computational studies performed above 100 GPa. Arrhenian extrapolation of diffusion coefficients for oxygen to P–T conditions of the Earth's outer core yields faster average diffusion rates (∼3×10−8 m2s−1) than for Si or Fe in silicon-rich liquid iron alloys or pure liquid iron (∼5×10−9 m2s−1) reported previously. Oxygen diffusion data are used to constrain the maximum size of descending liquid metal droplets in a magma ocean that is required for chemical equilibration to be achieved. Our results indicate that if the Earth's core composition is representative of equilibrium chemical exchange with a silicate magma ocean, then it could only have been accomplished by large-scale break-up of impactor cores to liquid iron droplet sizes no larger than a few tens of centimeters.

Atomic transport property of Fe–O liquid alloys in the Earth’s outer core P, T condition

The self-diffusion and tracer diffusion coefficients and viscosity of non-magnetic liquid Fe–O alloys were determined at 100–350GPa and 4000–8000K on the basis of the ab initio molecular dynamics simulation method. Those of pure liquid iron are found to be consistent with the results of former studies. We confirmed that the self-diffusivity of Fe monotonically increases with increasing oxygen fraction. Meanwhile, the viscosity monotonically decreases with increasing oxygen fraction. On the other hand, the depth dependence of diffusivities and viscosity in the outer core are not great along the geotherm because pressure and temperature conversely affect the transport coefficients. The calculated diffusivity suggests that an O-rich stratified layer with a thickness of ∼70km could be produced at the uppermost layer of the outer core if the layer was created from the diffusion of O from the mantle.

Molecular-dynamic modeling of thermodynamic properties of the lunar Fe-S core

The physical properties (density, modules of compression, heat capacity, and velocity of the P-waves) of the lunar Fe-S core are modeled with molecular dynamics and the Embedded Atom Model potential at 0–18 at % S content, temperatures up to 2500 K, and pressures up to 5 GPa. The thermodynamic calculations of the velocity of P-waves in the liquid lunar core are consistent with the interpreted results of seismic processing of the Apollo observations. According to calculations, the density of the liquid core may vary from 7.4 (pure liquid iron) to 6.75 (10 at % S at 1950 ± 50 K) g/cm3. This approach provides more reliable restrictions for the inner structure of the Moon.

Liquid iron‐sulfur alloys at outer core conditions by first‐principles calculations

AbstractWe perform first‐principles calculations to investigate liquid iron‐sulfur alloys (Fe, Fe56S8, Fe52S12, and Fe48S16) under high‐pressure and high‐temperature (150–300 GPa and 4000–6000 K) conditions corresponding to the Earth's outer core. Considering only the density profile, the best match with the preliminary reference Earth model is by liquid Fe–14 wt % S (Fe50S14), assuming sulfur is the only light element. However, its bulk sound velocity is too high, in particular in the deep outer core, suggesting that another light component such as oxygen is required. An experimental check using inelastic X‐ray scattering shows good agreement with the calculations. In addition, a present study demonstrates that the Birch's law does not hold for liquid iron‐sulfur alloy, consistent with a previous report on pure liquid iron.

The P‐V‐T equation of state and thermodynamic properties of liquid iron

AbstractThe equation of state (EoS) and thermodynamic properties of non‐magnetic liquid iron were investigated from energy (E)‐pressure (P)‐volume (V)‐temperature (T) relationships calculated by means of ab initio molecular dynamics simulations at 60–420 GPa and 4000–7000 K. Its internally consistent thermodynamic and elastic properties, in particular, density, adiabatic bulk modulus, and P wave velocity, were then analyzed. Compared to the seismological data of the Earth's outer core, pure liquid iron is found to have an 8–10% larger density and 3–10% larger bulk modulus than the Earth's values. Results also show that the P wave velocity of liquid iron has marginal temperature dependence as the bulk sound velocity of solid iron. The new EoS model and thermodynamic properties of liquid iron may serve as fundamental data for the thermochemical modeling of the Earth's core.

