Phase Transformations In Iron Research Articles

Molecular dynamics simulation of shock waves in Fe and Fe–C: Influence of system characteristics

Pressure-induced phase transformation in iron and its alloys is a classic research topic in solid-state physics, material science, and geophysics. The crystal structure of iron undergoes a phase transformation at a hydrostatic pressure of 13 GPa, changing from a body-centered cubic system to a hexagonal close-packed system. Although extensive research has been carried out on the transformation in iron by using molecular dynamics simulations, there is very limited literature that focuses on the contribution of parent phase orientations, system size, and impurities to the phase evolution. In this work, classic molecular dynamics simulations have been employed to investigate the effects of system size, lattice orientation, and impurity concentration on the pressure-induced phase transformation of iron and iron alloys for the first time. Our results show that the lattice orientation has a strong influence on the phase transition behavior, while the influence of carbon is small. The phase transition is slightly delayed with increasing carbon content, whereas the transition pressure increases from [001] to [011] to [111] orientation. The amount of twinning and stacking faults highly depends on the orientation. It is easiest for solitary waves to travel through [111] lattice orientation. The addition of carbon has a slow-down effect on shock velocities, and this effect increases with carbon content and lattice orientation of the samples from [001] to [011] to [111].

Molecular Dynamic Simulation of Kinetics of fcc–bcc Heterointerface in Phase Transformation of Iron and Carbon Steel

Molecular Dynamic Simulation of Kinetics of fcc–bcc Heterointerface in Phase Transformation of Iron and Carbon Steel

Highly efficient and green separation of iron from complex low-grade polymetallic ore via hydrogen-based mineral phase transformation

The Bayan Obo low - grade polymetallic ore (BOLPO) is a complex polymetallic resource that has accumulated over decades. Conventional mineral processing struggles to meet specifications. In this study, a hydrogen - based mineral phase transformation (HMPT) technique was introduced. Using HMPT at 425 °C for 30 min, CO and H2 in a ratio of 1:3, 20% concentration, and a product grind size below 38 μm with 95% content, an iron concentrate achieved 65.56% grade and 93.68% recovery. x - ray diffraction, x - ray photoelectron spectroscopy and scanning electron microscopy with energy dispersive x - ray spectroscopy analyses showed selective phase transformation of iron to magnetite in BOLPO during HMPT, avoiding excessive wüstite formation, and no phase transformation of quartz, rare earths or fluorite was observed in the products. Vibrating sample magnetometer analysis showed improved magnetic properties, with saturation magnetization increasing from 7.25 A·m2·kg−1 to 47.28 A·m2·kg−1. HMPT selectively separates high - grade iron from low - grade polymetallic ores without compromising the subsequent recovery of rare earths and fluorite in non - magnetic components.

Organic binding iron formation and its mitigation in cation exchange resin assisted anaerobic digestion of chemically enhanced primary sedimentation sludge

Fe based chemically enhanced primary sedimentation (CEPS) is an effective method of capturing the colloidal particles and inorganic phosphorous (P) from wastewater but also produces Fe-CEPS sludge. Anaerobic digestion is recommended to treat the sludge for energy and phosphorus recovery. However, the aggregated sludge flocs caused by the coagulation limited sludge hydrolysis and P release during anaerobic digestion process. In this study, cation exchange resin (CER) was employed during anaerobic digestion of Fe-CEPS sludge with aims of prompting P release and carbon recovery. CER addition effectively dispersed the sludge flocs. However, the greater dispersion of sludge flocs could not translate to higher sludge hydrolysis. The maximum hydrolysis and acidification achieved at lower CER dosage of 0.5 g CER/g TS. It was observed that the extents of sludge hydrolysis and acidification had a strongly negative correlation with the organic binding iron (OBI) concentration. The presence of CER during anaerobic digestion favored Fe(III) reduction to Fe(II), and then further induced iron phase transformation, leading to the OBI formation from the released organic matters. Meanwhile, higher CER dosage resulted in higher P release efficiency and the maximum efficiency at 4 g CER/g TS was four times than that of the control. The reduction of BD-P, NaOH-P and HCl-P in solid phase contributed most P release into the supernatant. A new two-stage treatment process was further developed to immigrate the OBI formation and improve the carbon recovery efficiency. Through this process, approximately 45% of P was released, and 63% of carbon was recovered as methane from Fe-CEPS sludge via CER pretreatment.

