Phase Transformations In Iron Research Articles

α ↔ γ phase transformation in iron: comparative study of the influence of the interatomic interaction potential

Only few available interatomic interaction potentials implement the α ↔ γ phase transformation in iron by featuring a stable low-temperature bcc and high-temperature fcc lattice structure. Among these are the potentials by Meyer and Entel (1998 Phys. Rev. B 57 5140), by Müller et al (2007 J. Phys.: Condens. Matter 19 326220) and by Lee et al (2012 J. Phys.: Condens. Matter 24 225404). We study how these potentials model the phase transformation during heating and cooling; in order to help initiating the transformation, the simulation volume contains a grain boundary. For the martensitic transformation occurring on cooling an fcc structure, we additionally study two potentials that only implement a stable bcc structure of iron, by Zhou et al (2004 Phys. Rev. B 69 144113) and by Mendelev et al (2003 Philos. Mag. 83 3977). We find that not only the transition temperature depends on the potential, but that also the height of the energy barrier between fcc and bcc phase governs whether the transformation takes place at all. In addition, details of the emerging microstructure depend on the potential, such as the fcc/hcp fraction formed in the α → γ transformation, or the twinning induced in and the lattice orientation of the bcc phase in the γ → α transformation.

Calorimetry of the Phase Transformations in Carbon Steels in the Intercritical Temperature Range

Data on the calorimetric effects during the structural phase transformations in iron–carbon alloys (steels) are presented. No relationships are revealed between temperature points A3 (Am) and the calorimetric effect temperatures during steel thermal cycling. The conclusions in favor of the mechanisms of the phase transformations resulting in the observation of these calorimetric effects are presented. The obtained results can be used to study the conditions of heat treatment with quenching of steels from the intercritical temperature range.

Effect of Carbon Content on the Structure and Microhardness of Steels Under Rapid Action by Deforming Cutting

The effect of deforming cutting (DC) on structural and phase transformations in iron and steels 35 and U8 is studied with the help of scanning optical and transmission electron microscopy. It is shown that with an increase in carbon content test material structure and phase composition become more complex.

Extraction of Nickel from Garnierite Laterite Ore Using Roasting and Magnetic Separation with Calcium Chloride and Iron Concentrate

In this study, segregation roasting and magnetic separation are used to extract nickel from a garnierite laterite ore. The garnierite laterite ore containing 0.72% Ni, 0.029% Co, 8.65% Fe, 29.66% MgO, and 37.86% SiO2 was collected in the Mojiang area of China. Garnierite was the Ni-bearing mineral; the other main minerals were potash feldspar, forsterite, tremolite, halloysite, quartz, and kaolinite in the garnierite laterite ore. The iron phase transformations show that nickel is transformed from (Ni,Mg)O·SiO2·nH2O to a new nickel mineral phase dominated by [Ni]Fe solid solution; and iron changed from Fe2O3 and FeOOH to a new iron mineral phase dominated by metal Fe and Fe3O4 after segregation roasting. Ferronickel concentrate with Ni of 16.16%, Fe of 73.67%, and nickel recovery of 90.33% was obtained under the comprehensive conditions used: A roasting temperature of 1100 °C, a roasting time of 90 min, a calcium chloride dosage of 15%, an iron concentrate dosage of 30%, a coke dosage of 15%, a coke size of −1 + 0.5 mm, a magnetic separation grinding fineness of <45 μm occupying 90%, and a magnetic separation magnetic field intensity of H = 0.10 T. The main minerals in ferronickel concentrate are Fe, [Ni]Fe, Fe3O4, and a small amount of gangue minerals, such as CaO·SiO2 and CaO·Al2O3·SiO2.

Atomistic Study of the Role of Defects on α → ϵ Phase Transformations in Iron under Hydrostatic Compression

It has long been known that iron undergoes a phase transformation from body-centered cubic/ α structure to the metastable hexagonal close-packed/ ε phase under high pressure. However, the interplay of line and planar defects in the parent material with the transformation process is still not fully understood. We investigated the role of twins, dislocations, and Cottrell atmospheres in changing the crystalline iron structure during this phase transformation by using Monte Carlo methods and classical molecular dynamics simulations. Our results confirm that embryos of ε -Fe nucleate at twins under hydrostatic compression. The nucleation of the hcp phase is observed for single crystals containing an edge dislocation. We observe that the buckling of the dislocation can help to nucleate the dense phase. The crystal orientations between the initial structure α -Fe and ε -Fe in these simulations are 110 b c c | | 0001 h c p . The presence of Cottrell atmospheres surrounding an edge dislocation in bcc iron retards the development of the hcp phase.