In situ droplet surface tension and viscosity measurements in gas metal arc welding

In this paper, we present an adaptation of a drop oscillation technique that enables in situ measurements of thermophysical properties of an industrial pulsed gas metal arc welding (GMAW) process. Surface tension, viscosity, density and temperature were derived expanding the portfolio of existing methods and previously published measurements of surface tension in pulsed GMAW. Natural oscillations of pure liquid iron droplets are recorded during the material transfer with a high-speed camera. Frame rates up to 30 000 fps were utilized to visualize iron droplet oscillations which were in the low kHz range. Image processing algorithms were employed for edge contour extraction of the droplets and to derive parameters such as oscillation frequencies and damping rates along different dimensions of the droplet. Accurate surface tension measurements were achieved incorporating the effect of temperature on density. These are compared with a second method that has been developed to accurately determine the mass of droplets produced during the GMAW process which enables precise surface tension measurements with accuracies up to 1% and permits the study of thermophysical properties also for metals whose density highly depends on temperature. Thermophysical properties of pure liquid iron droplets formed by a wire with 1.2 mm diameter were investigated in a pulsed GMAW process with a base current of 100 A and a pulse current of 600 A. Surface tension and viscosity of a sample droplet were 1.83 ± 0.02 N m−1 and 2.9 ± 0.3 mPa s, respectively. The corresponding droplet temperature and density are 2040 ± 50 K and 6830 ± 50 kg m−3, respectively.

Reduction of molybdenum oxide from steelmaking slags by pure liquid iron

The effects of reaction temperature, slag basicity and FeO concentration on the reduction of molybdenum oxide from steelmaking slags by pure liquid iron were investigated experimently. The reduction kinetics of molybdenum oxide by liquid iron was analysed. The reaction models were developed based on the condition that diffusion of [Mo] in liquid iron and CaMoO4 in slag is the control steps, respectively. These reaction models were tested using data from a series of experiments. The results indicate that under the present experimental conditions, the temperature and the FeO content, other than slag basicity, have some effects on the reduction of molybdenum oxide from steelmaking slags by pure liquid iron. Both the molybdenum oxide reduction rate and final reduction ratio increase with an increase of temperature and a decrease of FeO content. The diffusion of CaMoO4 in slag which dominated overall reduction process is the only one ratecontrolling step with its apparent activation energy 294 kJ/mol. The reduction of molybdenum oxide used directly as alloy additive can be further enhanced by strong stirring in the converter practice.

Structure of eutectic Fe–FeS melts to pressures up to 17 GPa: Implications for planetary cores

In situ X-ray diffraction experiments have been conducted using the Paris–Edinburgh press to study the Fe–FeS eutectic liquid structure from 3 to 17 GPa near the melting temperature. The eutectic temperature is steadily decreasing over the studied pressure range. The liquid structure evolves discontinuously towards a compact structure, with a non-uniform contraction at pressures between 13 and 17 GPa. We show that this evolution may be correlated to the Fe–S–Si miscibility gap closure (Sanloup, C., Fei, Y., Closure of the Fe–S–Si liquid miscibility gap at high pressure, Phys. Earth Planet. Inter. 147 (2004) 57.) and the magnetic transition observed in Fe 3S compound (Lin, J.F., Fei, Y., Sturhahn, W., Zhao, J., Mao, H.K., Hemley, R.J., Magnetic transition and sound velocities of Fe 3S at high pressure: implications for Earth and planetary cores, Earth Planet. Sci. Lett. 226 (2004) 33–40.). The occurrence of such structural change in liquids Fe-alloys show that the extrapolation of their physical properties up to Earth's core pressure might not be relevant. As Fe–S liquid alloy acquires a compact structure close to that of pure liquid iron at high pressure, sulfur could be considered as one of the possible light element for the Earth's core from a pure elastic point of view. In addition, this structural modification could affect the partitioning coefficients of elements between liquid silicates and S-bearing liquid metallic phase.

