Iron Segregation Research Articles

Topological grain boundary segregation transitions.

Engineering the structure of grain boundaries (GBs) by solute segregation is a promising strategy to tailor the properties of polycrystalline materials. Solute segregation triggering phase transitions at GBs has been suggested theoretically to offer different pathways to design interfaces, but an understanding of their intrinsic atomistic nature is missing. We combined atomic resolution electron microscopy and atomistic simulations to discover that iron segregation to GBs in titanium stabilizes icosahedral units ("cages") that form robust building blocks of distinct GB phases. Owing to their five-fold symmetry, the iron cages cluster and assemble into hierarchical GB phases characterized by a different number and arrangement of the constituent icosahedral units. Our advanced GB structure prediction algorithms and atomistic simulations validate the stability of these observed phases and the high excess of iron at the GB that is accommodated by the phase transitions.

Enhanced DC and AC Soft Magnetic Properties of Fe-Co-Ni-Al-Si High-Entropy Alloys via Texture and Iron Segregation

The microstructure and soft magnetic properties under direct current (DC) mode and alternating current (AC) mode of FeCoNiAl1−xSix (x = 0.2, 0.4, 0.6) high-entropy alloys (HEAs) are investigated. All the studied HEAs show body-centered cubic (BCC) structures, and the [100] texture is formed in the x = 0.4 HEA. The iron (Fe) segregation at the grain boundaries is helpful in increasing the soft magnetic properties under DC. The FeCoNiAl0.6Si0.4 (x = 0.4) HEA exhibits optimal DC and AC soft magnetic properties, primarily due to the formation of the texture along the easy magnetization axis. The x = 0.4 HEA shows the highest permeability (μi = 344 and μm = 1334) and the smallest coercivity (Hc = 51 A/m), remanence (Br = 132 mT), and hysteresis loss (Pu = 205 J/m3). In comparison to the x = 0.2 HEA and x = 0.6 HEA, the total loss (AC Ps) at 50 Hz of the x = 0.4 HEA is decreased by 15% and 18%, and it is reduced at 950 Hz by 13% and 7%. Our findings can provide a useful approach for developing novel HEAs with increased soft magnetic properties by tuning ferromagnetic elemental segregation and forming the texture along the easy magnetization axis.

Chemical and mineralogical constitution of redoximorphic features and mechanism of formation of Plinthosols from the Araguaia River plain, Brazil

ABSTRACT Currently in Brazil, large grain cultivation projects on Plinthosols are a reality, however, there is little or no knowledge of the real mechanism of formation of the plinthite feature, in addition to what is reported in the literature as being a product of oxidation-reduction processes of iron element. This study evaluates iron redoximorphic features and investigates their chemical and mineralogical composition in two profiles of Plinthosols from the Araguaia River plain (P1 and P2). The study strengthens the understanding of the pedogenetic processes involved in the formation of mottles and plinthite. In this sense, it assesses whether the formation mechanisms corroborate the literature. Soil features were sampled in the upper right and left position at the initial plinthic horizon, upper right and left position at the main plinthic horizon, and lower right position at the base horizon of the plinthite zone in the profile. Separated samples comprising the soil matrix, mottles, and plinthite under natural moisture conditions were ground into powder form for chemical determinations by X-ray fluorescence (XRF), sulfuric acid attack (H 2 SO 4 ), sodium dithionite-citrate-bicarbonate (DCB), and ammonium acid oxalate; and mineralogical determinations by X-ray diffraction. Iron contents in all determined forms were always higher in the plinthite feature, intermediate in the mottle feature, and lower in the soil matrix feature. Most of the Fe in all redoximorphic features is included in the structure of primary minerals and their derivatives (vermiculite, illite, and VHEs). Only part of the iron present (about 35.40 % in P1 and 41.98 % in P2) is detected in the form of oxides such as goethite and hematite, which could be formed in redox processes. The mottle and plinthite features under study are not the product of the classic process of segregation, mobilization, and accumulation of iron as a consequence of redox processes. These features were formed or emerged as a result of a relatively slow and constant weathering process of their source material, which is gradually decomposed in an aqueous medium, releasing most of its components. These components include iron and more mobile elements such as bases and silicon, which leave the system through drainage water, and of which a small part may eventually recombine to form new less complex minerals such as kaolinite and oxides.