Continuum nonlinear dynamics of unstable shock waves induced by structural phase transformations in iron

Iron is the predominant metal in the interior of the Earth and other rocky planets and has also received substantial consideration in materials science and engineering due to its technological importance. Here, a finite deformation continuum framework for combining nonlinear elasto-viscoplasticity with multivariant phase-field theory is used to investigate shock-induced polymorphic phase transitions in single-crystal iron. A large number of low- and high-pressure variants with explicit anisotropic pressure-dependent stiffnesses are included with respect to the point-symmetry groups of cubic and hexagonal crystal lattices. Using the element-free Galerkin method with high-performance computing resources, the three-dimensional nonlinear calculations accurately describe some important features reported by the experimental literature, and strongly complement our understanding of the phase-change dynamics in iron at larger time and length scales than hitherto explored by molecular dynamics simulations in the last two decades. The numerical model is able to reproduce unstable shock waves (which break up into elastic, plastic and phase-transition waves), providing new stress-informed insights into the coupling between the high strain-rate plasticity and microstructure evolution during the displacive phase transitions. The analyses of time-position diagrams indicate that the prompt plastic relaxation to a nearly hydrostatic state from uniaxial shock-compression is responsible for the peculiar multiphase microstructure with an unexpected gradient selection of high-pressure variants behind the phase-transition wave front. The existence of two-zone sequential sets of “release” and “reload” variants that results from a heterogeneous nucleation instability leads to a specific microstructural fingerprint of the nonlinear dynamics of phase transitions, thus offering novel guidelines for future experimental diagnostics of shock wave propagation in iron.

Effects of pressure and CO concentration on vanadium, nickel and iron phase transformations for petcoke slag viscosity correlation development

Entrained flow gasification occurs typically at temperatures above 1400 °C. Inorganic species present in feedstocks like coal/petcoke melt and form slag. Slag viscosity is an important property for the design, operation and modelling the behavior of flows in gasifiers. Existing correlations between the composition and viscosity do not work accurately for petcoke ash slag due to V and Ni. In entrained flow gasifiers, the gaseous environment can affect the slag viscosity based on oxidation states of transition elements (primarily V, Fe and Ni). So, the mineral matter transformations need to be understood for the development of proper correlations. Coal ash can be adequately represented by the oxides of the following major elements: Si, Ca, Al, and Fe. In addition to those, petcoke ash contains significant amounts of V and Ni. The objectives of this work are to determine the effects of reducing atmosphere and pressure on the mineral matter transformations, and subsequently develop a slag correlation. CO-CO2 mixtures in different proportions were used to determine the effect of reducing atmosphere on the reduction of oxides of the transition elements. A high temperature X-Ray Diffraction (XRD) technique was used to study the transformations. A high-pressure thermogravimetric analysis (TGA) equipment was used to study the effect of pressure on the reduction reactions. XRD and TGA results indicated that V, Ni and Fe oxides were getting reduced. Consequently, lower oxidation state phases (V2O3, NiO and FeO) were used to develop slag viscosity correlation for a petcoke ash by linear regression. Viscosity data of melts with the aforementioned six oxides were obtained from the study of Wang et al. (2004).TGA results showed that the reduction reactions occurring at 1.01 bar, 10 bar and 20 bar contributed to mass loss of the synthetic petcoke ash. With increase in pressure, the reduction reactions occurred at higher temperature. X-Ray diffractograms indicated that CO concentration in the reactant gas influenced the transformations. Predictions made by the viscosity correlation with proper phases for V, Ni and Fe were in reasonably good agreement with experimental data.

Effect of Alloying Elements on the α-γ Phase Transformation in Iron.

Small concentrations of alloying elements can modify the - phase transition temperature of Fe. We study this effect using an atomistic model based on a set of many-body interaction potentials for iron and several alloying elements. Free-energy calculations based on perturbation theory allow us to determine the change in introduced by the alloying element. The resulting changes are in semi-quantitative agreement with experiment. The effect is traced back to the shape of the pair potential describing the interaction between the Fe and the alloying atom.