Molecular dynamics calculation of the bulk viscosity of liquid iron–nickel alloy and the mechanisms for the bulk attenuation of seismic waves in the Earth’s outer core

Bulk viscosity is closely related to bulk dissipation of seismic waves. To understand the potential bulk dissipation in the Earth’s outer core, molecular dynamics simulations were conducted in the present study to calculate the bulk viscosity of liquid iron–nickel (Fe–Ni) alloy at the temperatures and pressures of the Earth’s outer core. Results from these simulations showed that bulk viscosities of liquid Fe–Ni alloy are in the order of 10 −3 Pa s. These values are in the same order of magnitude as those of pure liquid iron [Phys. Chem. Minerals 27 (2000) 164], indicating that nickel has a negligible effect on the bulk viscosity of liquid iron. Consequently, the large bulk viscosity of the outer core inferred from seismological studies should be accounted for through concentration fluctuations or other mechanisms with light elements such as sulfur or oxygen involved in [Nature 285 (1980) 204; Science 277 (1997) 219], or using the particle suspension model [J. geophys. Res. 88 (1983) 2445]. In addition, using the activation energy and volume calculated from these bulk viscosity data, it was found that the seismically induced phase change model for a partial melt system [J. Geophys. Res. 73 (1968) 7675] might not be capable to account for the bulk attenuation of the outer core.

Henrian ideality of iron in liquid uranium solvent at high temperatures

Equilibrium vapor pressures of iron from its dilute solutions in liquid uranium were measured at high temperatures (2250–2750 K). From the Fe vapor pressure data at known compositions ( x Fe), Henry’s coefficient ( K Fe= p Fe/ x Fe) of the solute was obtained as log K Fe (±0.07) (Pa)=−18975.6(±1098.1)/T+11.15(±0.44) . The behavior of K Fe( T) is shown to be marginally different from that of the equilibrium vapor pressures of pure liquid iron ( p Fe°). The partial molar excess Gibbs energy of iron at infinite dilution derived from the behavior is expressed as Δ E G ̄ Fe ∞=−1588+1.25T ( J mol −1) . The result is discussed in the light of the quasi-regular solution model and compared with data of the system available in the literature.

Constraints on the composition of the Earth's core from ab initio calculations

Knowledge of the composition of the Earth's core is important for understanding its melting point and therefore the temperature at the inner-core boundary and the temperature profile of the core and mantle. In addition, the partitioning of light elements between solid and liquid, as the outer core freezes at the inner-core boundary, is believed to drive compositional convection, which in turn generates the Earth's magnetic field. It is generally accepted that the liquid outer core and the solid inner core consist mainly of iron. The outer core, however, is also thought to contain a significant fraction of light elements, because its density--as deduced from seismological data and other measurements--is 6-10 per cent less than that estimated for pure liquid iron. Similar evidence indicates a smaller but still appreciable fraction of light elements in the inner core. The leading candidates for the light elements present in the core are sulphur, oxygen and silicon. Here we obtain a constraint on core composition derived from ab initio calculation of the chemical potentials of light elements dissolved in solid and liquid iron. We present results for the case of sulphur, which provide strong evidence against the proposal that the outer core is close to being a binary iron-sulphur mixture.

A molecular dynamics study of bulk and shear viscosity of liquid iron using embedded-atom potential

Molecular dynamics simulations employing a many-body embedding potential model have been conducted to calculate both bulk and shear viscosity of pure liquid iron at the Earth's outer core conditions. Liquid iron shear viscosity thus obtained is in the order of 10−2 Pa · s and is in close agreement with previous estimates. In contrast, liquid iron bulk viscosity is in the order of 10−3 to 10−4 Pa · s and is much smaller than previous estimates. Consequently, the ratio of bulk to shear viscosity is close to 0.1. This value disagrees with both the common speculation that bulk and shear viscosities are equal at ambient pressure, and the previous inference that bulk viscosity of liquid iron is much larger than shear viscosity at outer core conditions. Potential implications of present data are also briefly given for the dynamic state of the outer core.

Kinetics of the Reaction of C2H6, CH4-CO2 and CO-CO2-O2 Gases with Liquid Iron.