Grain boundary segregation in iron doped strontium titanate: From dilute to concentrated solid solutions

Strontium titanate, a perovskite oxide, is a frequently studied material for a large variety of applications. When acceptor-doped (with Fe, for example), the material is useful for its mixed oxygen and electronic conductivity, with potential use in oxygen transport membranes or as a cathode for solid oxide fuel cells. A barrier to conductivity in perovskites is the presence of space charge regions at the grain boundaries, which form due to the segregation of charged point defects. Typically, space charge theory assumes bulk dopant concentrations beneath the dilute limit, however concentrated solid-solutions are often utilized in applications. The current work aims to address this disparity: grain boundary segregation in strontium ferrite-strontium titanate solid-solutions is analyzed at three compositions, with Fe contents ranging from near the dilute limit to well above the dilute limit (Fe contents of 2 %, 5 %, and 25 % on the B-site of the perovskite). Electrochemical impedance spectroscopy shows an increase in material conductivity as Fe is added. High-resolution STEM imaging and spectral mapping is utilized, showing that Fe segregates to the grain boundary core, contrary to what is expected from space charge theory. As the Fe content is increased, the amount of Fe segregated to the boundary increases significantly, but the segregation width of Fe at the boundary remains consistent.

Effect of Iron Content on the Pitting Corrosion Behavior of Laser-Cladded Ni-Cr-Mo Alloy Coating in a Simulated Seawater Environment

In order to study the effect of iron content on the pitting corrosion behavior of a Ni-Cr-Mo alloy coating in a simulated seawater environment, a Ni-Cr-Mo-xFe (x = 0, 5, 10, 15, 20, 25) alloy coating was prepared through laser cladding technology. These coatings primarily consist of a γ-Ni solid solution phase, with observable iron segregation in the interdendritic regions when the iron content reaches 25 wt%. After 42 days of salt spray corrosion, it was found that pitting began to appear on the surface when the iron content in the coating increased to 10 wt%. The results of electrochemical behavior revealed that the coatings with iron contents in a range of 10–25 wt% exhibited metastable pitting characteristics, and the impedance modulus decreased with the increase in iron content. Pitting corrosion occurs due to selective corrosion of the dendritic regions. When the iron content exceeds 10 wt%, the accumulation of iron in the outer layer of the passivation film would lead to an excess of cationic vacancies, and the stability of the passive film is then reduced. This study provides a reference for the control of the iron content in a Ni-Cr-Mo alloy coating when applied in marine environments.

Computational Analysis of Grain Boundary Segregation in Iron and Steel

Computational Analysis of Grain Boundary Segregation in Iron and Steel

Temperature-induced atypical oxidation in Sm2(Co, Fe, Cu, Zr)17 alloy systems: Dependency on oxygen-induced elemental migration

This paper addresses temperature and/or oxygen-induced chemical segregation of elements in the growth of oxide scale on Sm2(Co, Fe, Cu, Zr)17. Whereas, oxidation at T ≤ 573 K leads to primarily iron and cobalt segregation and subsequently their oxides, that at > 573 K, yields to largely CuO because of copper segregation. Only combination of higher sample temperature and environmental oxygen are found to have driven copper from alloy interior to its surface. Moreover, oxidation mechanism is considerably slowed down at higher temperature and is attributed to semi passivating surface copper oxide formation, impeding passage of oxygen to layers underneath to it.

High reactivity of condensed amorphous silicate and implication for chondrites

Amorphous silicates are abundant in extraterrestrial objects such as interplanetary dust particles and primitive chondrites. They are thought to be formed through condensation and possibly later exposed to thermal processes in the nebula before being accreted within an asteroid and/or comet.We aim to constrain the conditions that prevailed during thermal events in the nebula, through experimental work on the chemical and structural evolution of condensed amorphous silicate.We conducted coupled condensation and heating experiments of Fe-Mg-silicate thin films using the pulsed laser deposition technique. We compared samples condensed at room temperature and annealed in a second step with samples directly condensed on heated substrate, at 450 °C and 700 °C.For both processes, at temperature as low as 450 °C, iron-rich nanoparticles and Mg-rich domains form, evidencing the high reactivity of the condensed amorphous silicate. This reactivity was found to be even higher for the process of condensation on heated substrate. We also evidence the persistence of amorphous silicate up to 700 °C, in spite of the chemical evolution and the demixion into MgO and SiO2 domains.These results imply that amorphous silicates condensed from a plasma (and possibly from any process producing atoms in an excited state) are more reactive than quenched glasses of similar composition. In complement to high temperature events that occurred at the time of solar system formation and that formed chondrules for instance, this work emphasizes the importance of mild heating on dust evolution before accretion within parent(s) body(ies). It helps to place chemical and structural constraints on the thermal evolution of amorphous silicate found in primitive chondrites: i) iron segregation as metallic nanoparticles can be generated within a silicate groundmass at temperature as low as 450 °C (and possibly even below) ii) iron-rich chondritic amorphous silicate can persist up to 700 °C.