Pressure-induced phase transformations in Fe-C: Molecular dynamics approach

Molecular dynamics simulations provide a large body of literature on pressure-induced phase transformations in iron. However, the role of interstitial carbon on phase transformations in iron under high pressure is still an open topic. We investigate the mechanical response of Fe-C single crystals and polycrystals using hydrostatic and uniaxial compression. We couple a recently developed Fe interatomic EAM potential, which faithfully describes the α→ε transformation in iron, with several well-known Fe-C potentials. Our results show a three-wave profile demonstrating that the phase transformation is preceded by plasticity. Carbon strongly increases the transformation pressure but decreases the Hugoniot elastic limit in agreement with experimental results.

Deformation mechanisms of mechanically induced phase transformations in iron

Effects of uniaxial tensile directions on the phase transformations in a nano-sized face centred cubic (fcc) crystal in iron were studied by molecular dynamics simulations. The results show that externally applied loading induced phase transformations from fcc phase to body centred cubic (bcc) phase, and variant selection took place depending on the tensile direction relative to the crystallographic orientation of the fcc crystal. For the loading along the close packed fcc plane, the uniaxial tensile direction perpendicular to the close packed direction led to the formation of flat-ellipsoid bcc nuclei composed of {0 1 1} twinning martensite, while that parallel to the close packed direction resulted in zigzag bcc plates with the {1 1 2} twinning sub-structure. When the tensile loading was perpendicular to the close packed fcc planes, a number of stacking faults as a result of the movement of Shockley partial dislocations appeared temporarily in the shape of “funnels”, which distorted the fcc lattices severely before it transformed into the bcc phase. The deformation mechanisms of the mechanically induced phase transformations through twinning or dislocations revealed in the present study may provide clues to develop iron or iron-based shape memory alloys with both high strength and good ductility.

Калориметрия эвтектоидного превращения в системе Fe–C

This article presents data on the calorimetric effects of structural phase transformations in iron and eutecticide in the Fe–C system

Ferrite-to-Austenite and Austenite-to-Martensite Phase Transformations in the Vicinity of a Cementite Particle: A Molecular Dynamics Approach

We used classical molecular dynamics simulation to study the ferrite–austenite phase transformation of iron in the vicinity of a phase boundary to cementite. When heating a ferrite–cementite bicrystal, we found that the austenitic transformation starts to nucleate at the phase boundary. Due to the variants nucleated, an extended poly-crystalline microstructure is established in the transformed phase. When cooling a high-temperature austenite–cementite bicrystal, the martensitic transformation is induced; the new phase again nucleates at the phase boundary obeying the Kurdjumov–Sachs orientation relations, resulting in a twinned microstructure.

Aqueous Fe(II)-Induced Phase Transformation of Ferrihydrite Coupled Adsorption/Immobilization of Rare Earth Elements

The phase transformation of iron minerals induced by aqueous Fe(II) (Fe(II)aq) is a critical geochemical reaction which greatly affects the geochemical behavior of soil elements. How the geochemical behavior of rare earth elements (REEs) is affected by the Fe(II)aq-induced phase transformation of iron minerals, however, is still unknown. The present study investigated the adsorption and immobilization of REEs during the Fe(II)aq-induced phase transformation of ferrihydrite. The results show that the heavy REEs of Ho(III) were more efficiently adsorbed and stabilized compared with the light REEs of La(III) by ferrihydrite and its transformation products, which was due to the higher adsorptive affinity and smaller atomic radius of Ho(III). Both La(III) and Ho(III) inhibited the Fe atom exchange between Fe(II)aq and ferrihydrite, and sequentially, the Fe(II)aq-induced phase transformation rates of ferrihydrite, because of the competitive adsorption with Fe(II)aq on the surface of iron (hydr)oxides. Owing to the larger amounts of adsorbed and stabilized Ho(III), the inhibition of the Fe(II)aq-induced phase transformation of ferrihydrite affected by Ho(III) was higher than that by La(III). Our findings suggest an important role for the Fe(II)aq-induced phase transformation of iron (hydr)oxides in assessing the mobility and transfer behavior of REEs, as well as for their occurrence in earth surface environments.

Mechanisms of Se(IV) Co-precipitation with Ferrihydrite at Acidic and Alkaline Conditions and Its Behavior during Aging.