The kinetics of several gas–metal reactions relevant to bath smelting have been investigated. The rate of carburization of liguid iron by C2H6 gas was measured between 1400 and 1600°C under conditions in which partial pressure of C2H6 was in the range of 0.016 to 0.04 atm and sulfur content in the iron was in the range of 0.003 to 0.5 wt%. The experimental results indicate that the rate is controlled by the dissociation of C2H6 on the surface of iron and gas phase mass transfer in series. The gas phase mass transfer can be corrected with reasonable accuracy and the chemical rate constants were obtained. The rate was retarded by sulfur in liquid iron and there was evidence of a large residual rate at high sulfur contents. The rate of carburization of pure liquid iron (as=0.01) by CH4 –CO2 gas mixture was measured at 1600°C under conditions at which the rate is controlled by gas phase mass transfer and chemical reaction in series. The gas was 6% CH4 and up to 2.5% CO2 in Ar. It was concluded that CH4 and CO2 reached the surface of the iron before they reacted with each other and carburization by CH4 and decarburization by CO2 occurred independently for the present experimental conditions. The rate of decarburization of carbon saturated liquid iron by CO–CO2 –O2 gas mixture was measured at 1600°C. The partial pressure of O2 in 90%CO/10%CO2 gas was in the range of 0 to 0.03 atm and sulfur content in the metal was 0.1 wt%. The measured rate shows that the gases reached the surface of metal before they reacted with each other and decarburization by CO2 and O2 proceeded independently at a high gas flow rate (5l/min), but there may have been some gas phase reaction at lower flow rate (2l/min).

Oxygen in the Earth's core: a first-principles study

First-principles electronic structure calculations based on DFT have been used to study the thermodynamic, structural and transport properties of solid solutions and liquid alloys of iron and oxygen at Earth's core conditions. Aims of the work are to determine the oxygen concentration needed to account for the inferred density in the outer core, to probe the stability of the liquid against phase separation, to interpret the bonding in the liquid, and to find out whether the viscosity differs significantly from that of pure liquid iron at the same conditions. It is shown that the required concentration of oxygen is in the region 25–30 mol%, and evidence is presented for phase stability at these conditions. The Fe/O bonding is partly ionic, but with a strong covalent component. The viscosity is lower than that of pure liquid iron at Earth's core conditions. It is shown that earlier first-principles calculations indicating very large enthalpies of formation of solid solutions may need reinterpretation, since the assumed crystal structures are not the most stable at the oxygen concentration of interest.

水素-アルゴンプラズマアーク溶解による鉄およびコバルトの脱酸，脱窒

Removal of oxygen and nitrogen of pure liquid iron and cobalt has been examined by Ar and (0.5% to 20%)H2+Ar plasma arc melting under atmospheric pressure. While the removal rates of oxygen and nitrogen were very low when the plasma gas was noly Ar, they rose remarkable with addition of a small amount of hydrogen such as 0.5 to 1%H2 and increased with hydrogen content of the plasma gas. The oxygen and nitrogen concentrations finally decreased to the very low levels as follows; O<2 and N<1 mass ppm for iron and O<5 and N<1 mass ppm for cobalt. This excellent refining effect is supposed to be caused by activated hydrogen atoms dissociated in the high temperature plasma arc and to be expressed as the following reactions; O+2H=H2O for deoxidation and N+3H=NH3 for nitrogen removal.The deoxidation of iron proceeded through two steps by the fast 1st step at the beginning of melting and, thereafter, the slow 2nd step. Both steps were of a first order rate low and the deoxidation rate of 1st step was found to increase in proportion to the about 1/2th power of the hydrogen content in the plasma generating gas. So, it is considered that, for the 1st step of deoxidation, the chemical reaction on the surface of the liquid iron is the rate determining step.

Thermodynamic properties of titanium and iron in molten silicon

Titanium and iron in silicon are known as harmful “lifetime killer” impurities, which shorten the lifetime of excited carriers in silicon solar cell and disturb power generation. Therefore, the removal of titanium and iron is one of the most important topics for the production of solar grade silicon. Thermodynamic properties of titanium and iron in molten silicon were determined at 1723 K by equilibrating molten silicon-titanium alloys or molten silicon-iron alloys with molten lead, which has a limited mutual solubility for both alloys. The activity coefficients of infinite dilution, self-interaction coefficients of titanium and iron in molten silicon, and the Gibbs energy change of mixing for silicon-titanium and silicon-iron at 1723 K relative to pure liquid silicon, titanium, and iron were determined.