Oxyanion Engineering Suppressed Iron Segregation in Nickel-Iron Catalysts Toward Stable Water Oxidation.

Nickel-iron catalysts represent an appealing platform for electrocatalytic oxygen evolution reaction (OER) in alkaline media because of their high adjustability in components and activity. However, their long-term stabilities under high current density still remain unsatisfactory due to undesirable Fe segregation. Herein, a nitrate ion (NO3 - ) tailored strategy is developed to mitigate Fe segregation, and thereby improve the OER stability of nickel-iron catalyst. X-ray absorption spectroscopy combined with theoretical calculations indicate that introducing Ni3 (NO3 )2 (OH)4 with stable NO3 - in the lattice is conducive to constructing the stable interface of FeOOH/Ni3 (NO3 )2 (OH)4 via the strong interaction between Fe and incorporated NO3 - . Time of flight secondary ion mass spectrometry and wavelet transformation analysis demonstrate that the NO3 - tailored nickel-iron catalyst greatly alleviates Fe segregation, exhibiting a considerably enhanced long-term stability with a six-fold improvement over FeOOH/Ni(OH)2 without NO3 - modification. This work represents a momentous step toward regulating Fe segregation for stabilizing the catalytic performances of nickel-iron catalysts.

Crystallite Phase Effect on the Redox Reactivity of Layered Double Hydroxide-Derived Iron-Based Oxygen Carriers for Chemical Looping CO2 Reduction

Layered double hydroxide (LDH) is an effective self-template to develop efficient iron-based oxygen carriers for chemical looping CO2 reduction. However, how the synthesis condition and metal ratio affect crystallite phase formation and redox reactivity is still unclear. In this work, a series of iron-based oxygen carriers are synthesized from thermal transformation of LDHs with varying metal ratios and calcination temperatures to reveal the crystallite phase formation mechanism and interpret the phase–activity relationship. Spinel MgAlδFe2−δO4 is formed below 800 °C, while MgFeδAl2−δO4 is transformed into MgAlδFe2−δO4 with increasing iron content at 800 °C. During H2–CO2 redox cycles, Fe3+/Fe2+ transition and iron segregation are observed for MgAlδFe2−δO4, but only the former occurs for MgFeδAl2−δO4. For a phase mixture, with a low MgAlδFe2−δO4 amount, Al migrates and substitutes Fe with the reincorporation of segregated iron to generate MgFeδAl2−δO4. MgAlδFe2−δO4 achieves higher oxygen storage capacity and weaker stability than MgFeδAl2−δO4 because the former favors the creation of more oxygen vacancies and induce a larger crystallite size. Due to the crystallite phase effect, Fe0.5Mg2Al0.5 exhibits highest redox rates (6 mol[O]·kgFe–1·min–1 and 10 mol[O]·kgFe–1·min–1 for the release and uptake of oxygen, respectively) and CO space-time yield (0.46 molCO·kgFe–1·s–1). These findings provide a new route to tailor efficient oxygen carriers for chemical looping applications.