Microstructure Evolution and Phase Transformation of Iron Produced from the Sustainable Carbothermic Reduction Process of High-Pressure Acid Leaching Residue.

This paper describes the microstructure evolution and phase transformation of residue from the lateritic nickel ore high-pressure acid leaching (HPAL) during carbothermic reduction using palm kernel shell (PKS) charcoal as a reductant. The study consisted of thermodynamic calculations combined with analysis of reduction experiments. The thermodynamic assessment predicted that the main phases formed during the reduction process were γ-Fe metal, liquid metal, slag, and spinel, with abundance of reducing gas (CO and H2) at the temperature range of 750-1500 °C. The experimental study shows that the resulting product upon cooling was sponge iron or direct reduced iron when HPAL residue-PKS charcoal composites were reacted up to 1400 °C for 45 min. The sponge iron had an average apparent density of 5.8 ± 0.1 g/cm3 and a 90.8 ± 0.4% metallization degree. The microstructure analysis revealed that as the reduction time was increased, small iron nuggets began forming on the surface of the reduced product. By addition of Na2CO3, the separation of iron nuggets from slag appeared to be improved, hence enhancing the overall reduction process. Furthermore, iron nuggets' highest apparent density and metallization degree were obtained at 7 ± 0.1 g/cm3 and 98 ± 0.5%, respectively, when adding Na2CO3 of 6 wt %. The phase and microstructure analyses also revealed that the iron nuggets comprised coarse pearlite, eutectic cementite, ledeburite, and sulfides. Thus, this study offers alternative sustainable process conditions for simultaneously handling the HPAL residue using PKS waste to produce metallic iron.

Phase-field Modeling and Simulation of Solid-state Phase Transformations in Steels

The phase-field method is used as a powerful and versatile computational method to simulate the microstructural evolution taking place during solid-state phase transformations in iron and steel. This review presents the basic theory of the phase-field method and reviews recent advances in the phase-field modeling and simulation of solid-state phase transformations in iron and steel, with particular attention being paid to the modeling of the austenite-to-ferrite, pearlitic, bainitic, and martensitic transformations. This review elucidates that the phase-field method is a promising computational approach to investigate the microstructural evolutions (e.g., interface migration, solute diffusion, and stress/strain evolutions) that take place during the phase transformations. It also indicates that further improvements are required to enhance the predictive accuracy of the phase-field models developed to date. Finally, this review discusses the critical challenges and perspectives for the further improvement of the phase-field modeling of solid-state phase transformations in steel, i.e., the modeling of heterogeneous nucleation, the abnormal effect of the diffusion interface, and material parameter identification.

The mineral phase reconstruction of copper slag as Fenton-like catalysts for catalytic oxidation of NOx and SO2: Variation in active site and radical formation pathway

Herein, the copper slag through CaO-thermal treatment was used as a heterogeneous catalyst to activate H2O2 into free radicals to remove NOx and SO2. The NOx removal capacity of modified copper slag exhibited a volcano-type tendency with increased calcination temperatures. Because of the iron phase transformation and decrease in surface exposure Cu species by adjusting surface properties, the copper slag delivers a higher NOx capacity and H2O2 utilization rate than raw copper slag and bare H2O2. In addition, the interaction of multi-phases of modified copper slag was illustrated by simulation experiments. Moreover, the possible formation pathway of free radicals in the alkaline medium was proposed based on the electron paramagnetic resonance (EPR) as well as chemical quenching experiments. The surface-bound hydroxyl radical through ≡Fe3+ sites is decisive for free radical formation in alkaline conditions. Our findings shed new light on the feasibility of SO2 and NOx removal by metallurgical slag via mineral phase reconstruction as a novel heterogeneous catalyst.

Allotropy in ultra high strength materials

Allotropic phase transformations may be driven by the application of stresses in many materials; this has been especially well-documented for pressure driven transformations. Recent advances in strengthening materials allow for the application of very large shear stresses as well – opening up vast new regions of stress space. This means that the stress space is six-dimensional (rather than one for pressure) and that phase transformations depend upon crystal/grain orientation. We propose a novel approach for predicting the role of the entire stress tensor on phase transformations in grains of all orientations in any material. This multiscale approach is density functional theory based and guided by nonlinear elasticity. We focus on stress tensor dependent allotropic phase transformations in iron at high pressure and ultra-fine grained nickel and titanium. The results are quantitatively consistent with a range of experimental observations in these disparate systems. This approach enables the balanced design of high strength-high ductility materials.