Understanding the form of Se(IV) co-precipitated with ferrihydrite and its subsequent behavior during phase transformation is critical to predicting its long-term fate in a range of natural and engineered settings. In this work, Se(IV)-ferrihydrite co-precipitates formed at different pH were characterized with chemical extraction, transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAS) to determine how Se(IV) is associated with ferrihydrite. Results show that despite efficient removal, the mode and stability of Se(IV) retention in the co-precipitates varied with pH. At pH 5, Se(IV) was removed dominantly as a ferric selenite-like phase intimately associated with ferrihydrite, while at pH 10, it was mostly present as a surface species on ferrihydrite. Similarly, the behavior of Se(IV) and the extent of its retention during phase transformation varied with pH. At pH 5, Se(IV) remained completely associated with the solid phase despite the phase change, whereas it was partially released back into solution at pH 10. Regardless of this difference in behavior, TEM and XAS results show that Se(IV) was retained within the crystalline post-aging products and possibly occluded in nanopore and defect structures. These results demonstrate a potential long-term immobilization pathway for Se(IV) even after phase transformation. This work presents one of the first direct insights on Se(IV) co-precipitation and its behavior in response to iron phase transformations.

Fe(II)/Cu(II) interaction on goethite stimulated by an iron-reducing bacteria Aeromonas Hydrophila HS01 under anaerobic conditions

Copper is a trace element essential for living creatures, but copper content in soil should be controlled, as it is toxic. The physical-chemical-biological features of Cu in soil have a significant correlation with the Fe(II)/Cu(II) interaction in soil. Of significant interest to the current study is the effect of Fe(II)/Cu(II) interaction conducted on goethite under anaerobic conditions stimulated by HS01 (a dissimilatory iron reduction (DIR) microbial). The following four treatments were designed: HS01 with α-FeOOH and Cu(II) (T1), HS01 with α-FeOOH (T2), HS01 with Cu(II) (T3), and α-FeOOH with Cu(II) (T4). HS01 presents a negligible impact on copper species transformation (T3), whereas the presence of α-FeOOH significantly enhanced copper aging contributing to the DIR effect (T1). Moreover, the violent reaction between adsorbed Fe(II) and Cu(II) leads to the decreased concentration of the active Fe(II) species (T1), further inhibiting reactions between Fe(II) and iron (hydr)oxides and decelerating the phase transformation of iron (hydr)oxides (T1). From this study, the effects of the Fe(II)/Cu(II) interaction on goethite under anaerobic conditions by HS01 are presented in three aspects: (1) the accelerating effect of copper aging, (2) the reductive transformation of copper, and (3) the inhibition effect of the phase transformation of iron (hydr)oxides.

Phase transformation of iron in limonite ore by microwave roasting with addition of alkali lignin and its effects on magnetic separation

Phase transformation and magnetic properties of limonite ore with 40.10% Fe via microwave roasting with addition of alkali lignin were investigated by chemical phase analysis and vibrating sample magnetometer (VSM) analysis, respectively. The results of chemical phase analysis indicated that phase compositions and contents of iron in microwave roasted limonite ore were varied with the dosage of alkali lignin, the roasting temperature, the roasting time and the microwave power, and among them the dosage of alkali lignin exerted more significant influence. Iron oxides in limonite ore could be reduced to magnetic iron oxides including γ-Fe2O3 and Fe3O4 in the following sequence during microwave roasting process by evenly distribution of alkali lignin below 5%: FeOOH/α-Fe2O3→γ-Fe2O3→Fe3O4, accompanied with small amount of FeO, and as the dosage was over 5%, γ-Fe2O3 and Fe3O4 could be in turn successively transformed into α-Fe2O3. Magnetic property studies demonstrated that an iron concentrate containing 88.72% magnetic iron oxides with a maximum saturation magnetization of 41.393 emu/g could be produced from roasted ore which was obtained by microwave roasting at 200 °C and 600 W with 5% alkali lignin for 30 min. In addition, the roasted ore was further used for magnetic separation, and the results showed that combining microwave roasting with addition of 5% alkali lignin could improve the iron recovery from the roasted ore distinctively. It was concluded that the microwave roasting process in the presence of alkali lignin could be a promising approach to effective utilization of limonite ore resources.

Towards the ab initio based theory of phase transformations in iron and steel

Despite of the appearance of numerous new materials, the iron based alloys and steels continue to play an essential role in modern technology. The properties of a steel are determined by its structural state (ferrite, cementite, pearlite, bainite, martensite, and their combination) that is formed under thermal treatment as a result of the shear lattice reconstruction "gamma" (fcc) -> "alpha" (bcc) and carbon diffusion redistribution. We present a review on a recent progress in the development of a quantitative theory of the phase transformations and microstructure formation in steel that is based on an ab initio parameterization of the Ginzburg-Landau free energy functional. The results of computer modeling describe the regular change of transformation scenario under cooling from ferritic (nucleation and diffusion-controlled growth of the "alpha" phase to martensitic (the shear lattice instability "gamma" -> "alpha"). It has been shown that the increase in short-range magnetic order with decreasing the temperature plays a key role in the change of transformation scenarios. Phase-field modeling in the framework of a discussed approach demonstrates the typical transformation patterns.