Highly aligned growth of carbon nanotube forests with in-situ catalyst generation: A route to multifunctional basalt fibres

Hierarchical fabrics are gaining ever-growing practical interest, thanks to the possibility of combining different properties into a single textile, particularly useful for applications such as high-performance composites showing in situ structural health monitoring capabilities. In this study, homogeneously aligned carbon nanotube forests are directly grown on basalt fabrics by a fast (∼15 min) chemical vapour deposition process without any external catalyst addition providing, for the first time, a highly dense coverage of the underlying substrate. It is demonstrated by transmission electron microscopy that the direct growth of nanotubes on basalt fibres is catalysed by the microstructural segregation of ferrous iron and its subsequent reduction in hydrogen atmosphere to nanocrystalline metallic iron. It is shown that in situ growth requires a pre-treatment etching which can be conducted in mild basic or acidic conditions, achieving optimum performance with an alkaline attack, without compromising the tensile strength of the fibres. Post-growth Raman analysis shows characteristic features associated with high graphitic ordering which turn an intrinsically insulating substrate into an electrically conductive (∼260 S/m) fabric, thus suggesting the possibility to employ the newly engineered hierarchical fabric in all those applications where the combination of good mechanical properties and electrical conductivity is a key requirement.

Microstructure, grain boundary evolution and anisotropic Fe segregation in (0001) textured Ti thin films

The structure and chemistry of grain boundaries (GBs) are crucial in determining polycrystalline materials’ properties. Faceting and solute segregation to minimize the GB energy is a commonly observed phenomenon. In this paper, a deposition process to obtain pure tilt GBs in titanium (Ti) thin films is presented. By increasing the power density, a transition from polycrystalline film growth to a maze bicrystalline Ti film on SrTiO3 (001) substrate is triggered. All the GBs in the bicrystalline thin film are characterized to be Σ13 [0001] coincident site lattice (CSL) boundaries. The GB planes are seen to distinctly facet into symmetric {7¯520} and {134¯0} and asymmetric {101¯0} // {112¯0} segments of 20–50 nm length. Additionally, EDS reveals preferential segregation of iron (Fe) in every symmetric {7¯520} segment. Both the faceting and the segregation are explained by a difference in the CSL density between the facet planes. Furthermore, in the GB plane containing Fe segregation, atom probe tomography is used to experimentally determine the GB excess solute to be 1.25 atoms/nm2. In summary, the study reveals for the first time a methodology to obtain bicrystalline Ti thin films with strong faceting and an anisotropy in Fe segregation behaviour within the neighbouring GB facets.

Atomistic investigation on grain boundary effect on helium segregation and clustering in iron

Molecular dynamics simulation was applied to study the behavior of helium segregation and clustering near different grain boundaries (GBs) in α-Fe. The 〈110〉 and 〈100〉 family symmetric tilt grain boundaries (STGBs) covering a wide range of GB energy and misorientation angle were investigated. The present results indicate that the major factor of GB characteristic effects on helium behavior at the grain boundary are the GB energy and structure. Generally, the proportion of helium that is bound to the grain boundary is higher for high-energy grain boundaries than for low-energy grain boundaries. However, the three grain boundaries with the highest energy lose their original structure units and become disordered under the combined effects of high temperature and helium bubble-induced stress field, forming a diffusion “barrier” for He interstitials to enter the GB plane, thus lead to the slight decrease of the proportion of segregated helium. Four typical He migration patterns at different grain boundaries are found, which depends on the GB structure and energy. In general, with the increase of GB energy, the average diameter of He-V clusters at the GB becomes smaller, and the cluster density becomes larger.

Corrosion Resistance of L120G13 Steel Castings Zone-Reinforced with Al2O3.

The aim of the study was to determine the effect of zone reinforcement of cast steel L120G13 with Al2O3 macro-particles on the corrosion resistance of the composite obtained in this way. The obtained results allow us to conclude that strengthening of cast steel with corundum, the aim of which was to significantly increase the abrasive wear resistance, did not significantly deteriorate corrosion resistance. SEM tests show that a permanent diffusion layer interface is formed at the boundary between cast steel and corundum. In this area, simple manganese segregation and reverse iron and chromium segregation were found. These elements pass from the liquid alloy to the surface layer of the corundum particles, causing the aluminium and oxygen to be pushed deep into the corundum grains. Corrosion tests indicate comparable corrosion resistance of cast steel L120G13 and the composite L120G13 + Al2O3. Moreover, no intergranular corrosion was found in the matrix of the composite and no signs of pitting corrosion were found in the areas of the interface between the phases of the composite. This information is extremely important from the point of view of the material’s service life. Observations of breakthrough of both materials obtained during fracture after potentiodynamic corrosion tests, immediately after freezing in liquid nitrogen, indicate cracking with plastic features and increased resistance to dynamic forces of cast steel L120G13 and the composite L120G13 + Al2O3.