Mechanisms of phase transformations in iron

Gibbsmolarvolumetricenergyandiron boundary molar energy are determined depending on temperature. It has been shown that phase transformations in iron occur under thermodynamically equilibrium conditions. The iron phase microcrystals are dendrites. Crystallization of the iron melt is a nanostructural process. Iron recrystallization has been shown to be a largely diffuse process.

Magnetic Properties and Washability of Roasted Suspended Siderite Ores.

Steel is one of the most important industrial materials, which mainly comes from the smelting of iron ore. In view of the huge steel consumption every year, the exploitation of vast reserves of siderite ores is significant for improving the self-sufficiency rate of iron ore resources and ensuring the strategic security of the iron and steel industries. This paper investigated the influence of temperature, time, and other parameters on the magnetic properties of roasted siderite ores using the method of suspended roasting and analyzed the washability of roasted ores under weak-magnetic-field conditions using the magnetic separation tube experiment. The findings of the study explained the iron phase transformation process, i.e., FeCO3 was transformed into Fe3O4 by suspension magnetization roasting. Furthermore, the saturation magnetization of the roasted ore increased in due time at a constant temperature range of 550–750 °C and a roasting time of less than 5 s. It also increased with increasing temperature and constant time. The roasted ore achieved the best magnetic characteristics after roasting at 750 °C for 5 s. After low-intensity magnetic separation, the iron grade of the concentrate changed to 55.12%, with a recovery rate of 90.34%. The study results provide a reference for the development and application of siderite suspension magnetization roasting technology.

Extraction and phase transformation of iron in fine-grained complex hematite ore by suspension magnetizing roasting and magnetic separation

Extraction and phase transformation of iron in fine-grained complex hematite ore by suspension magnetizing roasting and magnetic separation

Photoreductive Dissolution of Iron (Hydr)oxides and Its Geochemical Significance

Iron (hydr)oxides are the most abundant metal oxides, which are widespread on Earth’s surface in the major form of micro/nanoparticles. Dissolution of iron (hydr)oxides significantly controls their compositions on Earth’s surface and is a critical step for the global Fe cycling. Photoreductive dissolution of iron (hydr)oxides is recognized as one of the most important process for generating Fe2+ in surface water and is also a common pathway for transforming solar energy into chemical energy. This review article dissects the main characteristics of photoreductive dissolution of iron (hydr)oxides and discusses its geochemical and environmental significance. We categorize the mechanisms for photoreduction of iron (hydr)oxides into three types: reduction by intrinsic photogenerated electrons, reduction by ligand to metal electron transfer (LMCT), and reduction by direct injection of exogenous photoelectrons. The efficiency of photoreductive dissolution is constrained by both the structure of iron (hydr)oxides (e.g., crystal structure and particle size) and environmental conditions (e.g., light, pH, and concurrent chemicals). Therefore, different iron (hydr)oxides may exhibit quite distinctive photoreductive dissolution characteristics because of their unique crystal structures and physicochemical properties. Iron (hydr)oxides with low crystallinity (e.g., ferrihydrite, lepidocrocite) are subject to direct photoreductive dissolution, while those with high crystallinity (e.g., goethite, hematite) generally need ligands to proceed with photoreductive dissolution. The photoreductive dissolution of iron (hydr)oxides is involved in many important geochemical and environmental processes, such as the Fe availability to primary producers, the generation of reactive oxygen species, the transportation and fate of contaminants, and the phase transformation of iron (hydr)oxides. Given the ubiquitous occurrence of photoreductive dissolution of iron (hydr)oxides, this review will advance our understanding of the role of this process in Earth’s surface environments.

H3PO4 activation mediated the iron phase transformation and enhanced the removal of bisphenol A on iron carbide-loaded activated biochar

Zero valent iron-loaded biochar (Fe0-BC) has shown promise for the removal of various organic pollutants, but is restricted by reduced specific surface area, low utilization efficiency and limited production of reactive oxygen species (ROS). In this study, iron carbide-loaded activated biochar (Fe3C-AB) with a high surface area was synthesized through the pyrolysis of H3PO4 activated biochar with Fe(NO3)3, tested for removing bisphenol A (BPA) and elucidated the adsorption and degradation mechanisms. As a result, H3PO4 activated biochar was beneficial for the transformation of Fe0 to Fe3C. Fe3C-AB exhibited a significantly higher removal rate and removal capacity for BPA than that of Fe0-BC within a wide pH range of 5.0–11.0, and its performance was maintained even under extremely high salinity and different water sources. Moreover, X-ray photoelectron spectra and density functional theory calculations confirmed that hydrogen bonds were formed between the COOH groups and BPA. 1O2 was the major reactive species, constituting 37.0% of the removal efficiency in the degradation of BPA by Fe3C-AB. Density functional reactivity theory showed that degradation pathway 2 of BPA was preferentially attacked by ROS. Thus, Fe3C-AB with low cost and excellent recycling performance could be an alternative candidate for the efficient removal of contaminants.