Fe(II)-induced phase transformation of ferrihydrite: The inhibition effects and stabilization of divalent metal cations

The Fe(II)-induced phase transformation of iron (hydr)oxides is an important process in the geochemical iron cycle and is one in which the coexisting metal cations inhibit the phase transformation rates. However, the underlying affecting mechanisms by metal cations and the critical property of metal cations that is responsible for the inhibition remain unclear. In this study, we focus on the cation effect of seven divalent cations (denoted as Me(II), including Mg(II), Ca(II), Ba(II), Mn(II), Co(II), Ni(II), and Zn(II)) and their influencing mechanisms on Fe(II)-catalyzed transformation processes of ferrihydrite. At initial reaction conditions (i.e. pH6.5 and 2.0mM Fe(II)), the binding ability (the affinity of cations for ferrihydrite surface) of Me(II) was found to affect the ferrihydrite transformation. Me(II) with higher binding abilities reduced the bound-Fe(II) amount on ferrihydrite and decreased the redox potentials of the Fe(II)-catalyzed system to inhibit the transformation rates of ferrihydrite. In addition to the inhibition effect, the Me(II) was partly stabilized in the formed secondary iron minerals. The binding abilities of Me(II) also affected the Fe(II)-induced transformation pathways of ferrihydrite by affecting the amount of bound-Fe(II) on ferrihydrite. Ferrihydrite was first transformed to lepidocrocite and later to goethite and magnetite with Me(II) that had lower binding ability than Fe(II), whereas it was directly transformed to goethite and magnetite when with Me(II) that had higher binding ability than Fe(II).

High-temperature chlorination of gold with transformation of iron phase

Gold in cyanide tailings from Shandong Province is mainly encapsulated by hematite and magnetite at distribution rates of 76.49 % and 10.88 %, respectively. Chlorination–reduction one-step roasting of cyanide tailings was conducted under the following conditions: calcium chloride dosage of 6 %, bituminous coal dosage of 30 %, calcium oxide dosage of 10 % (all dosages are vs. the mass of cyanide tailings) at 1000 °C of roasting temperature. X-ray diffraction (XRD), scanning electron microscopy (SEM), and chemical-phase analysis were performed to investigate the effects of iron phase transformation on the high-temperature chlorination of gold. Results indicate that the lattice structure of hematite undergoes expansion, pulverization, and reorganization when hematite is reduced to magnetite, which leads to 42.03 % gold exposure, and the high-temperature chlorination rate of gold is 41.17 % at the same time. The structure of wustite formed by the reduction in magnetite is porous and loose, and thus 44.02 % of gold is exposed. The high-temperature chlorination rate of gold is increased by 41.98 percentage points. When wustite is reduced to metallic iron, 4.42 % of gold is exposed, and the high-temperature chlorination rate of gold is increased by 3.38 percentage points. Accordingly, the high-temperature chlorination of gold mainly occurs in two stages, in which Fe2O3 is reduced to Fe3O4, and Fe3O4 is reduced to Fe x O finally.

Comparisons of CTH Simulations with Measured Wave Profiles for Simple Flyer Plate Experiments

We have conducted detailed 2-dimensional hydrodynamics calculations to assess the quality of simulations commonly used to design and analyze simple shock compression experiments. Such simple shock experiments also contain data where dynamic properties of materials are integrated together. We wished to assess how well the chosen computer hydrodynamic code could do at capturing both the simple parts of the experiments and the integral parts. We began with very simple shock experiments, in which we examined the effects of the equation of state and the compressional and tensile strength models. We increased complexity to include spallation in copper and iron and a solid–solid phase transformation in iron to assess the quality of the damage and phase transformation simulations. For experiments with a window, the response of both the sample and the window are integrated together, providing a good test of the material models. While CTH physics models are not perfect and do not reproduce all experimental details well, we find the models are useful; the simulations are adequate for understanding much of the dynamic process and for planning experiments. However, higher complexity in the simulations, such as adding in spall, led to greater differences between simulation and experiment. This comparison of simulation to experiment may help guide future development of hydrodynamics codes so that they better capture the underlying physics.