Accurate quantification of phosphorus intergranular segregation in iron by STEM-EDX

This study describes a method to quantify phosphorus grain boundary segregation by Energy Dispersive X-ray Spectroscopy in Scanning Transmission Electron Microscope (STEM-EDX). A “box-type method” is employed, removing the long-discussed problems of interaction volume and the beam broadening effect. The proposed methodology also introduces a novel way of subtracting the spectrum background to remove the influence of coherent Bremsstrahlung and spurious peaks. A Fe-P model alloy was used to compare the box method to the quantification results previously obtained by atom probe tomography on two high angle grain boundaries. The results are specifically reported in surface concentration (atom/nm2) to avoid additional hypotheses and allow the results between the two techniques to be directly compared. The measurements show that the box-type method can accurately measure phosphorus intergranular segregation in iron.

Graphene Blocks Oxidative Segregation of Iron Dissolved in Platinum: A Model Study

AbstractIron–platinum (Fe–Pt) compounds are well known for their interesting magnetic and electrocatalytic properties. However, iron segregation and iron oxides formation under oxidative conditions may influence the characteristics of Fe–Pt systems. Several approaches are used to protect the Fe–Pt compounds from oxidation, the most promising of which involves covering the material with a protective graphitic layer. By performing model‐type ultrahigh vacuum (UHV) studies, it is shown that a layer of epitaxial graphene (Gr) grown on a [111]‐oriented single‐crystal platinum substrate with thermally dissolved iron (Fe–Pt(111) surface alloy) effectively blocks iron segregation and iron oxides formation under oxidative conditions, while still allowing for the adsorption of oxygen atoms underneath the carbon layer. The oxidation is monitored in real time and at the micrometer scale using low energy electron microscopy (LEEM) and local diffraction (μLEED). Notably, a similar result is obtained for a poorly ordered Gr‐like carbon layer grown directly on a Fe–Pt(111) substrate. The findings are rationalized in terms of a locally lowered partial oxygen pressure and inhibited iron oxide growth in a confined space between the carbon layer and the metal support.

Millisecond photonic sintering of iron oxide doped alumina ceramic coatings

The sintering of alumina (Al2O3) traditionally occurs at high temperatures (up to ca. 1700 °C) and in significantly long times (up to several hours), which are required for the consolidation of the material by diffusion processes. Here we investigate the photonic sintering of alumina particles using millisecond flash lamp irradiation with extreme heating rates up to 108 K/min. The limitation of the low visible light absorption of alumina is resolved by adding colored α-Fe2O3 nanoparticles, which initiated the grain growth during sintering. After the millisecond-long light pulses from a xenon flash lamp, a bimodal mixture of α-Al2O3 precursor particles was sintered and iron segregation at the grain boundaries was observed. The proposed photonic sintering approach based on doping with colored centers may be extended to other refractory ceramics with low absorption in the visible light range once appropriate high-absorbing dopants are identified.

Grain-refining and strengthening mechanisms of bulk ultrafine grained CP-Ti processed by L-ECAP and MDF

The microstructural evolution and mechanical properties of ultrafine-grained (UFG) CP-Ti after an innovative large-volume equal channel angular pressing (L-ECAP) and multi-directional forging (MDF) were systematically examined using monotonic tensile tests combined with transmission electron microscope (TEM) and electron backscatter diffraction (EBSD) techniques. Substantially refined and homogeneous microstructures were achieved after L-ECAP (8-pass and 12-pass) and MDF (2-cycle and 3-cycle), respectively, where the grain size distribution conformed to lognormal distribution. The grain refinement of 450 °C L-ECAP is dominated by dynamic recrystallization (DRX) and dynamic recovery (DRV), while that of MDF is dominated by DRX. The iron impurities promote recrystallization by pinning-induced dislocation accumulation so that DRX is prone to occur at iron segregation regions during L-ECAP. The monotonic tensile results show that the strain hardening rate of CP-Ti increases with the decrease of grain size, while the continuous strain hardening ability decreases. The relationship between the average grain size and yield strength is in accordance with Hall-Petch relationship. Meanwhile, the individual strengthening mechanisms were quantitatively examined by the modified model. The results indicate that the strengthening contribution of dislocation accumulation to yield strength is greater than that of grain refinement.