Firing Increases Arsenic Leaching from Ceramic Water Filters via Arsenic and Iron Phase Transformations.

Ceramic water filters (CWFs) are produced globally using local clay sources and can effectively remove bacterial pathogens during point-of-use water treatment. The ceramic production process involves firing clay mixed with burnout material at temperatures of 800-1100 °C, which induces mineralogical changes leading to increased arsenic (As) leaching from CWF material compared to source clay. Unfired clay and fired CWFs from Cambodia, Canada, and Mexico, CWF from Laos, and test-fired clay from the United States were analyzed to determine the extent of As leaching from CWFs that range in As (<1 to 16 mg kg-1) and iron (Fe) (0.6 to 5%) content. Deionized water, NaOH, HCl, and oxalate extractions showed that firing increased As solubility and decreased Fe solubility compared to unfired clay, with up to 8 mg kg-1 of water-soluble As in Cambodian CWFs. X-ray absorption spectra of the Cambodian clay and CWF showed a decrease in the Fe-O distance from 2.01 to 1.91 Å and decreased Fe coordination number from 6.3 to 4.6 after firing, indicating a decrease in Fe-O coordination. Arsenic(V) was the dominant species in Cambodia clay and CWF, existing primarily as a surface complex with average As-Fe distance of 3.28 Å in clay while in CWF As was either an outer-sphere As(V) phase or a discrete arsenate phase with no significant As-Fe scattering contribution within the resolution of the data. Improved understanding of molecular-scale processes that cause increased As leaching from CWFs provides a basis for assessing As leaching potential prior to CWF factory capital investment as well as engineered solutions (e.g., modified firing temperature, material amendments, and leaching prior to distribution) to mitigate As exposure from CWFs.

Effects of vacancies on plasticity and phase transformation in single-crystal iron under shock loading

A characteristic region with vacancy concentration ranging from 0% to 2% was introduced into the single-crystal iron to investigate the effects of vacancies on plasticity and phase transformation of single-crystal iron under shock loading. The simulations were implemented by applying non-equilibrium molecular dynamics simulations with an excellent modified analytic embedded-atom method (MAEAM) potential. A fixed piston velocity of vp = 0.5 km/s was applied in our simulations, under which no plasticity or phase transformation occurred in the perfect single-crystal iron based on the description of the used MAEAM potential. The plasticity and phase transformation in iron were observably influenced by the vacancies as shown in this work. Significant anisotropy of shock response was distinctly exhibited. The nucleation and growth of dislocation loops emitting from the vacancy region were clearly observed in the sample that was shocked along the [110] direction, and the activated slip systems were determined as (112¯)[111] and (112)[111¯]. The vacancies and the vacancies-induced dislocation loops provided preferential nucleation positions for the subsequent phase transformation, which resulted in the phenomenon that the phase transformation product (HCP phase) always preferentially appeared in the vacancy region. The influences of different vacancy concentrations on plasticity and phase transformation were also discussed.

Role of interface morphology on the martensitic transformation in pure Fe

Using classical molecular dynamics simulations, we study austenite to ferrite phase transformation in iron, focusing on the role of interface morphology. We compare two different morphologies; a flat interface in which the two phases are joined according to Nishiyama–Wasserman orientation relationship vs. a ledged one, having steps similar to the vicinal surface. We identify the atomic displacements along a misfit dislocation network at the interface leading to the phase transformation. In the case of the ledged interface, stacking faults are nucleated at the steps, which hinder the interface motion, leading to lower mobility of the inter-phase boundary, than that of a flat interface. Interestingly, we also find the temperature dependence of the interface mobility to show opposite trends in the case of flat vs. ledged boundary. We believe that our study is going to present a unified and comprehensive view of martensitic transformation in iron with different interface morphologies, which is lacking at present, as flat and ledged interfaces are treated separately in the existing literature.