Segregation of nickel, cobalt and iron from limonite: recoveries, fundamentals and in situ reduction comparison

ABSTRACT This paper describes a study of the segregation process in which nickel, cobalt and iron are extracted as volatile metal chlorides from a limonite laterite and collected as ferronickel onto carbon within a single step, through addition of 5% CaCl2 and 5% coke to calcined limonite at 1123 and 1173 K. Segregation of limonite yielded maximum nickel, cobalt and iron recoveries of 78%, 23% and 1.1%, respectively within ferronickel containing 57% nickel. All recoveries were quantified using rate equations. Estimations from evolved gas and recycle analyses revealed that segregation reduction required 40–45% of the stoichiometric hydrogen requirement and <0.1% moisture from calcine. Ferronickel recovery and grades were comparable when using 5% coke for segregation and chloride-free in situ reduction. Comparison of carbon consumption clarified the roles of the Boudouard reaction and the water-gas reaction during segregation. Segregation and in situ reduction (from this and other work) were compared as potential processes for concentration of ferronickel from limonite. Segregation appeared to have significant advantages because at lower temperatures, it produced a larger grained, higher grade ferronickel, more amenable to physical concentration. Cet article décrit une étude du processus de ségrégation dans lequel le nickel, le cobalt et le fer sont extraits sous forme de chlorures métalliques volatiles à partir d'une limonite latéritique et captés sous forme de ferronickel sur du carbone, en une seule étape, par addition de 5% de CaCl2 et 5% de coke à la limonite calcinée à 1123 et 1173 K. La ségrégation de la limonite a produit des récupérations maximales de nickel, de cobalt et de fer de 78%, 23% et 1.1%, respectivement, dans le ferronickel contenant 57% de nickel. On a quantifié toutes les récupérations à l'aide d'équations de vitesse. Les estimations des analyses de gaz dégagé et de recyclage ont révélé que la réduction de la ségrégation nécessitait de 40 à 45% de la demande stœchiométrique d'hydrogène et <0.1% d'humidité du calcinat. La récupération et les teneurs du ferronickel étaient comparables lorsqu'on utilisait 5% de coke pour la ségrégation et une réduction in situ sans chlorure. La comparaison de la consommation de carbone a clarifié les rôles de la réaction de Boudouard et de la réaction eau-gaz lors de la ségrégation. On a comparé la ségrégation et la réduction in situ (à partir de ce travail et d'autres travaux) en tant que processus potentiels de concentration de ferronickel à partir de la limonite. La ségrégation semblait présenter des avantages significatifs parce qu'à des températures inférieures, elle produisait du ferronickel à grain plus gros et de qualité supérieure, plus propice à la concentration physique.

Understanding the Interaction of Potassium Salts with an Ilmenite Oxygen Carrier Under Dry and Wet Conditions.

This study describes how potassium salts representative of those in bio ash affect the reactivity of the oxygen carrier ilmenite under moist and dry conditions. Ilmenite is a bench-mark oxygen carrier for chemical-looping combustion, a technique that can separate CO2 from flue gases with minimal energy penalty. Different potassium salts were mixed with ilmenite to a concentration of 4 wt % potassium. The salts used were K2CO3, K2SO4, KCl, and KH2PO4. Experiments were performed at 850 °C under alternately oxidizing and reducing conditions in a dry atmosphere or in the presence of steam. Analyses of the oxygen carrier regarding changes in reactivity, structure, and composition followed the exposures. This study showed that salts such as K2CO3, K2SO4, and KCl increase the reactivity of the ilmenite. For the samples mixed with KCl, most of the salt was evaporated. KH2PO4 decomposed into KPO3, forming layers around the ilmenite particles that lead to agglomeration. Additionally, the KPO3 layer was more or less nonpermeable for CO and decreased the reactivity toward H2 significantly in both dry and wet conditions. This decreased reactivity indicates that the concentration of phosphorus in biofuel may have a significant effect on oxygen carrier degradation. It was also observed that the presence of steam changed the chemistry drastically for the nonphosphorus-containing salts. Alkali salts may react with steam, forming volatile KOH that evaporates partly. KOH may also form K-titanates by reaction with the oxygen carrier, leading to segregation of iron and titanium phases in the ilmenite.