Body-centered-cubic to face-centered-cubic phase transformation of iron under compressive loading along [100] direction

Phase transformation of iron nanoplate and bulk under compressive loading along [100] direction is investigated by using the molecular dynamic simulation method. The simulation results show that the iron nanoplate undergoes body-centered-cubic to face-centered-cubic phase transformation; while iron bulk undergoes body-centered-cubic to hexagonal close-packed phase transformation. The two different phase transformation modes can be attributed to the free surface-induced stress effect of the nanoplate. By applying additional lateral stress, the iron bulk also transforms into face-centered-cubic phase. Moreover, the body-centered-cubic to face-centered-cubic phase transformation is further studied in term of elastic stability theory; it is found that the theoretical results are consistent with the simulation results very well. Therefore, it can be concluded that body-centered-cubic to face-centered-cubic phase transformation can be realized under both tensile and compressive loading along [100] direction.

Influence of hydrogen and carbon monoxide on reduction behavior of iron oxide at high temperature: Effect on reduction gas concentrations

The purposes of this study are to reduce Fe2O3 using hydrogen (H2) and carbon monoxide (CO) gases at a high temperature zone (500 °C–900 °C) by focusing on the influence of reduction gas concentrations. Reduction behavior of hematite (Fe2O3) at high temperature was examined using temperature programmed reduction (TPR) under non-isothermal conditions with the presence of 10% H2/N2, 20% H2/N2, 10% CO/N2, 20% CO/N2 and 40% CO/N2. The TPRCO results indicated that the first and second reduction peaks of Fe2O3 at a temperature below 660 °C appeared rapidly when compared to TPRH2. However, TPRH2 exhibited a better reduction in which Fe2O3 entirely reduced to Fe at temperature 800 °C (20% H2) without any remaining of wustite (FeO) whereas a temperature above 900 °C is needed for a complete reduction in 10% H2/N2, 10% and 20% CO/N2. Furthermore, the reduction of hematite could be improved when increasing CO and H2 concentrations since reduction profiles were shifted to a lower temperature. Thermodynamic calculation has shown that enthalpy change of reaction (ΔHr) for all phases transformation in CO atmosphere is significantly lower than in H2. This disclosed that CO is the best reductant as it is a more exothermic, more spontaneous reaction and able to initiate the reduction at a much lower temperature than H2 atmosphere. Nevertheless, the reduction of hematite using CO completed at a temperature slightly higher compared to H2. It is due to the presence of an additional carburization reaction which is a phase transformation of wustite to iron carbide (FeO → Fe3C). Carburization started at the end of the second stage reduction at 600 °C and 630 °C under 20% and 40% CO, respectively. Therefore, reduction by CO encouraged the formation of carbide, slower the reduction and completed at high temperature. XRD analysis disclosed the formation of austenite during the final stage of a reduction under further exposure with high CO concentration. Overall, less energy consumption needed during the first and second stages of reduction by CO, the formation of iron carbide and austenite were enhanced with the presence of higher CO concentration. Meanwhile, H2 has stimulated the formation of pure metallic iron (Fe), completed the reduction faster, considered as the strongest reducing agent than CO and these are effective at a higher temperature. Proposed iron phase transformation under different reducing agent concentrations are as followed: (a) 10% H2, 20% H2 and 10% C; Fe2O3 → Fe3O4 → FeO → Fe, (b) 20% CO; Fe2O3 → Fe3O4 → FeO → Fe3C → Fe and (c) 40% CO; Fe2O3 → Fe3O4 → FeO → Fe3C → Fe → F,C (austenite).

Shock-induced plasticity in nanocrystalline iron: Large-scale molecular dynamics simulations

Large-scale nonequilibrium molecular dynamics (MD) simulations of shock waves in nanocrystalline iron show evidence of plasticity before the polymorphic transformation takes place. The atomistic structure in the shock direction shows an elastic precursor, plastic deformation, and shock-induced phase transformation from bcc to hcp iron. In this Rapid Communication, large-scale MD models show that the shock response of iron is highly related to the ramp time of the applied shocks. For long ramp times we observe significant plastic relaxation and formation of microstructure defects. Pressure-induced phase transformations in iron are accompanied by stress relaxation achieving almost fully relaxed three-dimensional hydrostatic final states. The evolution of the stress relaxation is in agreement with theory and experiments. Analysis of the x-ray diffraction patterns calculated from the atomistic structure using the Debye equation revealed pronounced anisotropy of the line broadening that is caused by stacking faults in hcp Fe and by dislocations in bcc Fe.

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.